= OpenSSH Key Structure Guide
brent saner <bts@square-r00t.net>, https://r00t2.io
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== 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 https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.key[the OpenSSH source tree's `PROTOCOL.key`^], which is a little lacking.

== Basic Introduction
=== Legacy
==== Private Keys

In OpenSSH pre-7.8, private keys are stored in their respective PEM encodingfootnote:[https://datatracker.ietf.org/doc/html/rfc7468] with no modification. These legacy private keys should be entirely usable by OpenSSL/LibreSSL/GnuTLS etc. natively with no conversion necessary.

==== 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, Base64footnote:[https://datatracker.ietf.org/doc/html/rfc4648]-encoded
C:: The key's comment

The structures specified in the breakdowns later in this document describe the _decoded_ version of *B* *_only_*. They are specific to each keytype and format version starting with item `2.0`.


=== New "v1" Format
==== Private Keys

Private key structures have been retooled in the "v1" format. In recent OpenSSH versions, all new keys use the v1 format. They no longer are in straight PEM-compatible format.

Refer to https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.key[`PROTOCOL.key`^] for a (very) general description, or each key's specific breakdown for more detailed information.

The v1 format offers several benefits over the legacy format, including:

* customizable key derivation and encryption ciphers for encrypted private keys
* embedded comments
* embedded public key (no need to derive from the private key)
* "checksumming" to confirm proper decryption for encrypted keys

==== Public Keys

All public keys in v1 continue to use the same packed binary format as <<public_keys, the legacy format>>.

== Keytype-Specific Breakdowns

include::rsa/main.adoc[]

include::ed25519/main.adoc[]

== Further Information

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<span xmlns:dct="http://purl.org/dc/terms/" href="http://purl.org/dc/dcmitype/Text" property="dct:title" rel="dct:type">OpenSSH Key Structure Guide</span> by <a xmlns:cc="http://creativecommons.org/ns#" href="https://r00t2.io/" property="cc:attributionName" rel="cc:attributionURL">Brent Saner</a> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">Creative Commons Attribution-ShareAlike 4.0 International License</a>.<br />
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