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Specifically, why are the first 25 characters of ssh Ed25519 public keys always the same?

For example, if I make 5 keys with ssh-keygen -o -a 100 -t ed25519, the first 25 characters are always AAAAC3NzaC1lZDI1NTE5AAAAI.

I assume this is a preamble or header of some sort, but I would love to know the actual answer.

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    Crossdupe crypto.stackexchange.com/questions/5944/… . New format OpenSSH private key files similarly are the base64 encoding of data that begins with several mostly-constant XDR-formatted strings, although the key type/algorithm is some way in; new format is automatically used for ed25519 (because no old format exists for that type) and you only need to specify -o for other types. Jan 3, 2018 at 10:04

1 Answer 1

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The string is base64 encoded. When decoded, it produces the string \0\0\0\vssh-ed25519\0. This identifies the type of the key. In this case an EdDSA key.

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    The \0\0\0\v part is 11 written as a 32 bit number and is indicating the length of the string ssh-ed25519.
    – kasperd
    Jan 3, 2018 at 23:24

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