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

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Tue May 28, 2024 at 1:18 AM EEST, Jarkko Sakkinen wrote:
> On Tue May 28, 2024 at 12:58 AM EEST, Jarkko Sakkinen wrote:
> > 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@kernel.org>
> > > > ---
> > > > 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.
>
> 3rd trial, i.e. no octect encoding at all but with the undocumented
> (in the current mainline) prefix byte:
>
>         ptr = &data[0];
>         *ptr++ = 0x04; /* uncompressed */
>         memcpy(&ptr[0], &x[0], x_size);
>         memcpy(&ptr[x_size], &x[x_size + 2], x_size);
>         ret = crypto_akcipher_set_pub_key(tfm, data, 3 * x_size + 1);
>         crypto_free_akcipher(tfm);
>
> I added also kprobe:
>
> echo 'r:ecdsa ecdsa_set_pub_key $retval' >> /sys/kernel/tracing/kprobe_events
> echo 1 > /sys/kernel/tracing/events/kprobes/enable
> cat /sys/kernel/tracing/dynamic_events
>
> Results:
>
> ecdsa: (ecdsa_set_pub_key+0xc1/0xe0 <- ecc_is_pubkey_valid_full) arg1=0xffffffea
>
> So I guess this is on a right track reverse engineering the format used
> in this API at least :-)

AFAIK TPM2 x and y are big-endian but I have no idea what
ecc_digits_from_bytes() expects because it has no documentation.

I guess it expect little-endian format, is this correct?

BR, Jarkko