Re: [PATCH] crypto: ecc - update ecc_gen_privkey for FIPS 186-5

[Vitaly Chikunov <vt@altlinux.org>]

Joachim,

On Wed, Mar 20, 2024 at 12:13:38AM -0500, Joachim Vandersmissen wrote:
> FIPS 186-5 [1] was released approximately 1 year ago. The most
> interesting change for ecc_gen_privkey is the removal of curves with
> order < 224 bits. This is minimum is now checked in step 1. It is
> unlikely that there is still any benefit in generating private keys for
> curves with n < 224, as those curves provide less than 112 bits of
> security strength and are therefore unsafe for any modern usage.
> 
> This patch also updates the documentation for __ecc_is_key_valid and
> ecc_gen_privkey to clarify which FIPS 186-5 method is being used to
> generate private keys. Previous documentation mentioned that "extra
> random bits" was used. However, this did not match the code. Instead,
> the code currently uses (and always has used) the "rejection sampling"
> ("testing candidates" in FIPS 186-4) method.
> 
> [1]: https://doi.org/10.6028/NIST.FIPS.186-5
> 
> Signed-off-by: Joachim Vandersmissen <git@jvdsn.com>
> ---
>  crypto/ecc.c | 29 +++++++++++++++++------------
>  1 file changed, 17 insertions(+), 12 deletions(-)
> 
> diff --git a/crypto/ecc.c b/crypto/ecc.c
> index f53fb4d6af99..3581027e9f92 100644
> --- a/crypto/ecc.c
> +++ b/crypto/ecc.c
> @@ -1416,6 +1416,12 @@ void ecc_point_mult_shamir(const struct ecc_point *result,
>  }
>  EXPORT_SYMBOL(ecc_point_mult_shamir);
>  
> +/*
> + * This function performs checks equivalent to Appendix A.4.2 of FIPS 186-5.
> + * Whereas A.4.2 results in an integer in the interval [1, n-1], this function
> + * ensures that the integer is in the range of [2, n-3]. We are slightly
> + * stricter because of the currently used scalar multiplication algorithm.
> + */
>  static int __ecc_is_key_valid(const struct ecc_curve *curve,
>  			      const u64 *private_key, unsigned int ndigits)
>  {
> @@ -1455,16 +1461,11 @@ int ecc_is_key_valid(unsigned int curve_id, unsigned int ndigits,
>  EXPORT_SYMBOL(ecc_is_key_valid);
>  
>  /*
> - * ECC private keys are generated using the method of extra random bits,
> - * equivalent to that described in FIPS 186-4, Appendix B.4.1.
> - *
> - * d = (c mod(n–1)) + 1    where c is a string of random bits, 64 bits longer
> - *                         than requested
> - * 0 <= c mod(n-1) <= n-2  and implies that
> - * 1 <= d <= n-1

Looks like method of extra random bits was never used.

But I am not sure that reference to FIPS 186-5 is correct since it's
about DSS while this function is only used for ECDH, which is perhaps
regulated by NIST SP 800-186 and 800-56A. Still, 3.1.2 of 800-186
suggests that for 'EC key establishment' listed curves with bit lengths
from 224 are allowed to be used.

Thanks,


> + * ECC private keys are generated using the method of rejection sampling,
> + * equivalent to that described in FIPS 186-5, Appendix A.2.2.
>   *
>   * This method generates a private key uniformly distributed in the range
> - * [1, n-1].
> + * [2, n-3].
>   */
>  int ecc_gen_privkey(unsigned int curve_id, unsigned int ndigits, u64 *privkey)
>  {
> @@ -1474,12 +1475,15 @@ int ecc_gen_privkey(unsigned int curve_id, unsigned int ndigits, u64 *privkey)
>  	unsigned int nbits = vli_num_bits(curve->n, ndigits);
>  	int err;
>  
> -	/* Check that N is included in Table 1 of FIPS 186-4, section 6.1.1 */
> -	if (nbits < 160 || ndigits > ARRAY_SIZE(priv))
> +	/*
> +	 * Step 1 & 2: check that N is included in Table 1 of FIPS 186-5,
> +	 * section 6.1.1.
> +	 */
> +	if (nbits < 224 || ndigits > ARRAY_SIZE(priv))
>  		return -EINVAL;
>  
>  	/*
> -	 * FIPS 186-4 recommends that the private key should be obtained from a
> +	 * FIPS 186-5 recommends that the private key should be obtained from a
>  	 * RBG with a security strength equal to or greater than the security
>  	 * strength associated with N.
>  	 *
> @@ -1492,12 +1496,13 @@ int ecc_gen_privkey(unsigned int curve_id, unsigned int ndigits, u64 *privkey)
>  	if (crypto_get_default_rng())
>  		return -EFAULT;
>  
> +	/* Step 3: obtain N returned_bits from the DRBG. */
>  	err = crypto_rng_get_bytes(crypto_default_rng, (u8 *)priv, nbytes);
>  	crypto_put_default_rng();
>  	if (err)
>  		return err;
>  
> -	/* Make sure the private key is in the valid range. */
> +	/* Step 4: make sure the private key is in the valid range. */
>  	if (__ecc_is_key_valid(curve, priv, ndigits))
>  		return -EINVAL;
>  
> -- 
> 2.44.0