class RsaPem

# File lib/rsa_pem.rb, line 5
def from(mod, exp)
  encoded_pubkey = create_encoded_pubkey_from(mod, exp)
  der_b64 = hex_to_base64_digest encoded_pubkey

  pem_padding_for der_b64
end

# File lib/rsa_pem.rb, line 37
def add_newlines(der_b64)
  der_b64.scan(/.{1,64}/).join("\n")
end

# File lib/rsa_pem.rb, line 81
def base64_to_hex_digest(base64)
  # http://anthonylewis.com/2011/02/09/to-hex-and-back-with-ruby/
  base64.unpack('H*').first
end

# File lib/rsa_pem.rb, line 14
def create_encoded_pubkey_from(mod, exp)
  modulus = Base64.urlsafe_decode64(mod)
  exponent = Base64.urlsafe_decode64(exp)

  modulus_hex = prepad_signed(base64_to_hex_digest modulus)
  exponent_hex = prepad_signed(base64_to_hex_digest exponent)

  modlen = modulus_hex.length / 2
  explen = exponent_hex.length / 2

  encoded_modlen = encode_length_hex(modlen)
  encoded_explen = encode_length_hex(explen)

  full_hex_len = modlen + explen + encoded_modlen.length/2 + encoded_explen.length/2 + 2
  encoded_hex_len = encode_length_hex(full_hex_len)

  "30#{encoded_hex_len}02#{encoded_modlen}#{modulus_hex}02#{encoded_explen}#{exponent_hex}"
end

# File lib/rsa_pem.rb, line 50
def encode_length_hex(der_len)
  # encode ASN.1 DER length field

  if short_form? der_len
    int_to_hex(der_len)
  else
    number_hex = int_to_hex(der_len)
    length_of_length_byte = 128 + number_hex.length/2

    "#{int_to_hex(length_of_length_byte)}#{number_hex}"
  end
end

# File lib/rsa_pem.rb, line 76
def hex_to_base64_digest(hex)
  # https://stackoverflow.com/questions/9986971/converting-a-hexadecimal-digest-to-base64-in-ruby
  [[hex].pack("H*")].pack("m0")
end

# File lib/rsa_pem.rb, line 71
def int_to_hex(int)
  number_hex = int.to_s(16)
  number_hex.length.odd? ? pad(number_hex, '0') : number_hex
end

# File lib/rsa_pem.rb, line 67
def pad(hex, padding)
  "#{padding}#{hex}"
end

# File lib/rsa_pem.rb, line 33
def pem_padding_for(der_b64)
  "-----BEGIN RSA PUBLIC KEY-----\n#{add_newlines der_b64}\n-----END RSA PUBLIC KEY-----"
end

# File lib/rsa_pem.rb, line 41
def prepad_signed(hex)
  msb = hex[0]
  if (msb < '0' || msb > '7')
    pad(hex, '00')
  else
   hex
  end
end

# File lib/rsa_pem.rb, line 63
def short_form?(asn1_der_length)
  asn1_der_length <= 127
end