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rsa: extract helper function for CRT

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Was only used in one place so far, but will be used in
rsa_gen_rand_with_inverse()'s upcoming CRT-based implementation.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard
2025-11-18 11:39:40 +01:00
parent 3be31bf21b
commit b13033dd3e
1 changed files with 33 additions and 13 deletions
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46
library/rsa.c
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@@ -1268,6 +1268,38 @@ cleanup:
return 0;
}
#if !defined(MBEDTLS_RSA_NO_CRT)
/*
* Compute T such that T = TP mod P and T = TP mod Q.
* (This is the Chinese Remainder Theorem - CRT.)
*
* WARNING: uses TP as a temporary, so its value is lost!
*/
static int rsa_apply_crt(mbedtls_mpi *T,
mbedtls_mpi *TP,
const mbedtls_mpi *TQ,
const mbedtls_rsa_context *ctx)
{
int ret;
/*
* T = (TP - TQ) * (Q^-1 mod P) mod P
*/
MBEDTLS_MPI_CHK(mbedtls_mpi_sub_mpi(T, TP, TQ));
MBEDTLS_MPI_CHK(mbedtls_mpi_mul_mpi(TP, T, &ctx->QP));
MBEDTLS_MPI_CHK(mbedtls_mpi_mod_mpi(T, TP, &ctx->P));
/*
* T = TQ + T * Q
*/
MBEDTLS_MPI_CHK(mbedtls_mpi_mul_mpi(TP, T, &ctx->Q));
MBEDTLS_MPI_CHK(mbedtls_mpi_add_mpi(T, TQ, TP));
cleanup:
return ret;
}
#endif
/* Generate random A and B such that A^-1 = B mod N */
static int rsa_gen_rand_with_inverse(const mbedtls_rsa_context *ctx,
mbedtls_mpi *A,
@@ -1524,19 +1556,7 @@ int mbedtls_rsa_private(mbedtls_rsa_context *ctx,
MBEDTLS_MPI_CHK(mbedtls_mpi_exp_mod(&TP, &T, &DP_blind, &ctx->P, &ctx->RP));
MBEDTLS_MPI_CHK(mbedtls_mpi_exp_mod(&TQ, &T, &DQ_blind, &ctx->Q, &ctx->RQ));
/*
* T = (TP - TQ) * (Q^-1 mod P) mod P
*/
MBEDTLS_MPI_CHK(mbedtls_mpi_sub_mpi(&T, &TP, &TQ));
MBEDTLS_MPI_CHK(mbedtls_mpi_mul_mpi(&TP, &T, &ctx->QP));
MBEDTLS_MPI_CHK(mbedtls_mpi_mod_mpi(&T, &TP, &ctx->P));
/*
* T = TQ + T * Q
*/
MBEDTLS_MPI_CHK(mbedtls_mpi_mul_mpi(&TP, &T, &ctx->Q));
MBEDTLS_MPI_CHK(mbedtls_mpi_add_mpi(&T, &TQ, &TP));
MBEDTLS_MPI_CHK(rsa_apply_crt(&T, &TP, &TQ, ctx));
#endif /* MBEDTLS_RSA_NO_CRT */
/* Verify the result to prevent glitching attacks. */