HMAC function in ruby

hmac.rb

require 'openssl'

def hmac key, message
  sha = OpenSSL::Digest::SHA256.new
  blocksize = 64 # SHA256 uses a 64 byte (512 bit) block size

  def xor str0, str1 # assuming strings are of equal length
    b0 = str0.bytes.to_a
    b1 = str1.bytes.to_a
    result = []
    b0.each_with_index do |b, i|
      result << (b ^ b1[i]).chr
    end
    result.join ''
  end

  if key.length > blocksize
    key = sha.update(key).to_s
    sha.reset
  end
  if key.length < blocksize
    key += 0.chr * (blocksize - key.length)
  end

  o_key_pad = xor key, (92.chr * blocksize)
  i_key_pad = xor key, (54.chr * blocksize)

  hash = sha.update(i_key_pad).update(message).to_s
  sha.reset
  sha.update(o_key_pad).update(hash).to_s
end