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/*
* Encrypt credit cards numbers with valid checksums into other credit
* card numbers with valid checksums using format preserving encryption.
*/
#include <botan/fpe.h>
#include <botan/sha160.h>
#include <botan/init.h>
using namespace Botan;
#include <iostream>
#include <stdexcept>
byte luhn_checksum(u64bit cc_number)
{
byte sum = 0;
bool alt = false;
while(cc_number)
{
byte 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(u64bit cc_number)
{
return (luhn_checksum(cc_number) == 0);
}
u64bit cc_rank(u64bit cc_number)
{
// Remove Luhn checksum
return cc_number / 10;
}
u64bit cc_derank(u64bit cc_number)
{
for(u32bit i = 0; i != 10; ++i)
if(luhn_check(cc_number * 10 + i))
return (cc_number * 10 + i);
return 0;
}
/*
* Use the SHA-1 hash of the account name or ID as a tweak
*/
SecureVector<byte> sha1(const std::string& acct_name)
{
SHA_160 hash;
hash.update(acct_name);
return hash.final();
}
u64bit encrypt_cc_number(u64bit cc_number,
const SymmetricKey& key,
const std::string& acct_name)
{
BigInt n = 1000000000000000;
u64bit cc_ranked = cc_rank(cc_number);
BigInt c = fpe_encrypt(n, cc_ranked, key, sha1(acct_name));
if(c.bits() > 50)
throw std::runtime_error("FPE produced a number too large");
u64bit enc_cc = 0;
for(u32bit i = 0; i != 7; ++i)
enc_cc = (enc_cc << 8) | c.byte_at(6-i);
return cc_derank(enc_cc);
}
u64bit decrypt_cc_number(u64bit enc_cc,
const SymmetricKey& key,
const std::string& acct_name)
{
BigInt n = 1000000000000000;
u64bit cc_ranked = cc_rank(enc_cc);
BigInt c = fpe_decrypt(n, cc_ranked, key, sha1(acct_name));
if(c.bits() > 50)
throw std::runtime_error("FPE produced a number too large");
u64bit dec_cc = 0;
for(u32bit i = 0; i != 7; ++i)
dec_cc = (dec_cc << 8) | c.byte_at(6-i);
return cc_derank(dec_cc);
}
int main(int argc, char* argv[])
{
LibraryInitializer init;
if(argc != 4)
{
std::cout << "Usage: " << argv[0] << " cc-number acct-name passwd\n";
return 1;
}
u64bit cc_number = atoll(argv[1]);
std::string acct_name = argv[2];
std::string passwd = argv[3];
std::cout << cc_number << ' ' << luhn_check(cc_number) << '\n';
SymmetricKey key = sha1(passwd);
u64bit enc_cc = encrypt_cc_number(cc_number, key, acct_name);
std::cout << enc_cc << ' ' << luhn_check(enc_cc) << '\n';
u64bit dec_cc = decrypt_cc_number(enc_cc, key, acct_name);
std::cout << dec_cc << ' ' << luhn_check(dec_cc) << '\n';
if(dec_cc != cc_number)
std::cout << "Something went wrong :(\n";
}
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