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Stack usage optimization for mod_p521

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Instead of creating an mpi on the stack, reuse the unused part of the input mpi.

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
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Gabor Mezei 2023-02-08 16:27:03 +01:00
parent fe24e91a34
commit d10d429380
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GPG Key ID: FEE76C0CF8C6267D
1 changed files with 15 additions and 5 deletions
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20
library/ecp_curves.c
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@ -5262,12 +5262,22 @@ int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *X, size_t X_limbs)
/* Keep the top 9 bits and reduce the rest, using 2^521 = 1 mod P521. */
addend += (X[P521_WIDTH - 1] >> 9);
X[P521_WIDTH - 1] &= P521_MASK;
/* Declare a helper array for carrying out the addition. */
mbedtls_mpi_uint addend_arr[P521_WIDTH] = { 0 };
/* Resuse the top part of X (already zeroed) as a helper array for
* carrying out the addition. */
mbedtls_mpi_uint *addend_arr = X + P521_WIDTH;
addend_arr[0] = addend;
(void) mbedtls_mpi_core_add(X, X, addend_arr, P521_WIDTH);
/* Both addends were less than P521 therefore X < 2 P521. (This also means
* that the result fit in P521_WIDTH limbs and there won't be any carry.) */
/* The unused part of X is P521_WIDTH - 1 limbs in size and only that
* size can be used for addition. Due to the addend fit in a limb
* the limbs other the first in the helper array are only used for
* propagating the carry. By adding the carry of the P521_WIDTH - 1 limb
* addition to the last limb of X makes the addition of X and the addend
* complete. */
carry = mbedtls_mpi_core_add(X, X, addend_arr, P521_WIDTH - 1);
X[P521_WIDTH - 1] += carry;
/* Clear the reused part of X. */
addend_arr[0] = 0;
return 0;
}