]> Intuit

yaronf.ietf@gmail.com

independent

stpeter@stpeter.im

arm

thomas.fossati@arm.com

Applications UTA Working Group Internet-Draft Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are widely used to protect data exchanged over application protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the years, the industry has witnessed several serious attacks on TLS and DTLS, including attacks on the most commonly used cipher suites and their modes of operation. This document provides the latest recommendations for ensuring the security of deployed services that use TLS and DTLS. These recommendations are applicable to the majority of use cases. An earlier version of this document was published as RFC 7525 when the industry was in the midst of its transition to TLS 1.2. Years later this transition is largely complete and TLS 1.3 is widely available. This document updates the guidance given the new environment and obsoletes RFC 7525. In addition, the document updates RFC 5288 and RFC 6066 in view of recent attacks.

Introduction Transport Layer Security (TLS) and Datagram Transport Security Layer (DTLS) are widely used to protect data exchanged over application protocols such as HTTP , SMTP , IMAP , POP , SIP , and XMPP . Over the years leading to 2015, the industry has witnessed serious attacks on TLS and DTLS, including attacks on the most commonly used cipher suites and their modes of operation. For instance, both the AES-CBC and RC4 encryption algorithms, which together were once the most widely deployed ciphers, have been attacked in the context of TLS. A companion document provides detailed information about these attacks and will help the reader understand the rationale behind the recommendations provided here. That document has not been updated in concert with this one; instead, newer attacks are described in this document, as are mitigations for those attacks. The TLS community reacted to these attacks in several ways:

• Detailed guidance was published on the use of TLS 1.2 and DTLS 1.2 , along with earlier protocol versions. This guidance is included in the original and mostly retained in this revised version; note that this guidance was mostly adopted by the industry since the publication of RFC 7525 in 2015.
• Versions of TLS earlier than 1.2 were deprecated .
• Version 1.3 of TLS was released, followed by version 1.3 of DTLS ; these versions largely mitigate or resolve the described attacks.

Those who implement and deploy TLS and DTLS, in particular versions 1.2 or earlier of these protocols, need guidance on how TLS can be used securely. This document provides guidance for deployed services as well as for software implementations, assuming the implementer expects his or her code to be deployed in environments defined in . Concerning deployment, this document targets a wide audience -- namely, all deployers who wish to add authentication (be it one-way only or mutual), confidentiality, and data integrity protection to their communications. The recommendations herein take into consideration the security of various mechanisms, their technical maturity and interoperability, and their prevalence in implementations at the time of writing. Unless it is explicitly called out that a recommendation applies to TLS alone or to DTLS alone, each recommendation applies to both TLS and DTLS. This document attempts to minimize new guidance to TLS 1.2 implementations, and the overall approach is to encourage systems to move to TLS 1.3. However this is not always practical. Newly discovered attacks, as well as ecosystem changes, necessitated some new requirements that apply to TLS 1.2 environments. Those are summarized in . As noted, the TLS 1.3 specification resolves many of the vulnerabilities listed in this document. A system that deploys TLS 1.3 should have fewer vulnerabilities than TLS 1.2 or below. Therefore this document replaces , with an explicit goal to encourage migration of most uses of TLS 1.2 to TLS 1.3. These are minimum recommendations for the use of TLS in the vast majority of implementation and deployment scenarios, with the exception of unauthenticated TLS (see ). Other specifications that reference this document can have stricter requirements related to one or more aspects of the protocol, based on their particular circumstances (e.g., for use with a particular application protocol); when that is the case, implementers are advised to adhere to those stricter requirements. Furthermore, this document provides a floor, not a ceiling, so stronger options are always allowed (e.g., depending on differing evaluations of the importance of cryptographic strength vs. computational load). Community knowledge about the strength of various algorithms and feasible attacks can change quickly, and experience shows that a Best Current Practice (BCP) document about security is a point-in-time statement. Readers are advised to seek out any errata or updates that apply to this document.

Terminology A number of security-related terms in this document are used in the sense defined in . The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

General Recommendations This section provides general recommendations on the secure use of TLS. Recommendations related to cipher suites are discussed in the following section.

Protocol Versions

SSL/TLS Protocol Versions It is important both to stop using old, less secure versions of SSL/TLS and to start using modern, more secure versions; therefore, the following are the recommendations concerning TLS/SSL protocol versions:

• Implementations MUST NOT negotiate SSL version 2. Rationale: Today, SSLv2 is considered insecure .
• Implementations MUST NOT negotiate SSL version 3. Rationale: SSLv3 was an improvement over SSLv2 and plugged some significant security holes but did not support strong cipher suites. SSLv3 does not support TLS extensions, some of which (e.g., renegotiation_info ) are security-critical. In addition, with the emergence of the POODLE attack , SSLv3 is now widely recognized as fundamentally insecure. See for further details.
• Implementations MUST NOT negotiate TLS version 1.0 . Rationale: TLS 1.0 (published in 1999) does not support many modern, strong cipher suites. In addition, TLS 1.0 lacks a per-record Initialization Vector (IV) for CBC-based cipher suites and does not warn against common padding errors. This and other recommendations in this section are in line with .
• Implementations MUST NOT negotiate TLS version 1.1 . Rationale: TLS 1.1 (published in 2006) is a security improvement over TLS 1.0 but still does not support certain stronger cipher suites.
• Implementations MUST support TLS 1.2 and MUST prefer to negotiate TLS version 1.2 over earlier versions of TLS. Rationale: Several stronger cipher suites are available only with TLS 1.2 (published in 2008). In fact, the cipher suites recommended by this document for TLS 1.2 ( below) are not available in older versions of the protocol.
• Implementations SHOULD support TLS 1.3 and, if implemented, MUST prefer to negotiate TLS 1.3 over earlier versions of TLS. Rationale: TLS 1.3 is a major overhaul to the protocol and resolves many of the security issues with TLS 1.2. Even if a TLS implementation defaults to TLS 1.3, as long as it supports TLS 1.2 it MUST follow all the recommendations in this document.
• Implementations of newly-developed protocols SHOULD support TLS 1.3 only with no negotiation of earlier versions, since there is no need to allow legacy endpoints that support TLS 1.2. Similarly, we recommend basing new protocol designs that embed the TLS mechanisms on TLS 1.3, as was done for QUIC ). Rationale: secure deployment of TLS 1.3 is significantly easier and less error prone than the secure deployment of TLS 1.2.

This BCP applies to TLS 1.2, 1.3 and to earlier versions. It is not safe for readers to assume that the recommendations in this BCP apply to any future version of TLS.

DTLS Protocol Versions DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS 1.1 was published. The following are the recommendations with respect to DTLS:

• Implementations MUST NOT negotiate DTLS version 1.0 . Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).
• Implementations MUST support DTLS 1.2 and MUST prefer to negotiate DTLS version 1.2 over earlier versions of DTLS. Version 1.2 of DTLS correlates to version 1.2 of TLS (see above). (There is no version 1.1 of DTLS.)
• Implementations SHOULD support DTLS 1.3 and, if implemented, MUST prefer to negotiate DTLS version 1.3 over earlier versions of DTLS. Version 1.3 of DTLS correlates to version 1.3 of TLS (see above).

Fallback to Lower Versions TLS/DTLS 1.2 clients MUST NOT fall back to earlier TLS versions, since those versions have been deprecated . We note that as a result of that, the SCSV mechanism is no longer needed for clients. In addition, TLS 1.3 implements a new version negotiation mechanism.

Strict TLS The following recommendations are provided to help prevent SSL Stripping and STARTTLS Command Injection (attacks that are summarized in ):

• Many existing application protocols were designed before the use of TLS became common. These protocols typically support TLS in one of two ways: either via a separate port for TLS-only communication (e.g., port 443 for HTTPS) or via a method for dynamically upgrading a channel from unencrypted to TLS-protected (e.g., STARTTLS, which is used in protocols such as SMTP and XMPP). Because dynamic upgrade methods depend on negotiations that begin over an unencrypted channel (e.g., the server might send a flag indicating that TLS is supported or required), they are subject to downgrade attacks (e.g., an attacker could remove such indications); if the server does not indicate that it supports TLS, a client that insists on TLS protection would simply abort the connection, although the details might depend on the particular application protocol in use. In any case, whether the mechanism for protecting the communication channel is a TLS-only port or a dynamic upgrade method, what matters is the end state of the channel. When TLS-only communication is available for a certain protocol, it MUST be used by implementations and MUST be configured by administrators. When a protocol only supports dynamic upgrade, implementations MUST enable a strict local policy (a policy that forbids fallback to plaintext) and administrators MUST use this policy.
• HTTP client and server implementations MUST support the HTTP Strict Transport Security (HSTS) header , in order to allow Web servers to advertise that they are willing to accept TLS-only clients.
• Web servers SHOULD use HSTS to indicate that they are willing to accept TLS-only clients, unless they are deployed in such a way that using HSTS would in fact weaken overall security (e.g., it can be problematic to use HSTS with self-signed certificates, as described in ).

Rationale: Combining unprotected and TLS-protected communication opens the way to SSL Stripping and similar attacks, since an initial part of the communication is not integrity protected and therefore can be manipulated by an attacker whose goal is to keep the communication in the clear.

Compression In order to help prevent compression-related attacks (summarized in ), when using TLS 1.2 implementations and deployments SHOULD NOT support TLS-level compression (); the only exception is when the application protocol in question has been proved not to be open to such attacks, however even in this case extreme caution is warranted because of the potential for future attacks related to TLS compression. More specifically, the HTTP protocol is known to be vulnerable to compression-related attacks. Note: this recommendation applies to TLS 1.2 only, because compression has been removed from TLS 1.3. Rationale: TLS compression has been subject to security attacks, such as the CRIME attack. Implementers should note that compression at higher protocol levels can allow an active attacker to extract cleartext information from the connection. The BREACH attack is one such case. These issues can only be mitigated outside of TLS and are thus outside the scope of this document. See for further details.

TLS Session Resumption Session resumption drastically reduces the number of TLS handshakes and thus is an essential performance feature for most deployments. Stateless session resumption with session tickets is a popular strategy. For TLS 1.2, it is specified in . For TLS 1.3, a more secure PSK-based mechanism is described in . See for a quantitative study of TLS cryptographic "shortcuts", including session resumption. When it is used, the resumption information MUST be authenticated and encrypted to prevent modification or eavesdropping by an attacker. Further recommendations apply to session tickets:

• A strong cipher suite MUST be used when encrypting the ticket (as least as strong as the main TLS cipher suite).
• Ticket keys MUST be changed regularly, e.g., once every week, so as not to negate the benefits of forward secrecy (see for details on forward secrecy). Old ticket keys MUST be destroyed shortly after a new key version is made available.
• For similar reasons, session ticket validity MUST be limited to a reasonable duration (e.g., half as long as ticket key validity).
• TLS 1.2 does not roll the session key forward within a single session. Thus, to prevent an attack where a stolen ticket key is used to decrypt the entire content of a session (negating the concept of forward secrecy), a TLS 1.2 server SHOULD NOT resume sessions that are too old, e.g. sessions that have been open longer than two ticket key rotation periods. Note that this implies that some server implementations might need to abort sessions after a certain duration.

Rationale: session resumption is another kind of TLS handshake, and therefore must be as secure as the initial handshake. This document () recommends the use of cipher suites that provide forward secrecy, i.e. that prevent an attacker who gains momentary access to the TLS endpoint (either client or server) and its secrets from reading either past or future communication. The tickets must be managed so as not to negate this security property. TLS 1.3 provides the powerful option of forward secrecy even within a long-lived connection that is periodically resumed. recommends that clients SHOULD send a "key_share" when initiating session resumption. In order to gain forward secrecy, this document recommends that server implementations SHOULD respond with a "key_share", to complete an ECDHE exchange on each session resumption. As a more performant alternative, server implementations MAY refrain from responding with a "key_share" until a certain amount of time (e.g., measured in days) has passed since the last ECDHE exchange; this implies that the "key_share" operation would not occur for the presumed majority of session resumption requests occurring within a few hours, while still ensuring forward secrecy for longer-lived sessions. TLS session resumption introduces potential privacy issues where the server is able to track the client, in some cases indefinitely. See for more details.

Renegotiation in TLS 1.2 The recommendations in this section apply to TLS 1.2 only, because renegotiation has been removed from TLS 1.3. Renegotiation in TLS 1.2 is a handshake that establishes new cryptographic parameters for an existing session. The mechanism existed in TLS 1.2 and in earlier protocol versions, and was improved following several major attacks including a plaintext injection attack, CVE-2009-3555 . TLS 1.2 clients and servers MUST implement the renegotiation_info extension, as defined in . TLS 1.2 clients MUST send renegotiation_info in the Client Hello. If the server does not acknowledge the extension, the client MUST generate a fatal handshake_failure alert prior to terminating the connection. Rationale: It is not safe for a client to connect to a TLS 1.2 server that does not support renegotiation_info, regardless of whether either endpoint actually implements renegotiation. See also . A related attack resulting from TLS session parameters not properly authenticated is Triple Handshake . To address this attack, TLS 1.2 implementations SHOULD support the extended_master_secret extension defined in .

Post-Handshake Authentication Renegotiation in TLS 1.2 was replaced in TLS 1.3 by separate post-handshake authentication and key update mechanisms. In the context of protocols that multiplex requests over a single connection (such as HTTP/2 ), post-handshake authentication has the same problems as TLS 1.2 renegotiation. Multiplexed protocols SHOULD follow the advice provided for HTTP/2 in .

Server Name Indication (SNI) TLS implementations MUST support the Server Name Indication (SNI) extension defined in for those higher-level protocols that would benefit from it, including HTTPS. However, the actual use of SNI in particular circumstances is a matter of local policy. Implementers are strongly encouraged to support TLS Encrypted Client Hello once has been standardized. Rationale: SNI supports deployment of multiple TLS-protected virtual servers on a single address, and therefore enables fine-grained security for these virtual servers, by allowing each one to have its own certificate. However, SNI also leaks the target domain for a given connection; this information leak is closed by use of TLS Encrypted Client Hello. In order to prevent the attacks described in , a server that does not recognize the presented server name SHOULD NOT continue the handshake and instead SHOULD fail with a fatal-level unrecognized_name(112) alert. Note that this recommendation updates : "If the server understood the ClientHello extension but does not recognize the server name, the server SHOULD take one of two actions: either abort the handshake by sending a fatal-level unrecognized_name(112) alert or continue the handshake." Clients SHOULD abort the handshake if the server acknowledges the SNI extension, but presents a certificate with a different hostname than the one sent by the client.

Application-Layer Protocol Negotiation (ALPN) TLS implementations (both client- and server-side) MUST support the Application-Layer Protocol Negotiation (ALPN) extension . In order to prevent "cross-protocol" attacks resulting from failure to ensure that a message intended for use in one protocol cannot be mistaken for a message for use in another protocol, servers SHOULD strictly enforce the behavior prescribed in : "In the event that the server supports no protocols that the client advertises, then the server SHALL respond with a fatal no_application_protocol alert." Clients SHOULD abort the handshake if the server acknowledges the ALPN extension, but does not select a protocol from the client list. Failure to do so can result in attacks such those described in . Protocol developers are strongly encouraged to register an ALPN identifier for their protocols. This applies to new protocols, as well as well-established protocols.

Zero Round Trip Time (0-RTT) Data in TLS 1.3 The 0-RTT early data feature is new in TLS 1.3. It provides reduced latency when TLS connections are resumed, at the potential cost of security. As a result, it requires special attention from implementers on both the server and the client side. Typically this extends to both the TLS library as well as protocol layers above it. For use in HTTP-over-TLS, readers are referred to for guidance. For QUIC-on-TLS, refer to Sec. 9.2 of . For other protocols, generic guidance is given in Sec. 8 and Appendix E.5 of . To paraphrase Appendix E.5, applications MUST avoid this feature unless an explicit specification exists for the application protocol in question to clarify when 0-RTT is appropriate and secure. This can take the form of an IETF RFC, a non-IETF standard, or even documentation associated with a non-standard protocol.

Recommendations: Cipher Suites TLS 1.2 provided considerable flexibility in the selection of cipher suites. Unfortunately, the security of some of these cipher suites has degraded over time to the point where some are known to be insecure (this is one reason why TLS 1.3 restricted such flexibility). Incorrectly configuring a server leads to no or reduced security. This section includes recommendations on the selection and negotiation of cipher suites.

General Guidelines Cryptographic algorithms weaken over time as cryptanalysis improves: algorithms that were once considered strong become weak. Consequently, they need to be phased out over time and replaced with more secure cipher suites. This helps to ensure that the desired security properties still hold. SSL/TLS has been in existence for almost 20 years and many of the cipher suites that have been recommended in various versions of SSL/TLS are now considered weak or at least not as strong as desired. Therefore, this section modernizes the recommendations concerning cipher suite selection.

• Implementations MUST NOT negotiate the cipher suites with NULL encryption. Rationale: The NULL cipher suites do not encrypt traffic and so provide no confidentiality services. Any entity in the network with access to the connection can view the plaintext of contents being exchanged by the client and server.
  Nevertheless, this document does not discourage software from implementing NULL cipher suites, since they can be useful for testing and debugging.
• Implementations MUST NOT negotiate RC4 cipher suites. Rationale: The RC4 stream cipher has a variety of cryptographic weaknesses, as documented in . Note that DTLS specifically forbids the use of RC4 already.
• Implementations MUST NOT negotiate cipher suites offering less than 112 bits of security, including so-called "export-level" encryption (which provide 40 or 56 bits of security). Rationale: Based on , at least 112 bits of security is needed. 40-bit and 56-bit security are considered insecure today. TLS 1.2 never negotiates 40-bit or 56-bit export ciphers and such ciphers are not supported at all in TLS 1.3.
• Implementations SHOULD NOT negotiate cipher suites that use algorithms offering less than 128 bits of security. Rationale: Cipher suites that offer more than 112 bits but less than 128 bits of security are not considered weak at this time; however, it is expected that their useful lifespan is short enough to justify supporting stronger cipher suites at this time. 128-bit ciphers are expected to remain secure for at least several years, and 256-bit ciphers until the next fundamental technology breakthrough. Note that, because of so-called "meet-in-the-middle" attacks , some legacy cipher suites (e.g., 168-bit 3DES) have an effective key length that is smaller than their nominal key length (112 bits in the case of 3DES). Such cipher suites should be evaluated according to their effective key length.
• Implementations SHOULD NOT negotiate cipher suites based on RSA key transport, a.k.a. "static RSA". Rationale: These cipher suites, which have assigned values starting with the string "TLS_RSA_WITH_*", have several drawbacks, especially the fact that they do not support forward secrecy.
• Implementations SHOULD NOT negotiate cipher suites based on non-ephemeral (static) finite-field Diffie-Hellman key agreement. Rationale: These cipher suites, which have assigned values prefixed by "TLS_DH_*", have several drawbacks, especially the fact that they do not support forward secrecy.
• Implementations MUST support and prefer to negotiate cipher suites offering forward secrecy. However, TLS 1.2 implementations SHOULD NOT negotiate cipher suites based on ephemeral finite-field Diffie-Hellman key agreement (i.e., "TLS_DHE_*" suites). This is justified by the known fragility of the construction (see ) and the limitation around negotiation -- including using , which has seen very limited uptake. Rationale: Forward secrecy (sometimes called "perfect forward secrecy") prevents the recovery of information that was encrypted with older session keys, thus limiting how far back in time data can be decrypted when an attack is successful. See for a detailed discussion.

Cipher Suites for TLS 1.2 Given the foregoing considerations, implementation and deployment of the following cipher suites is RECOMMENDED:

• TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
• TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
• TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
• TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384

As these are authenticated encryption (AEAD) algorithms , these cipher suites are supported only in TLS 1.2 and not in earlier protocol versions. Typically, in order to prefer these suites, the order of suites needs to be explicitly configured in server software. (See for helpful deployment guidelines, but note that its recommendations differ from the current document in some details.) It would be ideal if server software implementations were to prefer these suites by default. Some devices have hardware support for AES-CCM but not AES-GCM, so they are unable to follow the foregoing recommendations regarding cipher suites. There are even devices that do not support public key cryptography at all, but these are out of scope entirely. When using ECDSA signatures for authentication of TLS peers, it is RECOMMENDED that implementations use the NIST curve P-256. In addition, to avoid predictable or repeated nonces (that would allow revealing the long term signing key), it is RECOMMENDED that implementations implement "deterministic ECDSA" as specified in and in line with the recommendations in .

Implementation Details Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the first proposal to any server, unless they have prior knowledge that the server cannot respond to a TLS 1.2 client_hello message. Servers MUST prefer this cipher suite over weaker cipher suites whenever it is proposed, even if it is not the first proposal. Clients are of course free to offer stronger cipher suites, e.g., using AES-256; when they do, the server SHOULD prefer the stronger cipher suite unless there are compelling reasons (e.g., seriously degraded performance) to choose otherwise. The previous version of this document implicitly allowed the old RFC 5246 mandatory-to-implement cipher suite, TLS_RSA_WITH_AES_128_CBC_SHA. At the time of writing, this cipher suite does not provide additional interoperability, except with extremely old clients. As with other cipher suites that do not provide forward secrecy, implementations SHOULD NOT support this cipher suite. Other application protocols specify other cipher suites as mandatory to implement (MTI). allows clients and servers to negotiate ECDH parameters (curves). Both clients and servers SHOULD include the "Supported Elliptic Curves" extension . Clients and servers SHOULD support the NIST P-256 (secp256r1) and X25519 (x25519) curves. Note that deprecates all but the uncompressed point format. Therefore, if the client sends an ec_point_formats extension, the ECPointFormatList MUST contain a single element, "uncompressed".

Cipher Suites for TLS 1.3 This document does not specify any cipher suites for TLS 1.3. Readers are referred to Sec. 9.1 of for cipher suite recommendations.

Limits on Key Usage All ciphers have an upper limit on the amount of traffic that can be securely protected with any given key. In the case of AEAD cipher suites, two separate limits are maintained for each key:

1. Confidentiality limit (CL), i.e., the number of records that can be encrypted.
2. Integrity limit (IL), i.e., the number of records that are allowed to fail authentication.

The latter only applies to DTLS since TLS connections are torn down on the first decryption failure. When a sender is approaching CL, the implementation SHOULD initiate a new handshake (or in TLS 1.3, a Key Update) to rotate the session key. When a receiver has reached IL, the implementation SHOULD close the connection. For all TLS 1.3 cipher suites, readers are referred to for the values of CL and IL. For all DTLS 1.3 cipher suites, readers are referred to . For all AES-GCM cipher suites recommended for TLS 1.2 and DTLS 1.2 in this document, CL can be derived by plugging the corresponding parameters into the inequalities in that apply to random, partially implicit nonces, i.e., the nonce construction used in TLS 1.2. Although the obtained figures are slightly higher than those for TLS 1.3, it is RECOMMENDED that the same limit of 2^24.5 records is used for both versions. For all AES-GCM cipher suites recommended for DTLS 1.2, IL (obtained from the same inequalities referenced above) is 2²⁸.

Public Key Length When using the cipher suites recommended in this document, two public keys are normally used in the TLS handshake: one for the Diffie-Hellman key agreement and one for server authentication. Where a client certificate is used, a third public key is added. With a key exchange based on modular exponential (MODP) Diffie-Hellman groups ("DHE" cipher suites), DH key lengths of at least 2048 bits are REQUIRED. Rationale: For various reasons, in practice, DH keys are typically generated in lengths that are powers of two (e.g., 2¹⁰ = 1024 bits, 2¹¹ = 2048 bits, 2¹² = 4096 bits). Because a DH key of 1228 bits would be roughly equivalent to only an 80-bit symmetric key , it is better to use keys longer than that for the "DHE" family of cipher suites. A DH key of 1926 bits would be roughly equivalent to a 100-bit symmetric key . A DH key of 2048 bits (equivalent to a 112-bit symmetric key) is the minimum allowed by the latest revision of , as of this writing (see in particular Appendix D). As noted in , correcting for the emergence of a TWIRL machine would imply that 1024-bit DH keys yield about 65 bits of equivalent strength and that a 2048-bit DH key would yield about 92 bits of equivalent strength. The Logjam attack further demonstrates that 1024-bit Diffie Hellman parameters should be avoided. With regard to ECDH keys, implementers are referred to the IANA "Supported Groups Registry" (former "EC Named Curve Registry"), within the "Transport Layer Security (TLS) Parameters" registry , and in particular to the "recommended" groups. Curves of less than 224 bits MUST NOT be used. This recommendation is in-line with the latest revision of . When using RSA, servers SHOULD authenticate using certificates with at least a 2048-bit modulus for the public key. In addition, the use of the SHA-256 hash algorithm is RECOMMENDED and SHA-1 or MD5 MUST NOT be used (, and see for more details). Clients MUST indicate to servers that they request SHA-256, by using the "Signature Algorithms" extension defined in TLS 1.2. For TLS 1.3, the same requirement is already specified by .

Truncated HMAC Implementations MUST NOT use the Truncated HMAC extension, defined in . Rationale: the extension does not apply to the AEAD cipher suites recommended above. However it does apply to most other TLS cipher suites. Its use has been shown to be insecure in .

Applicability Statement The recommendations of this document primarily apply to the implementation and deployment of application protocols that are most commonly used with TLS and DTLS on the Internet today. Examples include, but are not limited to:

• Web software and services that wish to protect HTTP traffic with TLS.
• Email software and services that wish to protect IMAP, POP3, or SMTP traffic with TLS.
• Instant-messaging software and services that wish to protect Extensible Messaging and Presence Protocol (XMPP) or Internet Relay Chat (IRC) traffic with TLS.
• Realtime media software and services that wish to protect Secure Realtime Transport Protocol (SRTP) traffic with DTLS.

This document does not modify the implementation and deployment recommendations (e.g., mandatory-to-implement cipher suites) prescribed by existing application protocols that employ TLS or DTLS. If the community that uses such an application protocol wishes to modernize its usage of TLS or DTLS to be consistent with the best practices recommended here, it needs to explicitly update the existing application protocol definition (one example is , which updates ). Designers of new application protocols developed through the Internet Standards Process are expected at minimum to conform to the best practices recommended here, unless they provide documentation of compelling reasons that would prevent such conformance (e.g., widespread deployment on constrained devices that lack support for the necessary algorithms).

Security Services This document provides recommendations for an audience that wishes to secure their communication with TLS to achieve the following:

• Confidentiality: all application-layer communication is encrypted with the goal that no party should be able to decrypt it except the intended receiver.
• Data integrity: any changes made to the communication in transit are detectable by the receiver.
• Authentication: an endpoint of the TLS communication is authenticated as the intended entity to communicate with.

With regard to authentication, TLS enables authentication of one or both endpoints in the communication. In the context of opportunistic security , TLS is sometimes used without authentication. As discussed in , considerations for opportunistic security are not in scope for this document. If deployers deviate from the recommendations given in this document, they need to be aware that they might lose access to one of the foregoing security services. This document applies only to environments where confidentiality is required. It recommends algorithms and configuration options that enforce secrecy of the data in transit. This document also assumes that data integrity protection is always one of the goals of a deployment. In cases where integrity is not required, it does not make sense to employ TLS in the first place. There are attacks against confidentiality-only protection that utilize the lack of integrity to also break confidentiality (see, for instance, in the context of IPsec). This document addresses itself to application protocols that are most commonly used on the Internet with TLS and DTLS. Typically, all communication between TLS clients and TLS servers requires all three of the above security services. This is particularly true where TLS clients are user agents like Web browsers or email software. This document does not address the rarer deployment scenarios where one of the above three properties is not desired, such as the use case described in below. As another scenario where confidentiality is not needed, consider a monitored network where the authorities in charge of the respective traffic domain require full access to unencrypted (plaintext) traffic, and where users collaborate and send their traffic in the clear.

Opportunistic Security There are several important scenarios in which the use of TLS is optional, i.e., the client decides dynamically ("opportunistically") whether to use TLS with a particular server or to connect in the clear. This practice, often called "opportunistic security", is described at length in and is often motivated by a desire for backward compatibility with legacy deployments. In these scenarios, some of the recommendations in this document might be too strict, since adhering to them could cause fallback to cleartext, a worse outcome than using TLS with an outdated protocol version or cipher suite.

IANA Considerations This document has no IANA actions.

Security Considerations This entire document discusses the security practices directly affecting applications using the TLS protocol. This section contains broader security considerations related to technologies used in conjunction with or by TLS.

Host Name Validation Application authors should take note that some TLS implementations do not validate host names. If the TLS implementation they are using does not validate host names, authors might need to write their own validation code or consider using a different TLS implementation. It is noted that the requirements regarding host name validation (and, in general, binding between the TLS layer and the protocol that runs above it) vary between different protocols. For HTTPS, these requirements are defined by Sections 4.3.3, 4.3.4 and 4.3.5 of . Readers are referred to for further details regarding generic host name validation in the TLS context. In addition, that RFC contains a long list of example protocols, some of which implement a policy very different from HTTPS. If the host name is discovered indirectly and in an insecure manner (e.g., by an insecure DNS query for an MX or SRV record), it SHOULD NOT be used as a reference identifier even when it matches the presented certificate. This proviso does not apply if the host name is discovered securely (for further discussion, see and ). Host name validation typically applies only to the leaf "end entity" certificate. Naturally, in order to ensure proper authentication in the context of the PKI, application clients need to verify the entire certification path in accordance with (see also ).

AES-GCM above recommends the use of the AES-GCM authenticated encryption algorithm. Please refer to for general security considerations when using TLS 1.2, and to for security considerations that apply specifically to AES-GCM when used with TLS.

 Nonce Reuse in TLS 1.2 The existence of deployed TLS stacks that mistakenly reuse the AES-GCM nonce is documented in , showing there is an actual risk of AES-GCM getting implemented in an insecure way and thus making TLS sessions that use an AES-GCM cipher suite vulnerable to attacks such as . (See records: CVE-2016-0270, CVE-2016-10213, CVE-2016-10212, CVE-2017-5933.) While this problem has been fixed in TLS 1.3, which enforces a deterministic method to generate nonces from record sequence numbers and shared secrets for all of its AEAD cipher suites (including AES-GCM), TLS 1.2 implementations could still choose their own (potentially insecure) nonce generation methods. It is therefore RECOMMENDED that TLS 1.2 implementations use the 64-bit sequence number to populate the nonce_explicit part of the GCM nonce, as described in the first two paragraphs of . This stronger recommendation updates , which specified that the use of 64-bit sequence numbers to populate the nonce_explicit field was optional. We note that at the time of writing there are no cipher suites defined for nonce reuse resistant algorithms such as AES-GCM-SIV .

Forward Secrecy Forward secrecy (also called "perfect forward secrecy" or "PFS" and defined in ) is a defense against an attacker who records encrypted conversations where the session keys are only encrypted with the communicating parties' long-term keys. Should the attacker be able to obtain these long-term keys at some point later in time, the session keys and thus the entire conversation could be decrypted. In the context of TLS and DTLS, such compromise of long-term keys is not entirely implausible. It can happen, for example, due to:

• A client or server being attacked by some other attack vector, and the private key retrieved.
• A long-term key retrieved from a device that has been sold or otherwise decommissioned without prior wiping.
• A long-term key used on a device as a default key .
• A key generated by a trusted third party like a CA, and later retrieved from it either by extortion or compromise .
• A cryptographic break-through, or the use of asymmetric keys with insufficient length .
• Social engineering attacks against system administrators.
• Collection of private keys from inadequately protected backups.

Forward secrecy ensures in such cases that it is not feasible for an attacker to determine the session keys even if the attacker has obtained the long-term keys some time after the conversation. It also protects against an attacker who is in possession of the long-term keys but remains passive during the conversation. Forward secrecy is generally achieved by using the Diffie-Hellman scheme to derive session keys. The Diffie-Hellman scheme has both parties maintain private secrets and send parameters over the network as modular powers over certain cyclic groups. The properties of the so-called Discrete Logarithm Problem (DLP) allow the parties to derive the session keys without an eavesdropper being able to do so. There is currently no known attack against DLP if sufficiently large parameters are chosen. A variant of the Diffie-Hellman scheme uses elliptic curves instead of the originally proposed modular arithmetic. Given the current state of the art, elliptic-curve Diffie-Hellman appears to be more efficient, permits shorter key lengths, and allows less freedom for implementation errors than finite-field Diffie-Hellman. Unfortunately, many TLS/DTLS cipher suites were defined that do not feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. This document therefore advocates strict use of forward-secrecy-only ciphers.

Diffie-Hellman Exponent Reuse For performance reasons, many TLS implementations reuse Diffie-Hellman and Elliptic Curve Diffie-Hellman exponents across multiple connections. Such reuse can result in major security issues:

• If exponents are reused for too long (in some cases, even as little as a few hours), an attacker who gains access to the host can decrypt previous connections. In other words, exponent reuse negates the effects of forward secrecy.
• TLS implementations that reuse exponents should test the DH public key they receive for group membership, in order to avoid some known attacks. These tests are not standardized in TLS at the time of writing, although general guidance in this area is provided by and available in many protocol implementations.
• Under certain conditions, the use of static finite-field DH keys, or of ephemeral finite-field DH keys that are reused across multiple connections, can lead to timing attacks (such as those described in ) on the shared secrets used in Diffie-Hellman key exchange.
• An "invalid curve" attack can be mounted against elliptic-curve DH if the victim does not verify that the received point lies on the correct curve. If the victim is reusing the DH secrets, the attacker can repeat the probe varying the points to recover the full secret (see and ).

To address these concerns:

• TLS implementations SHOULD NOT use static finite-field DH keys and SHOULD NOT reuse ephemeral finite-field DH keys across multiple connections.
• Server implementations that want to reuse elliptic-curve DH keys SHOULD either use a "safe curve" (e.g., X25519), or perform the checks described in on the received points.

Certificate Revocation The following considerations and recommendations represent the current state of the art regarding certificate revocation, even though no complete and efficient solution exists for the problem of checking the revocation status of common public key certificates :

• Certificate revocation is an important tool when recovering from attacks on the TLS implementation, as well as cases of misissued certificates. TLS implementations MUST implement a strategy to distrust revoked certificates.
• Although Certificate Revocation Lists (CRLs) are the most widely supported mechanism for distributing revocation information, they have known scaling challenges that limit their usefulness, despite workarounds such as partitioned CRLs and delta CRLs. The more modern and the follow-on Let's Revoke build on the availability of Certificate Transparency logs and aggressive compression to allow practical use of the CRL infrastructure, but at the time of writing, neither solution is deployed for client-side revocation processing at scale.
• Proprietary mechanisms that embed revocation lists in the Web browser's configuration database cannot scale beyond a small number of the most heavily used Web servers.
• The On-Line Certification Status Protocol (OCSP) in its basic form presents both scaling and privacy issues. In addition, clients typically "soft-fail", meaning that they do not abort the TLS connection if the OCSP server does not respond. (However, this might be a workaround to avoid denial-of-service attacks if an OCSP responder is taken offline.). For an up-to-date survey of the status of OCSP deployment in the Web PKI see .
• The TLS Certificate Status Request extension (), commonly called "OCSP stapling", resolves the operational issues with OCSP. However, it is still ineffective in the presence of a MITM attacker because the attacker can simply ignore the client's request for a stapled OCSP response.
• defines a certificate extension that indicates that clients must expect stapled OCSP responses for the certificate and must abort the handshake ("hard-fail") if such a response is not available.
• OCSP stapling as used in TLS 1.2 does not extend to intermediate certificates within a certificate chain. The Multiple Certificate Status extension addresses this shortcoming, but it has seen little deployment and had been deprecated by . As a result, we no longer recommend this extension for TLS 1.2.
• TLS 1.3 () allows the association of OCSP information with intermediate certificates by using an extension to the CertificateEntry structure. However using this facility remains impractical because many CAs either do not publish OCSP for CA certificates or publish OCSP reports with a lifetime that is too long to be useful.
• Both CRLs and OCSP depend on relatively reliable connectivity to the Internet, which might not be available to certain kinds of nodes. A common example is newly provisioned devices that need to establish a secure connection in order to boot up for the first time.

For the common use cases of public key certificates in TLS, servers SHOULD support the following as a best practice given the current state of the art and as a foundation for a possible future solution: OCSP and OCSP stapling using the status_request extension defined in . Note that the exact mechanism for embedding the status_request extension differs between TLS 1.2 and 1.3. As a matter of local policy, server operators MAY request that CAs issue must-staple certificates for the server and/or for client authentication, but we recommend to review the operational conditions before deciding on this approach. The considerations in this section do not apply to scenarios where the DANE-TLSA resource record is used to signal to a client which certificate a server considers valid and good to use for TLS connections.

Acknowledgments Thanks to Alexey Melnikov, Andrei Popov, Christian Huitema, Daniel Kahn Gillmor, David Benjamin, Eric Rescorla, Francesca Palombini, Hannes Tschofenig, Hubert Kario, Ilari Liusvaara, John Mattsson, John R Levine, Julien , Leif Johansson, Martin Thomson, Mohit Sahni, Nick Sullivan, Nimrod Aviram, Paul Wouters, Rich Salz, Ryan Sleevi, Sean Turner, Valery Smyslov, Viktor Dukhovni for helpful comments and discussions that have shaped this document. The authors gratefully acknowledge the contribution of Ralph Holz, who was a coauthor of RFC 7525, the previous version of this document. See RFC 7525 for additional acknowledgments for the previous revision of this document.

References Normative References This document requires that Transport Layer Security (TLS) clients and servers never negotiate the use of RC4 cipher suites when they establish connections. This applies to all TLS versions. This document updates RFCs 5246, 4346, and 2246. This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] This document specifies version 1.2 of the Datagram Transport Layer Security (DTLS) protocol. The DTLS protocol provides communications privacy for datagram protocols. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. The DTLS protocol is based on the Transport Layer Security (TLS) protocol and provides equivalent security guarantees. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document updates DTLS 1.0 to work with TLS version 1.2. [STANDARDS-TRACK] This document formally deprecates Transport Layer Security (TLS) versions 1.0 (RFC 2246) and 1.1 (RFC 4346). Accordingly, those documents have been moved to Historic status. These versions lack support for current and recommended cryptographic algorithms and mechanisms, and various government and industry profiles of applications using TLS now mandate avoiding these old TLS versions. TLS version 1.2 became the recommended version for IETF protocols in 2008 (subsequently being obsoleted by TLS version 1.3 in 2018), providing sufficient time to transition away from older versions. Removing support for older versions from implementations reduces the attack surface, reduces opportunity for misconfiguration, and streamlines library and product maintenance. This document also deprecates Datagram TLS (DTLS) version 1.0 (RFC 4347) but not DTLS version 1.2, and there is no DTLS version 1.1. This document updates many RFCs that normatively refer to TLS version 1.0 or TLS version 1.1, as described herein. This document also updates the best practices for TLS usage in RFC 7525; hence, it is part of BCP 195. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. This document specifies version 1.3 of the Datagram Transport Layer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. The DTLS 1.3 protocol is based on the Transport Layer Security (TLS) 1.3 protocol and provides equivalent security guarantees with the exception of order protection / non-replayability. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document obsoletes RFC 6347. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document requires that when Transport Layer Security (TLS) clients and servers establish connections, they never negotiate the use of Secure Sockets Layer (SSL) version 2.0. This document updates the backward compatibility sections found in the Transport Layer Security (TLS). [STANDARDS-TRACK] Secure Socket Layer (SSL) and Transport Layer Security (TLS) renegotiation are vulnerable to an attack in which the attacker forms a TLS connection with the target server, injects content of his choice, and then splices in a new TLS connection from a client. The server treats the client's initial TLS handshake as a renegotiation and thus believes that the initial data transmitted by the attacker is from the same entity as the subsequent client data. This specification defines a TLS extension to cryptographically tie renegotiations to the TLS connections they are being performed over, thus preventing this attack. [STANDARDS-TRACK] The Transport Layer Security (TLS) master secret is not cryptographically bound to important session parameters such as the server certificate. Consequently, it is possible for an active attacker to set up two sessions, one with a client and another with a server, such that the master secrets on the two sessions are the same. Thereafter, any mechanism that relies on the master secret for authentication, including session resumption, becomes vulnerable to a man-in-the-middle attack, where the attacker can simply forward messages back and forth between the client and server. This specification defines a TLS extension that contextually binds the master secret to a log of the full handshake that computes it, thus preventing such attacks. This document updates RFC 7540 by forbidding TLS 1.3 post-handshake authentication, as an analog to the existing TLS 1.2 renegotiation restriction. This document describes a Transport Layer Security (TLS) extension for application-layer protocol negotiation within the TLS handshake. For instances in which multiple application protocols are supported on the same TCP or UDP port, this extension allows the application layer to negotiate which protocol will be used within the TLS connection. Implementors of systems that use public key cryptography to exchange symmetric keys need to make the public keys resistant to some predetermined level of attack. That level of attack resistance is the strength of the system, and the symmetric keys that are exchanged must be at least as strong as the system strength requirements. The three quantities, system strength, symmetric key strength, and public key strength, must be consistently matched for any network protocol usage. While it is fairly easy to express the system strength requirements in terms of a symmetric key length and to choose a cipher that has a key length equal to or exceeding that requirement, it is harder to choose a public key that has a cryptographic strength meeting a symmetric key strength requirement. This document explains how to determine the length of an asymmetric key as a function of a symmetric key strength requirement. Some rules of thumb for estimating equivalent resistance to large-scale attacks on various algorithms are given. The document also addresses how changing the sizes of the underlying large integers (moduli, group sizes, exponents, and so on) changes the time to use the algorithms for key exchange. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document defines a deterministic digital signature generation procedure. Such signatures are compatible with standard Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA) digital signatures and can be processed with unmodified verifiers, which need not be aware of the procedure described therein. Deterministic signatures retain the cryptographic security features associated with digital signatures but can be more easily implemented in various environments, since they do not need access to a source of high-quality randomness. This document describes key exchange algorithms based on Elliptic Curve Cryptography (ECC) for the Transport Layer Security (TLS) protocol. In particular, it specifies the use of Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) key agreement in a TLS handshake and the use of the Elliptic Curve Digital Signature Algorithm (ECDSA) and Edwards-curve Digital Signature Algorithm (EdDSA) as authentication mechanisms. This document obsoletes RFC 4492. This memo specifies two elliptic curves over prime fields that offer a high level of practical security in cryptographic applications, including Transport Layer Security (TLS). These curves are intended to operate at the ~128-bit and ~224-bit security level, respectively, and are generated deterministically based on a list of required properties. The MD5 and SHA-1 hashing algorithms are increasingly vulnerable to attack, and this document deprecates their use in TLS 1.2 and DTLS 1.2 digital signatures. However, this document does not deprecate SHA-1 with Hashed Message Authentication Code (HMAC), as used in record protection. This document updates RFC 5246. Adobe Fastly greenbytes GmbH The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFC 2818, RFC 7231, RFC 7232, RFC 7233, RFC 7235, RFC 7538, RFC 7615, RFC 7694, and portions of RFC 7230. Many application technologies enable secure communication between two entities by means of Internet Public Key Infrastructure Using X.509 (PKIX) certificates in the context of Transport Layer Security (TLS). This document specifies procedures for representing and verifying the identity of application services in such interactions. [STANDARDS-TRACK] This memo describes the use of the Advanced Encryption Standard (AES) in Galois/Counter Mode (GCM) as a Transport Layer Security (TLS) authenticated encryption operation. GCM provides both confidentiality and data origin authentication, can be efficiently implemented in hardware for speeds of 10 gigabits per second and above, and is also well-suited to software implementations. This memo defines TLS cipher suites that use AES-GCM with RSA, DSA, and Diffie-Hellman-based key exchange mechanisms. [STANDARDS-TRACK] This document provides specifications for existing TLS extensions. It is a companion document for RFC 5246, "The Transport Layer Security (TLS) Protocol Version 1.2". The extensions specified are server_name, max_fragment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and status_request. [STANDARDS-TRACK] Informative References US-CERT CA/Browser Forum This memo describes a downgrade-resistant protocol for SMTP transport security between Message Transfer Agents (MTAs), based on the DNS-Based Authentication of Named Entities (DANE) TLSA DNS record. Adoption of this protocol enables an incremental transition of the Internet email backbone to one using encrypted and authenticated Transport Layer Security (TLS). The DNS-Based Authentication of Named Entities (DANE) specification (RFC 6698) describes how to use TLSA resource records secured by DNSSEC (RFC 4033) to associate a server's connection endpoint with its Transport Layer Security (TLS) certificate (thus enabling administrators of domain names to specify the keys used in that domain's TLS servers). However, application protocols that use SRV records (RFC 2782) to indirectly name the target server connection endpoints for a service domain name cannot apply the rules from RFC 6698. Therefore, this document provides guidelines that enable such protocols to locate and use TLSA records. Adobe Fastly greenbytes GmbH The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document specifies the HTTP/1.1 message syntax, message parsing, connection management, and related security concerns. This document obsoletes portions of RFC 7230. Mozilla Apple Inc. This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced latency by introducing field compression and allowing multiple concurrent exchanges on the same connection. This document obsoletes RFC 7540 and RFC 8740. IANA bettercrypto.org The Secure Sockets Layer version 3.0 (SSLv3), as specified in RFC 6101, is not sufficiently secure. This document requires that SSLv3 not be used. The replacement versions, in particular, Transport Layer Security (TLS) 1.2 (RFC 5246), are considerably more secure and capable protocols. This document updates the backward compatibility section of RFC 5246 and its predecessors to prohibit fallback to SSLv3. MITRE This document is a specification of the basic protocol for Internet electronic mail transport. It consolidates, updates, and clarifies several previous documents, making all or parts of most of them obsolete. It covers the SMTP extension mechanisms and best practices for the contemporary Internet, but does not provide details about particular extensions. Although SMTP was designed as a mail transport and delivery protocol, this specification also contains information that is important to its use as a "mail submission" protocol for "split-UA" (User Agent) mail reading systems and mobile environments. [STANDARDS-TRACK] The Internet Message Access Protocol Version 4rev2 (IMAP4rev2) allows a client to access and manipulate electronic mail messages on a server. IMAP4rev2 permits manipulation of mailboxes (remote message folders) in a way that is functionally equivalent to local folders. IMAP4rev2 also provides the capability for an offline client to resynchronize with the server. IMAP4rev2 includes operations for creating, deleting, and renaming mailboxes; checking for new messages; removing messages permanently; setting and clearing flags; parsing per RFCs 5322, 2045, and 2231; searching; and selective fetching of message attributes, texts, and portions thereof. Messages in IMAP4rev2 are accessed by the use of numbers. These numbers are either message sequence numbers or unique identifiers. IMAP4rev2 does not specify a means of posting mail; this function is handled by a mail submission protocol such as the one specified in RFC 6409. The Post Office Protocol - Version 3 (POP3) is intended to permit a workstation to dynamically access a maildrop on a server host in a useful fashion. [STANDARDS-TRACK] This document describes Session Initiation Protocol (SIP), an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. [STANDARDS-TRACK] The Extensible Messaging and Presence Protocol (XMPP) is an application profile of the Extensible Markup Language (XML) that enables the near-real-time exchange of structured yet extensible data between any two or more network entities. This document defines XMPP's core protocol methods: setup and teardown of XML streams, channel encryption, authentication, error handling, and communication primitives for messaging, network availability ("presence"), and request-response interactions. This document obsoletes RFC 3920. [STANDARDS-TRACK] This document describes the use of the Advanced Encryption Standard (AES) Cipher Algorithm in Cipher Block Chaining (CBC) Mode, with an explicit Initialization Vector (IV), as a confidentiality mechanism within the context of the IPsec Encapsulating Security Payload (ESP). Over the last few years, there have been several serious attacks on Transport Layer Security (TLS), including attacks on its most commonly used ciphers and modes of operation. This document summarizes these attacks, with the goal of motivating generic and protocol-specific recommendations on the usage of TLS and Datagram TLS (DTLS). Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are widely used to protect data exchanged over application protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the last few years, several serious attacks on TLS have emerged, including attacks on its most commonly used cipher suites and their modes of operation. This document provides recommendations for improving the security of deployed services that use TLS and DTLS. The recommendations are applicable to the majority of use cases. This Glossary provides definitions, abbreviations, and explanations of terminology for information system security. The 334 pages of entries offer recommendations to improve the comprehensibility of written material that is generated in the Internet Standards Process (RFC 2026). The recommendations follow the principles that such writing should (a) use the same term or definition whenever the same concept is mentioned; (b) use terms in their plainest, dictionary sense; (c) use terms that are already well-established in open publications; and (d) avoid terms that either favor a particular vendor or favor a particular technology or mechanism over other, competing techniques that already exist or could be developed. This memo provides information for the Internet community. This document is published as a historical record of the SSL 3.0 protocol. The original Abstract follows. This document specifies version 3.0 of the Secure Sockets Layer (SSL 3.0) protocol, a security protocol that provides communications privacy over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. This document defines a Historic Document for the Internet community. This document specifies Version 1.0 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications privacy over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. This document specifies Version 1.1 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. This document specifies Version 1.0 of the Datagram Transport Layer Security (DTLS) protocol. The DTLS protocol provides communications privacy for datagram protocols. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. The DTLS protocol is based on the Transport Layer Security (TLS) protocol and provides equivalent security guarantees. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document defines a Signaling Cipher Suite Value (SCSV) that prevents protocol downgrade attacks on the Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) protocols. It updates RFCs 2246, 4346, 4347, 5246, and 6347. Server update considerations are included. This specification defines a mechanism enabling web sites to declare themselves accessible only via secure connections and/or for users to be able to direct their user agent(s) to interact with given sites only over secure connections. This overall policy is referred to as HTTP Strict Transport Security (HSTS). The policy is declared by web sites via the Strict-Transport-Security HTTP response header field and/or by other means, such as user agent configuration, for example. [STANDARDS-TRACK] This document describes a mechanism that enables the Transport Layer Security (TLS) server to resume sessions and avoid keeping per-client session state. The TLS server encapsulates the session state into a ticket and forwards it to the client. The client can subsequently resume a session using the obtained ticket. This document obsoletes RFC 4507. [STANDARDS-TRACK] RTFM, Inc. Fastly Cloudflare Cloudflare This document describes a mechanism in Transport Layer Security (TLS) for encrypting a ClientHello message under a server public key. Discussion Venues This note is to be removed before publishing as an RFC. Source for this draft and an issue tracker can be found at https://github.com/tlswg/draft-ietf-tls-esni (https://github.com/tlswg/draft-ietf-tls-esni). Using TLS early data creates an exposure to the possibility of a replay attack. This document defines mechanisms that allow clients to communicate with servers about HTTP requests that are sent in early data. Techniques are described that use these mechanisms to mitigate the risk of replay. This document describes how Transport Layer Security (TLS) is used to secure QUIC. Traditional finite-field-based Diffie-Hellman (DH) key exchange during the Transport Layer Security (TLS) handshake suffers from a number of security, interoperability, and efficiency shortcomings. These shortcomings arise from lack of clarity about which DH group parameters TLS servers should offer and clients should accept. This document offers a solution to these shortcomings for compatible peers by using a section of the TLS "Supported Groups Registry" (renamed from "EC Named Curve Registry" by this document) to establish common finite field DH parameters with known structure and a mechanism for peers to negotiate support for these groups. This document updates TLS versions 1.0 (RFC 2246), 1.1 (RFC 4346), and 1.2 (RFC 5246), as well as the TLS Elliptic Curve Cryptography (ECC) extensions (RFC 4492). This document defines algorithms for Authenticated Encryption with Associated Data (AEAD), and defines a uniform interface and a registry for such algorithms. The interface and registry can be used as an application-independent set of cryptoalgorithm suites. This approach provides advantages in efficiency and security, and promotes the reuse of crypto implementations. [STANDARDS-TRACK] ETH Zurich Mozilla Cloudflare An Authenticated Encryption with Associated Data (AEAD) algorithm provides confidentiality and integrity. Excessive use of the same key can give an attacker advantages in breaking these properties. This document provides simple guidance for users of common AEAD functions about how to limit the use of keys in order to bound the advantage given to an attacker. It considers limits in both single- and multi-key settings. This document provides recommendations for the use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP). This document updates RFC 6120. This memo documents the process used by the Internet community for the standardization of protocols and procedures. It defines the stages in the standardization process, the requirements for moving a document between stages and the types of documents used during this process. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document defines the concept "Opportunistic Security" in the context of communications protocols. Protocol designs based on Opportunistic Security use encryption even when authentication is not available, and use authentication when possible, thereby removing barriers to the widespread use of encryption on the Internet. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This memo specifies two authenticated encryption algorithms that are nonce misuse resistant -- that is, they do not fail catastrophically if a nonce is repeated. This document is the product of the Crypto Forum Research Group. This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. This document obsoletes RFC 6962. It also specifies a new TLS extension that is used to send various CT log artifacts. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. This document specifies a protocol useful in determining the current status of a digital certificate without requiring Certificate Revocation Lists (CRLs). Additional mechanisms addressing PKIX operational requirements are specified in separate documents. This document obsoletes RFCs 2560 and 6277. It also updates RFC 5912. The purpose of the TLS feature extension is to prevent downgrade attacks that are not otherwise prevented by the TLS protocol. In particular, the TLS feature extension may be used to mandate support for revocation checking features in the TLS protocol such as Online Certificate Status Protocol (OCSP) stapling. Informing clients that an OCSP status response will always be stapled permits an immediate failure in the case that the response is not stapled. This in turn prevents a denial-of-service attack that might otherwise be possible. This document defines the Transport Layer Security (TLS) Certificate Status Version 2 Extension to allow clients to specify and support several certificate status methods. (The use of the Certificate Status extension is commonly referred to as "OCSP stapling".) Also defined is a new method based on the Online Certificate Status Protocol (OCSP) that servers can use to provide status information about not only the server's own certificate but also the status of intermediate certificates in the chain. Encrypted communication on the Internet often uses Transport Layer Security (TLS), which depends on third parties to certify the keys used. This document improves on that situation by enabling the administrators of domain names to specify the keys used in that domain's TLS servers. This requires matching improvements in TLS client software, but no change in TLS server software. [STANDARDS-TRACK]

Differences from RFC 7525 This revision of the Best Current Practices contains numerous changes, and this section is focused on the normative changes.

• High level differences:
  • Clarified items (e.g. renegotiation) that only apply to TLS 1.2.
  • Changed status of TLS 1.0 and 1.1 from SHOULD NOT to MUST NOT.
  • Added TLS 1.3 at a SHOULD level.
  • Similar changes to DTLS.
  • Specific guidance for multiplexed protocols.
  • MUST-level implementation requirement for ALPN, and more specific SHOULD-level guidance for ALPN and SNI.
  • Clarified discussion of strict TLS policies, including MUST-level recommendations.
  • Limits on key usage.
  • New attacks since : ALPACA, Raccoon, Logjam, "Nonce-Disrespecting Adversaries".
  • RFC 6961 (OCSP status_request_v2) has been deprecated.
• Differences specific to TLS 1.2:
  • SHOULD-level guidance on AES-GCM nonce generation.
  • SHOULD NOT use (static or ephemeral) finite-field DH key agreement.
  • SHOULD NOT reuse ephemeral finite-field DH keys across multiple connections.
  • 2048-bit DH now a MUST, ECDH minimal curve size is 224, vs. 192 previously.
  • Support for extended_master_secret is a SHOULD. Also removed other, more complicated, related mitigations.
  • MUST-level restriction on session ticket validity, replacing a SHOULD.
  • SHOULD-level restriction on the TLS session duration, depending on the rotation period of an ticket key.
  • Drop TLS_DHE_RSA_WITH_AES from the recommended ciphers
  • Add TLS_ECDHE_ECDSA_WITH_AES to the recommended ciphers
  • SHOULD NOT use the old MTI cipher suite, TLS_RSA_WITH_AES_128_CBC_SHA.
  • Recommend curve X25519 alongside NIST P-256
• Differences specific to TLS 1.3:
  • New TLS 1.3 capabilities: 0-RTT.
  • Removed capabilities: renegotiation, compression.
  • Added mention of TLS Encrypted Client Hello, but no recommendation to use until it is finalized.
  • SHOULD-level requirement for forward secrecy in TLS 1.3 session resumption.
  • Generic SHOULD-level guidance to avoid 0-RTT unless it is documented for the particular protocol.

Document History Note to RFC Editor: please remove before publication.

draft-ietf-uta-rfc7525bis-07

• Addressed AD reviews by Francesca and Paul.

draft-ietf-uta-rfc7525bis-06

• Addressed several I-D nits raised by the document shepherd.

draft-ietf-uta-rfc7525bis-05

• Addressed WG Last Call comments, specifically:
  • More clarity and guidance on session resumption.
  • Clarity on TLS 1.2 renegotiation.
  • Wording on the 0-RTT feature aligned with RFC 8446.
  • SHOULD NOT guidance on static and ephemeral finite field DH cipher suites.
  • Revamped the recommended TLS 1.2 cipher suites, removing DHE and adding ECDSA. The latter due to the wide adoption of ECDSA certificates and in line with RFC 8446.
  • Recommendation to use deterministic ECDSA.
  • Finally deprecated the old TLS 1.2 MTI cipher suite.
  • Deeper discussion of ECDH public key reuse issues, and as a result, recommended support of X25519.
  • Reworded the section on certificate revocation and OCSP following a long mailing list thread.

draft-ietf-uta-rfc7525bis-04

• No version fallback from TLS 1.2 to earlier versions, therefore no SCSV.

draft-ietf-uta-rfc7525bis-03

• Cipher integrity and confidentiality limits.
• Require extended_master_secret.

draft-ietf-uta-rfc7525bis-02

• Adjusted text about ALPN support in application protocols
• Incorporated text from draft-ietf-tls-md5-sha1-deprecate

draft-ietf-uta-rfc7525bis-01

• Many more changes, including:
  • SHOULD-level requirement for forward secrecy in TLS 1.3 session resumption.
  • Removed TLS 1.2 capabilities: renegotiation, compression.
  • Specific guidance for multiplexed protocols.
  • MUST-level implementation requirement for ALPN, and more specific SHOULD-level guidance for ALPN and SNI.
  • Generic SHOULD-level guidance to avoid 0-RTT unless it is documented for the particular protocol.
  • SHOULD-level guidance on AES-GCM nonce generation in TLS 1.2.
  • SHOULD NOT use static DH keys or reuse ephemeral DH keys across multiple connections.
  • 2048-bit DH now a MUST, ECDH minimal curve size is 224, up from 192.

draft-ietf-uta-rfc7525bis-00

• Renamed: WG document.
• Started populating list of changes from RFC 7525.
• General rewording of abstract and intro for revised version.
• Protocol versions, fallback.
• Reference to ECHO.

draft-sheffer-uta-rfc7525bis-00

• Renamed, since the BCP number does not change.
• Added an empty "Differences from RFC 7525" section.

draft-sheffer-uta-bcp195bis-00

• Initial release, the RFC 7525 text as-is, with some minor editorial changes to the references.