mbedtls writes ECDSA signatures in ASN1

[chrysn]

It seems (from code inspection while digging through what safe buffer sizes for signature buffers are and whether *siglen can be predetermined; I'm not at the point of using it yet) that the mbedtls ECDSA implementation uses ASN.1 encoding for signatures where COSE expects its own fixed-length encoding.

Code path:

• cose_crypto_sign_ecdsa at src/crypt/mbedtls.c calls mbedtls_ecdsa_write_signature
• mbedtls_ecdsa_write_signature at library/ecdsa.c calls (via mbedtls_ecdsa_write_signature_restartable) ecdsa_signature_to_asn1 into sig[*siglen]
• ecdsa_signature_to_asn1 does ASN.1 encoding

while https://tools.ietf.org/html/rfc8152#section-8.1 says:

Using the function defined in [RFC8017], the signature is:
Signature = I2OSP(R, n) | I2OSP(S, n)
where n = ceiling(key_length / 8)

As mbedtls is the only ECDSA implementation in libcose, it wouldn't run against common test vectors. Has this been tested against any external COSE implementation yet?