Calculate various key fingerprints

key_fingerprints.rb

require 'openssl'

# The key for the sample:
private_key = OpenSSL::PKey::RSA.new(1024)

# This method is for comparing an RSA key too an SSH server when it's using
# MD5. You can pass either a public or private key here.
def ssh_md5_fingerprint(key)
  OpenSSL::Digest::MD5.hexdigest(
    [7].pack("N") + 'ssh-rsa' + key.public_key.e.to_s(0) + key.public_key.n.to_s(0)
  ).scan(/../).join(':')
end

# This method is for comparing an RSA key too an SSH server when it's using
# SHA1. You can pass either a public or private key here.
def ssh_sha1_fingerprint(key)
  OpenSSL::Digest::SHA1.hexdigest(
    [7].pack("N") + 'ssh-rsa' + key.public_key.e.to_s(0) + key.public_key.n.to_s(0)
  ).scan(/../).join(':')
end

# This method is for comparing an RSA key to the hash identifier listed in x509
# (a lot of people refer to them as SSL) certificates.
def x509_sha1_fingerprint(key)
  sequence = OpenSSL::ASN1::Sequence([
    OpenSSL::ASN1::Integer.new(key.public_key.n),
    OpenSSL::ASN1::Integer.new(key.public_key.e)
  ])
  OpenSSL::Digest::SHA1.hexdigest(sequence.to_der).scan(/../).join(':')
end

puts ssh_md5_fingerprint(private_key)
puts ssh_md5_fingerprint(private_key.public_key)

puts ssh_sha1_fingerprint(private_key)
puts ssh_sha1_fingerprint(private_key.public_key)

puts x509_sha1_fingerprint(private_key)
puts x509_sha1_fingerprint(private_key.public_key)