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@xxxxxxxxx>
> ---
> 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
