Format Preserving Encryption

New in version 1.9.17.

Format preserving encryption (FPE) refers to a set of techniques for encrypting data such that the ciphertext has the same format as the plaintext. For instance, you can use FPE to encrypt credit card numbers with valid checksums such that the ciphertext is also an credit card number with a valid checksum, or similiarly for bank account numbers, US Social Security numbers, or even more general mappings like English words onto other English words.

The scheme currently implemented in botan is called FE1, and described in the paper Format Preserving Encryption by Mihir Bellare, Thomas Ristenpart, Phillip Rogaway, and Till Stegers. FPE is an area of ongoing standardization and it is likely that other schemes will be included in the future.

To use FE1, use these functions, from fpe_fe1.h:

BigInt FPE::fe1_encrypt(const BigInt &n, const BigInt &X, const SymmetricKey &key, const std::vector<uint8_t> &tweak)

Encrypts the value X modulo the value n using the key and tweak specified. Returns an integer less than n. The tweak is a value that does not need to be secret that parameterizes the encryption function. For instance, if you were encrypting a database column with a single key, you could use a per-row-unique integer index value as the tweak.

To encrypt an arbitrary value using FE1, you need to use a ranking method. Basically, the idea is to assign an integer to every value you might encrypt. For instance, a 16 digit credit card number consists of a 15 digit code plus a 1 digit checksum. So to encrypt a credit card number, you first remove the checksum, encrypt the 15 digit value modulo 1015, and then calculate what the checksum is for the new (ciphertext) number.

BigInt FPE::fe1_decrypt(const BigInt &n, const BigInt &X, const SymmetricKey &key, const std::vector<uint8_t> &tweak)

Decrypts an FE1 ciphertext produced by fe1_encrypt; the n, key and tweak should be the same as that provided to the encryption function. Returns the plaintext.

Note that there is not any implicit authentication or checking of data, so if you provide an incorrect key or tweak the result is simply a random integer.

This example encrypts a credit card number with a valid Luhn checksum to another number with the same format, including a correct checksum.

/*
* (C) 2014,2015 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#include "cli.h"
#include <botan/hex.h>

#if defined(BOTAN_HAS_FPE_FE1) && defined(BOTAN_HAS_PBKDF)

#include <botan/fpe_fe1.h>
#include <botan/pbkdf.h>

namespace Botan_CLI {

namespace {

uint8_t luhn_checksum(uint64_t cc_number)
   {
   uint8_t sum = 0;

   bool alt = false;
   while(cc_number)
      {
      uint8_t digit = cc_number % 10;
      if(alt)
         {
         digit *= 2;
         if(digit > 9)
            {
            digit -= 9;
            }
         }

      sum += digit;

      cc_number /= 10;
      alt = !alt;
      }

   return (sum % 10);
   }

bool luhn_check(uint64_t cc_number)
   {
   return (luhn_checksum(cc_number) == 0);
   }

uint64_t cc_rank(uint64_t cc_number)
   {
   // Remove Luhn checksum
   return cc_number / 10;
   }

uint64_t cc_derank(uint64_t cc_number)
   {
   for(size_t i = 0; i != 10; ++i)
      {
      if(luhn_check(cc_number * 10 + i))
         {
         return (cc_number * 10 + i);
         }
      }

   return 0;
   }

uint64_t encrypt_cc_number(uint64_t cc_number,
                           const Botan::secure_vector<uint8_t>& key,
                           const std::vector<uint8_t>& tweak)
   {
   const Botan::BigInt n = 1000000000000000;

   const uint64_t cc_ranked = cc_rank(cc_number);

   const Botan::BigInt c = Botan::FPE::fe1_encrypt(n, cc_ranked, key, tweak);

   if(c.bits() > 50)
      {
      throw Botan::Internal_Error("FPE produced a number too large");
      }

   uint64_t enc_cc = 0;
   for(size_t i = 0; i != 7; ++i)
      {
      enc_cc = (enc_cc << 8) | c.byte_at(6 - i);
      }
   return cc_derank(enc_cc);
   }

uint64_t decrypt_cc_number(uint64_t enc_cc,
                           const Botan::secure_vector<uint8_t>& key,
                           const std::vector<uint8_t>& tweak)
   {
   const Botan::BigInt n = 1000000000000000;

   const uint64_t cc_ranked = cc_rank(enc_cc);

   const Botan::BigInt c = Botan::FPE::fe1_decrypt(n, cc_ranked, key, tweak);

   if(c.bits() > 50)
      {
      throw CLI_Error("FPE produced a number too large");
      }

   uint64_t dec_cc = 0;
   for(size_t i = 0; i != 7; ++i)
      {
      dec_cc = (dec_cc << 8) | c.byte_at(6 - i);
      }
   return cc_derank(dec_cc);
   }

}

class CC_Encrypt final : public Command
   {
   public:
      CC_Encrypt() : Command("cc_encrypt CC passphrase --tweak=") {}

      void go() override
         {
         const uint64_t cc_number = std::stoull(get_arg("CC"));
         const std::vector<uint8_t> tweak = Botan::hex_decode(get_arg("tweak"));
         const std::string pass = get_arg("passphrase");

         std::unique_ptr<Botan::PBKDF> pbkdf(Botan::PBKDF::create("PBKDF2(SHA-256)"));
         if(!pbkdf)
            {
            throw CLI_Error_Unsupported("PBKDF", "PBKDF2(SHA-256)");
            }

         Botan::secure_vector<uint8_t> key = pbkdf->pbkdf_iterations(32, pass, tweak.data(), tweak.size(), 100000);

         output() << encrypt_cc_number(cc_number, key, tweak) << "\n";
         }
   };

BOTAN_REGISTER_COMMAND("cc_encrypt", CC_Encrypt);

class CC_Decrypt final : public Command
   {
   public:
      CC_Decrypt() : Command("cc_decrypt CC passphrase --tweak=") {}

      void go() override
         {
         const uint64_t cc_number = std::stoull(get_arg("CC"));
         const std::vector<uint8_t> tweak = Botan::hex_decode(get_arg("tweak"));
         const std::string pass = get_arg("passphrase");

         std::unique_ptr<Botan::PBKDF> pbkdf(Botan::PBKDF::create("PBKDF2(SHA-256)"));
         if(!pbkdf)
            {
            throw CLI_Error_Unsupported("PBKDF", "PBKDF2(SHA-256)");
            }

         Botan::secure_vector<uint8_t> key = pbkdf->pbkdf_iterations(32, pass, tweak.data(), tweak.size(), 100000);

         output() << decrypt_cc_number(cc_number, key, tweak) << "\n";
         }
   };

BOTAN_REGISTER_COMMAND("cc_decrypt", CC_Decrypt);

}

#endif // FPE && PBKDF