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/*
* Format Preserving Encryption (FE1 scheme)
* (C) 2009,2018 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
#include <botan/fpe_fe1.h>
#include <botan/numthry.h>
#include <botan/divide.h>
#include <botan/reducer.h>
#include <botan/mac.h>
namespace Botan {
namespace {
// Normally FPE is for SSNs, CC#s, etc, nothing too big
const size_t 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)
*/
void factor(BigInt n, BigInt& a, BigInt& b)
{
a = 1;
b = 1;
size_t 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(size_t 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 <= 1 || b <= 1)
throw Exception("Could not factor n for use in FPE");
}
}
FPE_FE1::FPE_FE1(const BigInt& n,
size_t rounds,
bool compat_mode,
const std::string& mac_algo) :
m_rounds(rounds)
{
if(m_rounds < 3)
throw Invalid_Argument("FPE_FE1 rounds too small");
m_mac = MessageAuthenticationCode::create_or_throw(mac_algo);
m_n_bytes = BigInt::encode(n);
if(m_n_bytes.size() > MAX_N_BYTES)
throw Exception("N is too large for FPE encryption");
factor(n, m_a, m_b);
if(compat_mode)
{
if(m_a < m_b)
std::swap(m_a, m_b);
}
else
{
if(m_a > m_b)
std::swap(m_a, m_b);
}
mod_a.reset(new Modular_Reducer(m_a));
}
FPE_FE1::~FPE_FE1()
{
// for ~unique_ptr
}
void FPE_FE1::clear()
{
m_mac->clear();
}
std::string FPE_FE1::name() const
{
return "FPE_FE1(" + m_mac->name() + "," + std::to_string(m_rounds) + ")";
}
Key_Length_Specification FPE_FE1::key_spec() const
{
return m_mac->key_spec();
}
void FPE_FE1::key_schedule(const uint8_t key[], size_t length)
{
m_mac->set_key(key, length);
}
BigInt FPE_FE1::F(const BigInt& R, size_t round,
const secure_vector<uint8_t>& tweak_mac,
secure_vector<uint8_t>& tmp) const
{
tmp = BigInt::encode_locked(R);
m_mac->update(tweak_mac);
m_mac->update_be(static_cast<uint32_t>(round));
m_mac->update_be(static_cast<uint32_t>(tmp.size()));
m_mac->update(tmp.data(), tmp.size());
tmp = m_mac->final();
return BigInt(tmp.data(), tmp.size());
}
secure_vector<uint8_t> FPE_FE1::compute_tweak_mac(const uint8_t tweak[], size_t tweak_len) const
{
m_mac->update_be(static_cast<uint32_t>(m_n_bytes.size()));
m_mac->update(m_n_bytes.data(), m_n_bytes.size());
m_mac->update_be(static_cast<uint32_t>(tweak_len));
m_mac->update(tweak, tweak_len);
return m_mac->final();
}
BigInt FPE_FE1::encrypt(const BigInt& input, const uint8_t tweak[], size_t tweak_len) const
{
const secure_vector<uint8_t> tweak_mac = compute_tweak_mac(tweak, tweak_len);
BigInt X = input;
secure_vector<uint8_t> tmp;
BigInt L, R, Fi;
for(size_t i = 0; i != m_rounds; ++i)
{
divide(X, m_b, L, R);
Fi = F(R, i, tweak_mac, tmp);
X = m_a * R + mod_a->reduce(L + Fi);
}
return X;
}
BigInt FPE_FE1::decrypt(const BigInt& input, const uint8_t tweak[], size_t tweak_len) const
{
const secure_vector<uint8_t> tweak_mac = compute_tweak_mac(tweak, tweak_len);
BigInt X = input;
secure_vector<uint8_t> tmp;
BigInt W, R, Fi;
for(size_t i = 0; i != m_rounds; ++i)
{
divide(X, m_a, R, W);
Fi = F(R, m_rounds-i-1, tweak_mac, tmp);
X = m_b * mod_a->reduce(W - Fi) + R;
}
return X;
}
BigInt FPE_FE1::encrypt(const BigInt& x, uint64_t tweak) const
{
uint8_t tweak8[8];
store_be(tweak, tweak8);
return encrypt(x, tweak8, sizeof(tweak8));
}
BigInt FPE_FE1::decrypt(const BigInt& x, uint64_t tweak) const
{
uint8_t tweak8[8];
store_be(tweak, tweak8);
return decrypt(x, tweak8, sizeof(tweak8));
}
namespace FPE {
BigInt fe1_encrypt(const BigInt& n, const BigInt& X,
const SymmetricKey& key,
const std::vector<uint8_t>& tweak)
{
FPE_FE1 fpe(n, 3, true, "HMAC(SHA-256)");
fpe.set_key(key);
return fpe.encrypt(X, tweak.data(), tweak.size());
}
BigInt fe1_decrypt(const BigInt& n, const BigInt& X,
const SymmetricKey& key,
const std::vector<uint8_t>& tweak)
{
FPE_FE1 fpe(n, 3, true, "HMAC(SHA-256)");
fpe.set_key(key);
return fpe.decrypt(X, tweak.data(), tweak.size());
}
}
}
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