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Stack usage optimization for mod_p521
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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@ -5262,12 +5262,22 @@ int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *X, size_t X_limbs)
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/* Keep the top 9 bits and reduce the rest, using 2^521 = 1 mod P521. */
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addend += (X[P521_WIDTH - 1] >> 9);
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X[P521_WIDTH - 1] &= P521_MASK;
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/* Declare a helper array for carrying out the addition. */
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mbedtls_mpi_uint addend_arr[P521_WIDTH] = { 0 };
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/* Resuse the top part of X (already zeroed) as a helper array for
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* carrying out the addition. */
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mbedtls_mpi_uint *addend_arr = X + P521_WIDTH;
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addend_arr[0] = addend;
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(void) mbedtls_mpi_core_add(X, X, addend_arr, P521_WIDTH);
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/* Both addends were less than P521 therefore X < 2 P521. (This also means
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* that the result fit in P521_WIDTH limbs and there won't be any carry.) */
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/* The unused part of X is P521_WIDTH - 1 limbs in size and only that
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* size can be used for addition. Due to the addend fit in a limb
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* the limbs other the first in the helper array are only used for
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* propagating the carry. By adding the carry of the P521_WIDTH - 1 limb
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* addition to the last limb of X makes the addition of X and the addend
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* complete. */
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carry = mbedtls_mpi_core_add(X, X, addend_arr, P521_WIDTH - 1);
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X[P521_WIDTH - 1] += carry;
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/* Clear the reused part of X. */
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addend_arr[0] = 0;
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return 0;
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}
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