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authorlloyd <[email protected]>2009-11-13 15:07:51 +0000
committerlloyd <[email protected]>2009-11-13 15:07:51 +0000
commit47032ae7a9070b1dfb3efdd6b491a3f8be958f69 (patch)
tree6d874f1d47c3bfd9ad36d0960d425078a9c8f891
parent4a68c67cc526a2223344ed12c3dc4728d6de3455 (diff)
Extend FPE example to encrypt credit card numbers with valid Luhn checksums
onto other CCNs with valid checksums.
-rw-r--r--doc/examples/fpe.cpp128
1 files changed, 120 insertions, 8 deletions
diff --git a/doc/examples/fpe.cpp b/doc/examples/fpe.cpp
index 9384a0d2d..cc9ee8093 100644
--- a/doc/examples/fpe.cpp
+++ b/doc/examples/fpe.cpp
@@ -1,22 +1,134 @@
+/*
+* 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();
+ }
-int main()
+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;
- BigInt n = 100000000;
- BigInt x = 49604394;
+ 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';
- SymmetricKey key("AAAAAAAAAAAAAAAA");
- MemoryVector<byte> tweak(4);
+ u64bit dec_cc = decrypt_cc_number(enc_cc, key, acct_name);
- BigInt c = fpe_encrypt(n, x, key, tweak);
- BigInt p = fpe_decrypt(n, c, key, tweak);
+ std::cout << dec_cc << ' ' << luhn_check(dec_cc) << '\n';
- std::cout << c << ' ' << p << ' ' << x << '\n';
+ if(dec_cc != cc_number)
+ std::cout << "Something went wrong :(\n";
}