1 files changed, 195 insertions, 0 deletions
diff --git a/src/constructs/fpe/fpe.cpp b/src/constructs/fpe/fpe.cpp
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+++ b/src/constructs/fpe/fpe.cpp
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+/*
+* Format Preserving Encryption using the scheme FE1 from the paper
+* "Format-Preserving Encryption" by Bellare, Rogaway, et al
+*    (http://eprint.iacr.org/2009/251)
+*
+* (C) 2009 Jack Lloyd
+*
+* Distributed under the terms of the Botan license
+*/
+
+#include <botan/fpe.h>
+#include <botan/numthry.h>
+#include <botan/hmac.h>
+#include <botan/sha2_32.h>
+#include <botan/loadstor.h>
+#include <stdexcept>
+
+namespace Botan {
+
+namespace {
+
+// Normally FPE is for SSNs, CC#s, etc, nothing too big
+const u32bit MAX_N_BYTES = 128/8;
+
+/*
+* Factor n into a and b which are as close together as possible.
+* Assumes n is composed mostly of small factors which is the case for
+* typical uses of FPE (typically, n is a power of 10)
+*
+* Want a >= b since the safe number of rounds is 2+log_a(b); if a >= b
+* then this is always 3
+*/
+void factor(BigInt n, BigInt& a, BigInt& b)
+   {
+   a = 1;
+   b = 1;
+
+   u32bit n_low_zero = low_zero_bits(n);
+
+   a <<= (n_low_zero / 2);
+   b <<= n_low_zero - (n_low_zero / 2);
+   n >>= n_low_zero;
+
+   for(u32bit i = 0; i != PRIME_TABLE_SIZE; ++i)
+      {
+      while(n % PRIMES[i] == 0)
+         {
+         a *= PRIMES[i];
+         if(a > b)
+            std::swap(a, b);
+         n /= PRIMES[i];
+         }
+      }
+
+   if(a > b)
+      std::swap(a, b);
+   a *= n;
+   if(a < b)
+      std::swap(a, b);
+
+   if(a <= 1 || b <= 1)
+      throw std::runtime_error("Could not factor n for use in FPE");
+   }
+
+/*
+* According to a paper by Rogaway, Bellare, etc, the min safe number
+* of rounds to use for FPE is 2+log_a(b). If a >= b then log_a(b) <= 1
+* so 3 rounds is safe. The FPE factorization routine should always
+* return a >= b, so just confirm that and return 3.
+*/
+u32bit rounds(const BigInt& a, const BigInt& b)
+   {
+   if(a < b)
+      throw std::logic_error("FPE rounds: a < b");
+   return 3;
+   }
+
+/*
+* A simple round function based on HMAC(SHA-256)
+*/
+class FPE_Encryptor
+   {
+   public:
+      FPE_Encryptor(const SymmetricKey& key,
+                    const BigInt& n,
+                    const MemoryRegion<byte>& tweak);
+
+      ~FPE_Encryptor() { delete mac; }
+
+      BigInt operator()(u32bit i, const BigInt& R);
+
+   private:
+      MessageAuthenticationCode* mac;
+      SecureVector<byte> mac_n_t;
+   };
+
+FPE_Encryptor::FPE_Encryptor(const SymmetricKey& key,
+                             const BigInt& n,
+                             const MemoryRegion<byte>& tweak)
+   {
+   mac = new HMAC(new SHA_256);
+   mac->set_key(key);
+
+   SecureVector<byte> n_bin = BigInt::encode(n);
+
+   if(n_bin.size() > MAX_N_BYTES)
+      throw std::runtime_error("N is too large for FPE encryption");
+
+   for(u32bit i = 0; i != 4; ++i)
+      mac->update(get_byte(i, n_bin.size()));
+   mac->update(&n_bin[0], n_bin.size());
+
+   for(u32bit i = 0; i != 4; ++i)
+      mac->update(get_byte(i, tweak.size()));
+   mac->update(&tweak[0], tweak.size());
+
+   mac_n_t = mac->final();
+   }
+
+BigInt FPE_Encryptor::operator()(u32bit round_no, const BigInt& R)
+   {
+   mac->update(mac_n_t);
+
+   for(u32bit i = 0; i != 4; ++i)
+      mac->update(get_byte(i, round_no));
+
+   SecureVector<byte> r_bin = BigInt::encode(R);
+
+   for(u32bit i = 0; i != 4; ++i)
+      mac->update(get_byte(i, r_bin.size()));
+   mac->update(&r_bin[0], r_bin.size());
+
+   SecureVector<byte> X = mac->final();
+   return BigInt(&X[0], X.size());
+   }
+
+}
+
+/**
+* Generic Z_n FPE encryption, FE1 scheme
+*/
+BigInt fpe_encrypt(const BigInt& n, const BigInt& X0,
+                   const SymmetricKey& key,
+                   const MemoryRegion<byte>& tweak)
+   {
+   FPE_Encryptor F(key, n, tweak);
+
+   BigInt a, b;
+   factor(n, a, b);
+
+   const u32bit r = rounds(a, b);
+
+   BigInt X = X0;
+
+   for(u32bit i = 0; i != r; ++i)
+      {
+      BigInt L = X / b;
+      BigInt R = X % b;
+
+      BigInt W = (L + F(i, R)) % a;
+      X = a * R + W;
+      }
+
+   return X;
+   }
+
+/**
+* Generic Z_n FPE decryption, FD1 scheme
+*/
+BigInt fpe_decrypt(const BigInt& n, const BigInt& X0,
+                   const SymmetricKey& key,
+                   const MemoryRegion<byte>& tweak)
+   {
+   FPE_Encryptor F(key, n, tweak);
+
+   BigInt a, b;
+   factor(n, a, b);
+
+   const u32bit r = rounds(a, b);
+
+   BigInt X = X0;
+
+   for(u32bit i = 0; i != r; ++i)
+      {
+      BigInt W = X % a;
+      BigInt R = X / a;
+
+      BigInt L = (W - F(r-i-1, R)) % a;
+      X = b * L + R;
+      }
+
+   return X;
+   }
+
+}