Refine signature buffer size in t_cose_crypto_sign

[gaborgvarga]

When using the PSA Crypto adapter layer, the signature buffer provided
to t_cose_crypto_sign() may be significantly larger than necessary for
the algorithm in use. Passing oversized buffers to psa_sign_hash()
can lead to performance inefficiencies and potential internal errors
in strict environments.

This change queries the expected signature size using
t_cose_crypto_sig_size() and checks it against the actual buffer
length before calling psa_sign_hash(). If the expected size exceeds
the available buffer, t_cose_crypto_sign() returns
with T_COSE_ERR_SIG_BUFFER_SIZE.

This improves robustness and avoids excessive memory usage
during signature generation.

[davidvincze]

Hi @laurencelundblade,
May I ask you to review this small change in the PSA crypto adaptor layer? We try to rely on the upstream version and avoid using patch files in TF-M as much as possible.

I noticed that the v2.0-alpha-3 and v2.0-alpha-4 were tagged on the master branch and not on the dev. Can I ask how the Branch strategy has changed compared to what is written in the contribution guide?

I saw that COSE_Mac is now available on master. Is the dev branch now completely "replaced" by the master (or provides almost the same functionalities at the moment)? Should Gabor retarget his PR against the master branch in the hope that we can bump the t_cose version in TF-M from alpha-2 to v2.0-alpha-4+x in the future?

Thank you!

[laurencelundblade]

That v2.0-alpha-4 was from the master branch was a big mistake on my part. I've remade the release from dev.

[gaborgvarga]

Hi @laurencelundblade,

I’ve updated the pull request based on our discussion.
When you have a moment, please take a look and let me know if you’re satisfied with the changes. Thanks for your feedback!

Best,
Gabor