Get public key from SSH server

Question

I have been looking for this for a long time.

I need to extract and get the modulus and the exponent from an SSH server.

For example, I know, that SSH is running on server xxx.xxx.xxx.xxx (I can connect to this server / ping) but I don't know the user name and the password so I cannot log in.

I need to get the modulus and the exponent of the public RSA key of this server.

I found out, that ssh-keyscan can get modulus + exponent (from documentation) but only if ssh-rsa1 is used. If I try to get ssh-rsa(2) public key with ssh-keyscan, I cannot retrieve the modulus and the exponent from the output.

Is it possible?

Answer 1

Sure you can!

Download public key from github and save it into github.pub file:

ssh-keyscan -t rsa github.com | sed "s/^[^ ]* //" > github.pub

Convert SSH public key format into X.509 public key format:

ssh-keygen -f github.pub -e -m pem > github.pem

Parse ASN.1 encoding of key to obtain public key modulus and exponent:

sed "/--/d" github.pem | openssl asn1parse | grep "INTEGER" | sed "s/.*://"

Result:

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
23

First line is a modulus and second line is an exponent.