psa_cipher_set_iv doesn't allow to set the counter value for chacha20

[dbuenzli]

According to the spec it should be possible to set the counter value for ChaCha20 by providing a 16 bytes IV.

This is currently not possible in the current implementation. A first check in mbedtls_psa_cipher_set_iv seems to assume IV lengths are fixed for a given operation which is not the case for ChaCha20. Then barring that, mbedtls_cipher_set_iv doesn't seem anyways to have the logic for setting up the counter to something else than 0.

P.S. Not that I need this I was just writing some test code for the cipher API and trying to match the result of this vector.

[gilles-peskine-arm]

Indeed that's a known limitation (we haven't yet migrated issues from the mbedtls repository). Do you need it in production, or just as part of testing effort?

[dbuenzli]

I was just testing bindings I'm making to PSA crypto.