Understanding SSH Certificate signatures

Digant Kasundra diggyk at gmail.com
Tue Feb 9 17:20:20 AEDT 2021

Hello OpenSSH community,

I haven't found a good Rust library to verify that a presented OpenSSH
public certificate is valid.  My plan is to compare the signature_key to my
trusted CA certs and verify the signature in the user's public certificate.

Here is what I tried but it isn't working:
* create an openssl RSA public key using the n and e from the signature_key
* decrypt the signature with that key to get the hash
* create a hash from all the base64 bytes up to but not including the
signature using SHA1
* compare the hashes, but they do not match

To create my own hash, I'm skipping the opening text identifier in the
cert, but using the "ssh-rsa-cert-v01 at openssh.com" string as part of the
octet string.

Am I on the right track?  If so, I'll give some details of what I'm doing
so we can hopefully pinpoint where I'm going wrong.  (Or if someone knows a
rust crate that actually verifies a certificate instead of just parsing it,
that would be awesome!)

Digant Chimanlal "DC" Kasundra

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