Re: [PATCH] crypto: ecdsa: Fix the public key format description

[Jarkko Sakkinen]

On Tue May 28, 2024 at 12:05 AM EEST, Jarkko Sakkinen wrote:
> On Mon May 27, 2024 at 11:28 PM EEST, Jarkko Sakkinen wrote:
> > Public key blob is not just x and y concatenated. It follows RFC5480
> > section 2.2. Address this by re-documenting the function with the
> > correct description of the format.
> >
> > Link: https://datatracker.ietf.org/doc/html/rfc5480
> > Fixes: 4e6602916bc6 ("crypto: ecdsa - Add support for ECDSA signature verification")
> > Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > ---
> > It is a bug fix that does not really need a stable backport. Still
> > categorizes as a bug because by following the existing documentation
> > you end up with an error code.
> >  crypto/ecdsa.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
> > index 258fffbf623d..55114146ff84 100644
> > --- a/crypto/ecdsa.c
> > +++ b/crypto/ecdsa.c
> > @@ -215,9 +215,8 @@ static int ecdsa_ecc_ctx_reset(struct ecc_ctx *ctx)
> >  }
> >  
> >  /*
> > - * Set the public key given the raw uncompressed key data from an X509
> > - * certificate. The key data contain the concatenated X and Y coordinates of
> > - * the public key.
> > + * Set the public ECC key as defined by RFC5480 section 2.2 "Subject Public
> > + * Key". Only the uncompressed format is supported.
> >   */
> >  static int ecdsa_set_pub_key(struct crypto_akcipher *tfm, const void *key, unsigned int keylen)
> >  {
>
> Based on this, is this now along the lines of correct format":
>
>        *ptr++ = 0x04; /* uncompressed */
>        ptr = asn1_encode_octet_string(&ptr[0], &in[sizeof(in)], &x[0], x_size);
>        ptr = asn1_encode_octet_string(&ptr[0], &in[sizeof(in)], &x[x_size + 2], x_size);
>        in_len = ptr - in;
>        ret = crypto_akcipher_set_pub_key(tfm, in, in_len);


I fixed up the above as it should be only single octect string to this:

	ptr = &in[0];
        *ptr++ = 0x04; /* uncompressed */
        ptr = asn1_encode_octet_string(&ptr[0], &in[sizeof(in)],
                                       &data[0], 2 * x_size);
        in_len = ptr - in;
        pr_info("in_len=%u\n", in_len);
        ret = crypto_akcipher_set_pub_key(tfm, in, in_len);
        crypto_free_akcipher(tfm);

It fails in:

	ndigits = DIV_ROUND_UP(digitlen, sizeof(u64));
	if (ndigits != ctx->curve->g.ndigits)
		return -EINVAL;

I checked that in_len=67.

The tfm is deleted at instant because the above code is part of *_query.
TPM2 ECDSA asymmetric key that way that signature verification will work
when it is needed. The key type signs with TPM and verifies with the
software cipher.

BR, Jarkko