RFC 5656

RFC 5656
Title	RFC 5656
Status	Standard Track
Authors	T. Ylonen, T. Kivinen
Date	March 2010
Pages	32
Category	Network Protocol

RFC 5656

RFC 5656 specifies extensions for the Secure Shell (SSH) Protocol to support additional public key algorithms and key exchange methods, primarily Elliptic Curve Cryptography (ECC). The document updates the SSH-2 Protocol by defining new algorithm names, message formats, and negotiation semantics to enable interoperable use of elliptic curve keys and signatures within SSH. It was published as part of the Request for Comments series and influenced implementations across major operating system vendors and network appliance manufacturers.

Introduction

RFC 5656 extends the SSH (Secure Shell) Protocol by introducing ECC-based authentication and key exchange, aligning SSH cryptography with advances adopted by standards bodies such as the Internet Engineering Task Force and institutions like the National Institute of Standards and Technology. The specification identifies algorithm identifiers, semantics for negotiation during session setup, and the format for ECC keys and signatures to ensure compatibility between clients and servers from projects such as OpenSSH, PuTTY, and products from vendors including Cisco Systems and Juniper Networks. It situates SSH enhancements within the broader ecosystem of Internet standards including work from the IETF Security Area, the Internet Architecture Board, and cryptographic research from universities like Stanford University and MIT.

Key Concepts and Protocol Extensions

The document introduces named algorithm strings used in the SSH-2 algorithm negotiation exchange, referencing curves and signature types associated with standards such as NIST FIPS 186-3 and the SEC (Standards for Efficient Cryptography). It defines how clients and servers advertise support for algorithms like ecc-based key exchange and public key authentication alongside legacy methods such as RSA and DSA. The extension clarifies interoperability with implementations from projects such as OpenSSL, GnuTLS, LibreSSL, and corporate stacks from Microsoft and Oracle Corporation by specifying wire formats and algorithm name canonicalization. The design also considers compatibility with centralized identity systems exemplified by Kerberos deployments and federated identity initiatives like SAML.

Algorithms and Key Exchange Methods

RFC 5656 standardizes support for ECC curves and signature schemes used in key exchange algorithms, including curve identifiers related to families documented by SECG and recommendations from NIST. It specifies use of elliptic-curve Diffie–Hellman variants as key exchange methods comparable to existing Diffie–Hellman groups and details signature algorithms that map to standards such as ECDSA and their relationship to certificate frameworks like X.509 and PKCS#11. The specification deliberates algorithm name precedence and interaction with host key algorithms employed by implementations from OpenSSH, client libraries like libssh2, and commercial products from IBM and Hewlett-Packard.

Message Formats and Packet Details

The RFC defines binary formats for ECC public keys, signatures, and key exchange messages within the SSH binary packet protocol specified in earlier RFCs. It prescribes how to encode curve parameters, Q-points, and signature components into SSH string and mpint fields used by stacks such as OpenSSH and toolkits like WolfSSL. Message flow diagrams in the text map to transport layer behavior commonly implemented alongside TCP stacks in Linux, FreeBSD, and Windows Server environments. The document also describes negotiation failure semantics and backward compatibility behaviors observed in deployments from vendors such as Apple and Google.

Security Considerations

RFC 5656 assesses security properties of ECC within SSH, discussing resistance to known attacks compared to RSA and DSA and referencing cryptanalytic concerns addressed by organizations such as NSA and research groups at University of California, Berkeley. It recommends curve choices and key sizes consistent with contemporary guidance from NIST and international standards bodies like the International Organization for Standardization. The specification warns implementers about pitfalls in random number generation and signature validation that have affected implementations in projects like OpenSSL and utility libraries used by Amazon Web Services and Microsoft Azure.

Implementation and Deployment

The RFC influenced widespread implementation across open-source and commercial SSH clients and servers, with early adoption by OpenSSH and integration into TLS/crypto stacks such as OpenSSL and LibreSSL. Vendors including Cisco Systems, Juniper Networks, IBM, and Hewlett-Packard incorporated the extensions into routers, switches, and enterprise servers, enabling ECC-based host keys and authentication in cloud platforms operated by companies like Google, Amazon, and Microsoft. The document also guided interoperability testing in standards events organized by the IETF and conformance efforts coordinated by consortiums like the Open Group.

History and Standards Process

Authored by T. Ylonen and T. Kivinen, the RFC progressed through the IETF working group process with input from implementers and cryptographers associated with projects such as OpenSSH, IETF SSH discussions, and vendors including Cisco and Sun Microsystems. Publication followed review cycles and adoption of elliptic curve recommendations from bodies such as NIST and the IETF Security Area, situating the RFC within the lineage of SSH-related standards like earlier protocol specifications that shaped secure remote administration across platforms ranging from UNIX distributions to enterprise systems. The RFC remains part of the historical evolution of SSH and cryptographic practice in networked systems.

Category:Internet Standards