tree 8b9f6757f11925830612f17b1c0136a2e517ea2f
parent a0832d47f7eb859f75301ae321be5dea4ccb17f3
author Gilles Peskine <Gilles.Peskine@arm.com> 1611758685 +0100
committer Gilles Peskine <Gilles.Peskine@arm.com> 1612806206 +0100

ECC import: more useful choice of INVALID_ARGUMENT vs NOT_SUPPORTED

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>