Deprecating TLS 1.0 and TLS 1.1

Deprecating TLS 1.0 and TLS 1.1 Center for Internet Security (CIS)

East Greenbush NY United States of America Kathleen.Moriarty.ietf@gmail.com

Trinity College Dublin

Dublin 2 Ireland +353-1-896-2354 stephen.farrell@cs.tcd.ie

Security Area Internet Engineering Task Force TLS deprecate TLSv1.0 TLSv1.1 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.

Introduction Transport Layer Security (TLS) versions 1.0 and 1.1 were superseded by TLS 1.2 in 2008, which has now itself been superseded by TLS 1.3 . Datagram Transport Layer Security (DTLS) version 1.0 was superseded by DTLS 1.2 in 2012. Therefore, it is timely to further deprecate TLS 1.0, TLS 1.1, and DTLS 1.0. Accordingly, the aforementioned documents have been moved to Historic status. Technical reasons for deprecating these versions include:

They require the implementation of older cipher suites that are no longer desirable for cryptographic reasons, e.g., TLS 1.0 makes TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA mandatory to implement.
There is a lack of support for current recommended cipher suites, especially authenticated encryption with associated data (AEAD) ciphers, which were not supported prior to TLS 1.2. Note that registry entries for no-longer-desirable ciphersuites remain in the registries, but many TLS registries were updated by , which indicates that such entries are not recommended by the IETF.
The integrity of the handshake depends on SHA-1 hash.
The authentication of the peers depends on SHA-1 signatures.
Support for four TLS protocol versions increases the likelihood of misconfiguration.
At least one widely used library has plans to drop TLS 1.1 and TLS 1.0 support in upcoming releases; products using such libraries would need to use older versions of the libraries to support TLS 1.0 and TLS 1.1, which is clearly undesirable.

Deprecation of these versions is intended to assist developers as additional justification to no longer support older (D)TLS versions and to migrate to a minimum of (D)TLS 1.2. Deprecation also assists product teams with phasing out support for the older versions, to reduce the attack surface and the scope of maintenance for protocols in their offerings.

RFCs Updated This document updates the following RFCs that normatively reference TLS 1.0, TLS 1.1, or DTLS 1.0. The update is to obsolete usage of these older versions. Fallback to these versions is prohibited through this update. Specific references to mandatory minimum protocol versions of TLS 1.0 or TLS 1.1 are replaced by TLS 1.2, and references to minimum protocol version DTLS 1.0 are replaced by DTLS 1.2. Statements that "TLS 1.0 is the most widely deployed version and will provide the broadest interoperability" are removed without replacement. The status of , , , , , and will be updated with permission of the Independent Submissions Editor. In addition, these RFCs normatively refer to TLS 1.0 or TLS 1.1 and have already been obsoleted; they are still listed here and marked as updated by this document in order to reiterate that any usage of the obsolete protocol should use modern TLS: , , , , , , , , , , , , , , , , , , and . Note that has already been updated by , which makes an overlapping, but not quite identical, update as this document. has a requirement for TLS 1.1 or later, although it only makes an informative reference to . This requirement is updated to be for TLS 1.2 or later. , , and are already Historic; they are still listed here and marked as updated by this document in order to reiterate that any usage of the obsolete protocol should use modern TLS. This document updates DTLS . had allowed for negotiating the use of DTLS 1.0, which is now forbidden. The DES and International Data Encryption Algorithm (IDEA) cipher suites specified in were specifically removed from TLS 1.2 by ; since the only versions of TLS for which their usage is defined are now Historic, has been moved to Historic as well. The version-fallback Signaling Cipher Suite Value specified in was defined to detect when a given client and server negotiate a lower version of (D)TLS than their highest shared version. TLS 1.3 () incorporates a different mechanism that achieves this purpose, via sentinel values in the ServerHello.Random field. With (D)TLS versions prior to 1.2 fully deprecated, the only way for (D)TLS implementations to negotiate a lower version than their highest shared version would be to negotiate (D)TLS 1.2 while supporting (D)TLS 1.3; supporting (D)TLS 1.3 implies support for the ServerHello.Random mechanism. Accordingly, the functionality from has been superseded, and this document marks it as Obsolete.

Terminology 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.

Support for Deprecation Specific details on attacks against TLS 1.0 and TLS 1.1, as well as their mitigations, are provided in , , and other RFCs referenced therein. Although mitigations for the current known vulnerabilities have been developed, any future issues discovered in old protocol versions might not be mitigated in older library versions when newer library versions do not support those old protocols. For example, NIST has provided the following rationale, copied with permission from Section 1.1, "History of TLS", of :

TLS 1.1, specified in RFC 4346 [24], was developed to address weaknesses discovered in TLS 1.0, primarily in the areas of initialization vector selection and padding error processing. Initialization vectors were made explicit to prevent a certain class of attacks on the Cipher Block Chaining (CBC) mode of operation used by TLS. The handling of padding errors was altered to treat a padding error as a bad message authentication code rather than a decryption failure. In addition, the TLS 1.1 RFC acknowledges attacks on CBC mode that rely on the time to compute the message authentication code (MAC). The TLS 1.1 specification states that to defend against such attacks, an implementation must process records in the same manner regardless of whether padding errors exist. Further implementation considerations for CBC modes (which were not included in RFC 4346 [24]) are discussed in Section 3.3.2. TLS 1.2, specified in RFC 5246 [25], made several cryptographic enhancements, particularly in the area of hash functions, with the ability to use or specify the SHA-2 family of algorithms for hash, MAC, and Pseudorandom Function (PRF) computations. TLS 1.2 also adds authenticated encryption with associated data (AEAD) cipher suites. TLS 1.3, specified in RFC 8446 [57], represents a significant change to TLS that aims to address threats that have arisen over the years. Among the changes are a new handshake protocol, a new key derivation process that uses the HMAC-based Extract-and-Expand Key Derivation Function (HKDF) [37], and the removal of cipher suites that use RSA key transport or static Diffie-Hellman ( DH) [sic] key exchanges, the CBC mode of operation, or SHA-1. Many extensions defined for use with TLS 1.2 and previous versions cannot be used with TLS 1.3.

SHA-1 Usage Problematic in TLS 1.0 and TLS 1.1 The integrity of both TLS 1.0 and TLS 1.1 depends on a running SHA-1 hash of the exchanged messages. This makes it possible to perform a downgrade attack on the handshake by an attacker able to perform 2⁷⁷ operations, well below the acceptable modern security margin. Similarly, the authentication of the handshake depends on signatures made using a SHA-1 hash or a concatenation of MD5 and SHA-1 hashes that is not appreciably stronger than a SHA-1 hash, allowing the attacker to impersonate a server when it is able to break the severely weakened SHA-1 hash. Neither TLS 1.0 nor TLS 1.1 allows the peers to select a stronger hash for signatures in the ServerKeyExchange or CertificateVerify messages, making the only upgrade path the use of a newer protocol version. See for additional details.

Do Not Use TLS 1.0 TLS 1.0 MUST NOT be used. Negotiation of TLS 1.0 from any version of TLS MUST NOT be permitted. Any other version of TLS is more secure than TLS 1.0. While TLS 1.0 can be configured to prevent some types of interception, using the highest version available is preferred. Pragmatically, clients MUST NOT send a ClientHello with ClientHello.client_version set to {03,01}. Similarly, servers MUST NOT send a ServerHello with ServerHello.server_version set to {03,01}. Any party receiving a Hello message with the protocol version set to {03,01} MUST respond with a "protocol_version" alert message and close the connection. Historically, TLS specifications were not clear on what the record layer version number (TLSPlaintext.version) could contain when sending a ClientHello message. notes that TLSPlaintext.version could be selected to maximize interoperability, though no definitive value is identified as ideal. That guidance is still applicable; therefore, TLS servers MUST accept any value {03,XX} (including {03,00}) as the record layer version number for ClientHello, but they MUST NOT negotiate TLS 1.0.

Do Not Use TLS 1.1 TLS 1.1 MUST NOT be used. Negotiation of TLS 1.1 from any version of TLS MUST NOT be permitted. Pragmatically, clients MUST NOT send a ClientHello with ClientHello.client_version set to {03,02}. Similarly, servers MUST NOT send a ServerHello with ServerHello.server_version set to {03,02}. Any party receiving a Hello message with the protocol version set to {03,02} MUST respond with a "protocol_version" alert message and close the connection. Any newer version of TLS is more secure than TLS 1.1. While TLS 1.1 can be configured to prevent some types of interception, using the highest version available is preferred. Support for TLS 1.1 is dwindling in libraries and will impact security going forward if mitigations for attacks cannot be easily addressed and supported in older libraries. Historically, TLS specifications were not clear on what the record layer version number (TLSPlaintext.version) could contain when sending a ClientHello message. notes that TLSPlaintext.version could be selected to maximize interoperability, though no definitive value is identified as ideal. That guidance is still applicable; therefore, TLS servers MUST accept any value {03,XX} (including {03,00}) as the record layer version number for ClientHello, but they MUST NOT negotiate TLS 1.1.

Updates to RFC 7525 "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)" is BCP 195, which is the most recent Best Current Practice for implementing TLS and was based on TLS 1.2. At the time of publication, TLS 1.0 and TLS 1.1 had not yet been deprecated. As such, BCP 195 is called out specifically to update text implementing the deprecation recommendations of this document. This document updates by changing SHOULD NOT to MUST NOT as follows:

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.
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.

This document updates by changing SHOULD NOT to MUST NOT and adding a reference to RFC 6347 as follows:

Implementations MUST NOT negotiate DTLS version 1.0 . Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).

Operational Considerations This document is part of BCP 195 and, as such, reflects the understanding of the IETF (at the time of this document's publication) as to the best practices for TLS and DTLS usage. Though TLS 1.1 has been obsolete since the publication of in 2008, and DTLS 1.0 has been obsolete since the publication of in 2012, there may remain some systems in operation that do not support (D)TLS 1.2 or higher. Adopting the practices recommended by this document for any systems that need to communicate with the aforementioned class of systems will cause failure to interoperate. However, disregarding the recommendations of this document in order to continue to interoperate with the aforementioned class of systems incurs some amount of risk. The nature of the risks incurred by operating in contravention to the recommendations of this document are discussed in Sections and , and knowledge of those risks should be used along with any potential mitigating factors and the risks inherent to updating the systems in question when deciding how quickly to adopt the recommendations specified in this document.

Security Considerations This document deprecates two older TLS protocol versions and one older DTLS protocol version for security reasons already described. The attack surface is reduced when there are a smaller number of supported protocols and fallback options are removed.

IANA Considerations This document has no IANA actions.

References Normative References Informative References Guidelines for the Selection, Configuration, and Use of Transport Layer Security (TLS) Implementations NIST SP800-52r2 National Institute of Standards and Technology Transcript Collision Attacks: Breaking Authentication in TLS, IKE, and SSH INRIA INRIA

Acknowledgements Thanks to those that provided usage data and reviewed and/or improved this document, including: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , and .