SSH public key format: The Secure Shell (SSH) Public Key File Format
A plain-text file format for sharing SSH public keys between different SSH programs, plus a standard fingerprint.
Galbraith, J. and R. Thayer, "The Secure Shell (SSH) Public Key File Format", RFC 4716, DOI 10.17487/RFC4716, November 2006, <https://www.rfc-editor.org/info/rfc4716>.
@misc{rfc4716,
series = {Request for Comments},
number = 4716,
howpublished = {RFC 4716},
publisher = {RFC Editor},
doi = {10.17487/RFC4716},
url = {https://www.rfc-editor.org/info/rfc4716},
author = {J. Galbraith and R. Thayer},
title = {{The Secure Shell (SSH) Public Key File Format}},
pagetotal = 10,
year = 2006,
month = nov,
} RFC text
Contents
Network Working Group J. Galbraith
Request for Comments: 4716 VanDyke Software
Category: Informational R. Thayer
Canola & Jones
November 2006
The Secure Shell (SSH) Public Key File Format
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2006).
Abstract
This document formally documents an existing public key file format
in use for exchanging public keys between different Secure Shell
(SSH) implementations.
In addition, this document defines a standard textual representation
for SSH public key fingerprints.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. Key File Format .................................................2
3.1. Line Termination Characters ................................2
3.2. Begin and End Markers ......................................3
3.3. Key File Header ............................................3
3.3.1. Subject Header ......................................3
3.3.2. Comment Header ......................................4
3.3.3. Private Use Headers .................................4
3.4. Public Key File Body .......................................4
3.5. Differences with RFC 1421 PEM Formats ......................4
3.6. Examples ...................................................5
4. Public Key Fingerprints .........................................6
5. IANA Considerations .............................................6
6. Security Considerations .........................................7
7. References ......................................................8
7.1. Normative References .......................................8
7.2. Informative References .....................................8
1. Introduction
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.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Key File Format
In order to implement public key authentication, SSH implementations
must share public key files between the client and the server in
order to interoperate.
A key file is a text file, containing a sequence of lines. Each line
in the file MUST NOT be longer than 72 8-bit bytes excluding line
termination characters.
3.1. Line Termination Characters
Implementations SHOULD generate public key files using their system's
local text file representation.
In the event that public key files are not transferred as text files,
implementations SHOULD be prepared to read files using any of the
common line termination sequence, <CR>, <LF>, or <CR><LF>.
3.2. Begin and End Markers
The first line of a conforming key file MUST be a begin marker, which
is the literal text:
---- BEGIN SSH2 PUBLIC KEY ----
The last line of a conforming key file MUST be an end marker, which
is the literal text:
---- END SSH2 PUBLIC KEY ----
3.3. Key File Header
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.
3.3.1. Subject Header
This field is used to store the login-name that the key was generated
under. For example:
Subject: user
3.3.2. Comment Header
The comment header contains a user-specified comment. The comment
SHOULD be displayed when using the key.
It is suggested that this field default to user@hostname for the user
and machine used to generate the key. For example:
Comment: user@example.com
Currently, common practice is to quote the Header-value of the
Comment by prefixing and suffixing it with '"' characters, and some
existing implementations fail if these quotation marks are omitted.
Compliant implementations MUST function correctly if the quotation
marks are omitted.
Implementations MAY include the quotation marks. If the first and
last characters of the Header-value are matching quotation marks,
implementations SHOULD remove them before using the value.
3.3.3. Private Use Headers
Headers with header-tags beginning with "x-" are reserved for private
use.
3.4. Public Key File Body
The body of a public key file is the base64 encoded ([RFC2045])
public key data as specified by [RFC4253], Section 6.6:
string certificate or public key format identifier
byte[n] key/certificate data
As with all other lines, each line in the body MUST NOT be longer
than 72 8-bit bytes excluding line termination characters.
3.5. Differences with RFC 1421 PEM Formats
Implementers should take care to notice that while the format is
superficially similar to those specified by PEM [RFC1421] and OpenPGP
[RFC2440], it is not identical; most notably:
o The other specifications use different BEGIN/END delimiters (five
dashes, no space rather than four dashes and a space).
o There is no blank line before the start of the base64-encoded
contents.
o There is no Cyclic Redundancy Check (CRC) at the end of the
base64-encoded block.
o Header continuation uses a backslash at the end of the continued
line rather than whitespace at the start of the next line.
3.6. Examples
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 me@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.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---- END SSH2 PUBLIC KEY ----
---- BEGIN SSH2 PUBLIC KEY ----
Comment: DSA Public Key for use with MyIsp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---- END SSH2 PUBLIC KEY ----
---- BEGIN SSH2 PUBLIC KEY ----
Subject: me
Comment: 1024-bit rsa, created by me@example.com Mon Jan 15 \
08:31:24 2001
AAAAB3NzaC1yc2EAAAABJQAAAIEAiPWx6WM4lhHNedGfBpPJNPpZ7yKu+dnn1SJejgt4
596k6YjzGGphH2TUxwKzxcKDKKezwkpfnxPkSMkuEspGRt/aZZ9wa++Oi7Qkr8prgHc4
soW6NUlfDzpvZK2H5E7eQaSeP3SAwGmQKUFHCddNaP0L+hM7zhFNzjFvpaMgJw0=
---- END SSH2 PUBLIC KEY ----
4. Public Key Fingerprints
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"
5. IANA Considerations
Section 3.3 defines a new namespace of "Header-tags". These are
US-ASCII strings of maximum length 64 characters and are
case-insensitive.
IANA has created and maintains a registry of these header-tags. The
registry maps each header-tag to a reference defining the header.
The initial contents of the registry are as follows:
subject defined in Section 3.3.1
comment defined in Section 3.3.2
Header-tags beginning with "x-" are reserved for private use, as
defined in [RFC2434].
All other allocations are to be made by IETF consensus, as defined in
[RFC2434].
6. Security Considerations
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.
MD5 is used here for historical reasons.
7. References
7.1. Normative References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
7.2. Informative References
[RFC1421] Linn, J., "Privacy Enhancement for Internet Electronic
Mail: Part I: Message Encryption and Authentication
Procedures", RFC 1421, February 1993.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
Authors' Addresses
Joseph Galbraith
VanDyke Software
4848 Tramway Ridge Blvd
Suite 101
Albuquerque, NM 87111
US
Phone: +1 505 332 5700
EMail: galb@vandyke.com
Rodney Thayer
Canola & Jones
650 Castro Street Suite 120-205
Mountain View CA 94041
US
Phone: +1 650 704 8389
EMail: rodney@canola-jones.com
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AI ELI5
On this page
§1Why a Key File Format Is Needed
The SSH protocol can authenticate with public/private key pairs. Before public key authentication can work, the public keys must first be exchanged between client and server.
RFC 4716 formally describes an existing public key file format that can travel over any of the common file transfer mechanisms. The memo is Informational and states that it does not specify an Internet standard of any kind.
The same keys are also used to authenticate the server, so the RFC adds a way to make a short text string, a fingerprint, that represents a particular public key.
Before two computers can trust each other, they have to swap keys. This gives everyone the same kind of envelope to put the key in.
§3The Key File Layout
A key file is a text file made of lines. Each line MUST NOT be longer than 72 8-bit bytes, not counting the line ending. Files should be written with the system's local text line ending, and readers SHOULD accept <CR>, <LF> or <CR><LF> in case a file was not transferred as text.
The first line MUST be the literal begin marker ---- BEGIN SSH2 PUBLIC KEY ---- and the last line MUST be the end marker ---- END SSH2 PUBLIC KEY ----.
Between the markers come header lines, then the body. The body is the base64 encoding of the public key data as defined in RFC 4253, Section 6.6: a string naming the certificate or public key format, followed by the key or certificate data. Body lines obey the same 72-byte limit. A line with no colon that is not a continuation line is the first line of the body.
The key sits between a "start here" line and an "end here" line, like a letter in an envelope, and no line may be longer than a short, fixed length.
---- BEGIN SSH2 PUBLIC KEY ----
Subject: me
Comment: "1024-bit RSA, created by me@example.com"
AAAAB3NzaC1yc2EAAAABJQAAAIEAiPWx6WM4lhHNedGfBpPJNPpZ7yKu+dnn1SJejgt4
...
---- END SSH2 PUBLIC KEY ----§3.3Header Fields
Headers are RFC 822-style lines of the form Header-tag: Header-value. The tag is case-insensitive, US-ASCII and at most 64 bytes. The value is UTF-8 and at most 1024 bytes. A line ending in a backslash is continued: the backslash and line ending are removed and the next line is appended.
Two headers are defined. Subject holds the login name the key was generated under. Comment holds a user-specified comment that SHOULD be displayed when the key is used, and is suggested to default to user@hostname. Existing practice quotes the comment, and some implementations fail without the quotes. Compliant implementations MUST work when the quotes are omitted, MAY include them, and SHOULD strip matching quotes from the first and last characters.
Tags beginning with x- are reserved for private use. Implementations MUST ignore headers with tags they do not recognise and SHOULD preserve them when editing the file.
The format looks like PEM and OpenPGP armor but is not identical: the delimiters use four dashes and a space instead of five dashes, there is no blank line before the base64, there is no CRC at the end, and header continuation uses a trailing backslash rather than leading whitespace on the next line.
Above the key you can stick labels like "whose key" and "a note". If a program meets a label it does not know, it just leaves it alone and keeps it.
§4Public Key Fingerprints
SSH security depends on checking public host keys, but keys are long and hard for a person to compare. A fingerprint is a short stand-in for the key.
The fingerprint is the output of the MD5 message digest (RFC 1321) computed over the public key data as specified in RFC 4253, which is the same data that is base64-encoded to form the file body. The 16 output octets are printed as lowercase hexadecimal, separated by colons. The RFC's example is c1:b1:30:29:d7:b8:de:6c:97:77:10:d7:46:41:63:87.
A key is a very long number, so people check a short nickname made from it instead, a bit like checking the last digits of a long ID number.
§5The Header-Tag Registry
Section 3.3 creates a new namespace of header-tags: case-insensitive US-ASCII strings of at most 64 characters. IANA maintains a registry mapping each tag to the reference that defines it. The initial entries are subject and comment.
Tags starting with x- are private use. All other allocations need IETF consensus.
There is an official list of label names so two people never use the same label for different things, and a few names are kept free for your own use.
§6Security Considerations
The format has no way to check integrity or detect tampering, so files SHOULD be protected with system measures such as access control lists or UNIX permissions, and SHOULD be moved only over a trusted channel.
Header data is open-ended and can leak information about a user, a key or its use, for example a comment containing a name and address. Sites that deploy the format SHOULD review how headers are used.
The fingerprint uses MD5, which has known collision weaknesses. The RFC argues this use depends only on 2nd-preimage resistance, not collision resistance, and says MD5 is used for historical reasons.
The file has no lock on it, so keep it somewhere safe, do not put private details in the notes, and remember the nickname trick uses an older method kept for history.