how is the sha fingerprint generated?

[Johannes Löthberg]

On 30/06, shawn wilson wrote:
>% cat ext_rsa.pub| sed -r 's/.*(AAAA[^ ]+).*/\1/' | sha256sum
>
>  ~/.ssh swlap1
>d4bf8b06f2d9d9af7a11583a5367205ed310a84f0dee68d062e2ddca1e85c3ff  -
> % ssh-keygen -lf ext_rsa.pub
>
>   ~/.ssh swlap1
>8192 SHA256:FgrfxmdjTM/j4wwRa7nVdPSUaJdqHYMJtJ6aciPl9ug swilson at swlap1 (RSA)
>
>Why do those differ and how would i generate the equivalent (mainly
>just curious)? I've also tried base64 and a few other substitutions at
>the end and I can't get them to match (probably would save time to
>just look at the code, but...).

It's not simply a checksum of the key file. You need to extract the 
exponent and prime from the public key, then append those to a specific 
string of bits, then get a SHA256 digest of that, and then base64 encode 
that.

https://github.com/kyrias/bin/blob/master/ssh-gen-fprint has an example 
implementation of `ssh-keygen -lf` in Ruby.

-- 
Sincerely,
  Johannes Löthberg
  PGP Key ID: 0x50FB9B273A9D0BB5
  https://theos.kyriasis.com/~kyrias/
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