Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)

Internet-Draft	TLS Recommendations	May 2022
Sheffer, et al.	Expires 28 November 2022	[Page]

Abstract

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.¶

1. Introduction

Transport Layer Security (TLS) and Datagram Transport Security Layer (DTLS) are widely used to protect data exchanged over application protocols such as HTTP [HTTP1.1] [HTTP2], SMTP [RFC5321], IMAP [RFC9051], POP [STD53], SIP [RFC3261], and XMPP [RFC6120]. 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 [RFC3602] and RC4 [RFC7465] encryption algorithms, which together were once the most widely deployed ciphers, have been attacked in the context of TLS. A companion document [RFC7457] 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 [RFC5246] and DTLS 1.2 [RFC6347], along with earlier protocol versions. This guidance is included in the original [RFC7525] 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 [RFC8996].¶
Version 1.3 of TLS [RFC8446] was released, followed by version 1.3 of DTLS [RFC9147]; 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 Section 5. 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 Appendix A.¶

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 [RFC7525], 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 Section 5). 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.¶

3. General Recommendations

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

3.1. Protocol Versions

3.1.1. 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 [RFC6176].¶
Implementations MUST NOT negotiate SSL version 3.¶

Rationale: SSLv3 [RFC6101] 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 [RFC5746]) are security-critical. In addition, with the emergence of the POODLE attack [POODLE], SSLv3 is now widely recognized as fundamentally insecure. See [DEP-SSLv3] for further details.¶
Implementations MUST NOT negotiate TLS version 1.0 [RFC2246].¶

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 [RFC8996].¶
Implementations MUST NOT negotiate TLS version 1.1 [RFC4346].¶

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 [RFC5246] 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 (Section 4.2 below) are not available in older versions of the protocol.¶
Implementations SHOULD support TLS 1.3 [RFC8446] 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 [RFC9001]).¶

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.¶

3.1.2. 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 [RFC4347].¶

Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).¶
Implementations MUST support DTLS 1.2 [RFC6347] 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 [RFC9147] 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).¶

3.1.3. Fallback to Lower Versions

TLS/DTLS 1.2 clients MUST NOT fall back to earlier TLS versions, since those versions have been deprecated [RFC8996]. We note that as a result of that, the SCSV mechanism [RFC7507] is no longer needed for clients. In addition, TLS 1.3 implements a new version negotiation mechanism.¶

3.2. Strict TLS

The following recommendations are provided to help prevent SSL Stripping and STARTTLS Command Injection (attacks that are summarized in [RFC7457]):¶

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 [RFC6797], 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 Section 11.3 of [RFC6797]).¶

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.¶

3.3. Compression

In order to help prevent compression-related attacks (summarized in Section 2.6 of [RFC7457]), when using TLS 1.2 implementations and deployments SHOULD NOT support TLS-level compression (Section 6.2.2 of [RFC5246]); 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 Section 2.6 of [RFC7457] for further details.¶

3.4. 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 [RFC5077]. For TLS 1.3, a more secure PSK-based mechanism is described in Section 4.6.1 of [RFC8446]. See [Springall16] 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 Section 7.3 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 (Section 4) 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. Section 2.2 of [RFC8446] 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 [Sy2018] for more details.¶

3.5. 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 [CVE].¶

TLS 1.2 clients and servers MUST implement the renegotiation_info extension, as defined in [RFC5746].¶

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 Section 4.1 of [RFC5746].¶

A related attack resulting from TLS session parameters not properly authenticated is Triple Handshake [triple-handshake]. To address this attack, TLS 1.2 implementations SHOULD support the extended_master_secret extension defined in [RFC7627].¶

3.6. 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 [HTTP2]), post-handshake authentication has the same problems as TLS 1.2 renegotiation. Multiplexed protocols SHOULD follow the advice provided for HTTP/2 in [RFC8740].¶

3.7. Server Name Indication (SNI)

TLS implementations MUST support the Server Name Indication (SNI) extension defined in Section 3 of [RFC6066] 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 [I-D.ietf-tls-esni] 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 [ALPACA], 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 Section 3 of [RFC6066]: "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.¶

3.8. Application-Layer Protocol Negotiation (ALPN)

TLS implementations (both client- and server-side) MUST support the Application-Layer Protocol Negotiation (ALPN) extension [RFC7301].¶

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 Section 3.2 of [RFC7301]: "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 [ALPACA].¶

Protocol developers are strongly encouraged to register an ALPN identifier for their protocols. This applies to new protocols, as well as well-established protocols.¶

3.9. 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 [RFC8470] for guidance.¶

For QUIC-on-TLS, refer to Sec. 9.2 of [RFC9001].¶

For other protocols, generic guidance is given in Sec. 8 and Appendix E.5 of [RFC8446]. 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.¶

4. 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.¶

4.1. 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 [RFC7465]. 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 [RFC3766], 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 [Multiple-Encryption], 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 [RACCOON]) and the limitation around negotiation -- including using [RFC7919], 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 Section 7.3 for a detailed discussion.¶

4.2. 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 [RFC5116], 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 [BETTERCRYPTO] 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 [RFC6979] and in line with the recommendations in [RFC8446].¶

4.2.1. 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).¶

[RFC8422] allows clients and servers to negotiate ECDH parameters (curves). Both clients and servers SHOULD include the "Supported Elliptic Curves" extension [RFC8422]. Clients and servers SHOULD support the NIST P-256 (secp256r1) [RFC8422] and X25519 (x25519) [RFC7748] curves. Note that [RFC8422] 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".¶

4.3. 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 [RFC8446] for cipher suite recommendations.¶

4.4. 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:¶

Confidentiality limit (CL), i.e., the number of records that can be encrypted.¶
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 Section 5.5 of [RFC8446] for the values of CL and IL. For all DTLS 1.3 cipher suites, readers are referred to Section 4.5.3 of [RFC9147].¶

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 Section 6.1 of [I-D.irtf-cfrg-aead-limits] 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.¶

4.5. 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 [RFC3766], 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 [RFC3766]. A DH key of 2048 bits (equivalent to a 112-bit symmetric key) is the minimum allowed by the latest revision of [NIST.SP.800-56A], as of this writing (see in particular Appendix D).¶

As noted in [RFC3766], 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 [Logjam] 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 [IANA_TLS], 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 [NIST.SP.800-56A].¶

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 ([RFC9155], and see [CAB-Baseline] 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 [RFC8446].¶

4.6. Truncated HMAC

Implementations MUST NOT use the Truncated HMAC extension, defined in Section 7 of [RFC6066].¶

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 [PatersonRS11].¶

5. 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 [RFC7590], which updates [RFC6120]).¶

Designers of new application protocols developed through the Internet Standards Process [RFC2026] 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).¶

5.1. 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 [RFC7435], TLS is sometimes used without authentication. As discussed in Section 5.2, 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, [DegabrieleP07] 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 Section 5.2 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.¶

5.2. 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 [RFC7435] 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.¶

7. 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.¶

7.1. 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 [I-D.ietf-httpbis-semantics].¶

Readers are referred to [RFC6125] 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 [RFC6125] even when it matches the presented certificate. This proviso does not apply if the host name is discovered securely (for further discussion, see [DANE-SRV] and [DANE-SMTP]).¶

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 [RFC5280] (see also [RFC6125]).¶

7.2. AES-GCM

Section 4.2 above recommends the use of the AES-GCM authenticated encryption algorithm. Please refer to Section 11 of [RFC5246] for general security considerations when using TLS 1.2, and to Section 6 of [RFC5288] for security considerations that apply specifically to AES-GCM when used with TLS.¶

7.2.1. Nonce Reuse in TLS 1.2

The existence of deployed TLS stacks that mistakenly reuse the AES-GCM nonce is documented in [Boeck2016], 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 [Joux2006]. (See [CVE] 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 Section 5.3 of [RFC8446]. This stronger recommendation updates Section 3 of [RFC5288], 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 [RFC8452].¶

7.3. Forward Secrecy

Forward secrecy (also called "perfect forward secrecy" or "PFS" and defined in [RFC4949]) 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 [Heninger2012].¶
A key generated by a trusted third party like a CA, and later retrieved from it either by extortion or compromise [Soghoian2011].¶
A cryptographic break-through, or the use of asymmetric keys with insufficient length [Kleinjung2010].¶
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.¶

7.4. 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 [NIST.SP.800-56A] 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 [RACCOON]) 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 [Antipa2003] and [Jager2015]).¶

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" [SAFECURVES] (e.g., X25519), or perform the checks described in [NIST.SP.800-56A] on the received points.¶

7.5. 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 [RFC5280]:¶

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 [CRLite] and the follow-on Let's Revoke [LetsRevoke] build on the availability of Certificate Transparency [RFC9162] 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) [RFC6960] 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 [Chung18].¶
The TLS Certificate Status Request extension (Section 8 of [RFC6066]), 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.¶
[RFC7633] 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 [RFC6961] addresses this shortcoming, but it has seen little deployment and had been deprecated by [RFC8446]. As a result, we no longer recommend this extension for TLS 1.2.¶
TLS 1.3 (Section 4.4.2.1 of [RFC8446]) 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 [RFC6960] and OCSP stapling using the status_request extension defined in [RFC6066]. 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 [RFC7633] 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 [RFC6698] is used to signal to a client which certificate a server considers valid and good to use for TLS connections.¶

9. References

9.1. Normative References

[I-D.ietf-httpbis-semantics]: Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP Semantics", Work in Progress, Internet-Draft, draft-ietf-httpbis-semantics-19, 12 September 2021, <https://www.ietf.org/archive/id/draft-ietf-httpbis-semantics-19.txt>.
[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC3766]: Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, DOI 10.17487/RFC3766, April 2004, <https://www.rfc-editor.org/info/rfc3766>.
[RFC5246]: Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <https://www.rfc-editor.org/info/rfc5246>.
[RFC5288]: Salowey, J., Choudhury, A., and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, DOI 10.17487/RFC5288, August 2008, <https://www.rfc-editor.org/info/rfc5288>.
[RFC5746]: Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, "Transport Layer Security (TLS) Renegotiation Indication Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010, <https://www.rfc-editor.org/info/rfc5746>.
[RFC6066]: Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, <https://www.rfc-editor.org/info/rfc6066>.
[RFC6125]: Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6176]: Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer (SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March 2011, <https://www.rfc-editor.org/info/rfc6176>.
[RFC6347]: Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6979]: Pornin, T., "Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2013, <https://www.rfc-editor.org/info/rfc6979>.
[RFC7301]: Friedl, S., Popov, A., Langley, A., and E. Stephan, "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7465]: Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, DOI 10.17487/RFC7465, February 2015, <https://www.rfc-editor.org/info/rfc7465>.
[RFC7627]: Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A., Langley, A., and M. Ray, "Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension", RFC 7627, DOI 10.17487/RFC7627, September 2015, <https://www.rfc-editor.org/info/rfc7627>.
[RFC7748]: Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8422]: Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS) Versions 1.2 and Earlier", RFC 8422, DOI 10.17487/RFC8422, August 2018, <https://www.rfc-editor.org/info/rfc8422>.
[RFC8446]: Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.
[RFC8740]: Benjamin, D., "Using TLS 1.3 with HTTP/2", RFC 8740, DOI 10.17487/RFC8740, February 2020, <https://www.rfc-editor.org/info/rfc8740>.
[RFC8996]: Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS 1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021, <https://www.rfc-editor.org/info/rfc8996>.
[RFC9147]: Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022, <https://www.rfc-editor.org/info/rfc9147>.
[RFC9155]: Velvindron, L., Moriarty, K., and A. Ghedini, "Deprecating MD5 and SHA-1 Signature Hashes in TLS 1.2 and DTLS 1.2", RFC 9155, DOI 10.17487/RFC9155, December 2021, <https://www.rfc-editor.org/info/rfc9155>.

9.2. Informative References

[ALPACA]: Brinkmann, M., Dresen, C., Merget, R., Poddebniak, D., Müller, J., Somorovsky, J., Schwenk, J., and S. Schinzel, "ALPACA: Application Layer Protocol Confusion - Analyzing and Mitigating Cracks in TLS Authentication", 30th USENIX Security Symposium (USENIX Security 21) , 2021, <https://www.usenix.org/conference/usenixsecurity21/presentation/brinkmann>.
[Antipa2003]: Antipa, A., Brown, D. R. L., Menezes, A., Struik, R., and S. A. Vanstone, "Validation of Elliptic Curve Public Keys", Public Key Cryptography - PKC 2003 , 2003.
[BETTERCRYPTO]: bettercrypto.org, "Applied Crypto Hardening", April 2015, <https://bettercrypto.org/>.
[Boeck2016]: Böck, H., Zauner, A., Devlin, S., Somorovsky, J., and P. Jovanovic, "Nonce-Disrespecting Adversaries: Practical Forgery Attacks on GCM in TLS", May 2016, <https://eprint.iacr.org/2016/475.pdf>.
[CAB-Baseline]: CA/Browser Forum, "Baseline Requirements for the Issuance and Management of Publicly-Trusted Certificates Version 1.1.6", 2013, <https://www.cabforum.org/documents.html>.
[Chung18]: Chung, T., Lok, J., Chandrasekaran, B., Choffnes, D., Levin, D., Maggs, B., Mislove, A., Rula, J., Sullivan, N., and C. Wilson, "Is the Web Ready for OCSP Must-Staple?", Proceedings of the Internet Measurement Conference 2018, DOI 10.1145/3278532.3278543, October 2018, <https://doi.org/10.1145/3278532.3278543>.
[CRLite]: Larisch, J., Choffnes, D., Levin, D., Maggs, B., Mislove, A., and C. Wilson, "CRLite: A Scalable System for Pushing All TLS Revocations to All Browsers", 2017 IEEE Symposium on Security and Privacy (SP), DOI 10.1109/sp.2017.17, May 2017, <https://doi.org/10.1109/sp.2017.17>.
[CVE]: MITRE, "Common Vulnerabilities and Exposures", <https://cve.mitre.org>.
[DANE-SMTP]: Dukhovni, V. and W. Hardaker, "SMTP Security via Opportunistic DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10.17487/RFC7672, October 2015, <https://www.rfc-editor.org/info/rfc7672>.
[DANE-SRV]: Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-Based Authentication of Named Entities (DANE) TLSA Records with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October 2015, <https://www.rfc-editor.org/info/rfc7673>.
[DegabrieleP07]: Degabriele, J. and K. Paterson, "Attacking the IPsec Standards in Encryption-only Configurations", 2007 IEEE Symposium on Security and Privacy (SP '07), DOI 10.1109/sp.2007.8, May 2007, <https://doi.org/10.1109/sp.2007.8>.
[DEP-SSLv3]: Barnes, R., Thomson, M., Pironti, A., and A. Langley, "Deprecating Secure Sockets Layer Version 3.0", RFC 7568, DOI 10.17487/RFC7568, June 2015, <https://www.rfc-editor.org/info/rfc7568>.
[Heninger2012]: Heninger, N., Durumeric, Z., Wustrow, E., and J. A. Halderman, "Mining Your Ps and Qs: Detection of Widespread Weak Keys in Network Devices", Usenix Security Symposium 2012, 2012.
[HTTP1.1]: Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-httpbis-messaging-19, 12 September 2021, <https://www.ietf.org/archive/id/draft-ietf-httpbis-messaging-19.txt>.
[HTTP2]: Thomson, M. and C. Benfield, "HTTP/2", Work in Progress, Internet-Draft, draft-ietf-httpbis-http2bis-07, 24 January 2022, <https://www.ietf.org/archive/id/draft-ietf-httpbis-http2bis-07.txt>.
[I-D.ietf-tls-esni]: Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS Encrypted Client Hello", Work in Progress, Internet-Draft, draft-ietf-tls-esni-14, 13 February 2022, <https://www.ietf.org/archive/id/draft-ietf-tls-esni-14.txt>.
[I-D.irtf-cfrg-aead-limits]: Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on AEAD Algorithms", Work in Progress, Internet-Draft, draft-irtf-cfrg-aead-limits-04, 7 March 2022, <https://www.ietf.org/archive/id/draft-irtf-cfrg-aead-limits-04.txt>.
[IANA_TLS]: IANA, "Transport Layer Security (TLS) Parameters", <https://www.iana.org/assignments/tls-parameters>.
[Jager2015]: Jager, T., Schwenk, J., and J. Somorovsky, "Practical Invalid Curve Attacks on TLS-ECDH", European Symposium on Research in Computer Security (ESORICS) 2015 , 2015.
[Joux2006]: Joux, A., "Authentication Failures in NIST version of GCM", 2006, <https://csrc.nist.gov/csrc/media/projects/block-cipher-techniques/documents/bcm/comments/800-38-series-drafts/gcm/joux_comments.pdf>.
[Kleinjung2010]: Kleinjung, T., Aoki, K., Franke, J., Lenstra, A., Thomé, E., Bos, J., Gaudry, P., Kruppa, A., Montgomery, P., Osvik, D., te Riele, H., Timofeev, A., and P. Zimmermann, "Factorization of a 768-Bit RSA Modulus", Advances in Cryptology - CRYPTO 2010 pp. 333-350, DOI 10.1007/978-3-642-14623-7_18, 2010, <https://doi.org/10.1007/978-3-642-14623-7_18>.
[LetsRevoke]: Smith, T., Dickinson, L., and K. Seamons, "Let's Revoke: Scalable Global Certificate Revocation", Proceedings 2020 Network and Distributed System Security Symposium, DOI 10.14722/ndss.2020.24084, 2020, <https://doi.org/10.14722/ndss.2020.24084>.
[Logjam]: Adrian, D., Bhargavan, K., Durumeric, Z., Gaudry, P., Green, M., Halderman, J., Heninger, N., Springall, D., Thomé, E., Valenta, L., VanderSloot, B., Wustrow, E., Zanella-Béguelin, S., and P. Zimmermann, "Imperfect Forward Secrecy: How Diffie-Hellman Fails in Practice", Proceedings of the 22nd ACM SIGSAC Conference on Computer and Communications Security, DOI 10.1145/2810103.2813707, October 2015, <https://doi.org/10.1145/2810103.2813707>.
[Multiple-Encryption]: Merkle, R. and M. Hellman, "On the security of multiple encryption", Communications of the ACM Vol. 24, pp. 465-467, DOI 10.1145/358699.358718, July 1981, <https://doi.org/10.1145/358699.358718>.
[NIST.SP.800-56A]: Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R. Davis, "Recommendation for pair-wise key-establishment schemes using discrete logarithm cryptography", National Institute of Standards and Technology report, DOI 10.6028/nist.sp.800-56ar3, April 2018, <https://doi.org/10.6028/nist.sp.800-56ar3>.
[PatersonRS11]: Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag Size Does Matter: Attacks and Proofs for the TLS Record Protocol", Lecture Notes in Computer Science pp. 372-389, DOI 10.1007/978-3-642-25385-0_20, 2011, <https://doi.org/10.1007/978-3-642-25385-0_20>.
[POODLE]: US-CERT, "SSL 3.0 Protocol Vulnerability and POODLE Attack", October 2014, <https://www.us-cert.gov/ncas/alerts/TA14-290A>.
[RACCOON]: Merget, R., Brinkmann, M., Aviram, N., Somorovsky, J., Mittmann, J., and J. Schwenk, "Raccoon Attack: Finding and Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)", 30th USENIX Security Symposium (USENIX Security 21) , 2021, <https://www.usenix.org/conference/usenixsecurity21/presentation/merget>.
[RFC2026]: Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996, <https://www.rfc-editor.org/info/rfc2026>.
[RFC2246]: Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, DOI 10.17487/RFC2246, January 1999, <https://www.rfc-editor.org/info/rfc2246>.
[RFC3261]: Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, <https://www.rfc-editor.org/info/rfc3261>.
[RFC3602]: Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher Algorithm and Its Use with IPsec", RFC 3602, DOI 10.17487/RFC3602, September 2003, <https://www.rfc-editor.org/info/rfc3602>.
[RFC4346]: Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, DOI 10.17487/RFC4346, April 2006, <https://www.rfc-editor.org/info/rfc4346>.
[RFC4347]: Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security", RFC 4347, DOI 10.17487/RFC4347, April 2006, <https://www.rfc-editor.org/info/rfc4347>.
[RFC4949]: Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, <https://www.rfc-editor.org/info/rfc4949>.
[RFC5077]: Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, DOI 10.17487/RFC5077, January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5116]: McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, <https://www.rfc-editor.org/info/rfc5116>.
[RFC5280]: Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <https://www.rfc-editor.org/info/rfc5280>.
[RFC5321]: Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008, <https://www.rfc-editor.org/info/rfc5321>.
[RFC6101]: Freier, A., Karlton, P., and P. Kocher, "The Secure Sockets Layer (SSL) Protocol Version 3.0", RFC 6101, DOI 10.17487/RFC6101, August 2011, <https://www.rfc-editor.org/info/rfc6101>.
[RFC6120]: Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120, March 2011, <https://www.rfc-editor.org/info/rfc6120>.
[RFC6698]: Hoffman, P. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 2012, <https://www.rfc-editor.org/info/rfc6698>.
[RFC6797]: Hodges, J., Jackson, C., and A. Barth, "HTTP Strict Transport Security (HSTS)", RFC 6797, DOI 10.17487/RFC6797, November 2012, <https://www.rfc-editor.org/info/rfc6797>.
[RFC6960]: Santesson, S., Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 6960, DOI 10.17487/RFC6960, June 2013, <https://www.rfc-editor.org/info/rfc6960>.
[RFC6961]: Pettersen, Y., "The Transport Layer Security (TLS) Multiple Certificate Status Request Extension", RFC 6961, DOI 10.17487/RFC6961, June 2013, <https://www.rfc-editor.org/info/rfc6961>.
[RFC7435]: Dukhovni, V., "Opportunistic Security: Some Protection Most of the Time", RFC 7435, DOI 10.17487/RFC7435, December 2014, <https://www.rfc-editor.org/info/rfc7435>.
[RFC7457]: Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing Known Attacks on Transport Layer Security (TLS) and Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457, February 2015, <https://www.rfc-editor.org/info/rfc7457>.
[RFC7507]: Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015, <https://www.rfc-editor.org/info/rfc7507>.
[RFC7525]: Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7590]: Saint-Andre, P. and T. Alkemade, "Use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP)", RFC 7590, DOI 10.17487/RFC7590, June 2015, <https://www.rfc-editor.org/info/rfc7590>.
[RFC7633]: Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS) Feature Extension", RFC 7633, DOI 10.17487/RFC7633, October 2015, <https://www.rfc-editor.org/info/rfc7633>.
[RFC7919]: Gillmor, D., "Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for Transport Layer Security (TLS)", RFC 7919, DOI 10.17487/RFC7919, August 2016, <https://www.rfc-editor.org/info/rfc7919>.
[RFC8452]: Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption", RFC 8452, DOI 10.17487/RFC8452, April 2019, <https://www.rfc-editor.org/info/rfc8452>.
[RFC8470]: Thomson, M., Nottingham, M., and W. Tarreau, "Using Early Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September 2018, <https://www.rfc-editor.org/info/rfc8470>.
[RFC9001]: Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, <https://www.rfc-editor.org/info/rfc9001>.
[RFC9051]: Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message Access Protocol (IMAP) - Version 4rev2", RFC 9051, DOI 10.17487/RFC9051, August 2021, <https://www.rfc-editor.org/info/rfc9051>.
[RFC9162]: Laurie, B., Messeri, E., and R. Stradling, "Certificate Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162, December 2021, <https://www.rfc-editor.org/info/rfc9162>.
[SAFECURVES]: Bernstein, D. J. and T. Lange, "SafeCurves: Choosing Safe Curves for Elliptic-Curve Cryptography", December 2014, <https://safecurves.cr.yp.to>.
[Soghoian2011]: Soghoian, C. and S. Stamm, "Certified Lies: Detecting and Defeating Government Interception Attacks Against SSL", SSRN Electronic Journal, DOI 10.2139/ssrn.1591033, 2010, <https://doi.org/10.2139/ssrn.1591033>.
[Springall16]: Springall, D., Durumeric, Z., and J. Halderman, "Measuring the Security Harm of TLS Crypto Shortcuts", Proceedings of the 2016 Internet Measurement Conference, DOI 10.1145/2987443.2987480, November 2016, <https://doi.org/10.1145/2987443.2987480>.
[STD53]: Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996.
<https://www.rfc-editor.org/info/std53>
[Sy2018]: Sy, E., Burkert, C., Federrath, H., and M. Fischer, "Tracking Users across the Web via TLS Session Resumption", Proceedings of the 34th Annual Computer Security Applications Conference, DOI 10.1145/3274694.3274708, December 2018, <https://doi.org/10.1145/3274694.3274708>.
[triple-handshake]: Bhargavan, K., Lavaud, A., Fournet, C., Pironti, A., and P. Strub, "Triple Handshakes and Cookie Cutters: Breaking and Fixing Authentication over TLS", 2014 IEEE Symposium on Security and Privacy, DOI 10.1109/sp.2014.14, May 2014, <https://doi.org/10.1109/sp.2014.14>.