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ECC import: more useful choice of INVALID_ARGUMENT vs NOT_SUPPORTED

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Attempting to create an ECC key with a curve specification that is not
valid can plausibly fail with PSA_ERROR_INVALID_ARGUMENT ("this is not
a curve specification at all") or PSA_ERROR_NOT_SUPPORTED ("this may
be a curve specification, but not one I support"). The choice of error
is somewhat subjective.

Before this commit, due to happenstance in the implementation, an
attempt to use a curve that is declared in the PSA API but not
implemented in Mbed TLS returned PSA_ERROR_INVALID_ARGUMENT, whereas
an attempt to use a curve that Mbed TLS supports but for which support
was disabled at compile-time returned PSA_ERROR_NOT_SUPPORTED. This
inconsistency made it difficult to write negative tests that could
work whether the curve is implemented via Mbed TLS code or via a
driver.

After this commit, any attempt to use parameters that are not
recognized fails with NOT_SUPPORTED, whether a curve with the
specified size might plausibly exist or not, because "might plausibly
exist" is not something Mbed TLS can determine.

To keep returning INVALID_ARGUMENT when importing an ECC key with an
explicit "bits" attribute that is inconsistent with the size of the
key material, this commit changes the way mbedtls_ecc_group_of_psa()
works: it now works on a size in bits rather than bytes, with an extra
flag indicating whether the bit-size must be exact or not.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine
2021-01-27 15:44:45 +01:00
parent a0832d47f7
commit 2fa6b5f503
7 changed files with 109 additions and 51 deletions
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11
include/psa/crypto_extra.h
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@ -637,16 +637,21 @@ static inline psa_ecc_family_t mbedtls_ecc_group_to_psa( mbedtls_ecp_group_id gr
*
* \param curve A PSA elliptic curve identifier
* (`PSA_ECC_FAMILY_xxx`).
* \param byte_length The byte-length of a private key on \p curve.
* \param bits The bit-length of a private key on \p curve.
* \param bits_is_sloppy If true, \p bits may be the bit-length rounded up
* to the nearest multiple of 8. This allows the caller
* to infer the exact curve from the length of a key
* which is supplied as a byte string.
*
* \return The corresponding Mbed TLS elliptic curve identifier
* (`MBEDTLS_ECP_DP_xxx`).
* \return #MBEDTLS_ECP_DP_NONE if \c curve is not recognized.
* \return #MBEDTLS_ECP_DP_NONE if \p byte_length is not
* \return #MBEDTLS_ECP_DP_NONE if \p bits is not
* correct for \p curve.
*/
mbedtls_ecp_group_id mbedtls_ecc_group_of_psa( psa_ecc_family_t curve,
size_t byte_length );
size_t bits,
int bits_is_sloppy );
#endif /* MBEDTLS_ECP_C */
/**@}*/