Add fall-back to hash-based KDF for internal ECP DRBG
The dependency on a DRBG module was perhaps a bit strict for LTS branches, so
let's have an option that works with no DRBG when at least one SHA module is
present.
This changes the internal API of ecp_drbg_seed() by adding the size of the
MPI as a parameter. Re-computing the size from the number of limbs doesn't
work too well here as we're writing out to a fixed-size buffer and for some
curves (P-521) that would round up too much. Using mbedtls_mpi_get_len() is
not entirely satisfactory either as it would mean using a variable-length
encoding, with could open side channels.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 935248f..473488c 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -127,8 +127,11 @@
defined(MBEDTLS_ECP_ALT) || \
defined(MBEDTLS_CTR_DRBG_C) || \
defined(MBEDTLS_HMAC_DRBG_C) || \
+ defined(MBEDTLS_SHA512_C) || \
+ defined(MBEDTLS_SHA256_C) || \
+ defined(MBEDTLS_SHA1_C) || \
defined(MBEDTLS_ECP_NO_INTERNAL_RNG))
-#error "MBEDTLS_ECP_C requires a DRBG module unless MBEDTLS_ECP_NO_INTERNAL_RNG is defined or an alternative implementation is used"
+#error "MBEDTLS_ECP_C requires a DRBG or SHA module unless MBEDTLS_ECP_NO_INTERNAL_RNG is defined or an alternative implementation is used"
#endif
#if defined(MBEDTLS_PK_PARSE_C) && !defined(MBEDTLS_ASN1_PARSE_C)