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increase resistance against timing attacks

pull/2/head
weidai 2003-02-28 21:28:28 +00:00
parent 5ed4a7daec
commit 1d4b38de7f
1 changed files with 5 additions and 5 deletions
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10
integer.cpp
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@ -2146,11 +2146,11 @@ void MontgomeryReduce(word *R, word *T, const word *X, const word *M, const word
{ {
MultiplyBottom(R, T, X, U, N); MultiplyBottom(R, T, X, U, N);
MultiplyTop(T, T+N, X, R, M, N); MultiplyTop(T, T+N, X, R, M, N);
if (Subtract(R, X+N, T, N)) word borrow = Subtract(T, X+N, T, N);
{ // defend against timing attack by doing this Add even when not needed
word carry = Add(R, R, M, N); word carry = Add(T+N, T, M, N);
assert(carry); assert(carry || !borrow);
} CopyWords(R, T + (borrow ? N : 0), N);
} }
// R[N] --- result = X/(2**(WORD_BITS*N/2)) mod M // R[N] --- result = X/(2**(WORD_BITS*N/2)) mod M