AES Decryption logic for implicit IVs

decrypt.rb

require "openssl"
require "digest"
require "base64"

passphrase = '<key>'
encrypted_data = '<base64 encoded value>'

def derive_key_and_iv(passphrase, salt)
    dx = di = ""
    enc_pass = passphrase.bytes.pack('c*')
    for _ in 1...4
      di = Digest::MD5.digest(di + enc_pass + salt)
      dx += di
    end
    return dx[0..31], dx[32..47]
end

data = Base64.strict_decode64(encrypted_data)

salted = data[0..7]
if salted != "Salted__"
  raise "Invalid data"
end

salt = data[8..15]
crypted = data[16..-1]

cipher = OpenSSL::Cipher::AES256.new(:CBC)
cipher.decrypt
cipher.key, cipher.iv = derive_key_and_iv(passphrase, salt)
decrypted = cipher.update(crypted) + cipher.final

puts decrypted