CODEWITHSUNDEEP
rubyrsapublic-key-encryption
mjwhitta
mjwhitta

Reputation: 71

How to read id_rsa.pub into ruby Bignum?

I'm trying to read in some public key files generated with:

for i in $(seq 1 100); do
    ssh-keygen -t rsa -f keys/$i -q -N ""
done

I'm using ruby and my code is below. The problem I'm having is that I can't be sure I'm doing it correctly. I thought that the public key was a large number generated by multiplying 2 large prime numbers. I'm getting numbers that are divisible by 3, 15, 33, and/or some other numbers. These are all multiples of prime numbers but I was expecting only 2 prime numbers, not 3 or 4. It may be that I am using ruby incorrectly or that I am misunderstanding the format of a public key. Any help would be greatly appreciated.

#!/usr/bin/env ruby
pubhash = Hash.new
# Read in public key files
pubfiles = File.join("**", "*.pub")
Dir.glob(pubfiles) do |filename|
    File.open(filename) do |file|
        file.each do |line|
            base64 = line.chomp.split[1]
            bytes = base64.unpack("m").first.unpack("C*").reverse
            key = bytes.each_with_index.inject(0) do
                |sum, (byte, index)|
                sum + byte * (256 ** index)
            end
            pubhash[filename] = key
        end
        file.close
    end
end

EDIT solution thanks to the link that Charlie provided:

File.open(filename) do |file|
    file.each do |line|
        base64 = line.chomp.split[1]
        keydata = base64.unpack("m").first
        parts = Array.new
        while (keydata.length > 0)
            dlen = keydata[0, 4].bytes.inject(0) do |a, b|
                (a << 8) + b
            end
            data = keydata[4, dlen]
            keydata = keydata[(dlen + 4)..-1]
            parts.push(data)
        end
        @type = parts[0]
        @e = parts[1].bytes.inject do |a, b|
            (a << 8) + b
        end
        @n = parts[2].bytes.inject do |a, b|
            (a << 8) + b
        end
    end
    file.close
end

Upvotes: 0

Views: 704

Answers (1)

Charlie
Charlie

Reputation: 7349

The reason you're finding other divisors of this number is likely because the base64 block contains more than just the key.

For example, I used ssh-keygen -t rsa -b 768 -C so-is-cool to generate a keypair and have this as my .pub: 

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAYQDK92Q/FMsaOuBE9NL7OufMYWVPWj62F6QXl4ADcYgFMrGMJRF1njg5UGujgqaIrouGjoqudt23fykUNG7HRZV4M4Plxknj4TSvFIG5hi+6x/AQzzPP7xnLkYBKDOxSs+M= so-is-cool

Fire up IRB:

base64='AAAAB3NzaC1yc2EAAAADAQABAAAAYQDK92Q/FMsaOuBE9NL7OufMYWVPWj62F6QXl4ADcYgFMrGMJRF1njg5UGujgqaIrouGjoqudt23fykUNG7HRZV4M4Plxknj4TSvFIG5hi+6x/AQzzPP7xnLkYBKDOxSs+M='
base64.unpack('m').first

You will see that the first bytes of it are: 

\x00\x00\x00\assh-rsa\x00\x00\x00\

or in other words my key algorithm. You probably need to parse this value a bit more to be of actual value. I found this blog that discusses the format of the OpenSSH .pub file a bit: http://blog.oddbit.com/2011/05/08/converting-openssh-public-keys/

Upvotes: 1

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