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Diffstat (limited to 'comm/third_party/botan/src/lib/math/numbertheory/reducer.cpp')
-rw-r--r-- | comm/third_party/botan/src/lib/math/numbertheory/reducer.cpp | 119 |
1 files changed, 119 insertions, 0 deletions
diff --git a/comm/third_party/botan/src/lib/math/numbertheory/reducer.cpp b/comm/third_party/botan/src/lib/math/numbertheory/reducer.cpp new file mode 100644 index 0000000000..deb3874d3e --- /dev/null +++ b/comm/third_party/botan/src/lib/math/numbertheory/reducer.cpp @@ -0,0 +1,119 @@ +/* +* Modular Reducer +* (C) 1999-2011,2018 Jack Lloyd +* +* Botan is released under the Simplified BSD License (see license.txt) +*/ + +#include <botan/reducer.h> +#include <botan/internal/ct_utils.h> +#include <botan/internal/mp_core.h> +#include <botan/divide.h> + +namespace Botan { + +/* +* Modular_Reducer Constructor +*/ +Modular_Reducer::Modular_Reducer(const BigInt& mod) + { + if(mod < 0) + throw Invalid_Argument("Modular_Reducer: modulus must be positive"); + + // Left uninitialized if mod == 0 + m_mod_words = 0; + + if(mod > 0) + { + m_modulus = mod; + m_mod_words = m_modulus.sig_words(); + + // Compute mu = floor(2^{2k} / m) + m_mu.set_bit(2 * BOTAN_MP_WORD_BITS * m_mod_words); + m_mu = ct_divide(m_mu, m_modulus); + } + } + +BigInt Modular_Reducer::reduce(const BigInt& x) const + { + BigInt r; + secure_vector<word> ws; + reduce(r, x, ws); + return r; + } + +namespace { + +/* +* Like if(cnd) x.rev_sub(...) but in const time +*/ +void cnd_rev_sub(bool cnd, BigInt& x, const word y[], size_t y_sw, secure_vector<word>& ws) + { + if(x.sign() != BigInt::Positive) + throw Invalid_State("BigInt::sub_rev requires this is positive"); + + const size_t x_sw = x.sig_words(); + + const size_t max_words = std::max(x_sw, y_sw); + ws.resize(std::max(x_sw, y_sw)); + clear_mem(ws.data(), ws.size()); + x.grow_to(max_words); + + const int32_t relative_size = bigint_sub_abs(ws.data(), x.data(), x_sw, y, y_sw); + + x.cond_flip_sign((relative_size > 0) && cnd); + bigint_cnd_swap(cnd, x.mutable_data(), ws.data(), max_words); + } + +} + +void Modular_Reducer::reduce(BigInt& t1, const BigInt& x, secure_vector<word>& ws) const + { + if(&t1 == &x) + throw Invalid_State("Modular_Reducer arguments cannot alias"); + if(m_mod_words == 0) + throw Invalid_State("Modular_Reducer: Never initalized"); + + const size_t x_sw = x.sig_words(); + + if(x_sw > 2*m_mod_words) + { + // too big, fall back to slow boat division + t1 = ct_modulo(x, m_modulus); + return; + } + + t1 = x; + t1.set_sign(BigInt::Positive); + t1 >>= (BOTAN_MP_WORD_BITS * (m_mod_words - 1)); + + t1.mul(m_mu, ws); + t1 >>= (BOTAN_MP_WORD_BITS * (m_mod_words + 1)); + + // TODO add masked mul to avoid computing high bits + t1.mul(m_modulus, ws); + t1.mask_bits(BOTAN_MP_WORD_BITS * (m_mod_words + 1)); + + t1.rev_sub(x.data(), std::min(x_sw, m_mod_words + 1), ws); + + /* + * If t1 < 0 then we must add b^(k+1) where b = 2^w. To avoid a + * side channel perform the addition unconditionally, with ws set + * to either b^(k+1) or else 0. + */ + const word t1_neg = t1.is_negative(); + + if(ws.size() < m_mod_words + 2) + ws.resize(m_mod_words + 2); + clear_mem(ws.data(), ws.size()); + ws[m_mod_words + 1] = t1_neg; + + t1.add(ws.data(), m_mod_words + 2, BigInt::Positive); + + // Per HAC this step requires at most 2 subtractions + t1.ct_reduce_below(m_modulus, ws, 2); + + cnd_rev_sub(t1.is_nonzero() && x.is_negative(), t1, m_modulus.data(), m_modulus.size(), ws); + } + +} |