The SSH protocol supports the use of public/private key pairs in order to perform authentication based on public key cryptography. However, in order to use public key authentication in the SSH protocol, public keys must first be exchanged between client and server.
This document formally describes an existing public key file format that can be used with any of the common existing file transfer mechanisms in order to exchange public keys.
The SSH protocol also uses public/private key pairs to authenticate the server. In this scenario, it is important to verify that the public key provided by the server is indeed the server's public key. This document describes a mechanism for creating a short text string that uniquely represents a particular public key, called fingerprinting.
The key file header section consists of multiple RFC822-style header fields. Each field is a line of the following format:
Header-tag ':' ' ' Header-value
The Header-tag MUST NOT be more than 64 8-bit bytes and is case- insensitive. The Header-value MUST NOT be more than 1024 8-bit bytes. Each line in the header MUST NOT be more than 72 8-bit bytes.
A line is continued if the last character in the line is a '\'. If the last character of a line is a '\', then the logical contents of the line are formed by removing the '\' and the line termination characters, and appending the contents of the next line.
The Header-tag MUST be encoded in US-ASCII. The Header-value MUST be encoded in UTF-8 [RFC3629].
A line that is not a continuation line that has no ':' in it is the first line of the base64-encoded body. (See Section 3.4.)
The space of header-tags is managed as described in Section 5.
Compliant implementations MUST ignore headers with unrecognized header-tags. Implementations SHOULD preserve such unrecognized headers when manipulating the key file.
The following are some examples of public key files that are compliant (note that the examples all wrap before 72 bytes to meet IETF document requirements; however, they are still compliant.)
---- BEGIN SSH2 PUBLIC KEY ---- Comment: "1024-bit RSA, converted from OpenSSH by email@example.com" x-command: /home/me/bin/lock-in-guest.sh AAAAB3NzaC1yc2EAAAABIwAAAIEA1on8gxCGJJWSRT4uOrR13mUaUk0hRf4RzxSZ1zRb YYFw8pfGesIFoEuVth4HKyF8k1y4mRUnYHP1XNMNMJl1JcEArC2asV8sHf6zSPVffozZ 5TT4SfsUu/iKy9lUcCfXzwre4WWZSXXcPff+EHtWshahu3WzBdnGxm5Xoi89zcE= ---- END SSH2 PUBLIC KEY ----
---- BEGIN SSH2 PUBLIC KEY ---- Comment: This is my public key for use on \ servers which I don't like. AAAAB3NzaC1kc3MAAACBAPY8ZOHY2yFSJA6XYC9HRwNHxaehvx5wOJ0rzZdzoSOXxbET W6ToHv8D1UJ/z+zHo9Fiko5XybZnDIaBDHtblQ+Yp7StxyltHnXF1YLfKD1G4T6JYrdH YI14Om1eg9e4NnCRleaqoZPF3UGfZia6bXrGTQf3gJq2e7Yisk/gF+1VAAAAFQDb8D5c vwHWTZDPfX0D2s9Rd7NBvQAAAIEAlN92+Bb7D4KLYk3IwRbXblwXdkPggA4pfdtW9vGf J0/RHd+NjB4eo1D+0dix6tXwYGN7PKS5R/FXPNwxHPapcj9uL1Jn2AWQ2dsknf+i/FAA vioUPkmdMc0zuWoSOEsSNhVDtX3WdvVcGcBq9cetzrtOKWOocJmJ80qadxTRHtUAAACB AN7CY+KKv1gHpRzFwdQm7HK9bb1LAo2KwaoXnadFgeptNBQeSXG1vO+JsvphVMBJc9HS n24VYtYtsMu74qXviYjziVucWKjjKEb11juqnF0GDlB3VVmxHLmxnAz643WK42Z7dLM5 sY29ouezv4Xz2PuMch5VGPP+CDqzCM4loWgV ---- END SSH2 PUBLIC KEY ----
---- BEGIN SSH2 PUBLIC KEY ---- Comment: DSA Public Key for use with MyIsp AAAAB3NzaC1kc3MAAACBAPY8ZOHY2yFSJA6XYC9HRwNHxaehvx5wOJ0rzZdzoSOXxbET W6ToHv8D1UJ/z+zHo9Fiko5XybZnDIaBDHtblQ+Yp7StxyltHnXF1YLfKD1G4T6JYrdH YI14Om1eg9e4NnCRleaqoZPF3UGfZia6bXrGTQf3gJq2e7Yisk/gF+1VAAAAFQDb8D5c vwHWTZDPfX0D2s9Rd7NBvQAAAIEAlN92+Bb7D4KLYk3IwRbXblwXdkPggA4pfdtW9vGf J0/RHd+NjB4eo1D+0dix6tXwYGN7PKS5R/FXPNwxHPapcj9uL1Jn2AWQ2dsknf+i/FAA vioUPkmdMc0zuWoSOEsSNhVDtX3WdvVcGcBq9cetzrtOKWOocJmJ80qadxTRHtUAAACB AN7CY+KKv1gHpRzFwdQm7HK9bb1LAo2KwaoXnadFgeptNBQeSXG1vO+JsvphVMBJc9HS n24VYtYtsMu74qXviYjziVucWKjjKEb11juqnF0GDlB3VVmxHLmxnAz643WK42Z7dLM5 sY29ouezv4Xz2PuMch5VGPP+CDqzCM4loWgV ---- END SSH2 PUBLIC KEY ----
Galbraith & Thayer Informational [Page 5]
RFC 4716 SSH Public Key File Format November 2006
---- BEGIN SSH2 PUBLIC KEY ---- Subject: me Comment: 1024-bit rsa, created by firstname.lastname@example.org Mon Jan 15 \ 08:31:24 2001 AAAAB3NzaC1yc2EAAAABJQAAAIEAiPWx6WM4lhHNedGfBpPJNPpZ7yKu+dnn1SJejgt4 596k6YjzGGphH2TUxwKzxcKDKKezwkpfnxPkSMkuEspGRt/aZZ9wa++Oi7Qkr8prgHc4 soW6NUlfDzpvZK2H5E7eQaSeP3SAwGmQKUFHCddNaP0L+hM7zhFNzjFvpaMgJw0= ---- END SSH2 PUBLIC KEY ----
The security of the SSH protocols relies on the verification of public host keys. Since public keys tend to be very large, it is difficult for a human to verify an entire host key. Even with a Public Key Infrastructure (PKI) in place, it is useful to have a standard for exchanging short fingerprints of public keys.
This section formally describes the method of generating public key fingerprints that is in common use in the SSH community.
The fingerprint of a public key consists of the output of the MD5 message-digest algorithm [RFC1321]. The input to the algorithm is the public key data as specified by [RFC4253]. (This is the same data that is base64 encoded to form the body of the public key file.)
The output of the algorithm is presented to the user as a sequence of 16 octets printed as hexadecimal with lowercase letters and separated by colons.
For example: "c1:b1:30:29:d7:b8:de:6c:97:77:10:d7:46:41:63:87"
The file format described by this document provides no mechanism to verify the integrity or otherwise detect tampering with the data stored in such files. Given the potential of adversarial tampering with this data, system-specific measures (e.g., Access Control Lists, UNIX permissions, other Discretionary and/or Mandatory Access Controls) SHOULD be used to protect these files. Also, if the contents of these files are transferred it SHOULD be done over a trusted channel.
The header data allowed by this file format could contain an unlimited range of information. While in many environments the information conveyed by this header data may be considered innocuous public information, it may constitute a channel through which information about a user, a key, or its use may be disclosed intentionally or otherwise (e.g., "Comment: Mary E. Jones, 123 Main St, Home Phone:..."). The presence and use of this header data SHOULD be reviewed by sites that deploy this file format.
The public key fingerprint method presented here relies on the MD5 one-way hash function, which is known to have certain weaknesses regarding its collision resistance; however, the particular use made of MD5 here depends solely on its 2nd-preimage resistance, not on its collision resistance.
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