Message-ID: <25213093.1r3eYUQgxm@positron.chronox.de>
Date:   Sun, 05 Dec 2021 06:52:28 +0100
From:   Stephan Müller <smueller@...onox.de>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Nicolai Stange <nstange@...e.de>
Cc:     Hannes Reinecke <hare@...e.de>, Torsten Duwe <duwe@...e.de>,
        Zaibo Xu <xuzaibo@...wei.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        qat-linux@...el.com, keyrings@...r.kernel.org,
        Nicolai Stange <nstange@...e.de>
Subject: Re: [PATCH 09/18] crypto: dh - implement private key generation primitive

Am Mittwoch, 1. Dezember 2021, 01:48:49 CET schrieb Nicolai Stange:

Hi Nicolai,

> The support for NVME in-band authentication currently in the works ([1])
> needs to generate ephemeral DH keys.
> 
> Implement crypto_dh_gen_privkey() which is intended to be used from
> the DH implementations just in analogy to how ecc_gen_privkey() is used
> for ECDH.
> 
> Make the new crypto_dh_gen_privkey() to follow the approach specified
> in SP800-56Arev3, sec. 5.6.1.1.3 ("Key-Pair Generation Using Extra Random
> Bits").
> 
> SP800-56Arev3 specifies a lower as well as an upper bound on the generated
> key's length:
> - it must be >= two times the maximum supported security strength of
>   the group in question and
> - it must be <= the length of the domain parameter Q.
> Both of these are available only for the safe-prime groups from
> RFC 3526 or RFC 7919, which had been introduced to the kernel with previous
> patches: for any safe-prime group Q = (P - 1)/2 by definition and the
> individual maximum supported security strength as specified by
> SP800-56Arev3 has already been made available alongside the resp. domain
> parameters with said previous patches. Restrict crypto_dh_gen_privkey() to
> these safe-prime groups, i.e. to those groups with any group_id but
> dh_group_id_unknown. Make it pick twice the maximum supported strength
> rounded up to the next power of two for the output key size. This choice
> respects both, the lower and upper bounds given by SP800-90Arev3 for
> all safe-prime groups known to the kernel by now and is also in line with
> the NVME base spec 2.0, which requires the key size to be >= 256bits.
> 
> [1] https://lkml.kernel.org/r/20211122074727.25988-4-hare@suse.de
> 
> Signed-off-by: Nicolai Stange <nstange@...e.de>
> ---
>  crypto/Kconfig      |   1 +
>  crypto/dh_helper.c  | 128 ++++++++++++++++++++++++++++++++++++++++++++
>  include/crypto/dh.h |  22 ++++++++
>  3 files changed, 151 insertions(+)
> 
> diff --git a/crypto/Kconfig b/crypto/Kconfig
> index fcb044bdc90a..578711b02bb3 100644
> --- a/crypto/Kconfig
> +++ b/crypto/Kconfig
> @@ -228,6 +228,7 @@ menuconfig CRYPTO_DH
>  	tristate "Diffie-Hellman algorithm"
>  	select CRYPTO_KPP
>  	select MPILIB
> +	select CRYPTO_RNG_DEFAULT
>  	help
>  	  Generic implementation of the Diffie-Hellman algorithm.
> 
> diff --git a/crypto/dh_helper.c b/crypto/dh_helper.c
> index fb8df4734dc1..5a8c9c50297f 100644
> --- a/crypto/dh_helper.c
> +++ b/crypto/dh_helper.c
> @@ -9,6 +9,7 @@
>  #include <linux/string.h>
>  #include <crypto/dh.h>
>  #include <crypto/kpp.h>
> +#include <crypto/rng.h>
> 
>  #define DH_KPP_SECRET_MIN_SIZE (sizeof(struct kpp_secret) + \
>  				sizeof(enum dh_group_id) + 3 * sizeof(int))
> @@ -592,3 +593,130 @@ int crypto_dh_decode_key(const char *buf, unsigned int
> len, struct dh *params) return 0;
>  }
>  EXPORT_SYMBOL_GPL(crypto_dh_decode_key);
> +
> +static u64 __add_u64_to_be(__be64 *dst, unsigned int n, u64 val)
> +{
> +	unsigned int i;
> +
> +	for (i = n; val && i > 0; --i) {
> +		u64 tmp = be64_to_cpu(dst[i - 1]);
> +
> +		tmp += val;
> +		val = tmp >= val ? 0 : 1;
> +		dst[i - 1] = cpu_to_be64(tmp);
> +	}
> +
> +	return val;
> +}
> +
> +int crypto_dh_gen_privkey(enum dh_group_id group_id,
> +			  char key[CRYPTO_DH_MAX_PRIVKEY_SIZE],
> +			  unsigned int *key_size)
> +{
> +	const struct safe_prime_group *g;
> +	unsigned int n, tmp_size;
> +	__be64 *tmp;
> +	int err;
> +	u64 h, o;
> +
> +	/*
> +	 * Generate a private key following NIST SP800-56Ar3,
> +	 * sec. 5.6.1.1.1 and 5.6.1.1.3 resp.. This is supported only
> +	 * for the (approved) safe-prime groups.
> +	 */
> +	g = get_safe_prime_group(group_id);
> +	if (!g)

What about

if (g == dh_group_id_unknown)

?

> +		return -EINVAL;
> +
> +	/*
> +	 * 5.6.1.1.1: choose key length N such that
> +	 * 2 * ->max_strength <= N <= log2(q) + 1 = ->p_size * 8 - 1
> +	 * with q = (p - 1) / 2 for the safe-prime groups.
> +	 * Choose the lower bound's next power of two for N in order to
> +	 * avoid excessively large private keys while still
> +	 * maintaining some extra reserve beyond the bare minimum in
> +	 * most cases. Note that for each entry in safe_prime_groups[],
> +	 * the following holds for such N:
> +	 * - N >= 256, in particular it is a multiple of 2^6 = 64
> +	 *   bits and
> +	 * - N < log2(q) + 1, i.e. N respects the upper bound.
> +	 */
> +	n = roundup_pow_of_two(2 * g->max_strength);
> +	WARN_ON_ONCE(n & ((1u << 6) - 1));
> +	n >>= 6; /* Convert N into units of u64. */

Couldn't we pre-compute that value for each of the safeprime groups? This 
value should be static for each of them.
> +
> +	/*
> +	 * Reserve one extra u64 to hold the extra random bits
> +	 * required as per 5.6.1.1.3.
> +	 */
> +	tmp_size = (n + 1) * sizeof(__be64);

Maybe add this to the pre-computed value?

> +	tmp = kmalloc(tmp_size, GFP_KERNEL);
> +	if (!tmp)
> +		return -ENOMEM;
> +
> +	/*
> +	 * 5.6.1.1.3, step 3 (and implicitly step 4): obtain N + 64
> +	 * random bits and interpret them as a big endian integer.
> +	 */
> +	err = -EFAULT;
> +	if (crypto_get_default_rng())
> +		goto out;
> +
> +	err = crypto_rng_get_bytes(crypto_default_rng, (u8 *)tmp, tmp_size);
> +	crypto_put_default_rng();
> +	if (err)
> +		goto out;
> +
> +	/*
> +	 * 5.6.1.1.3, step 5 is implicit: 2^N < q and thus,
> +	 * M = min(2^N, q) = 2^N.
> +	 *
> +	 * For step 6, calculate
> +	 * key = (tmp[] mod (M - 1)) + 1 = (tmp[] mod (2^N - 1)) + 1.
> +	 *
> +	 * In order to avoid expensive divisions, note that
> +	 * 2^N mod (2^N - 1) = 1 and thus, for any integer h,
> +	 * 2^N * h mod (2^N - 1) = h mod (2^N - 1) always holds.
> +	 * The big endian integer tmp[] composed of n + 1 64bit words
> +	 * may be written as tmp[] = h * 2^N + l, with h = tmp[0]
> +	 * representing the 64 most significant bits and l
> +	 * corresponding to the remaining 2^N bits. With the remark
> +	 * from above,
> +	 * h * 2^N + l mod (2^N - 1) = l + h mod (2^N - 1).
> +	 * As both, l and h are less than 2^N, their sum after
> +	 * this first reduction is guaranteed to be <= 2^(N + 1) - 2.
> +	 * Or equivalently, that their sum can again be written as
> +	 * h' * 2^N + l' with h' now either zero or one and if one,
> +	 * then l' <= 2^N - 2. Thus, all bits at positions >= N will
> +	 * be zero after a second reduction:
> +	 * h' * 2^N + l' mod (2^N - 1) = l' + h' mod (2^N - 1).
> +	 * At this point, it is still possible that
> +	 * l' + h' = 2^N - 1, i.e. that l' + h' mod (2^N - 1)
> +	 * is zero. This condition will be detected below by means of
> +	 * the final increment overflowing in this case.
> +	 */
> +	h = be64_to_cpu(tmp[0]);
> +	h = __add_u64_to_be(tmp + 1, n, h);
> +	h = __add_u64_to_be(tmp + 1, n, h);
> +	WARN_ON_ONCE(h);
> +
> +	/* Increment to obtain the final result. */
> +	o = __add_u64_to_be(tmp + 1, n, 1);
> +	/*
> +	 * The overflow bit o from the increment is either zero or
> +	 * one. If zero, tmp[1:n] holds the final result in big-endian
> +	 * order. If one, tmp[1:n] is zero now, but needs to be set to
> +	 * one, c.f. above.
> +	 */
> +	if (o)
> +		tmp[n] = cpu_to_be64(1);
> +
> +	/* n is in units of u64, convert to bytes. */
> +	*key_size = n << 3;
> +	memcpy(key, &tmp[1], *key_size);
> +
> +out:
> +	kfree_sensitive(tmp);
> +	return err;
> +}
> +EXPORT_SYMBOL_GPL(crypto_dh_gen_privkey);
> diff --git a/include/crypto/dh.h b/include/crypto/dh.h
> index 7eb1fad93d02..182100395bbb 100644
> --- a/include/crypto/dh.h
> +++ b/include/crypto/dh.h
> @@ -99,4 +99,26 @@ int crypto_dh_encode_key(char *buf, unsigned int len,
> const struct dh *params); */
>  int crypto_dh_decode_key(const char *buf, unsigned int len, struct dh
> *params);
> 
> +/*
> + * The maximum key length is two times the max. sec. strength of the
> + * safe-prime groups, rounded up to the next power of two.
> + */
> +#define CRYPTO_DH_MAX_PRIVKEY_SIZE (512 / 8)
> +
> +/**
> + * crypto_dh_gen_privkey() - generate a DH private key
> + * @buf:	The DH group to generate a key for
> + * @key:	Buffer provided by the caller to receive the generated
> + *		key
> + * @key_size:	Pointer to an unsigned integer the generated key's length
> + *		will be stored in
> + *
> + * This function is intended to generate an ephemeral DH key.
> + *
> + * Return:	Negative error code on failure, 0 on success
> + */
> +int crypto_dh_gen_privkey(enum dh_group_id group_id,
> +			  char key[CRYPTO_DH_MAX_PRIVKEY_SIZE],
> +			  unsigned int *key_size);
> +
>  #endif


Ciao
Stephan

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