FW: seeking info on source of DH key requirements

Can anyone tell me what the source (RFC, NIST, FIPS, whatever) of this test in diffie-hellman key generation (dh_key.c) is?

        /* Is it an approved safe prime ?*/
        if (DH_get_nid(dh) != NID_undef) {
            int max_strength =
                    ossl_ifc_ffc_compute_security_bits(BN_num_bits(dh->params.p));

            if (dh->params.q == NULL
                || dh->length > BN_num_bits(dh->params.q))
                goto err;

https://github.com/openssl/openssl/blob/d88c43a64408616572941e5d0b127194d80f562f/crypto/dh/dh_key.c#L318

We’re having an issue connecting via SSH to a vendor device, and I’ve tracked it to `dh->length` being 512 when `dh->params.q` is 256.  I’d like to have some document to point the vendor if possible.

Thank you much.

--
Stephen Wall
Senior Staff Software Engineer
585.924.7550

REDCOM Laboratories, Inc.
Research, Engineering, & Development in Communications
One Redcom Center, Victor, NY 14564-0995

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