commands.txt

#1 Generate Public & Private Keypair
 
#2 Create receipt.json - eg below - careful with line-endings, etc if line breaks
 
#3 Create signature & Encode
openssl dgst -binary -sha1 -sign private.pem receipt.json | openssl base64 > signature.txt
 
#4 Verify using OpenSSL & public key
openssl base64 -d -in signature.txt -out signature.sha1 | openssl dgst -sha1 -verify public.pem -signature signature.sha1 receipt.json
 
#5 Verify with Ruby script (see below) & public key
ruby ver.rb receipt.json signature.txt public.pem

receipt.json

{ 
   "orderId":"12999763169054705758.1371079406387615", 
   "packageName":"com.example.app",
   "productId":"exampleSku",
   "purchaseTime":1345678900000,
   "purchaseState":0,
   "developerPayload":"bGoa+V7g/yqDXvKRqq+JTFn4uQZbPiQJo4pf9RzJ",
   "purchaseToken":"rojeslcdyyiapnqcynkjyyjh"
 }

ver.rb

require 'rubygems'
require 'openssl'
require 'base64'

def main

  raise ArgumentError, 'Require receipt, signature and public key to run' unless ARGV.length == 3

  #read files
  receipt = read_file(ARGV[0])
  signature = read_file(ARGV[1])
  base64_encoded_public_key = read_file(ARGV[2])

  #decode public key from Base64 string
  public_key = OpenSSL::PKey::RSA.new base64_encoded_public_key

  #verify the signature digest was encrypted with priv key & matches digest for receipt string
  verified = public_key.verify( OpenSSL::Digest::SHA1.new, Base64.decode64(signature), receipt )

  print "Wow... thats #{verified}-ly amazing\n"

end

def read_file(file_name)
  file = File.open(file_name, "r")
  data = file.read
  file.close
  return data
end

if __FILE__ == $0
    main()
end