Internet Engineering Task Force M. D. Baushke Internet-Draft Juniper Networks, Inc. Updates: 4250 (if approved) 23 November 2020 Intended status: Standards Track Expires: 27 May 2021 Key Exchange (KEX) Method Updates and Recommendations for Secure Shell (SSH) draft-ietf-curdle-ssh-kex-sha2-12 Abstract This document is intended to update the recommended set of key exchange methods for use in the Secure Shell (SSH) protocol to meet evolving needs for stronger security. This document updates RFC 4250. 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 27 May 2021. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. 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. Baushke Expires 27 May 2021 [Page 1] Internet-Draft KEX Method Updates/Recommendations for S November 2020 Table of Contents 1. Overview and Rationale . . . . . . . . . . . . . . . . . . . 2 1.1. Selecting an appropriate hashing algorithm . . . . . . . 3 1.2. Selecting an appropriate Public Key Algorithm . . . . . . 3 1.2.1. Elliptic Curve Cryptography (ECC) . . . . . . . . . . 4 1.2.2. Finite Field Cryptography (FFC) . . . . . . . . . . . 4 1.2.3. Integer Factorization Cryptography (IFC) . . . . . . 5 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5 3. Key Exchange Methods . . . . . . . . . . . . . . . . . . . . 5 3.1. SHA-1 and SHA-2 Hashing . . . . . . . . . . . . . . . . . 6 3.2. Elliptic Curve Cryptography (ECC) . . . . . . . . . . . . 6 3.2.1. curve25519-sha256 and gss-curve25519-sha256-* . . . . 6 3.2.2. curve448-sha512 and gss-curve448-sha512-* . . . . . . 7 3.2.3. ECC diffie-hellman using ecdh-*, ecmqv-sha2, and gss-nistp* . . . . . . . . . . . . . . . . . . . . . 7 3.3. Finite Field Cryptography (FFC) . . . . . . . . . . . . . 8 3.3.1. FFC diffie-hellman using generated MODP groups . . . 8 3.3.2. FFC diffie-hellman using named MODP groups . . . . . 8 3.4. Integer Factorization Cryptography (IFC) . . . . . . . . 9 3.5. Secure Shell Extension Negotiation . . . . . . . . . . . 9 4. Summary Guidance for Key Exchange Method Names Implementations . . . . . . . . . . . . . . . . . . . . . 9 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 8.1. Normative References . . . . . . . . . . . . . . . . . . 12 8.2. Informative References . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 1. Overview and Rationale Secure Shell (SSH) is a common protocol for secure communication on the Internet. In [RFC4253], SSH originally defined two Key Exchange (KEX) Method Names that MUST be implemented. Over time what was once considered secure is no longer considered secure. The purpose of this RFC is to recommend that some published key exchanges be deprecated as well as recommending some that SHOULD and one that MUST be adopted. This document updates [RFC4250]. A key exchange has two components, a hashing algorithm and a public key algorithm. The following subsections describe how to select each component. Baushke Expires 27 May 2021 [Page 2] Internet-Draft KEX Method Updates/Recommendations for S November 2020 1.1. Selecting an appropriate hashing algorithm The SHA-1 hash is in the process of being deprecated for many reasons. There have been attacks against SHA-1 that have shown there are weaknesses in the algorithm. Therefore, it is desirable to move away from using it before attacks become more serious. At present, the attacks against SHA-1 are collision attacks that rely on human help rather than a pre-image attack. So, it is still possible to allow time backward compatibility use of SHA-1 during a SSH key-exchange for a transition to stronger hashing. However, any such key exchanges should be listed last in the preference list. Use of the SHA-2 family of hashes found in [RFC6234] rather than the SHA-1 hash is strongly advised. When it comes to the SHA-2 family of Secure Hashing functions, SHA2-224 has 112 bits of security strength; SHA2-256 has 128 bits of security strength; SHA2-384 has 192 bits of security strength; and SHA2-512 has 256 bits of security strength. As the same compute power is needed for both SHA2-224 and SHA2-256 and currently no KeX uses SHA2-224, it is suggested that the minimum secure hashing function that should be used for Key Exchange Methods is SHA2-256. To avoid combinatorial explosion of key exchange names, newer key exchanges are restricting to the use of *-sha256 and *-sha512. 1.2. Selecting an appropriate Public Key Algorithm SSH uses mathematically hard problems for doing Key Exchange: * Elliptic Curve Cryptography (ECC) has families of curves for Key Exchange Methods for SSH. NIST prime curves with names and other curves are available using an object identifier (OID) with Elliptic Curve Diffie-Hellman (ECDH) via [RFC5656]. Curve25519 and Curve448 key exchanges are used with ECDH via [RFC8731]. * Finite Field Cryptography (FFC) is used for Diffie-Hellman (DH) key exchange with "safe primes" either from a specified list found in [RFC3526] or generated dynamically via [RFC4419] as updated by [RFC8270]. * Integer Factorization Cryptography (IFC) using the RSA algorithm is provided for in [RFC4432]. Baushke Expires 27 May 2021 [Page 3] Internet-Draft KEX Method Updates/Recommendations for S November 2020 It is desirable for the security strength of the key exchange be chosen to be comparable with the security strength of the other elements of the SSH handshake. Attackers can target the weakest element of the SSH handshake. It is desirable to select a minimum of 112 bits of security strength. Based on implementer security needs, a stronger minimum may be desired. 1.2.1. Elliptic Curve Cryptography (ECC) For ECC, it is recommended to select one with approximately 128 bits of security strength. +============+=============================+ | Curve Name | Estimated Security Strength | +============+=============================+ | nistp256 | 128 bits | +------------+-----------------------------+ | nistp384 | 192 bits | +------------+-----------------------------+ | nistp521 | 512 bits | +------------+-----------------------------+ | Curve25519 | 128 bits | +------------+-----------------------------+ | Curve448 | 224 bits | +------------+-----------------------------+ Table 1: ECC Security Strengths 1.2.2. Finite Field Cryptography (FFC) For FFC, a modulus 2048 bits (112 bits of security strength). Baushke Expires 27 May 2021 [Page 4] Internet-Draft KEX Method Updates/Recommendations for S November 2020 +==================+=============================+============+ | Prime Field Size | Estimated Security Strength | Example | | | | MODP Group | +==================+=============================+============+ | 2048-bit | 112 bits | group14 | +------------------+-----------------------------+------------+ | 3072-bit | 128 bits | group15 | +------------------+-----------------------------+------------+ | 4096-bit | 152 bits | group16 | +------------------+-----------------------------+------------+ | 6144-bit | 176 bits | group17 | +------------------+-----------------------------+------------+ | 8192-bit | 200 bits | group18 | +------------------+-----------------------------+------------+ Table 2: FFC MODP Security Strengths The minimum MODP group that MAY be used is the 2048-bit MODP group14. Implementations SHOULD support a 3072-bit MODP group or larger. 1.2.3. Integer Factorization Cryptography (IFC) The only IFC algorithm for key exchange is the RSA algorithm via [RFC4432]. The minimum modulus size is 2048 bits. The use of a SHA-2 Family hash with RSA 2048-bit keys has sufficient security. +=====================+=============================+ | Key Exchange Method | Estimated Security Strength | +=====================+=============================+ | rsa1024-sha1 | 80 bits | +---------------------+-----------------------------+ | rsa2048-sha256 | 112 bits | +---------------------+-----------------------------+ Table 3: IFC Security Strengths 2. Requirements Language 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. Key Exchange Methods This memo adopts the style and conventions of [RFC4253] in specifying how the use of data key exchange is indicated in SSH. Baushke Expires 27 May 2021 [Page 5] Internet-Draft KEX Method Updates/Recommendations for S November 2020 This RFC also collects key exchange method names in various existing RFCs [RFC4253], [RFC4419], [RFC4432], [RFC4462], [RFC5656], [RFC8268], [RFC8731], [RFC8732], and [RFC8308], and provides a suggested suitability for implementation of MUST, SHOULD, SHOULD NOT, and MUST NOT. Any method not explicitly listed MAY be implemented. This document is intended to provide guidance as to what key exchange algorithms are to be considered for new or updated SSH implementations. 3.1. SHA-1 and SHA-2 Hashing All of the key exchanges using the SHA-1 hashing algorithm should be deprecated and phased out of use because SHA-1 has security concerns provided in [RFC6194]. The SHA-2 Family of hashes [RFC6234] is the only one which is more secure than SHA-1 and has been standardized for use with SSH key exchanges. diffie-hellman-group1-sha1 and diffie-hellman-group14-sha1 are currently mandatory to implement (MTI). diffie-hellman-group14-sha1 is the stronger of the two. Group14 (a 2048-bit MODP group) is defined in [RFC3526]. It is reasonable to retain the diffie-hellman- group14-sha1 exchange for interoperability with legacy implementations. Therefore, diffie-hellman-group14-sha1 SHOULD be implemented and all other *-sha1 key exchanges SHOULD NOT be implemented. 3.2. Elliptic Curve Cryptography (ECC) 3.2.1. curve25519-sha256 and gss-curve25519-sha256-* Curve25519 is efficient on a wide range of architectures with properties that allow higher performance implementations compared to traditional elliptic curves. The use of SHA2-256 (also known as SHA-256 and sha256) as defined in [RFC6234] for integrity is a reasonable one for this method. These key exchange methods are described in [RFC8731] and [RFC8732] and is similar to the IKEv2 Key Agreement described in [RFC8031]. The curve25519-sha256 key exchange method has multiple implementations and SHOULD be implemented. The gss-curve25519-sha256-* key exchange method SHOULD also be implemented because it shares the same performance and security characteristics as curve25519-sha2. Baushke Expires 27 May 2021 [Page 6] Internet-Draft KEX Method Updates/Recommendations for S November 2020 3.2.2. curve448-sha512 and gss-curve448-sha512-* Curve448 provides more security strength than Curve25519 at a higher computational and bandwidth cost. It uses SHA2-512 (also known as SHA-512) defined in [RFC6234] for integrity. This Key Exchange Method is described in [RFC8731] and is similar to the IKEv2 Key Agreement described in [RFC8031]. This method MAY be implemented. The gss-curve448-sha512-* key exchange method MAY also be implemented because it shares the same performance and security characteristics as curve448-sha512. 3.2.3. ECC diffie-hellman using ecdh-*, ecmqv-sha2, and gss-nistp* The ecdh-sha2-* name-space allows for other curves to be defined for the elliptic curve Diffie Hellman key exchange. At present, there are three named curves in this name-space which SHOULD be supported. They appear in [RFC5656] in section 10.1 Required Curves all of the NISTP curves named are mandatory to implement if any of this RFC is implemented. This set of methods MAY be implemented. If implemented, the named curves SHOULD always be enabled unless specifically disabled by local security policy. In [RFC5656], section 6.1, the method to name other ECDH curves using OIDs is specified. These other curves MAY be implemented. The GSS-API name-space with gss-nistp*-sha* mirrors the algorithms used by ecdh-sha2-* names. The table provides guidance for implementation. ECDH reduces bandwidth of key exchanges compared to FFC DH at a similar security strength. The following table lists algorithms as SHOULD where implementations may be more efficient or widely deployed. The items listed as MAY are potentially less efficient. Baushke Expires 27 May 2021 [Page 7] Internet-Draft KEX Method Updates/Recommendations for S November 2020 +==========================+==========+ | Key Exchange Method Name | Guidance | +==========================+==========+ | ecdh-sha2-* | MAY | +--------------------------+----------+ | ecdh-sha2-nistp256 | SHOULD | +--------------------------+----------+ | gss-nistp256-sha256-* | SHOULD | +--------------------------+----------+ | ecdh-sha2-nistp384 | SHOULD | +--------------------------+----------+ | gss-nistp384-sha384-* | SHOULD | +--------------------------+----------+ | ecdh-sha2-nistp521 | SHOULD | +--------------------------+----------+ | gss-nistp521-sha512-* | SHOULD | +--------------------------+----------+ | ecmqv-sha2 | MAY | +--------------------------+----------+ Table 4: ECDH Implementation Guidance It is advisable to match the ECDSA and ECDH algorithms to use the same curve for both to maintain the same security strength in the connection. 3.3. Finite Field Cryptography (FFC) 3.3.1. FFC diffie-hellman using generated MODP groups This random selection from a set of pre-generated moduli for key exchange uses SHA2-256 as defined in [RFC4419]. [RFC8270] mandates implementations avoid any MODP group whose modulus size is less than 2048 bits. Care should be taken in the pre-generation of the moduli P and generator G such that the generator provides a Q-ordered subgroup of P. Otherwise, the parameter set may leak one bit of the shared secret leaving the MODP group half as strong. This key exchange MAY be used. 3.3.2. FFC diffie-hellman using named MODP groups diffie-hellman-group14-sha256 represents the smallest FFC DH key exchange method considered to be secure. It is a reasonably simple transition from SHA-1 to SHA-2. diffie-hellman-group14-sha256 method MUST be implemented. The rest of the FFC MODP groups MAY be implemented. The table below provides explicit guidance by name. Baushke Expires 27 May 2021 [Page 8] Internet-Draft KEX Method Updates/Recommendations for S November 2020 +===============================+==========+ | Key Exchange Method Name | Guidance | +===============================+==========+ | diffie-hellman-group14-sha256 | MUST | +-------------------------------+----------+ | gss-group14-sha256-* | SHOULD | +-------------------------------+----------+ | diffie-hellman-group15-sha512 | MAY | +-------------------------------+----------+ | gss-group15-sha512-* | MAY | +-------------------------------+----------+ | diffie-hellman-group16-sha512 | SHOULD | +-------------------------------+----------+ | gss-group16-sha512-* | MAY | +-------------------------------+----------+ | diffie-hellman-group17-sha512 | MAY | +-------------------------------+----------+ | gss-group17-sha512-* | MAY | +-------------------------------+----------+ | diffie-hellman-group18-sha512 | MAY | +-------------------------------+----------+ | gss-group18-sha512-* | MAY | +-------------------------------+----------+ Table 5: FFC Implementation Guidance 3.4. Integer Factorization Cryptography (IFC) The rsa2048-sha256 key exchange method is defined in [RFC4432]. Uses an RSA 2048-bit modulus with a SHA2-256 hash. This key exchange meets 112 bit minimum security strength. This method MAY be implemented. 3.5. Secure Shell Extension Negotiation There are two key exchange methods, ext-info-c and ext-info-s, defined in [RFC8308] which are not actually key exchanges. They provide a method to support other Secure Shell negotiations. Being able to extend functionality is desirable. This method SHOULD be implemented. 4. Summary Guidance for Key Exchange Method Names Implementations The Implement column is the current recommendations of this RFC. Key Exchange Method Names are listed alphabetically. Baushke Expires 27 May 2021 [Page 9] Internet-Draft KEX Method Updates/Recommendations for S November 2020 +======================================+===========+============+ | Key Exchange Method Name | Reference | Implement | +======================================+===========+============+ | curve25519-sha256 | RFC8731 | SHOULD | +--------------------------------------+-----------+------------+ | curve448-sha512 | RFC8731 | MAY | +--------------------------------------+-----------+------------+ | diffie-hellman-group-exchange-sha1 | RFC4419 | SHOULD NOT | +--------------------------------------+-----------+------------+ | diffie-hellman-group-exchange-sha256 | RFC4419 | MAY | +--------------------------------------+-----------+------------+ | diffie-hellman-group1-sha1 | RFC4253 | SHOULD NOT | +--------------------------------------+-----------+------------+ | diffie-hellman-group14-sha1 | RFC4253 | SHOULD | +--------------------------------------+-----------+------------+ | diffie-hellman-group14-sha256 | RFC8268 | MUST | +--------------------------------------+-----------+------------+ | diffie-hellman-group15-sha512 | RFC8268 | MAY | +--------------------------------------+-----------+------------+ | diffie-hellman-group16-sha512 | RFC8268 | SHOULD | +--------------------------------------+-----------+------------+ | diffie-hellman-group17-sha512 | RFC8268 | MAY | +--------------------------------------+-----------+------------+ | diffie-hellman-group18-sha512 | RFC8268 | MAY | +--------------------------------------+-----------+------------+ | ecdh-sha2-* | RFC5656 | MAY | +--------------------------------------+-----------+------------+ | ecdh-sha2-nistp256 | RFC5656 | SHOULD | +--------------------------------------+-----------+------------+ | ecdh-sha2-nistp384 | RFC5656 | SHOULD | +--------------------------------------+-----------+------------+ | ecdh-sha2-nistp521 | RFC5656 | SHOULD | +--------------------------------------+-----------+------------+ | ecmqv-sha2 | RFC5656 | MAY | +--------------------------------------+-----------+------------+ | ext-info-c | RFC8308 | SHOULD | +--------------------------------------+-----------+------------+ | ext-info-s | RFC8308 | SHOULD | +--------------------------------------+-----------+------------+ | gss-* | RFC4462 | MAY | +--------------------------------------+-----------+------------+ | gss-curve25519-sha256-* | RFC8732 | SHOULD | +--------------------------------------+-----------+------------+ | gss-curve448-sha512-* | RFC8732 | MAY | +--------------------------------------+-----------+------------+ | gss-gex-sha1-* | RFC4462 | SHOULD NOT | +--------------------------------------+-----------+------------+ | gss-group1-sha1-* | RFC4462 | SHOULD NOT | Baushke Expires 27 May 2021 [Page 10] Internet-Draft KEX Method Updates/Recommendations for S November 2020 +--------------------------------------+-----------+------------+ | gss-group14-sha256-* | RFC8732 | SHOULD | +--------------------------------------+-----------+------------+ | gss-group15-sha512-* | RFC8732 | MAY | +--------------------------------------+-----------+------------+ | gss-group16-sha512-* | RFC8732 | SHOULD | +--------------------------------------+-----------+------------+ | gss-group17-sha512-* | RFC8732 | MAY | +--------------------------------------+-----------+------------+ | gss-group18-sha512-* | RFC8732 | MAY | +--------------------------------------+-----------+------------+ | gss-nistp256-sha256-* | RFC8732 | SHOULD | +--------------------------------------+-----------+------------+ | gss-nistp384-sha384-* | RFC8732 | SHOULD | +--------------------------------------+-----------+------------+ | gss-nistp521-sha512-* | RFC8732 | MAY | +--------------------------------------+-----------+------------+ | rsa1024-sha1 | RFC4432 | MUST NOT | +--------------------------------------+-----------+------------+ | rsa2048-sha256 | RFC4432 | MAY | +--------------------------------------+-----------+------------+ Table 6: IANA guidance for key exchange method name implementations The full set of official [IANA-KEX] key algorithm method names not otherwise mentioned in this document MAY be implemented. [TO BE REMOVED: This registration should take place at the following location URL: http://www.iana.org/assignments/ssh-parameters/ssh- parameters.xhtml#ssh-parameters-16 ] 5. Acknowledgements Thanks to the following people for review and comments: Denis Bider, Peter Gutmann, Damien Miller, Niels Moeller, Matt Johnston, Iwamoto Kouichi, Simon Josefsson, Dave Dugal, Daniel Migault, Anna Johnston, Tero Kivinen, and Travis Finkenauer. Thanks to the following people for code to implement interoperable exchanges using some of these groups as found in an this draft: Darren Tucker for OpenSSH and Matt Johnston for Dropbear. And thanks to Iwamoto Kouichi for information about RLogin, Tera Term (ttssh) and Poderosa implementations also adopting new Diffie-Hellman groups based on this draft. Baushke Expires 27 May 2021 [Page 11] Internet-Draft KEX Method Updates/Recommendations for S November 2020 6. Security Considerations This SSH protocol provides a secure encrypted channel over an insecure network. It performs server host authentication, key exchange, encryption, and integrity checks. It also derives a unique session ID that may be used by higher-level protocols. Full security considerations for this protocol are provided in [RFC4251]. It is desirable to deprecate or remove key exchange method name that are considered weak. A key exchange method may be weak because too few bits are used, or the hashing algorithm is considered too weak. 7. IANA Considerations IANA is requested to annotate entries in [IANA-KEX] which MUST NOT be implemented as being deprecated by this document. 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)", RFC 3526, DOI 10.17487/RFC3526, May 2003, . [RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Assigned Numbers", RFC 4250, DOI 10.17487/RFC4250, January 2006, . [RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006, . [RFC8031] Nir, Y. and S. Josefsson, "Curve25519 and Curve448 for the Internet Key Exchange Protocol Version 2 (IKEv2) Key Agreement", RFC 8031, DOI 10.17487/RFC8031, December 2016, . Baushke Expires 27 May 2021 [Page 12] Internet-Draft KEX Method Updates/Recommendations for S November 2020 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . [RFC8270] Velvindron, L. and M. Baushke, "Increase the Secure Shell Minimum Recommended Diffie-Hellman Modulus Size to 2048 Bits", RFC 8270, DOI 10.17487/RFC8270, December 2017, . [RFC8308] Bider, D., "Extension Negotiation in the Secure Shell (SSH) Protocol", RFC 8308, DOI 10.17487/RFC8308, March 2018, . 8.2. Informative References [IANA-KEX] Internet Assigned Numbers Authority (IANA), "Secure Shell (SSH) Protocol Parameters: Key Exchange Method Names", July 2020, . [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, January 2006, . [RFC4419] Friedl, M., Provos, N., and W. Simpson, "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol", RFC 4419, DOI 10.17487/RFC4419, March 2006, . [RFC4432] Harris, B., "RSA Key Exchange for the Secure Shell (SSH) Transport Layer Protocol", RFC 4432, DOI 10.17487/RFC4432, March 2006, . [RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch, "Generic Security Service Application Program Interface (GSS-API) Authentication and Key Exchange for the Secure Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May 2006, . [RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer", RFC 5656, DOI 10.17487/RFC5656, December 2009, . Baushke Expires 27 May 2021 [Page 13] Internet-Draft KEX Method Updates/Recommendations for S November 2020 [RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security Considerations for the SHA-0 and SHA-1 Message-Digest Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011, . [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, May 2011, . [RFC8731] Adamantiadis, A., Josefsson, S., and M. Baushke, "Secure Shell (SSH) Key Exchange Method Using Curve25519 and Curve448", RFC 8731, DOI 10.17487/RFC8731, February 2020, . [RFC8732] Sorce, S. and H. Kario, "Generic Security Service Application Program Interface (GSS-API) Key Exchange with SHA-2", RFC 8732, DOI 10.17487/RFC8732, February 2020, . Author's Address Mark D. Baushke Juniper Networks, Inc. Email: mdb@juniper.net Baushke Expires 27 May 2021 [Page 14]