1-before.rb

require 'base64'

def public_base64(key)
  ["ssh-rsa", Base64.encode64(ssh_public_key_conversion(key)).gsub("\n", "")].join(" ").strip
end

def ssh_public_key_conversion(public_key)
  out = [0, 0, 0, 7].pack("C*")
  out += "ssh-rsa"

  ["e", "n"].each do |method|
    value = public_key.send(method).to_i
    byte_array = to_byte_array(value)
    out += encode_unsigned_int_32(byte_array.length).pack("c*")
    out += byte_array.pack("C*")
  end

  out
end

def encode_unsigned_int_32(value)
  out = []
  out[0] = value >> 24 & 0xff
  out[1] = value >> 16 & 0xff
  out[2] = value >> 8 & 0xff
  out[3] = value & 0xff
  out
end

def to_byte_array(num)
  result = []
  begin
    result << (num & 0xff)
    num >>= 8
  end until (num == 0 || num == -1) && (result.last[7] == num[7])
  result.reverse
end

2-after.rb

def public_base64(key)
  ["ssh-rsa ", "\0\0\0\assh-rsa#{ sized_bytes(key.e) }#{ sized_bytes(key.n) }"].pack("a*m0")
end

def sized_bytes(value)
  bytes = to_byte_array(value.to_i)
  [bytes.size, *bytes].pack('NC*')
end

def to_byte_array(num, *significant)
  return significant if num.between?(-1, 0) and significant[0][7] == num[7]
  to_byte_array(*num.divmod(256)) + significant
end

Ah, it generates the encoded public SSH key (you can for instance then add to a repo on Github) from a pair of prime numbers (ie generated by the Ruby openssl lib). The pre-refactoring code is pretty much copied from the sshkey gem.

Usage:

require 'openssl'

key = OpenSSL::PKey::RSA.generate(2048)
puts public_base64(key.public_key)

soo nice. thanks!!