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In our chapters, you can find interesting and important information about SSL and TLS certificates. You will understand what is a differences between certificates and train to setup and configure your security certificates on your servers.

Many thanks for public sources and Ilya Grigorik from Google (High Performance Browser Networking)

We wish to create a really helpful and interesting section, so we invite you to be our co-authors.

Please, send us your articles to classroom@certdb.com and we will publish most interesting of them under your copyright.

Table of content

An SSL Certificate is a set of files that enable the server to provide "asymmetric encryption" with a connecting computer, device, or other server (known as the "client"). These files include:

Signing Request. A file ending in ".csr" which is presented to a 3rd party and which is used by that 3rd party to generate the "public key".

Private Key: A file ending in ".key" which is installed on the server and which is kept secret and secure.

Public Key: A file ending in ".crt" which is installed on the server, but which is given freely to any connecting device (client). Anyone can have a copy of this file without interfering with the security of the system.

Intermediate Certificates. Additional files provided by the 3rd party that generated and signed your "public key". These are public files and need to be installed on the server so that connecting devices can properly verify your site's identity. If you forget to install this, your users connecting to your web site may get SSL warnings and be wary of trusting your site.

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The "public key" file for your SSL certificate contains several important pieces of information:
  • Information for encrypting data
  • Information about your domain name and possibly also your organization
  • A date range where the certificate is valid (e.g. when it expires)
  • Information about what organization (if any) "signed" your public key.
  • The "private key" file for your SSL certificate contains only information for encrypting data - it does not expire or have any details regarding your organization or domain name. The "signing request" contains all of the same information as the "public key" except for information about who has signed it. What is a "self-signed" certificate and when would you want one?
    A "self-signed" certificate is one in which you yourself have "signed" the certificate (e.g. when you or your hosting company created it). Self-signed certificates are free and anyone can make them with any information in them regarding organization and domain name. E.g. you could make one yourself that is for the computers of the NSA (National Security Agency). However, anyone who connects to a server using a self-signed certificate will immediately get a significant warning message in their connecting program (e.g. web browser, email program, etc.) that the server it is connecting to is not trusted and that you should proceed with extreme caution.
    Self-signed certificates are typically used only for:
  • Services hosted inside of your own corporate network where you control who accesses the server and the server itself. E.g. your staff may trust your own certificates in that case.
  • Public services where the users care only about the fact that the data is encrypted and which do not care who is on the other end of the data stream (this is rare).
  • Everyone else wants a "signed" certificate.

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    A company that signs your certificate must first verify your right to the certificate in question. Then, they add stuff to the certificate that allows others to see that they indeed have verified your ownership of and right to use this certificate. What does that mean?
    They check that the domain name in the certificate is actually owned by you and the people in charge of the domain approve the creation of this SSL.
    If there is information about your organization (e.g. your company name) in the certificate, then this must also be verified. People in charge of this company must approve the certificate. So
  • You can't get a certificate signed for domains that you do not have control over
  • You can't get an organization's name in a certificate unless you have permission for that
  • You'll never get that "NSA" self-signed certificate to be signed by a trusted third party (unless you happen to work for the NSA, that is).
  • The expense of the certificate covers:
  • The time and effort needed to verify
  • The more things that must be verified and the better job a third party does, the higher the cost and the higher the trust of the result

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    An "EV" certificate is an "Extended Validation" certificate.
    For these, the 3rd party signer goes through a significant effort to verify the company and domain in the certificate. Visiting a site that has an EV certificate generates a high degree of trust that the site is really the one you are trying to connect to (and not some fraudster). As a result, most web browsers identify this with a green address bar.

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    Standard SSL Certificates are issued by an Certificate Authority (CA) after the CA performs some basic standard validation on the identity of the certificate request to ensure that the certificate is not issued to "the wrong hands".
    The types of validation performed for standard SSL certificates vary by the type and cost of the certificate, but include:
  • A confirmation email message sent to the domain administrator as specified in the domain’s entry in the WHOIS database
  • A confirmation email message sent to a standard administrative email address at the domain itself, such as "admin@domain.com".
  • The name of the organization owning the domain name may be validated.
    You should purchase SSL Certificates that use the above forms of validation in order to:
  • Provide excellent encryption
  • Demonstrate a level of trust to the end user based on the amount of validation performed and the reputation of the Certificate Authority

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    In order to make it more clear to end users that a site is legitimate, Extended Validation certificates were introduced. These:
    Require the Certificate Authority to perform a much more detailed validation of the request and the requesting people.
    Make it visually obvious to the end user that an Extended Validation SSL Certificate is in use.
    The net result is that the end user can differentiate sites that use these more trusted certificates and thus
  • Feel confident in the quality of the SSL certificate
  • Be sure that they are viewing the desired web site and not a phishing site trying to trick the end user
  • The end user does not need any technical knowledge to feel "more secure".

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    Certificate Authorities issuing Extended Validation SSL Certificates must:
  • Establish the legal identity as well as the operational and physical presence (i.e. address) of the website owner;
  • Establish that the applicant is the domain name owner or has exclusive control over the domain name; and
  • Confirm the identity and authority of the individuals acting for the website owner, and that documents pertaining to legal obligations are signed by an authorized officer.
  • These validation checks require that the CA:
  • Look up details of the organization in several online databases.
  • Contact the applicant via email and phone to ask questions and validate the answers.
  • Have the applicant verify his/her identity and authority in the organization to request the certificate.
  • Have the applicant provide contact information for Human Resources in the organization to verify the applicant's authority, if the applicant is not already a senior staff member.
  • Have the applicant FAX back written confirmations.
  • Optionally perform other validations.
  • In the end, the CA will know for sure that the request originated from the organization owning the domain and from an individual authorized to request the certificate. This is far more detail that is afforded by other types of SSL certificate validation. As a result, end users can be much more sure that a site using an EV SSL Certificate is the desired site and not a phishing site. They can also know that the site they are visiting takes security seriously.

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    When visiting a web site that is using an Extended Validation SSL Certificate, the "address bar" of the web browser will clearly indicate that by:
    Displaying the full name of the organization owning the SSL certificate in green. If you mouse over or click on the name of the organization, the name of the Certificate Authority issuing the certificate will be displayed.
    When an end user goes to a secure site that is using an EV certificate, they will see "green" in the address bar. End users then associate "green sites" with "very secure sites". Simple, clear, concise, no technical knowledge needed.

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    Extended Validation SSL Certificates cost more than other types of certificates - significantly more. The reason is clear - it takes considerably more work on the part of the Certificate Authority to perform all of the required validation steps.
    If your budget allows, an EV SSL Certificate may be good investment. Your decision depends upon how much your organization or website depends on the image of security it portrays to its users.
    If it is very important that users feel safe and secure when visiting your site, (e.g. financial institutions, tax preparers, legal and medical firms), you should take steps to protect them from any kind of phishing attack. Extended Validation SSL Certificates offer a visible acknowledgment that you value your end users' privacy and security on your website. If you need SSL for basic security and your users do not pay much attention to the degree of trust or the possibility of phishing, or if cost is a constraint, then a standard SSL certificate, from a reputable CA, is acceptable.

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    The way that SSL (and TLS) works, the establishment of a secured communication channel comes first, before any information is sent between your computer and the server. When establishing this secure channel, your computer connects to the Internet address (IP Address) of the server and "does the encryption stuff". Telling the server what domain name you are trying to get to is not part of this process.
    So, the server cannot choose what SSL certificate to present to you based on what site you are trying to connect to…. it has to give the same certificate every time for every connection … the certificate configured for the IP address. Thus, every SSL certificate needs to reside on a different IP address. When a web browser connects to your dedicated IP address to access your web site securely, it will absolutely be given your SSL certificate. If the wrong certificate was presented, then the web browser would complain that the certificate received doesn’t match the domain name it is trying to connect to. The same process applies to connecting to email servers over SSL and any other service over SSL.

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    A wild card certificate covers a wide range of domains. E.g., the certificate for "*.yourdomain.com" covers all direct subdomains of "yourdomain.com". For example, it covers "smtp-5.yourdomain.com" and "webmail.yourdomain.com" and "www.yourdomain.com".
    However, such a wild card certificate does not cover plain old "yourdomain.com". It also does not cover lower-level subdomains like "a.b.yourdomain.com". The benefits of a wild card certificate are:
  • You only need to buy 1 SSL Certificate to cover a large number of similar domains
  • You only need 1 IP address for that certificate
  • While wild card certificates are more expensive, in general, they are a good value if you have many domains to protect.

    Can be use the same SSL Certificate for different things like email and web sites?
    That depends on if those services are located on the same server (IP Address). If your domain was "secure.com" and you wanted to use this for both secure email (e.g. POP and IMAP) and your web site, then these things would generally need to be served on the same server. So that the IP address of your web site and your email is the same and thus you can configure the address of "secure.com" with that address.

    If your mail and web sites were on different servers or IP addresses, then you would not be able to setup your domain "secure.com" so that it goes to one place for web and another place for email (without special hardware firewalls and other types of traffic redirecting rules that are not generally in place in common configurations).

    If you have your email on one server, your web sites on another server, and your WebMail on a third set of servers - this is for security and performance reasons. So, if you wanted to secure all three things with your own branded domain name and SSL, you would need up to 3 certificates (or 1 wild card certificate) - one for each server. Dedicated servers can have all of your services located on your single machine and can use 1 regular certificate for all of them.

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    Merely adding an SSL certificate to a site does not make the site secure.
    Once you have an SSL Certificate for your site you need to ensure that web pages that require security are only accessed over SSL (e.g. you need to link to them with https:// and not http:// links). You may also want to construct your site so that secure pages cannot be accessed via insecure links (e.g. http://).

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    It is a common misconception that ".shtml" web pages are "Secure HTML". This is not so. The "S" stands for "Server-Parsed" and is related to server-side includes. This has no bearing on security of encryption and does not mean that the page is secure.
    The page is only secure if the URL in the address bar starts with "https://" and your "lock icon" in your browser is closed (locked). Also look to be sure that the lock icon does not have a warning sign or "X" on it and that the address bar is not "red" as these can indicate that there is either some problem with your SSL certificate (like it expired) or that there are items (like images or JavaScript) in your web page that are being loaded without SSL and thus defeating some of the reasons for having the page secured in the first place.

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    The SSL protocol was originally developed at Netscape to enable ecommerce transaction security on the Web, which required encryption to protect customers' personal data, as well as authentication and integrity guarantees to ensure a safe transaction.
    To achieve this, the SSL protocol was implemented at the application layer, directly on top of TCP (Figure 1), enabling protocols above it (HTTP, email, instant messaging, and many others) to operate unchanged while providing communication security when communicating across the network. When SSL is used correctly, a third-party observer can only infer the connection endpoints, type of encryption, as well as the frequency and an approximate amount of data sent, but cannot read or modify any of the actual data.
    Figure 1. Transport Layer Security (TLS)

    When the SSL protocol was standardized by the IETF, it was renamed to Transport Layer Security (TLS). Many use the TLS and SSL names interchangeably, but technically, they are different, since each describes a different version of the protocol. SSL 2.0 was the first publicly released version of the protocol, but it was quickly replaced by SSL 3.0 due to a number of discovered security flaws. Because the SSL protocol was proprietary to Netscape, the IETF formed an effort to standardize the protocol, resulting in RFC 2246, which was published in January 1999 and became known as TLS 1.0. Since then, the IETF has continued iterating on the protocol to address security flaws, as well as to extend its capabilities: TLS 1.1 (RFC 4346) was published in April 2006, TLS 1.2 (RFC 5246) in August 2008, and now TLS 1.3 was drafted in March 2018. That said, don't let the abundance of versions numbers mislead you: your servers should always prefer and negotiate the latest stable version of the TLS protocol to ensure the best security, capability, and performance guarantees. In fact, some performance-critical features, such as HTTP/2, explicitly require the use of TLS 1.2 or higher and will abort the connection otherwise. Good security and performance go hand in hand. TLS was designed to operate on top of a reliable transport protocol such as TCP. However, it has also been adapted to run over datagram protocols such as UDP. The Datagram Transport Layer Security (DTLS) protocol, defined in RFC 6347, is based on the TLS protocol and is able to provide similar security guarantees while preserving the datagram delivery model.

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    The TLS protocol is designed to provide three essential services to all applications running above it: encryption, authentication, and data integrity. Technically, you are not required to use all three in every situation. You may decide to accept a certificate without validating its authenticity, but you should be well aware of the security risks and implications of doing so. In practice, a secure web application will leverage all three services.
    Encryption
    A mechanism to obfuscate what is sent from one host to another.
    Authentication
    A mechanism to verify the validity of provided identification material.
    Integrity
    A mechanism to detect message tampering and forgery.

    In order to establish a cryptographically secure data channel, the connection peers must agree on which ciphersuites will be used and the keys used to encrypt the data. The TLS protocol specifies a well-defined handshake sequence to perform this exchange, which we will examine in detail in TLS Handshake. The ingenious part of this handshake, and the reason TLS works in practice, is due to its use of public key cryptography (also known as asymmetric key cryptography), which allows the peers to negotiate a shared secret key without having to establish any prior knowledge of each other, and to do so over an unencrypted channel. As part of the TLS handshake, the protocol also allows both peers to authenticate their identity. When used in the browser, this authentication mechanism allows the client to verify that the server is who it claims to be (e.g., your bank) and not someone simply pretending to be the destination by spoofing its name or IP address. This verification is based on the established chain of trust - see Chain of Trust and Certificate Authorities. In addition, the server can also optionally verify the identity of the client - e.g., a company proxy server can authenticate all employees, each of whom could have their own unique certificate signed by the company.

    Finally, with encryption and authentication in place, the TLS protocol also provides its own message framing mechanism and signs each message with a message authentication code (MAC). The MAC algorithm is a one-way cryptographic hash function (effectively a checksum), the keys to which are negotiated by both connection peers. Whenever a TLS record is sent, a MAC value is generated and appended for that message, and the receiver is then able to compute and verify the sent MAC value to ensure message integrity and authenticity.

    Combined, all three mechanisms serve as a foundation for secure communication on the Web. All modern web browsers provide support for a variety of ciphersuites, are able to authenticate both the client and server, and transparently perform message integrity checks for every record.

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    The extensibility and the success of HTTP created a vibrant ecosystem of various proxies and intermediaries on the Web: cache servers, security gateways, web accelerators, content filters, and many others. In some cases we are aware of their presence (explicit proxies), and in others they are completely transparent to the end user.
    Unfortunately, the very success and the presence of these servers has created a problem for anyone who tries to deviate from the HTTP/1.x protocol in any way: some proxy servers may simply relay new HTTP extensions or alternative wire formats they cannot interpret, others may continue to blindly apply their logic even when they shouldn't, and some, such as security appliances, may infer malicious intent where there is none.

    In other words, in practice, deviating from the well-defined semantics of HTTP/1.x on port 80 often leads to unreliable deployments: some clients have no problems, while others fail with unpredictable and hard-to-reproduce behaviors - e.g., the same client may see different behaviors as it migrates between different networks.

    Due to these behaviors, new protocols and extensions to HTTP, such as WebSocket, HTTP/2, and others, have to rely on establishing an HTTPS tunnel to bypass the intermediate proxies and provide a reliable deployment model: the encrypted tunnel obfuscates the data from all intermediaries. If you have ever wondered why most WebSocket guides will tell you to use HTTPS to deliver data to mobile clients, this is why.

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    Unencrypted communication - via HTTP and other protocols - creates a large number of privacy, security, and integrity vulnerabilities. Such exchanges are susceptible to interception, manipulation, and impersonation, and can reveal users credentials, history, identity, and other sensitive information. Our applications need to protect themselves, and our users, against these threats by delivering data over HTTPS.
    HTTPS protects the integrity of the website
    Encryption prevents intruders from tampering with exchanged data - e.g. rewriting content, injecting unwanted and malicious content, and so on.
    HTTPS protects the privacy and security of the user
    Encryption prevents intruders from listening in on the exchanged data. Each unprotected request can reveal sensitive information about the user, and when such data is aggregated across many sessions, can be used to de-anonymize their identities and reveal other sensitive information. All browsing activity, as far as the user is concerned, should be considered private and sensitive.
    HTTPS enables powerful features on the web
    A growing number of new web platform features, such as accessing users geolocation, taking pictures, recording video, enabling offline app experiences, and more, require explicit user opt-in that, in turn, requires HTTPS. The security and integrity guarantees provided by HTTPS are critical components for delivering a secure user permission workflow and protecting their preferences.

    To further the point, both the Internet Engineering Task Force (IETF) and the Internet Architecture Board (IAB) have issued guidance to developers and protocol designers that strongly encourages adoption of HTTPS:
  • IETF: Pervasive Monitoring Is an Attack
  • IAB: Statement on Internet Confidentiality
  • As our dependency on the Internet has grown, so have the risks and the stakes for everyone that is relying on it. As a result, it is our responsibility, both as the application developers and users, to ensure that we protect ourselves by enabling HTTPS everywhere.
    The HTTPS-Only Standard published by the White House’s Office of Management and Budget is a great resource for additional information on the need for HTTPS, and hands-on advice for deploying it.

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    A common objection and roadblock towards widespread adoption of HTTPS has been the requirement to purchase certificates from one of the trusted authorities - see Chain of Trust and Certificate Authorities. The Let's Encrypt project launched in 2015 solves this particular problem:
    "Let's Encrypt is a free, automated, and open certificate authority brought to you by the Internet Security Research Group (ISRG). The objective of Let's Encrypt and the ACME protocol is to make it possible to set up an HTTPS server and have it automatically obtain a browser-trusted certificate, without any human intervention." Visit the project website to learn how to set it up on your own site. There are no restrictions, now anyone can obtain a trusted certificate for their site, free of charge.

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    Before the client and the server can begin exchanging application data over TLS, the encrypted tunnel must be negotiated: the client and the server must agree on the version of the TLS protocol, choose the ciphersuite, and verify certificates if necessary.
    Unfortunately, each of these steps requires new packet roundtrips (Figure 2) between the client and the server, which adds startup latency to all TLS connections.


    Figure 2. TLS handshake protocol

    Figure 2 assumes the same (optimistic) 28 millisecond one-way "light in fiber" delay between New York and London as used in previous TCP connection establishment examples.
    0 ms
    TLS runs over a reliable transport (TCP), which means that we must first complete the TCP three-way handshake, which takes one full roundtrip.
    56 ms
    With the TCP connection in place, the client sends a number of specifications in plain text, such as the version of the TLS protocol it is running, the list of supported ciphersuites, and other TLS options it may want to use.
    84 ms
    The server picks the TLS protocol version for further communication, decides on a ciphersuite from the list provided by the client, attaches its certificate, and sends the response back to the client. Optionally, the server can also send a request for the client's certificate and parameters for other TLS extensions.
    112 ms
    Assuming both sides are able to negotiate a common version and cipher, and the client is happy with the certificate provided by the server, the client initiates either the RSA or the Diffie-Hellman key exchange, which is used to establish the symmetric key for the ensuing session.
    140 ms
    The server processes the key exchange parameters sent by the client, checks message integrity by verifying the MAC, and returns an encrypted Finished message back to the client.
    168 ms
    The client decrypts the message with the negotiated symmetric key, verifies the MAC, and if all is well, then the tunnel is established and application data can now be sent.
    As the above exchange illustrates, new TLS connections require two roundtrips for a "full handshake" - that's the bad news. However, in practice, optimized deployments can do much better and deliver a consistent 1-RTT TLS handshake:
  • False Start is a TLS protocol extension that allows the client and server to start transmitting encrypted application data when the handshake is only partially complete—i.e., once ChangeCipherSpec and Finished messages are sent, but without waiting for the other side to do the same. This optimization reduces handshake overhead for new TLS connections to one roundtrip; see Enable TLS False Start.
  • If the client has previously communicated with the server, an "abbreviated handshake" can be used, which requires one roundtrip and also allows the client and server to reduce the CPU overhead by reusing the previously negotiated parameters for the secure session; see TLS Session Resumption.
  • The combination of both of the above optimizations allows us to deliver a consistent 1-RTT TLS handshake for new and returning visitors, plus computational savings for sessions that can be resumed based on previously negotiated session parameters. Make sure to take advantage of these optimizations in your deployments. One of the design goals for TLS 1.3 is to reduce the latency overhead for setting up the secure connection: 1-RTT for new, and 0-RTT for resumed sessions!

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    Due to a variety of historical and commercial reasons the RSA handshake has been the dominant key exchange mechanism in most TLS deployments: the client generates a symmetric key, encrypts it with the server's public key, and sends it to the server to use as the symmetric key for the established session.
    In turn, the server uses its private key to decrypt the sent symmetric key and the key-exchange is complete. From this point forward the client and server use the negotiated symmetric key to encrypt their session. The RSA handshake works, but has a critical weakness: the same public-private key pair is used both to authenticate the server and to encrypt the symmetric session key sent to the server. As a result, if an attacker gains access to the server's private key and listens in on the exchange, then they can decrypt the the entire session. Worse, even if an attacker does not currently have access to the private key, they can still record the encrypted session and decrypt it at a later time once they obtain the private key. By contrast, the Diffie-Hellman key exchange allows the client and server to negotiate a shared secret without explicitly communicating it in the handshake: the server's private key is used to sign and verify the handshake, but the established symmetric key never leaves the client or server and cannot be intercepted by a passive attacker even if they have access to the private key.

    Best of all, Diffie-Hellman key exchange can be used to reduce the risk of compromise of past communication sessions: we can generate a new "ephemeral" symmetric key as part of each and every key exchange and discard the previous keys. As a result, because the ephemeral keys are never communicated and are actively renegotiated for each the new session, the worst-case scenario is that an attacker could compromise the client or server and access the session keys of the current and future sessions. However, knowing the private key, or the current ephemeral key, does not help the attacker decrypt any of the previous sessions! Combined, the use of Diffie-Hellman key exchange and ephemeral sessions keys enables "perfect forward secrecy" (PFS): the compromise of long-term keys (e.g. server's private key) does not compromise past session keys and does not allow the attacker to decrypt previously recorded sessions. A highly desirable property, to say the least!

    As a result, and this should not come as a surprise, the RSA handshake is now being actively phased out: all the popular browsers prefer ciphers that enable forward secrecy (i.e., rely on Diffie-Hellman key exchange), and as an additional incentive, may enable certain protocol optimizations only when forward secrecy is available - e.g. 1-RTT handshakes via TLS False Start.

    Which is to say, consult your server documentation on how to enable and deploy forward secrecy! Once again, good security and performance go hand in hand.

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    Public-key cryptography is used only during initial setup of the TLS tunnel: the certificates are authenticated and the key exchange algorithm is executed.
    Symmetric key cryptography, which uses the established symmetric key is then used for all further communication between the client and the server within the session. This is done, in large part, to improve performance—public key cryptography is much more computationally expensive. To illustrate the difference, if you have OpenSSL installed on your computer, you can run the following tests:
    $> openssl speed ecdh
    $> openssl speed aes
    Note that the units between the two tests are not directly comparable: the Elliptic Curve Diffie-Hellman (ECDH) test provides a summary table of operations per second for different key sizes, while AES performance is measured in bytes per second. Nonetheless, it should be easy to see that the ECDH operations are much more computationally expensive. The exact performance numbers vary significantly based on used hardware, number of cores, TLS version, server configuration, and other factors. Don’t fall for an outdated benchmark! Always run the performance tests on your own hardware and refer to Reduce Computational Costs for additional context.

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    Two network peers may want to use a custom application protocol to communicate with each other. One way to resolve this is to determine the protocol upfront, assign a well-known port to it (e.g., port 80 for HTTP, port 443 for TLS), and configure all clients and servers to use it.
    However, in practice, this is a slow and impractical process: each port assignment must be approved and, worse, firewalls and other intermediaries often permit traffic only on ports 80 and 443. As a result, to enable easy deployment of custom protocols, we must reuse ports 80 or 443 and use an additional mechanism to negotiate the application protocol. Port 80 is reserved for HTTP, and the HTTP specification provides a special Upgrade flow for this very purpose. However, the use of Upgrade can add an extra network roundtrip of latency, and in practice is often unreliable in the presence of many intermediaries;

    The solution is, you guessed it, to use port 443, which is reserved for secure HTTPS sessions running over TLS. The use of an end-to-end encrypted tunnel obfuscates the data from intermediate proxies and enables a quick and reliable way to deploy new application protocols. However, we still need another mechanism to negotiate the protocol that will be used within the TLS session. Application Layer Protocol Negotiation (ALPN), as the name implies, is a TLS extension that addresses this need. It extends the TLS handshake (Figure 2) and allows the peers to negotiate protocols without additional roundtrips. Specifically, the process is as follows:
  • The client appends a new ProtocolNameList field, containing the list of supported application protocols, into the ClientHello message.
  • The server inspects the ProtocolNameList field and returns a ProtocolNamefield indicating the selected protocol as part of the ServerHello message.
  • The server may respond with only a single protocol name, and if it does not support any that the client requests, then it may choose to abort the connection. As a result, once the TLS handshake is finished, both the secure tunnel is established, and the client and server are in agreement as to which application protocol will be used; the client and server can immediately begin exchanging messages via the negotiated protocol.

    History and Relationship of NPN and ALPN
    Next Protocol Negotiation (NPN) is a TLS extension, which was developed as part of the SPDY effort at Google to enable efficient application protocol negotiation during the TLS handshake. Sound familiar? The end result is functionally equivalent to ALPN. ALPN is a revised and IETF approved version of the NPN extension. In NPN, the server advertised which protocols it supports, and the client then chose and confirmed the protocol. In ALPN, this exchange was reversed: the client now specifies which protocols it supports, and the server then selects and confirms the protocol. The rationale for the change is that this brings ALPN into closer alignment with other protocol negotiation standards. In short, ALPN is a successor to NPN.

    Server Name Indication (SNI)
    An encrypted TLS tunnel can be established between any two TCP peers: the client only needs to know the IP address of the other peer to make the connection and perform the TLS handshake. However, what if the server wants to host multiple independent sites, each with its own TLS certificate, on the same IP address — how does that work? Trick question; it doesn't. To address the preceding problem, the Server Name Indication (SNI) extension was introduced to the TLS protocol, which allows the client to indicate the hostname the client is attempting to connect to as part of the TLS handshake. In turn, the server is able to inspect the SNI hostname sent in the ClientHello message, select the appropriate certificate, and complete the TLS handshake for the desired host.

    TLS, HTTP, and Dedicated IPs
    The TLS+SNI workflow is identical to Host header advertisement in HTTP, where the client indicates the hostname of the site it is requesting: the same IP address may host many different domains, and both SNI and Host are required to disambiguate between them. Unfortunately, some older clients (e.g., most IE versions running on Windows XP, Android 2.2, and others) do not support SNI. As a result, if you need to provide TLS to such clients, then you may need a dedicated IP address for each and every host.

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    The extra latency and computational costs of the full TLS handshake impose a serious performance penalty on all applications that require secure communication. To help mitigate some of the costs, TLS provides a mechanism to resume or share the same negotiated secret key data between multiple connections.
    Session Identifiers
    The first Session Identifiers (RFC 5246) resumption mechanism was introduced in SSL 2.0, which allowed the server to create and send a 32-byte session identifier as part of its ServerHello message during the full TLS negotiation we saw earlier. With the session ID in place, both the client and server can store the previously negotiated session parameters - keyed by session ID and reuse them for a subsequent session.

    Specifically, the client can include the session ID in the ClientHello message to indicate to the server that it still remembers the negotiated cipher suite and keys from previous handshake and is able to reuse them. In turn, if the server is able to find the session parameters associated with the advertised ID in its cache, then an abbreviated handshake (Figure 3) can take place. Otherwise, a full new session negotiation is required, which will generate a new session ID.

    Figure 3. Abbreviated TLS handshake protocol
    Leveraging session identifiers allows us to remove a full round trip, as well as the overhead of public key cryptography, which is used to negotiate the shared secret key. This allows a secure connection to be established quickly and with no loss of security, since we are reusing the previously negotiated session data. Session resumption is an important optimization both for HTTP/1.x and HTTP/2 deployments. The abbreviated handshake eliminates a full roundtrip of latency and significantly reduces computational costs for both sides. In fact, if the browser requires multiple connections to the same host (e.g. when HTTP/1.x is in use), it will often intentionally wait for the first TLS negotiation to complete before opening additional connections to the same server, such that they can be "resumed" and reuse the same session parameters. If you've ever looked at a network trace and wondered why you rarely see multiple same-host TLS negotiations in flight, that's why!

    However, one of the practical limitations of the Session Identifiers mechanism is the requirement for the server to create and maintain a session cache for every client. This results in several problems on the server, which may see tens of thousands or even millions of unique connections every day: consumed memory for every open TLS connection, a requirement for a session ID cache and eviction policies, and nontrivial deployment challenges for popular sites with many servers, which should, ideally, use a shared TLS session cache for best performance.

    None of the preceding problems are impossible to solve, and many high-traffic sites are using session identifiers successfully today. But for any multi-server deployment, session identifiers will require some careful thinking and systems architecture to ensure a well operating session cache.

    Session Tickets
    To address this concern for server-side deployment of TLS session caches, the "Session Ticket" (RFC 5077) replacement mechanism was introduced, which removes the requirement for the server to keep per-client session state. Instead, if the client indicates that it supports session tickets, the server can include a New Session Ticket record, which includes all of the negotiated session data encrypted with a secret key known only by the server. This session ticket is then stored by the client and can be included in the SessionTicketextension within the ClientHello message of a subsequent session. Thus, all session data is stored only on the client, but the ticket is still safe because it is encrypted with a key known only by the server. The session identifiers and session ticket mechanisms are respectively commonly referred to as session caching and stateless resumption mechanisms. The main improvement of stateless resumption is the removal of the server-side session cache, which simplifies deployment by requiring that the client provide the session ticket on every new connection to the server—that is, until the ticket has expired.

    In practice, deploying session tickets across a set of load-balanced servers also requires some careful thinking and systems architecture: all servers must be initialized with the same session key, and an additional mechanism is required to periodically and securely rotate the shared key across all servers.

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    Authentication is an integral part of establishing every TLS connection. After all, it is possible to carry out a conversation over an encrypted tunnel with any peer, including an attacker, and unless we can be sure that the host we are speaking to is the one we trust, then all the encryption work could be for nothing.
    To understand how we can verify the peer's identity, let's examine a simple authentication workflow between Alice and Bob:
  • Both Alice and Bob generate their own public and private keys.
  • Both Alice and Bob hide their respective private keys.
  • Alice shares her public key with Bob, and Bob shares his with Alice.
  • Alice generates a new message for Bob and signs it with her private key.
  • Bob uses Alice's public key to verify the provided message signature.
  • Trust is a key component of the preceding exchange. Specifically, public key encryption allows us to use the public key of the sender to verify that the message was signed with the right private key, but the decision to approve the sender is still one that is based on trust. In the exchange just shown, Alice and Bob could have exchanged their public keys when they met in person, and because they know each other well, they are certain that their exchange was not compromised by an impostor—perhaps they even verified their identities through another, secret (physical) handshake they had established earlier!

    Next, Alice receives a message from Charlie, whom she has never met, but who claims to be a friend of Bob's. In fact, to prove that he is friends with Bob, Charlie asked Bob to sign his own public key with Bob's private key and attached this signature with his message (Figure 4). In this case, Alice first checks Bob's signature of Charlie's key. She knows Bob's public key and is thus able to verify that Bob did indeed sign Charlie's key. Because she trusts Bob's decision to verify Charlie, she accepts the message and performs a similar integrity check on Charlie's message to ensure that it is, indeed, from Charlie.

    Figure 4. Chain of trust for Alice, Bob, and Charlie
    What we have just done is established a chain of trust: Alice trusts Bob, Bob trusts Charlie, and by transitive trust, Alice decides to trust Charlie. As long as nobody in the chain is compromised, this allows us to build and grow the list of trusted parties. Authentication on the Web and in your browser follows the exact same process as shown. Which means that at this point you should be asking: whom does your browser trust, and whom do you trust when you use the browser? There are at least three answers to this question:
    Manually specified certificates Every browser and operating system provides a mechanism for you to manually import any certificate you trust. How you obtain the certificate and verify its integrity is completely up to you.

    Certificate authorities A certificate authority (CA) is a trusted third party that is trusted by both the subject (owner) of the certificate and the party relying upon the certificate.

    The browser and the operating system Every operating system and most browsers ship with a list of well-known certificate authorities. Thus, you also trust the vendors of this software to provide and maintain a list of trusted parties.

    In practice, it would be impractical to store and manually verify each and every key for every website (although you can, if you are so inclined). Hence, the most common solution is to use certificate authorities (CAs) to do this job for us (Figure 5): the browser specifies which CAs to trust (root CAs), and the burden is then on the CAs to verify each site they sign, and to audit and verify that these certificates are not misused or compromised. If the security of any site with the CA's certificate is breached, then it is also the responsibility of that CA to revoke the compromised certificate.

    Figure 5. CA signing of digital certificates
    Every browser allows you to inspect the chain of trust of your secure connection (Figure 6), usually accessible by clicking on the lock icon beside the URL.

    Figure 6. Certificate chain of trust for igvita.com (Google Chrome, v25)
  • igvita.com certificate is signed by StartCom Class 1 Primary Intermediate Server.
  • StartCom Class 1 Primary Intermediate Server certificate is signed by the StartCom Certification Authority.
  • StartCom Certification Authority is a recognized root certificate authority.
  • The "trust anchor" for the entire chain is the root certificate authority, which in the case just shown, is the StartCom Certification Authority. Every browser ships with a pre-initialized list of trusted certificate authorities ("roots"), and in this case, the browser trusts and is able to verify the StartCom root certificate. Hence, through a transitive chain of trust in the browser, the browser vendor, and the StartCom certificate authority, we extend the trust to our destination site.

    Certificate Transparency
    Every operating system vendor and every browser provide a public listing of all the certificate authorities they trust by default. Use your favorite search engine to find and investigate these lists. In practice, yo'’ll find that most systems rely on hundreds of trusted certificate authorities, which is also a common complaint against the system: the large number of trusted CAs creates a large attack surface area against the chain of trust in your browser. The good news is, the Certificate Transparency project is working to address these flaws by providing a framework—a public log—for monitoring and auditing of issuance of all new certificates.

    Certificate Revocation
    Occasionally the issuer of a certificate will need to revoke or invalidate the certificate due to a number of possible reasons: the private key of the certificate has been compromised, the certificate authority itself has been compromised, or due to a variety of more benign reasons such as a superseding certificate, change in affiliation, and so on. To address this, the certificates themselves contain instructions (Figure 7) on how to check if they have been revoked. Hence, to ensure that the chain of trust is not compromised, each peer can check the status of each certificate by following the embedded instructions, along with the signatures, as it verifies the certificate chain.

    Figure 7. CRL and OCSP instructions for igvita.com (Google Chrome, v25)

    Certificate Revocation List (CRL)
    Certificate Revocation List (CRL) is defined by RFC 5280 and specifies a simple mechanism to check the status of every certificate: each certificate authority maintains and periodically publishes a list of revoked certificate serial numbers. Anyone attempting to verify a certificate is then able to download the revocation list, cache it, and check the presence of a particular serial number within it - if it is present, then it has been revoked.

    This process is simple and straightforward, but it has a number of limitations:
  • The growing number of revocations means that the CRL list will only get longer, and each client must retrieve the entire list of serial numbers.
  • There is no mechanism for instant notification of certificate revocation - if the CRL was cached by the client before the certificate was revoked, then the CRL will deem the revoked certificate valid until the cache expires.
  • The need to fetch the latest CRL list from the CA may block certificate verification, which can add significant latency to the TLS handshake.
  • The CRL fetch may fail due to variety of reasons, and in such cases the browser behavior is undefined. Most browsers treat such cases as "soft fail", allowing the verification to proceed - yes, yikes.
  • Online Certificate Status Protocol (OCSP)
    To address some of the limitations of the CRL mechanism, the Online Certificate Status Protocol (OCSP) was introduced by RFC 2560, which provides a mechanism to perform a real-time check for status of the certificate. Unlike the CRL file, which contains all the revoked serial numbers, OCSP allows the client to query the CA’s certificate database directly for just the serial number in question while validating the certificate chain.

    As a result, the OCSP mechanism consumes less bandwidth and is able to provide real-time validation. However, the requirement to perform real-time OCSP queries creates its own set of problems:
  • The CA must be able to handle the load of the real-time queries.
  • The CA must ensure that the service is up and globally available at all times.
  • Real-time OCSP requests may impair the client's privacy because the CA knows which sites the client is visiting.
  • The client must block on OCSP requests while validating the certificate chain.
  • The browser behavior is, once again, undefined and typically results in a "soft fail" if the OCSP fetch fails due to a network timeout or other errors.
  • As a real-world data point, Firefox telemetry shows that OCSP requests time out as much as 15% of the time, and add approximately 350 ms to the TLS handshake when successful.

    OCSP Stapling
    For the reasons listed above, neither CRL or OSCP revocation mechanisms offer the security and performance guarantees that we desire for our applications. However, don't despair, because OCSP stapling (RFC 6066, "Certificate Status Request" extension) addresses most of the issues we saw earlier by allowing the validation to be performed by the server and be sent ("stapled") as part of the TLS handshake to the client:
  • Instead of the client making the OCSP request, it is the server that periodically retrieves the signed and timestamped OCSP response from the CA.
  • The server then appends (i.e. "staples") the signed OCSP response as part of the TLS handshake, allowing the client to validate both the certificate and the attached OCSP revocation record signed by the CA.
  • This role reversal is secure, because the stapled record is signed by the CA and can be verified by the client, and offers a number of important benefits:
  • The client does not leak its navigation history.
  • The client does not have to block and query the OCSP server.
  • The client may "hard-fail" revocation handling if the server opts-in (by advertising the OSCP "Must-Staple" flag) and the verification fails.
  • In short, to get both the best security and performance guarantees, make sure to configure and test OCSP stapling on your servers.

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    Not unlike the IP or TCP layers below it, all data exchanged within a TLS session is also framed using a well-defined protocol (Figure 8). The TLS Record protocol is responsible for identifying different types of messages (handshake, alert, or data via the "Content Type" field), as well as securing and verifying the integrity of each message.

    Figure 8. TLS record structure

    A typical workflow for delivering application data is as follows:
  • Record protocol receives application data.
  • Received data is divided into blocks: maximum of 214 bytes, or 16 KB per record.
  • Message authentication code (MAC) or HMAC is added to each record.
  • Data within each record is encrypted using the negotiated cipher.
  • Once these steps are complete, the encrypted data is passed down to the TCP layer for transport. On the receiving end, the same workflow, but in reverse, is applied by the peer: decrypt record using negotiated cipher, verify MAC, extract and deliver the data to the application above it. The good news is that all the work just shown is handled by the TLS layer itself and is completely transparent to most applications. However, the record protocol does introduce a few important implications that we need to be aware of:
  • Maximum TLS record size is 16 KB
  • Each record contains a 5-byte header, a MAC (up to 20 bytes for SSLv3, TLS 1.0, TLS 1.1, and up to 32 bytes for TLS 1.2), and padding if a block cipher is used.
  • To decrypt and verify the record, the entire record must be available.
  • Picking the right record size for your application, if you have the ability to do so, can be an important optimization. Small records incur a larger CPU and byte overhead due to record framing and MAC verification, whereas large records will have to be delivered and reassembled by the TCP layer before they can be processed by the TLS layer and delivered to your application—skip ahead to Optimize TLS Record Size for full details.

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    Deploying your application over TLS will require some additional work, both within your application (e.g. migrating resources to HTTPS to avoid mixed content), and on the configuration of the infrastructure responsible for delivering the application data over TLS.
    A well tuned deployment can make an enormous positive difference in the observed performance, user experience, and overall operational costs. Let's dive in. Reduce Computational Costs

    Establishing and maintaining an encrypted channel introduces additional computational costs for both peers. Specifically, first there is the asymmetric (public key) encryption used during the TLS handshake. Then, once a shared secret is established, it is used as a symmetric key to encrypt all TLS records.

    As we noted earlier, public key cryptography is more computationally expensive when compared with symmetric key cryptography, and in the early days of the Web often required additional hardware to perform "SSL offloading." The good news is, this is no longer necessary and what once required dedicated hardware can now be done directly on the CPU. Large organizations such as Facebook, Twitter, and Google, which offer TLS to billions of users, perform all the necessary TLS negotiation and computation in software and on commodity hardware.

    To get the best results in your own deployments, make the best of TLS Session Resumption - deploy, measure, and optimize its success rate. Eliminating the need to perform the costly public key cryptography operations on every handshake will significantly reduce both the computational and latency costs of TLS; there is no reason to spend CPU cycles on work that you don't need to do. Speaking of optimizing CPU cycles, make sure to keep your servers up to date with the latest version of the TLS libraries! In addition to the security improvements, you will also often see performance benefits. Security and performance go hand-in-hand.

    Enable 1-RTT TLS Handshakes

    An unoptimized TLS deployment can easily add many additional round trips and introduce significant latency for the user - e.g. multi-RTT handshakes, slow and ineffective certificate revocation checks, large TLS records that require multiple round trips, and so on. Don't be that site, you can do much better. A well-tuned TLS deployment should add at most one extra roundtrip for negotiating the TLS connection, regardless of whether it is new or resumed, and avoid all other latency pitfalls: configure session resumption, and enable forward secrecy to enable TLS False Start.

    To get the best end-to-end performance, make sure to audit both own and third-party services and servers used by your application, including your CDN provider.

    Optimize Connection Reuse
    The best way to minimize both latency and computational overhead of setting up new TCP+TLS connections is to optimize connection reuse. Doing so amortizes the setup costs across requests and delivers a much faster experience to the user. Verify that your server and proxy configurations are setup to allow keepalive connections, and audit your connection timeout settings. Many popular servers set aggressive connection timeouts (e.g. some Apache versions default to 5s timeouts) that force a lot of unnecessary renegotiations. For best results, use your logs and analytics to determine the optimal timeout values.

    Leverage Early Termination
    As we discussed in Primer on Latency and Bandwidth, we may not be able to make our packets travel faster, but we can make them travel a shorter distance. By placing our "edge" servers closer to the user (Figure 9), we can significantly reduce the round trip times and the total costs of the TCP and TLS handshakes.

    Figure 9. Early termination of client connections

    A simple way to accomplish this is to leverage the services of a content delivery network (CDN) that maintains pools of edge servers around the globe, or to deploy your own. By allowing the user to terminate their connection with a nearby server, instead of traversing across oceans and continental links to your origin, the client gets the benefit of "early termination" with shorter roundtrips. This technique is equally useful and important for static and dynamic content: static content can also be cached and served by the edge servers, whereas dynamic requests can be routed over established connections from the edge to origin.

    Uncached Origin Fetch
    The technique of using a CDN or a proxy server to fetch a resource, which may need to be customized per user or contains other private data, and hence is not a globally cacheable resource at the edge, is commonly known as an "uncached origin fetch." While CDNs work best when the data is cached in geo-distributed servers around the world, the uncached origin fetch still provides a very important optimization: the client connection is terminated with the nearby server, which can dramatically reduce the handshake latency costs. In turn, the CDN, or your own proxy server, can maintain a "warm connection pool" to relay the data to the origin servers, allowing you to return a fast response back to the client.

    In fact, as an additional layer of optimization, some CDN providers will use nearby servers on both sides of the connection! The client connection is terminated at a nearby CDN node, which then relays the request to the CDN node close to the origin, and the request is then routed to the origin. The hop within the CDN network allows the traffic to be routed over the optimized CDN backbone, which can help to further reduce latency between client and origin servers.

    Configure Session Caching and Stateless Resumption
    Terminating the connection closer to the user is an optimization that will help decrease latency for your users in all cases, but once again, no bit is faster than a bit not sent - send fewer bits. Enabling TLS session caching and stateless resumption allows us to eliminate an entire roundtrip of latency and reduce computational overhead for repeat visitors. Session identifiers, on which TLS session caching relies, were introduced in SSL 2.0 and have wide support among most clients and servers. However, if you are configuring TLS on your server, do not assume that session support will be on by default. In fact, it is more common to have it off on most servers by default—but you know better! Double-check and verify your server, proxy, and CDN configuration:
  • Servers with multiple processes or workers should use a shared session cache.
  • Size of the shared session cache should be tuned to your levels of traffic.
  • A session timeout period should be provided.
  • In a multi-server setup, routing the same client IP, or the same TLS session ID, to the same server is one way to provide good session cache utilization.
  • Where "sticky" load balancing is not an option, a shared cache should be used between different servers to provide good session cache utilization, and a secure mechanism needs to be established to share and update the secret keys to decrypt the provided session tickets.
  • Check and monitor your TLS session cache statistics for best performance.
  • In practice, and for best results, you should configure both session caching and session ticket mechanisms. These mechanisms work together to provide best coverage both for new and older clients.

    Enable TLS False Start
    Session resumption provides two important benefits: it eliminates an extra handshake roundtrip for returning visitors and reduces the computational cost of the handshake by allowing reuse of previously negotiated session parameters. However, it does not help in cases where the visitor is communicating with the server for the first time, or if the previous session has expired. To get the best of both worlds - a one round trip handshake for new and repeat visitors, and computational savings for repeat visitors - we can use TLS False Start, which is an optional protocol extension that allows the sender to send application data (Figure 10) when the handshake is only partially complete.

    Figure 10. TLS handshake with False Start

    False Start does not modify the TLS handshake protocol, rather it only affects the protocol timing of when the application data can be sent. Intuitively, once the client has sent theClientKeyExchange record, it already knows the encryption key and can begin transmitting application data - the rest of the handshake is spent confirming that nobody has tampered with the handshake records, and can be done in parallel. As a result, False Start allows us to keep the TLS handshake at one round trip regardless of whether we are performing a full or abbreviated handshake.

    Deploying TLS False Start
    Because False Start is only modifying the timing of the handshake protocol, it does not require any updates to the TLS protocol itself and can be implemented unilaterally - i.e., the client can simply begin transmitting encrypted application data sooner. Well, that's the theory. In practice, even though TLS False Start should be backwards compatible with all existing TLS clients and servers, enabling it by default for all TLS connections proved to be problematic due to some poorly implemented servers. As a result, all modern browsers are capable of using TLS False Start, but will only do so when certain conditions are met by the server:
  • Chrome and Firefox require an ALPN protocol advertisement to be present in the server handshake, and that the cipher suite chosen by the server enables forward secrecy.
  • Safari requires that the cipher suite chosen by the server enables forward secrecy.
  • Internet Explorer uses a combination of a blacklist of known sites that break when TLS False Start is enabled, and a timeout to repeat the handshake if the TLS False Start handshake failed.
  • To enable TLS False Start across all browsers the server should advertise a list of supported protocols via the ALPN extension - e.g., "h2, http/1.1" - and be configured to support and prefer cipher suites that enable forward secrecy.

    Optimize TLS Record Size
    All application data delivered via TLS is transported within a record protocol (Figure 8). The maximum size of each record is 16 KB, and depending on the chosen cipher, each record will add anywhere from 20 to 40 bytes of overhead for the header, MAC, and optional padding. If the record then fits into a single TCP packet, then we also have to add the IP and TCP overhead: 20-byte header for IP, and 20-byte header for TCP with no options. As a result, there is potential for 60 to 100 bytes of overhead for each record. For a typical maximum transmission unit (MTU) size of 1,500 bytes on the wire, this packet structure translates to a minimum of 6% of framing overhead.

    The smaller the record, the higher the framing overhead. However, simply increasing the size of the record to its maximum size (16 KB) is not necessarily a good idea. If the record spans multiple TCP packets, then the TLS layer must wait for all the TCP packets to arrive before it can decrypt the data (Figure 11). If any of those TCP packets get lost, reordered, or throttled due to congestion control, then the individual fragments of the TLS record will have to be buffered before they can be decoded, resulting in additional latency. In practice, these delays can create significant bottlenecks for the browser, which prefers to consume data in a streaming fashion.

    Figure 11. WireShark capture of 11,211-byte TLS record split over 8 TCP segments
    Small records incur overhead, large records incur latency, and there is no one value for the "optimal" record size. Instead, for web applications, which are consumed by the browser, the best strategy is to dynamically adjust the record size based on the state of the TCP connection:
  • When the connection is new and TCP congestion window is low, or when the connection has been idle for some time (see Slow-Start Restart), each TCP packet should carry exactly one TLS record, and the TLS record should occupy the full maximum segment size (MSS) allocated by TCP.
  • When the connection congestion window is large and if we are transferring a large stream (e.g., streaming video), the size of the TLS record can be increased to span multiple TCP packets (up to 16KB) to reduce framing and CPU overhead on the client and server.
  • If the TCP connection has been idle, and even if Slow-Start Restart is disabled on the server, the best strategy is to decrease the record size when sending a new burst of data: the conditions may have changed since last transmission, and our goal is to minimize the probability of buffering at the application layer due to lost packets, reordering, and retransmissions. Using a dynamic strategy delivers the best performance for interactive traffic: small record size eliminates unnecessary buffering latency and improves the time-to-first-{HTML byte,..., video frame}, and a larger record size optimizes throughput by minimizing the overhead of TLS for long-lived streams.

    To determine the optimal record size for each state let's start with the initial case of a new or idle TCP connection where we want to avoid TLS records from spanning multiple TCP packets:
  • Allocate 20 bytes for IPv4 framing overhead and 40 bytes for IPv6.
  • Allocate 20 bytes for TCP framing overhead.
  • Allocate 40 bytes for TCP options overhead (timestamps, SACKs).
  • Assuming a common 1,500-byte starting MTU, this leaves 1,420 bytes for a TLS record delivered over IPv4, and 1,400 bytes for IPv6. To be future-proof, use the IPv6 size, which leaves us with 1,400 bytes for each TLS record, and adjust as needed if your MTU is lower.

    Next, the decision as to when the record size should be increased and reset if the connection has been idle, can be set based on pre-configured thresholds: increase record size to up to 16 KB after X KB of data have been transferred, and reset the record size after Y milliseconds of idle time. Typically, configuring the TLS record size is not something we can control at the application layer. Instead, often this is a setting and sometimes a compile-time constant for your TLS server. Check the documentation of your server for details on how to configure these values.

    Optimize the Certificate Chain
    Verifying the chain of trust requires that the browser traverse the chain, starting from the site certificate, and recursively verify the certificate of the parent until it reaches a trusted root. Hence, it is critical that the provided chain includes all the intermediate certificates. If any are omitted, the browser will be forced to pause the verification process and fetch the missing certificates, adding additional DNS lookups, TCP handshakes, and HTTP requests into the process.

    How does the browser know from where to fetch the missing certificates? Each child certificate typically contains a URL for the parent. If the URL is omitted and the required certificate is not included, then the verification will fail. Conversely, do not include unnecessary certificates, such as the trusted roots in your certificate chain - they add unnecessary bytes. Recall that the server certificate chain is sent as part of the TLS handshake, which is likely happening over a new TCP connection that is in the early stages of its slow-start algorithm. If the certificate chain size exceeds TCP's initial congestion window, then we will inadvertently add additional roundtrips to the TLS handshake: certificate length will overflow the congestion window and cause the server to stop and wait for a client ACK before proceeding.

    In practice, the size and depth of the certificate chain was a much bigger concern and problem on older TCP stacks that initialized their initial congestion window to 4 TCP segments—see Slow-Start. For newer deployments, the initial congestion window has been raised to 10 TCP segments and should be more than sufficient for most certificate chains. That said, verify that your servers are using the latest TCP stack and settings, and optimize and reduce the size of your certificate chain. Sending fewer bytes is always a good and worthwhile optimization.

    Configure OCSP Stapling
    Every new TLS connection requires that the browser must verify the signatures of the sent certificate chain. However, there is one more critical step that we can’t forget: the browser also needs to verify that the certificates have not been revoked. To verify the status of the certificate the browser can use one of several methods: Certificate Revocation List (CRL), Online Certificate Status Protocol (OCSP), or OCSP Stapling. Each method has its own limitations, but OCSP Stapling provides, by far, the best security and performance guarantees-refer to earlier sections for details. Make sure to configure your servers to include (staple) the OCSP response from the CA to the provided certificate chain. Doing so allows the browser to perform the revocation check without any extra network roundtrips and with improved security guarantees.
  • OCSP responses can vary from 400 to 4,000 bytes in size. Stapling this response to your certificate chain will increase its size—pay close attention to the total size of the certificate chain, such that it doesn’t overflow the initial congestion window for new TCP connections.
  • Current OCSP Stapling implementations only allow a single OCSP response to be included, which means that the browser may have to fallback to another revocation mechanism if it needs to validate other certificates in the chain - reduce the length of your certificate chain. In the future, OCSP Multi-Stapling should address this particular problem.
  • Most popular servers support OCSP stapling. Check the relevant documentation for support and configuration instructions. Similarly, if using or deciding on a CDN, check that their TLS stack supports and is configured to use OCSP stapling.

    Enable HTTP Strict Transport Security (HSTS)
    HTTP Strict Transport Security is an important security policy mechanism that allows an origin to declare access rules to a compliant browser via a simple HTTP header - e.g., "Strict-Transport-Security: max-age=31536000". Specifically, it instructs the user-agent to enforce the following rules:
  • All requests to the origin should be sent over HTTPS. This includes both navigation and all other same-origin subresource requests—e.g. if the user types in a URL without the https prefix the user agent should automatically convert it to an https request; if a page contains a reference to a non-https resource, the user agent should automatically convert it to request the https version.
  • If a secure connection cannot be established, the user is not allowed to circumvent the warning and request the HTTP version - i.e. the origin is HTTPS-only.
  • max-age specifies the lifetime of the specified HSTS ruleset in seconds (e.g., max-age=31536000 is equal to a 365-day lifetime for the advertised policy).
  • includeSubdomains indicates that the policy should apply to all subdomains of the current origin.
  • HSTS converts the origin to an HTTPS-only destination and helps protect the application from a variety of passive and active network attacks. As an added bonus, it also offers a nice performance optimization by eliminating the need for HTTP-to-HTTPS redirects: the client automatically rewrites all requests to the secure origin before they are dispatched!

    Make sure to thoroughly test your TLS deployment before enabling HSTS. Once the policy is cached by the client, failure to negotiate a TLS connection will result in a hard-fail - i.e. the user will see the browser error page and won't be allowed to proceed. This behavior is an explicit and necessary design choice to prevent network attackers from tricking clients into accessing your site without HTTPS.

    HSTS Preload List
    The HSTS mechanism leaves the very first request to an origin unprotected from active attacks - e.g. a malicious party could downgrade the client's request and prevent it from registering the HSTS policy. To address this, most browsers provide a separate "HSTS preload list" mechanism that allows an origin to request to be included in the list of HSTS-enabled sites that ships with the browser.

    Enable HTTP Public Key Pinning (HPKP)
    One of the shortcomings of the current system - as discussed in Chain of Trust and Certificate Authorities - is our reliance on a large number of trusted Certificate Authorities (CA's). On the one hand, this is convenient, because it means that we can obtain a valid certificate from a wide pool of entities. However, it also means that any one of these entities is also able to issue a valid certificate for our, and any other, origin without their explicit consent. The compromise of the DigiNotar certificate authority is one of several high-profile examples where an attacker was able to issue and use fake-but valid-certificates against hundreds of high profile sites. Public Key Pinning enables a site to send an HTTP header that instructs the browsers to remember ("pin") one or more certificates in its certificate chain. By doing so, it is able to scope which certificates, or issuers, should be accepted by the browser on subsequent visits:
  • The origin can pin it's leaf certificate. This is the most secure strategy because you are, in effect, hard-coding a small set of specific certificate signatures that should be accepted by the browser.
  • The origin can pin one of the parent certificates in the certificate chain. For example, the origin can pin the intermediate certificate of its CA, which tells the browser that, for this particular origin, it should only trust certificates signed by that particular certificate authority.
  • Picking the right strategy for which certificates to pin, which and how many backups to provide, duration, and other criteria for deploying HPKP are important, nuanced, and beyond the scope of our discussion. Consult your favorite search engine, or your local security guru, for more information. HPKP also exposes a "report only" mode that does not enforce the provided pin but is able to report detected failures. This can be a great first step towards validating your deployment, and serve as a mechanism to detect violations.

    Update Site Content to HTTPS
    To get the best security and performance guarantees it is critical that the site actually uses HTTPS to fetch all of its resources. Otherwise, we run into a number of issues that will compromise both, or worse, break the site:
  • Mixed "active" content (e.g. scripts and stylesheets delivered over HTTP) will be blocked by the browser and may break the functionality of the site.
  • Mixed "passive" content (e.g. images, video, audio, etc., delivered over HTTP) will be fetched, but will allow the attacker to observe and infer user activity, and degrade performance by requiring additional connections and handshakes.
  • Audit your content and update your resources and links, including third-party content, to use HTTPS. The Content Security Policy (CSP) mechanism can be of great help here, both to identify HTTPS violations and to enforce the desired policies.

    Content-Security-Policy: upgrade-insecure-requests
    Content-Security-Policy-Report-Only: default-src https:;
    report-uri https://example.com/reporting/endpoint

    1. Tells the browser to upgrade all (own and third-party) requests to HTTPS.
    2. Tells the browser to report any non-HTTPS violations to designated endpoint.
    CSP provides a highly configurable mechanism to control which asset are allowed to be used, and how and from where they can be fetched. Make use of these capabilities to protect your site and your users.

    Performance Checklist
    As application developers we are shielded from most of the complexity of the TLS protocol - the client and server do most of the hard work on our behalf. However, as we saw in this chapter, this does not mean that we can ignore the performance aspects of delivering our applications over TLS. Tuning our servers to enable critical TLS optimizations and configuring our applications to enable the client to take advantage of such features pays high dividends: faster handshakes, reduced latency, better security guarantees, and more.
    With that in mind, a short checklist to put on the agenda:
  • Get best performance from TCP; see Optimizing for TCP.
  • Upgrade TLS libraries to latest release, and (re)build servers against them.
  • Enable and configure session caching and stateless resumption.
  • Monitor your session caching hit rates and adjust configuration accordingly.
  • Configure forward secrecy ciphers to enable TLS False Start.
  • Terminate TLS sessions closer to the user to minimize roundtrip latencies.
  • Use dynamic TLS record sizing to optimize latency and throughput.
  • Audit and optimize the size of your certificate chain.
  • Configure OCSP stapling.
  • Configure HSTS and HPKP.
  • Configure CSP policies.
  • Enable HTTP/2; see HTTP/2.

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    Finally, to verify and test your configuration, you can use an online service, such as the Qualys SSL Server Test to scan your public server for common configuration and security flaws. Additionally, you should familiarize yourself with the openssl command-line interface, which will help you inspect the entire handshake and configuration of your server locally.
    $> openssl s_client -state -CAfile root.ca.crt -connect yoursite.com:443
    CONNECTED(00000003)
    SSL_connect:before/connect initialization
    SSL_connect:SSLv2/v3 write client hello A
    SSL_connect:SSLv3 read server hello A
    depth=2 /C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing
       /CN=StartCom Certification Authority
    verify return:1
    depth=1 /C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing
       /CN=StartCom Class 1 Primary Intermediate Server CA
    verify return:1
    depth=0 /description=ABjQuqt3nPv7ebEG/C=US
        /CN=www.yoursite.com/emailAddress=admin@yoursite.com
    verify return:1
    SSL_connect:SSLv3 read server certificate A
    SSL_connect:SSLv3 read server done A
    SSL_connect:SSLv3 write client key exchange A
    SSL_connect:SSLv3 write change cipher spec A
    SSL_connect:SSLv3 write finished A
    SSL_connect:SSLv3 flush data
    SSL_connect:SSLv3 read finished A
    ---
    Certificate chain
    0 s:/description=ABjQuqt3nPv7ebEG/C=US
       /CN=www.igvita.com/emailAddress=admin@yoursite.com
      i:/C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing
      /CN=StartCom Class 1 Primary Intermediate Server CA
    1 s:/C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing
      /CN=StartCom Class 1 Primary Intermediate Server CA
      i:/C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing
      /CN=StartCom Certification Authority
    ---
    Server certificate
    -----BEGIN CERTIFICATE-----
    ... snip ...
    --- No client certificate CA names sent
    ---
    SSL handshake has read 3571 bytes and written 444 bytes
    ---
    New, TLSv1/SSLv3, Cipher is RC4-SHA
    Server public key is 2048 bit
    Secure Renegotiation IS supported
    Compression: NONE
    Expansion: NONE
    SSL-Session:
      Protocol : TLSv1
      Cipher : RC4-SHA
      Session-ID: 269349C84A4702EFA7 ...
      Session-ID-ctx:
      Master-Key: 1F5F5F33D50BE6228A ...
      Key-Arg : None
      Start Time: 1354037095
      Timeout : 300 (sec)
      Verify return code: 0 (ok)
    ---

    1. Client completed verification of received certificate chain.
    2. Received certificate chain (two certificates).
    3. Size of received certificate chain.
    4. Issued session identifier for stateful TLS resume.

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    TLS: Transport Layer Security
    SSL: Secure Sockets Layer
    SSL and TLS are protocols for data encryption and authentication between a client and a computer server. They create a secure connection for private communication across an unsecure network.
    While SSL and TLS are not the only protocols of their kind, they are the two most common protocols for communications of sensitive data such as passwords, personal information, financial information, etc. Protect your company's systems from cyberattack, secure your communication, and stay HIPAA compliant using the right security protocol.

    Protocol Overview
    SSL v3.0 laid the foundation for TLS v1.0
    Both use similar ciphers and message digests (though there are newer and better ciphers that are only available with newer versions of TLS such as TLS v1.0 and or v1.2). Differences in how the secure communications are established are also evident and make TLS v1.0 (and higher) clearly stronger than SSL v3.0. Which protocol is used TLS v1.x or SSL v3.0 is determined by a negotiation between the client and server, based on what software is installed on each and how each is configured.

    SSL V3.0
    Was exploited by the POODLE attack and is now obsolete

    TLS V1.2
    The newest, most widely-used TLS protocol
    Enables better use of more secure ciphers
    Features enhanced negotiation of the encrypted connections

    Server Connection: How It Works
    By Port (explicit) -"SSL"
    Client connects to a secure port like:
  • 443 for https (secure web)
  • 993 for secure IMAP
  • 995 for secure POP
  • Note: Even though this explicit method can be called "SSL" in many places, it still may use TLS v1.0+ or SSL v3.0 for the actual connection, depending on what is negotiated.

    By Protocol (implicit) -"TLS"
    Client reaches out to an insecure server with a "handshake" and begins secure communication once the connection is successful ("client hello" and "server hello" messages are exchanged)
    If the handshake proves to be unsuccessful, the connection is severed
    This handshake is often called "STARTTLS"
    If the client and server support multiple protocols, they must negotiate to agree upon which one they will use (note: websites that use SSL v3.0 have outdated security and should be updated ASAP).

    Configuration: Which Protocol to Use
    Server Configuration
    Use the latest version of TLS and configure it properly
    Extremely strong security (made even more secure with a high-quality security certificate)
    Stay away from SSL v3.0 as it does not provide the proper level of security
    Use only strong ciphers (especially if the server requires compliance)

    Program Configuration
    In terms of explicit vs. implicit connections, both SSL and TLS are fine options for program configuration (as long as they are supported by the server)
    However, TLS should be used for the actual protocol
    If no SSL to TLS protocol is selected, your connection to the server becomes easily accessible to prying eyes.

    HIPAA Compliance -Requirements
    SSL: SSL v3.0 must not be used (must be disabled along with SSL v2.0)
    TLS: TLS v1.0 is allowed (newer versions are better)

    Which is more secure -SSL or TLS or TLS v1.x?
    It used to be believed that TLS v1.0 was only marginally more secure than SSL v3.0, its predecessor. However, SSL v3.0 is very old and attacks such as the POODLE vulnerability have shown that SSL v3.0 is now completely insecure (especially for web sites using it). Even before the POODLE was set loose, the US Government had already mandated that SSL v3 not be used for sensitive government communications or for HIPAA-compliant communications. If that was not enough, POODLE certainly was. In fact, as a result of POODLE, SSL v3 was disabled on web sites all over the world and for many other services as well.

    SSL v3.0 is effectively "dead" as a useful security protocol. Places that still allow its use for web hosting are placing their "secure web sites" at extreme risk. Organizations that allow SSL v3 use to persist for other protocols (e.g., IMAP) should take steps to remove that support at the soonest software update maintenance window.

    Subsequent versions of TLS -v1.1, v1.2, and v1.3 are significantly more secure and fix many vulnerabilities present in SSL v3.0 and TLS v1.0. For example, the BEAST attack that can completely break web sites running the older SSL v3.0 and TLS v1.0 protocols. The newer TLS versions, if properly configured, prevent the BEAST, other attack vectors, and provide many stronger ciphers and encryption methods.

    As of the time of this writing (May, 2018), 11% of web sites still support SSL v3.0. However, a great majority 92% already support TLS v1.2+. Check the latest statics over at SSLLabs. The trend is, of course, to deprecate the older protocols in favor of the new ones. For example, use of TLS 1.0 by web sites that accept credit cards (and services used by the US government) must stop by June 30th, 2018. Instead, they must use TLS 1.1 with TLS 1.2+ strongly encouraged). As early as 2014, NIST (National Institute of Standards and Technology) revised its guidelines and recommends only use of TLS 1.1+ for government communications. NIST does indicate that TLS v1.0 is OK for non-government communications even in its latest 2018 draft updates. Everyone should use TLS 1.2 and 1.3 when possible.

    However, so many organizations still have older computers (more than 5 years old) that completely turning off TLS 1.0 and 1.1 support could cause a large disruption. As a result, we are seeing the transition to TLS 1.2 as being more gradual where sites and systems that need it (for PCI and government work) leading the way, followed by those whose users are likely to have more recent computed systems. Likely, in the next several years, TLS 1.0 support across the internet will steadily decline. However, as we see with SSL 3.0, it will not go away in general for some time to come.

    But wait: are not TLS and SSL different encryption mechanisms?
    If you setup an email program you may see separate options for "no encryption", "SSL", or "TLS" encryption of you transmission. This leads one to assume that TLS and SSL are very different things. In truth, this labeling is a misnomer. You are not actually selecting which method to use (SSL v3 or TLS v1.x) when making this choice. You are merely selecting between options that dictate how the secure connection will be initiated. No matter which "method" you choose for initiating the connection, TLS or SSL, the same level of encryption will be obtained when talking to the server and that level is determined by the software installed on the server, how that is configured, and what your program actually supports.

    If the SSL vs TLS choice is not SSLv3 vs TLS v1.0+, what is it?
    There are two distinct ways that a program can initiate a secure connection with a server:

    By Port (a.k.a. explicit): Connecting to a specific port means that a secure connection should be used. For example, port 443 for https (secure web), 993 for secure IMAP, 995 for secure POP, etc. These ports are setup on the server ready to negotiate a secure connection first, and do whatever else you want second.

    By Protocol (a.k.a. implicit): These connections first begin with an insecure "hello" to the server and only then switch to secured communications after the handshake between the client and the server is successful. If this handshake fails for any reason, the connection is severed. A good example of this is the command "STARTTLS" used in outbound email (SMTP) connections.

    The "By Port" method is commonly referred to as "SSL" or "explicit" and the "By Protocol" method is commonly referred to as "TLS" or «implicit» in many program configuration areas.
    Sometimes, you have only the option to specify the port and if you should be making a secure connection or not and the program itself guesses from that what method should be used many old email programs like Outlook and Mac Mail did that. In such cases, you need to know if the program will try and explicit or implicit connection to initiate security, and choose your port appropriately (or else the connection could fail).

    To Review: In email programs and other systems where you can select between SSL or TLS and choose the port a connection will be made on: 1. SSL means a "by port" explicit connection to a port that expects to the session to start with security negotiation
    2. TLS means a "by protocol" connection where the program will connect «insecurely» first and use special commands to enable encryption (implicit).
    3. Use of either could result in a connection encrypted with either SSL v3 or TLS v1.0+, based on what is installed on the sever and what is supported by your program.
    4. Both methods of connection (implicit and explicit) result in equally secure (or insecure) communications.

    Sidebar: It is unclear why the "By Protocol" method is referred to as «TLS» as it could result in either TLS or SSL actually being used. It is likely because the folks who designed the SMTP protocol decided to name their command to switch to SSL/TLS in the SMTP protocol to "STARTTLS" (using "TLS" in the name as that is the newer protocol name). Then email programs started listing "TLS" next to this and "SSL" next to the old "By Port" option which came first. Once they started labeling things this way, that expanded to general use in the configuration of other protocols (like POP and IMAP) for "consistency". I am not certain if this is the real reason, but based on my experience dealing with all versions of email programs and servers over the last 15 years, it seems very plausible.

    Both methods ensure that your data is encrypted as it is transmitted across the Internet. They also both enable you to be sure that the server that you are communication with is the server you intend to contact and not some "middle man eavesdropper". This is possible because servers that support SSL and TLS must have certificates issued to them by a trusted third party, like Comodo. These certificates verify that the domain name they are issued for really belongs to the server. Your computer will issue warnings to you if you try to connect to a server and the certificate that it gets back is not trusted or doesn't match the site you are trying to connect to.

    What about TLS v1.3?
    TLS v1.3 is the latest and greatest version of TLS. It became an Internet standard on March 25th, 2018. According to NIST, organizations should make plans to support TLS v1.3 by January 1st, 2020 or sooner.
    TLS v1.3 brings many significant changes over TLS v1.2. Some of these include:
  • All new ciphers. The ciphers that will be used with TLS v1.3 are incompatible with previous versions of TLS
  • Drop weak security. Many things knows known to be cryptographically weak such as MD5, RC4, and weak elliptic curves have been completely dropped, so that it will be impossible to use them with TLS v1.3.
  • Drop seldom-used features. Features that are little used, like compression and "change cipher" ciphers, have been dropped to simplify and strengthen the protocol.
  • Faster. TLS v1.3 speeds up the client-server negotiation of security, making secure connections faster to initiate.
  • No Corporate/ISP Eavesdropping. With TLS v1.3 is it no longer possible for organizations to seamlessly monitor secure connections by passively decrypting and re-encrypting them.

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