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Revert changes to mod_p521 flow

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It is not necessary to save the middle limb upfront as overwriting it is
the desired result: in the first step we are reducing modulo
2^{512+biL}.

Arguably, the original flow is more intuitive and easier to see the idea
behind it.

Signed-off-by: Janos Follath <janos.follath@arm.com>
Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
This commit is contained in:
Janos Follath 2023-02-07 15:24:57 +00:00 committed by Gabor Mezei
parent b50aeb8f05
commit 13c3aa13af
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1 changed files with 5 additions and 8 deletions
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13
library/ecp_curves.c
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@ -5222,12 +5222,6 @@ int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *N_p, size_t N_n)
return 0;
}
/* Save and clear the A1 content of the shared limb to prevent it
from overwrite. */
mbedtls_mpi_uint remainder[P521_WIDTH] = { 0 };
remainder[0] = N_p[P521_WIDTH - 1] >> 9;
N_p[P521_WIDTH - 1] &= P521_MASK;
if (N_n > P521_WIDTH) {
/* Helper references for top part of N */
mbedtls_mpi_uint *NT_p = N_p + P521_WIDTH;
@ -5236,14 +5230,17 @@ int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *N_p, size_t N_n)
/* Split N as A0 + 2^(512 + biL) A1 and compute A0 + 2^(biL - 9) * A1.
* This can be done in place. */
mbedtls_mpi_uint shift = ((mbedtls_mpi_uint) 1u) << (biL - 9);
carry = mbedtls_mpi_core_mla(N_p, P521_WIDTH - 1, NT_p, NT_n, shift);
carry = mbedtls_mpi_core_mla(N_p, P521_WIDTH, NT_p, NT_n, shift);
/* Clear top part */
memset(NT_p, 0, sizeof(mbedtls_mpi_uint) * NT_n);
}
mbedtls_mpi_uint remainder[P521_WIDTH] = { 0 };
remainder[0] = carry << (biL - 9);
remainder[0] += (N_p[P521_WIDTH - 1] >> 9);
N_p[P521_WIDTH - 1] &= P521_MASK;
(void) mbedtls_mpi_core_add(N_p, N_p, remainder, P521_WIDTH);
N_p[P521_WIDTH - 1] += carry;
return 0;
}