ssh-gen-fprint

#!/usr/bin/env ruby

require 'openssl'
require 'base64'

# Parse SSH keys to be used by OpenSSL lib
# Taken from Zerg Support project.
# See: https://github.com/pwnall/zerg_support/blob/faaa5dd140c95588a1db2a25f6c9d9cacb4f9b0a/lib/zerg_support/open_ssh.rb
module OpenSSHKeyConverter
	# The components in a openssh .pub / known_host RSA public key.
	RSA_COMPONENTS = ['ssh-rsa', :e, :n]
	# The components in a openssh .pub / known_host DSA public key.
	DSA_COMPONENTS = ['ssh-dss', :p, :q, :g, :pub_key]

	# Decodes an openssh public key from the format of .pub & known_hosts files.
	def self.decode_pubkey(string)
		components = unpack_pubkey_components Base64.decode64(string)
		case components.first
		when RSA_COMPONENTS.first
			ops = RSA_COMPONENTS.zip components
			key = OpenSSL::PKey::RSA.new
		when DSA_COMPONENTS.first
			ops = DSA_COMPONENTS.zip components
			key = OpenSSL::PKey::DSA.new
		else
			fail "Unsupported key type #{components.first}"
		end
		ops.each do |o|
			next unless o.first.is_a? Symbol
			key.send "#{o.first}=", decode_mpi(o.last)
		end
		key
	end

	# Loads a serialized key from an IO instance (File, StringIO).
	def self.load_key(io)
		serialized_key = io.read
		header = first_line serialized_key
		if header.index 'RSA'
			OpenSSL::PKey::RSA.new serialized_key
		elsif header.index 'DSA'
			OpenSSL::PKey::DSA.new serialized_key
		else
			fail 'Unknown key type'
		end
	end

	# Extracts the first line of a string.
	def self.first_line(string)
		string[0, string.index(/\r|\n/) || string.len]
	end

	# Unpacks the string components in an openssh-encoded pubkey.
	def self.unpack_pubkey_components(str)
		cs = []
		i = 0
		while i < str.length
			len = str[i, 4].unpack('N').first
			cs << str[i + 4, len]
			i += 4 + len
		end
		cs
	end

	# Decodes an openssh-mpi-encoded integer.
	def self.decode_mpi(mpi_str)
		mpi_str.unpack('C*').inject(0) { |a, e| (a << 8) | e }
	end
end

path_to_key = ARGV[0]

key = File.read(path_to_key).split[1]
key = OpenSSHKeyConverter.decode_pubkey(key)
key = OpenSSL::PKey::RSA.new(key)

data_string = [7].pack('N') + 'ssh-rsa' + key.public_key.e.to_s(0) + key.public_key.n.to_s(0)

sha_digest = OpenSSL::Digest::SHA256.digest(data_string)
sha_fingerprint = Base64.encode64(sha_digest)

md5_fingerprint = OpenSSL::Digest::MD5.hexdigest(data_string).scan(/../).join(':')

puts('MD5:    ' + md5_fingerprint)
puts('SHA256: ' + sha_fingerprint)