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bignum: compact primality check for small values

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Signed-off-by: Manuel Pégourié-Gonnard <[email protected]>
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Manuel Pégourié-Gonnard
2026-05-22 00:49:08 +02:00
parent 1a3cfa5fe7
commit 075e76b65b
1 changed files with 10 additions and 33 deletions
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library/bignum.c
+10 -33
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@@ -2092,31 +2092,6 @@ static const mbedtls_mpi small_primes_product = {
.s = 1,
.n = sizeof(small_primes_product_limbs) / sizeof(mbedtls_mpi_uint),
};
/* Gaps between primes, starting at 3. https://oeis.org/A001223 */
static const unsigned char small_prime_gaps[] = {
2, 2, 4, 2, 4, 2, 4, 6,
2, 6, 4, 2, 4, 6, 6, 2,
6, 4, 2, 6, 4, 6, 8, 4,
2, 4, 2, 4, 14, 4, 6, 2,
10, 2, 6, 6, 4, 6, 6, 2,
10, 2, 4, 2, 12, 12, 4, 2,
4, 6, 2, 10, 6, 6, 6, 2,
6, 4, 2, 10, 14, 4, 2, 4,
14, 6, 10, 2, 4, 6, 8, 6,
6, 4, 6, 8, 4, 8, 10, 2,
10, 2, 6, 4, 6, 8, 4, 2,
4, 12, 8, 4, 8, 4, 6, 12,
2, 18, 6, 10, 6, 6, 2, 6,
10, 6, 6, 2, 6, 6, 4, 2,
12, 10, 2, 4, 6, 6, 2, 12,
4, 6, 8, 10, 8, 10, 8, 6,
6, 4, 8, 6, 4, 8, 4, 14,
10, 12, 2, 10, 2, 4, 2, 10,
14, 4, 2, 4, 14, 4, 2, 4,
20, 4, 8, 10, 8, 4, 6, 6,
14, 4, 6, 6, 8, 6, /*reaches 997*/
0 /* the last entry is effectively unused */
};
/*
* Small divisors test (X must be positive)
@@ -2138,16 +2113,18 @@ static int mpi_check_small_factors(const mbedtls_mpi *X)
return MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
}
/* The GCD test below only works if X > small_primes_limit. */
/* The GCD test below only works if X > small_primes_limit.
* Below this limit, use trial division: numbers that small are of no
* interest for cryptography, so we don't care about performance or side
* channels. We're supporting them only for backwards compatibility, so
* let's not waste code size on those. */
if (mbedtls_mpi_cmp_int(X, small_primes_limit) <= 0) {
unsigned p = 3;
for (size_t i = 0; i < sizeof(small_prime_gaps); i++) {
if (mbedtls_mpi_cmp_int(X, p) == 0) {
return 1;
}
p += small_prime_gaps[i];
mbedtls_mpi_uint x = X->p[0];
mbedtls_mpi_uint d = 2;
while (x % d != 0) {
++d;
}
return MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
return x == d ? 1 : MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
}
/* We can't directly use mbedtls_mpi_gcd_modinv_odd() because we don't know