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RSA: document Montgomery trick in unblind

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Signed-off-by: Janos Follath <janos.follath@arm.com>
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Janos Follath 2023-12-27 10:22:59 +00:00 committed by Dave Rodgman
parent a62a554071
commit e6750b2a0b
1 changed files with 7 additions and 2 deletions
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9
library/rsa.c
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@ -991,9 +991,14 @@ static int rsa_unblind(mbedtls_mpi *T, mbedtls_mpi *Vf, mbedtls_mpi *N)
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(T, nlimbs));
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(Vf, nlimbs));
// T = T * R mod N
/* T = T * Vf mod N
* Reminder: montmul(A, B, N) = A * B * R^-1 mod N
* Usually both operands are multiplied by R mod N beforehand (by calling
* `to_mont_rep()` on them), yielding a result that's also * R mod N (aka
* "in the Montgomery domain"). Here we only multiply one operand by R mod
* N, so the result is directly what we want - no need to call
* `from_mont_rep()` on it. */
mbedtls_mpi_core_to_mont_rep(T->p, T->p, N->p, nlimbs, mm, RR.p, M_T.p);
// T = T * Vf mod N
mbedtls_mpi_core_montmul(T->p, T->p, Vf->p, nlimbs, N->p, nlimbs, mm, M_T.p);
cleanup: