linux-kernel - Re: [PATCH v7 5/5] keys: asymmetric: Add tpm2_key

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Message-Id: <D1LNNE1FMDBS.3T9J17BAX1D04@kernel.org>
Date: Wed, 29 May 2024 02:09:45 +0300
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Jarkko Sakkinen" <jarkko@...nel.org>, "Herbert Xu"
 <herbert@...dor.apana.org.au>
Cc: <linux-integrity@...r.kernel.org>, <keyrings@...r.kernel.org>,
 <Andreas.Fuchs@...ineon.com>, "James Prestwood" <prestwoj@...il.com>,
 "David Woodhouse" <dwmw2@...radead.org>, "Eric Biggers"
 <ebiggers@...nel.org>, "James Bottomley"
 <James.Bottomley@...senpartnership.com>, <linux-crypto@...r.kernel.org>,
 "Stefan Berger" <stefanb@...ux.ibm.com>, "Lennart Poettering"
 <lennart@...ttering.net>, "David S. Miller" <davem@...emloft.net>, "open
 list" <linux-kernel@...r.kernel.org>, "David Howells"
 <dhowells@...hat.com>, "Peter Huewe" <peterhuewe@....de>, "Jason Gunthorpe"
 <jgg@...pe.ca>, "James Bottomley" <James.Bottomley@...senPartnership.com>,
 "Ard Biesheuvel" <ardb@...nel.org>, "Mario Limonciello"
 <mario.limonciello@....com>
Subject: Re: [PATCH v7 5/5] keys: asymmetric: Add tpm2_key_ecdsa

On Wed May 29, 2024 at 12:42 AM EEST, Jarkko Sakkinen wrote:
> On Wed May 29, 2024 at 12:08 AM EEST, Jarkko Sakkinen wrote:
> > +	/* Encode the ASN.1 signature: */
> > +#define TPM2_KEY_ECDSA_SIG_SIZE		(2 + 2 * (2 + SHA256_DIGEST_SIZE) + r_0 + s_0)
> > +	pr_info("sig_size=%d\n", TPM2_KEY_ECDSA_SIG_SIZE);
> > +	ptr[0] = 0x30; /* SEQUENCE */
> > +	ptr[1] = TPM2_KEY_ECDSA_SIG_SIZE - 2;
> > +#define TPM2_KEY_ECDSA_SIG_R_TAG	2
> > +#define TPM2_KEY_ECDSA_SIG_R_SIZE	3
> > +#define TPM2_KEY_ECDSA_SIG_R_BODY	4
> > +	ptr[TPM2_KEY_ECDSA_SIG_R_TAG] = 0x02; /* INTEGER */
> > +	ptr[TPM2_KEY_ECDSA_SIG_R_SIZE] = SHA256_DIGEST_SIZE + r_0;
> > +	ptr[TPM2_KEY_ECDSA_SIG_R_BODY] = 0x00; /* maybe dummy write */
> > +	memcpy(&ptr[TPM2_KEY_ECDSA_SIG_R_BODY + r_0], r, SHA256_DIGEST_SIZE);
> > +#define TPM2_KEY_ECDSA_SIG_S_TAG	(4 + r_0 + SHA256_DIGEST_SIZE)
> > +#define TPM2_KEY_ECDSA_SIG_S_SIZE	(5 + r_0 + SHA256_DIGEST_SIZE)
> > +#define TPM2_KEY_ECDSA_SIG_S_BODY	(6 + r_0 + SHA256_DIGEST_SIZE)
> > +	ptr[TPM2_KEY_ECDSA_SIG_S_TAG] = 0x02; /* INTEGER */
> > +	ptr[TPM2_KEY_ECDSA_SIG_S_SIZE] = SHA256_DIGEST_SIZE + s_0;
> > +	ptr[TPM2_KEY_ECDSA_SIG_S_BODY] = 0x00; /* maybe dummy write */
> > +	memcpy(&ptr[TPM2_KEY_ECDSA_SIG_S_BODY + s_0], s, SHA256_DIGEST_SIZE);
> > +	ret = TPM2_KEY_ECDSA_SIG_SIZE;
>
> Stefan, so this how I realized the signature encoding, thanks to
> your earlier remarks [1]! I found out based on that a few glitches
> and ended up with this better structured ECDSA signature encoder,
> so thank you for doing that.

1. SHA384 would fit without any significant changes.
2. SHA512 will require a single additional byte, i.e. prefix byte 0x81
   for the sequence (not for coeffients).
3. SM3 similarly is also trivial to add.

Both will be also easy to add later on. I would not enlarge this patch
set from what it is now.

So I think this is pretty well along the lines of:

https://datatracker.ietf.org/doc/draft-woodhouse-cert-best-practice/

BR, Jarkko