1 files changed, 134 insertions, 0 deletions
diff --git a/doc/examples/fpe.cpp b/doc/examples/fpe.cpp
new file mode 100644
index 000000000..cc9ee8093
--- /dev/null
+++ b/doc/examples/fpe.cpp
@@ -0,0 +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();
+   }
+
+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";
+   }