Network Working Group N. Gajcowski Internet-Draft M. Jenkins Intended status: Informational NSA Expires: December 11, 2021 June 9, 2021 Commercial National Security Algorithm (CNSA) Suite Cryptography for Secure Shell (SSH) draft-gajcowski-cnsa-ssh-profile-02 Abstract The United States Government has published the NSA Commercial National Security Algorithm (CNSA) Suite, which defines cryptographic algorithm policy for national security applications. This document specifies the conventions for using the United States National Security Agency's CNSA Suite algorithms with the Secure Shell Transport Layer Protocol and the Secure Shell Authentication Protocol. It applies to the capabilities, configuration, and operation of all components of US National Security Systems that employ IPSec. US National Security Systems are described in NIST Special Publication 800-59. It is also appropriate for all other US Government systems that process high-value information. It is made publicly available for use by developers and operators of these and any other system deployments. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on December 11, 2021. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. Gajcowski & Jenkins Expires December 11, 2021 [Page 1]
Internet-Draft CNSA Suite SSH Profile June 2021 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. The Commercial National Security Algorithm Suite . . . . . . 3 4. CNSA and Secure Shell . . . . . . . . . . . . . . . . . . . . 3 4.1. Hash Functions . . . . . . . . . . . . . . . . . . . . . 4 4.2. Digital Signatures . . . . . . . . . . . . . . . . . . . 4 5. Security Mechanism Negotiation and Initialization . . . . . . 5 6. Key Exchange and Server Authentication . . . . . . . . . . . 6 6.1. ECDH Key Exchange . . . . . . . . . . . . . . . . . . . . 6 6.2. DH Key Exchange . . . . . . . . . . . . . . . . . . . . . 6 7. User Authentication . . . . . . . . . . . . . . . . . . . . . 7 8. Confidentiality and Data Integrity of SSH Binary Packets . . 7 8.1. Galois/Counter Mode . . . . . . . . . . . . . . . . . . . 7 8.2. Data Integrity . . . . . . . . . . . . . . . . . . . . . 8 9. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . . . 8 10. Security Considerations . . . . . . . . . . . . . . . . . . . 8 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 12.1. Normative References . . . . . . . . . . . . . . . . . . 8 12.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction This document specifies conventions for using the United States National Security Agency's CNSA Suite algorithms [CNSA] with Secure Shell Transport Layer Protocol [RFC4253] and the Secure Shell Authentication Protocol [RFC4252]. It applies to the capabilities, configuration, and operation of all components of US National Security Systems that employ IPSec. US National Security Systems are described in NIST Special Publication 800-59 [SP80059]. It is also appropriate for all other US Government systems that process high- value information. It is made publicly available for use by developers and operators of these and any other system deployments. Gajcowski & Jenkins Expires December 11, 2021 [Page 2]
Internet-Draft CNSA Suite SSH Profile June 2021 2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. The Commercial National Security Algorithm Suite The National Security Agency (NSA) profiles commercial cryptographic algorithms and protocols as part of its mission to support secure, interoperable communications for US Government National Security Systems. To this end, it publishes guidance both to assist with the US Government transition to new algorithms, and to provide vendors - and the Internet community in general - with information concerning their proper use and configuration. Recently, cryptographic transition plans have become overshadowed by the prospect of the development of a cryptographically-relevant quantum computer. NSA has established the Commercial National Security Algorithm (CNSA) Suite to provide vendors and IT users near- term flexibility in meeting their IA interoperability requirements. The purpose behind this flexibility is to avoid vendors and customers making two major transitions in a relatively short timeframe, as we anticipate a need to shift to quantum-resistant cryptography in the near future. NSA is authoring a set of RFCs, including this one, to provide updated guidance concerning the use of certain commonly available commercial algorithms in IETF protocols. These RFCs can be used in conjunction with other RFCs and cryptographic guidance (e.g., NIST Special Publications) to properly protect Internet traffic and data- at-rest for US Government National Security Systems. 4. CNSA and Secure Shell Several RFCs have documented how each of the CNSA components are to be integrated into Secure Shell (SSH): kex algorithms ecdh-sha2-nistp384 [RFC5656] diffie-hellman-group15-sha512 [RFC8268] diffie-hellman-group16-sha512 [RFC8268] Gajcowski & Jenkins Expires December 11, 2021 [Page 3]
Internet-Draft CNSA Suite SSH Profile June 2021 public key algorithms ecdsa-sha2-nistp384 [RFC5656] rsa-sha2-512 [RFC8332] encryption algorithms (both client_to_server and server_to_client) AEAD_AES_256_GCM [RFC5647] MAC algorithms (both client_to_server and server_to_client) AEAD_AES_256_GCM [RFC5647] The purpose of this document is to draw upon all of these documents to provide guidance for CNSA compliant implementations of Secure Shell. Note that while compliant Secure Shell implementations MUST follow the guidance in this document, that requirement does not in and of itself imply that a given implementation of Secure Shell is suitable for use national security systems. An implementation must be validated by the appropriate authority before such usage is permitted. 4.1. Hash Functions The approved CNSA hash function for all purposes is SHA-384, as defined in [FIPS180]. However, commercial products are more likely to incorporate the SHA-512 (sha2-512) based kex algorithms and public key algorithms defined in [RFC8268] and [RFC8332]. Therefore, the SHA-512 based kex and public key algorithms SHOULD be used; SHA-384 based algorithms MAY be used where they are available. Any hash algorithm other than SHA-384 or SHA-512 MUST NOT be used. 4.2. Digital Signatures Servers MUST be authenticated using digital signatures. The public key algorithm implemented MUST be ecdsa-sha2-nistp384 or rsa- sha2-512. The RSA public key modulus MUST be 3072 or 4096 bits in size; clients MUST NOT accept RSA signatures from a public key modulus of any other size. Implementations MUST NOT employ a trust on first use (TOFU) security model where a client accepts the first public host key presented to it from a not yet verified server. Use of a TOFU model would allow an intermediate adversary to present itself to the client as the server. Gajcowski & Jenkins Expires December 11, 2021 [Page 4]
Internet-Draft CNSA Suite SSH Profile June 2021 The public host keys presented MUST be verified as belonging to the presenting party before the signature is accepted. This certification SHOULD be done using certificates, provided the use of certificates has been approved for that environment. Otherwise, the user MUST validate the presented public key (and/or certificate) by some other means, possibly through an offline mechanism. Certificates MUST be X.509v3 certificates and their use MUST comply with [RFC8603]. 5. Security Mechanism Negotiation and Initialization As described in [RFC4253], the exchange of SSH_MSG_KEXINIT between the server and the client establishes which key agreement algorithm, MAC algorithm, host key algorithm (server authentication algorithm), and encryption algorithm are to be used. This section specifies the use of CNSA components in the Secure Shell algorithm negotiation, key agreement, server authentication, and user authentication. The choice of all but the user authentication methods are determined by the exchange of SSH_MSG_KEXINIT between the client and the server. The SSH_MSG_KEXINIT name lists can be used to constrain the choice of cryptographic algorithms in accordance with the guidance given in Section 2. One of the following kex_algorithms MUST be used. ecdh-sha2-nistp384 [RFC5656] diffie-hellman-group15-sha512 [RFC8268] diffie-hellman-group16-sha512 [RFC8268] One of the name lists from the following list MUST be used for the encryption algorithms and mac algorithm. This option MUST be used. encryption_algorithm name_list := { AEAD_AES_256_GCM } mac_algorithm name_list := { AEAD_AES_256_GCM } One of the following public key algorithms MUST be used. rsa-sha2-512 [RFC8332] ecdsa-sha2-nistp384 [RFC5656] Gajcowski & Jenkins Expires December 11, 2021 [Page 5]
Internet-Draft CNSA Suite SSH Profile June 2021 6. Key Exchange and Server Authentication Either elliptic curve Diffie-Hellman (ECDH) or Diffie-Hellman (DH) MUST be used to establish a shared secret value between the client and the server. A signature on the exchange hash value derived from the newly established shared secret value is used to authenticate the server to the client. The key exchange to be used is determined by the name lists exchanged in the SSH_MSG_KEXINT packets as described in [RFC4253]. A compliant system MUST NOT allow the reuse of ephemeral/exchange values in a key exchange algorithm due to security concerns related to this practice. Section 5.6.3.3 of [SP80056A] states that an ephemeral private key must be used in exactly one key establishment transaction and must be destroyed (zeroized) as soon as possible. Section 5.8 of [SP80056A] states that such shared secrets must be destroyed (zeroized) immediately after its use. CNSA compliant systems MUST follow these mandates. 6.1. ECDH Key Exchange The key exchange begins with the SSH_MSG_KEXECDH_INIT message which contains the client's ephemeral public key used to generate a shared secret value. The server responds to a SSH_MSG_KEXECDH_INIT message with a SSH_MSG_KEXECDH_REPLY message which contains the server's ephemeral public key, the server's public host key, and a signature of the exchange hash value formed from the newly established shared secret value. The public key algorithm MUST be ecdsa-sha2-nistp384 or rsa- sha2-512. 6.2. DH Key Exchange The key exchange begins with the SSH_MSG_KEXDH_INIT message which contains the client's DH exchange value used to generate a shared secret value. The server responds to a SSH_MSG_KEXDH_INIT message with a SSH_MSG_KEXDH_REPLY message. The SSH_MSG_KEXDH_REPLY contains the server's DH exchange value, the server's public host key, and a signature of the exchange hash value formed from the newly established shared secret value. The public key algorithm MUST be ecdsa-sha2-nistp384 or rsa-sha2-512. Gajcowski & Jenkins Expires December 11, 2021 [Page 6]
Internet-Draft CNSA Suite SSH Profile June 2021 7. User Authentication The Secure Shell Transport Layer Protocol authenticates the server to the host but does not authenticate the user (or the user's host) to the server. All users MUST be authenticated, MUST follow [RFC4252], and SHOULD be authenticated using a public key method. Users MAY authenticate using passwords. Other methods of authentication MUST not be used, including "none". When authenticating with public key, the following public key algorithms MUST be used: ecdsa-sha2-nistp384 [RFC5656] rsa-sha2-512 [RFC8332] The server MUST verify that the presented key is a valid authenticator for the user. This SHOULD be done using certificates. Certificates MUST be X.509v3 certificates and their use MUST comply with [RFC8603]. If authenticating with RSA, the client's public key modulus MUST be 3072 or 4096 bits in size, and the server MUST NOT accept signatures from an RSA public key modulus of any other size. If authenticating by passwords, it is essential that passwords have sufficient entropy to protect against dictionary attacks. During authentication, the password MUST be protected in the established encrypted communications channel. Additional guidelines are provided in [SP80063]. 8. Confidentiality and Data Integrity of SSH Binary Packets Secure Shell transfers data between the client and the server using its own binary packet structure. The SSH binary packet structure is independent of any packet structure on the underlying data channel. The contents of each binary packet and portions of the header are encrypted, and each packet is authenticated with its own message authentication code. Use of the Advanced Encryption Standard in Galois Counter Mode (AES GCM) will both encrypt the packet and form a 16-octet authentication tag to ensure data integrity. 8.1. Galois/Counter Mode Use of AES GCM in Secure Shell is described in [RFC5647]. CNSA complaint SSH implementations MUST support AEAD_AES_GCM_256 to provide confidentiality and ensure data integrity. No other confidentiality or data integrity algorithms are permitted. Gajcowski & Jenkins Expires December 11, 2021 [Page 7]
Internet-Draft CNSA Suite SSH Profile June 2021 The AES GCM invocation counter is incremented mod 2^64. That is, after processing a binary packet: invocation_counter = invocation_counter + 1 mod 2^64 The invocation counter MUST NOT repeat a counter value. 8.2. Data Integrity As specified in [RFC5647], all 16 octets of the authentication tag MUST be used as the SSH data integrity value of the SSH binary packet. 9. Rekeying Secure Shell allows either the server or client to request that the Secure Shell connection be rekeyed. The cipher suite being employed MUST NOT be changed when a rekey occurs. 10. Security Considerations The security considerations of [RFC4251], [RFC4252], [RFC4253], [RFC5647], and [RFC5656] apply. 11. IANA Considerations No IANA actions are requested. 12. References 12.1. Normative References [CNSA] Committee for National Security Systems, "Use of Public Standards for Secure Information Sharing", CNSSP 15, October 2016, <https://www.cnss.gov/CNSS/Issuances/Policies.htm>. [FIPS180] National Institute of Standards and Technology, "Secure Hash Standard (SHS)", Federal Information Processing Standard 180-4, August 2015, <https://csrc.nist.gov/publications/detail/fips/180/4/ final>. [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>. Gajcowski & Jenkins Expires December 11, 2021 [Page 8]
Internet-Draft CNSA Suite SSH Profile June 2021 [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, January 2006, <https://www.rfc-editor.org/info/rfc4251>. [RFC4252] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252, January 2006, <https://www.rfc-editor.org/info/rfc4252>. [RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006, <https://www.rfc-editor.org/info/rfc4253>. [RFC5647] Igoe, K. and J. Solinas, "AES Galois Counter Mode for the Secure Shell Transport Layer Protocol", RFC 5647, DOI 10.17487/RFC5647, August 2009, <https://www.rfc-editor.org/info/rfc5647>. [RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer", RFC 5656, DOI 10.17487/RFC5656, December 2009, <https://www.rfc-editor.org/info/rfc5656>. [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>. [RFC8268] Baushke, M., "More Modular Exponentiation (MODP) Diffie- Hellman (DH) Key Exchange (KEX) Groups for Secure Shell (SSH)", RFC 8268, DOI 10.17487/RFC8268, December 2017, <https://www.rfc-editor.org/info/rfc8268>. [RFC8332] Bider, D., "Use of RSA Keys with SHA-256 and SHA-512 in the Secure Shell (SSH) Protocol", RFC 8332, DOI 10.17487/RFC8332, March 2018, <https://www.rfc-editor.org/info/rfc8332>. [RFC8603] Jenkins, M. and L. Zieglar, "Commercial National Security Algorithm (CNSA) Suite Certificate and Certificate Revocation List (CRL) Profile", RFC 8603, DOI 10.17487/RFC8603, May 2019, <https://www.rfc-editor.org/info/rfc8603>. 12.2. Informative References Gajcowski & Jenkins Expires December 11, 2021 [Page 9]
Internet-Draft CNSA Suite SSH Profile June 2021 [SP80056A] National Institute of Standards and Technology, "Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography", NIST Special Publication 800-56A, Revision 3, April 2018, <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ NIST.SP.800-56Ar3.pdf>. [SP80059] National Institute of Standards and Technology, "Guideline for Identifying an Information System as a National Security System", Special Publication 800-59 , August 2003, <https://csrc.nist.gov/publications/detail/sp/800- 59/final>. [SP80063] National Institute of Standards and Technology, "Digital Identity Guidelines", NIST Special Publication 800-63, Revision 3, June 2017, <https://www.nist.gov/itl/tig/projects/special- publication-800-63>. Authors' Addresses Nicholas Gajcowski National Security Agency Email: nhgajco@uwe.nsa.gov Michael Jenkins National Security Agency Email: mjjenki@cyber.nsa.gov Gajcowski & Jenkins Expires December 11, 2021 [Page 10]