1. Purpose
This document attempts to present a much more detailed, thorough, and easily-understood form of the key formats used by OpenSSH. The extent of those formats' canonical documentation is the OpenSSH source tree’s PROTOCOL.key, which is a little lacking.
2. Basic Introduction
2.1. Legacy
2.1.1. Private Keys
In OpenSSH pre-7.8, private keys are stored in their respective PEM encoding[1] with no modification. These legacy private keys should be entirely usable by OpenSSL/LibreSSL/GnuTLS etc. natively with no conversion necessary.
2.1.2. Public Keys
Each public key file (*.pub) is written out in the following format:
A B C
Where:
- A
-
The key type (e.g.
ssh-rsa,ssh-ed25519, etc.) - B
-
The public key itself, Base64[2]-encoded
- C
-
The key’s comment
The structures specified in the breakdowns later in this document describe the decoded version of B only.
3. Keytype-Specific Breakdowns
3.1. RSA
RSA[3] is a widely-supported PKI system. It is ubiquitous, but it is recommended to use newer systems (e.g. ED25519) for OpenSSH if all clients and destinations support it.
The key structures have references to the RSA notations in single quotes. You can find these enumerated in RFC 8017 ยง 2. See also the Wikipedia article.
3.1.1. Public
3.1.1.1. Structure
Public keys are stored in the following structure:
1
2
3
4
5
6
0 uint32 allocator for 0.0 (4 bytes)
0.0 Public key type string (ASCII bytes; length defined above)
1 uint32 allocator for 1.0 (4 bytes)
1.0 Public exponent ('e')
2 uint32 allocator for 2.0 (4 bytes)
2.0 modulus ('n')
3.1.1.2. Example
.pub format1
ssh-rsa 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 This is a comment string
AAA…PBw==) 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
0 00000007 (7)
0.0 7373682d727361 ("ssh-rsa")
1 00000003 (3)
1.0 010001 (65537)
2 00000201 (513)
2.0 00b7cec04601ce2a12f0c924cb9a30eb990066812cb14369193f30b2b9fdd4af
cb300c918f2a77d64410f3617ae7c8ca318c257d3c4df4e2c4108bbbe93a8689
4ba14b3575f2f72150bc381dcbfb742c7a196866fd3184ace96761adda0fc299
2f6c866d7569919fc22d9c4bf0de405a8c76d519aa2a5329dc6825777229a5d0
b753a7825a89b95275f9c025e215343c6c88cd6690a221f8ae9ef675ee464dc7
d118da410507ea5d6b6489dd60afd8a6646492db3e279f1a78240db8abbda6c5
0714c9636650a72081e7fa5d472c1428b07eae5d15b64ea1e2a7508512fe9ab6
55f86a313486d3cca1dd8e90acc5c9fba4d6e767507fbab9f3a7f68c748142af
2a3701d31a8a9b7511958aa77187ba702ed934d385afcee42380e95e0e7e9bc0
f4d23367fc770374167b7f0926fb6fdb6d05aad1cfd191824845b014e18153bf
0d1d3c3b1fadbb25a3f1d151f9b684633d8c1690fcd8cad05aac2aeb23dbf19a
37e480a008910319c116d47bd924b39942543b88a0f6127952b2d8e1290f3029
f542aebe9c0c8e36cf3296865cd6643c8924d566ebf4971809399a1ac096fe1e
dc3b5f871bf5ef0b4d44e0ea27620d205142e0bfcf677b4db025532121a3f074
5aa4d0586331733257855a5cecbe3ac4403d04ff0cc0c58b7c04904b402125c2
bc2a63a20ebb309cc6f3e65db301a058b8dace07e71b38f3f3595433f69b198f
07