pkce

#!/usr/bin/env ruby
#
# OVERVIEW
# --------
#
#   This script generates a code verifier with 256-bit entropy
#   (43 characters in base64url) and computes the code challenge
#   corresponding to the code verifier.
#
# REFERENCES
# ----------
#
#   RFC 7636 Proof Key for Code Exchange by OAuth Public Clients
#     https://www.rfc-editor.org/rfc/rfc7636.html
#


require "base64"
require "digest/sha2"
require "securerandom"


#------------------------------------------------------------
# main
#------------------------------------------------------------
def main(args)
  # Generate a code verifier.
  verifier = base64url_encode(random_bytes(32))

  # Generate a code challenge.
  challenge = base64url_encode(sha256(verifier))

  # Print the generated values.
  puts "CODE_VERIFIER=#{verifier}"
  puts "CODE_CHALLENGE=#{challenge}"
end


#------------------------------------------------------------
# Generate a random bytes of the specified size.
#------------------------------------------------------------
def random_bytes(size)
  SecureRandom.random_bytes(size)
end


#------------------------------------------------------------
# Compute the SHA-256 digest of the input data.
#------------------------------------------------------------
def sha256(input)
  Digest::SHA256.digest(input)
end


#------------------------------------------------------------
# Base64url-encode the input data.
#------------------------------------------------------------
def base64url_encode(input)
  Base64.urlsafe_encode64(input, padding: false)
end


#------------------------------------------------------------
# Entry Point
#------------------------------------------------------------
main(ARGV)