wolfSSL - Embedded SSL Library — R, S components of ECDSA signature as binary string

Re: R, S components of ECDSA signature as binary string

null@example.com (werchter) — Thu, 16 Dec 2021 07:30:35 +0000

kareem_wolfssl wrote:

Hi werchter,
You should build with WOLFSSL_PUBLIC_MP defined and use mp_read_unsigned_bin, which will be built as a public API.
Check out our example here: https://github.com/wolfSSL/wolfssl-exam … ecc_sign.c
Thanks,
Kareem

Thanks! That helped a lot, building with WOLFSSL_PUBLIC_MP was an important part of the puzzle that I was missing.

BTW: When looking at the example, I think for exporting the key as binary string, mp_to_unsigned_bin needs to be used, instead of mp_read_unsigned_bin (for importing)?
At least mp_read_unsigned_bin seems to work for me

Re: R, S components of ECDSA signature as binary string

null@example.com (kareem_wolfssl) — Wed, 15 Dec 2021 18:36:36 +0000

Hi werchter,

You should build with WOLFSSL_PUBLIC_MP defined and use mp_read_unsigned_bin, which will be built as a public API.
Check out our example here: https://github.com/wolfSSL/wolfssl-exam … ecc_sign.c

Thanks,
Kareem

R, S components of ECDSA signature as binary string

null@example.com (werchter) — Wed, 15 Dec 2021 17:42:56 +0000

Hi,

I’m using

wc_ecc_sign_hash_ex

to get generate an ECDSA signature, and get the R and S components.

I need these to encode a CBOR COSE message (RFC 8152). The signature in a COSE message is a concatenation of the R and S components as a byte string. From RFC 8152:

The signature algorithm results in a pair of integers (R, S). These
integers will be the same length as the length of the key used for
the signature process. The signature is encoded by converting the
integers into byte strings of the same length as the key size. The
length is rounded up to the nearest byte and is left padded with zero
bits to get to the correct length. The two integers are then
concatenated together to form a byte string that is the resulting
signature.

But, I can’t figure out how to get from R and S as (mp_int), to a byte string, with wolfSSL, without using the internal mp_read_unsigned_bin.
Is there a way?

Thanks for any help!