* In the PSA API, forbid zero-length keys. To pass a zero-length input to a key derivation function, use a buffer instead (this is now always possible).
### New features
* Key derivation inputs in the PSA API can now either come from a key object or from a buffer regardless of the step type.
### Bug fixes
* Fix a buffer overflow in the PSA HMAC code when using a long key with an unsupported algorithm. Fixes #254.
* Fix `mbedtls_asn1_get_int` to support any number of leading zeros.
* Fix `mbedtls_asn1_get_bitstring_null` to correctly parse bitstrings of at most 2 bytes.