[openssl-dev] [openssl.org #4055] FIPS Object Module User Guide corrections needed for (*get_entropy)()

[Gibbons, Lee D via RT]

This is to highlight a bug in the FIPS Object Module 2.10 and corrective documentation in its User Guide.

The User Guide for the FIPS Object Module 2.10 describes the (*get_entropy)() callback:

                size_t (*get_entropy)(DRBG_CTX *ctx, unsigned char **pout,
           int entropy, size_t min_len, size_t max_len)

                "A call to this function requests entropy bits of entropy in a buffer of between min_len and
                max_len size bytes inclusive. The values of these are mechanism specific and taken from
                SP800-90 tables. This callback should then return the amount of data in the buffer *pout and the
                length in the return value, or zero in case of being unable to retrieve sufficient entropy."

The caller of (*get_entropy)() is the static function fips_get_entropy(). Notice how it constructs the <entropy> value, which should be in bits:

     rv = dctx->get_entropy(dctx, &tout, entropy + bl,
                min_len + bl, max_len + bl);
     *pout = tout + bl;
     if (rv < (min_len + bl) || (rv % bl))
           return 0;

The "entropy + bl" expression is mixing types, adding bits and bytes together. Anyone defining a (*get_entropy)() callback had better ignore the <entropy> parameter. What's more, the callback had better return <min_len> rounded up to a dctx->entropy_blocklen boundary or face failure. The User Guide mentions none of this.

I realize the FIPS Object Module is frozen. The documentation should be corrected to expose the real restrictions on the callback.

Doug Gibbons | Consulting Engineer | Avaya Inc. | 12121 Grant St | 2S-237 | Thornton, CO 80241 | 303-538-3538 | ldgibbons at avaya.com<mailto:ldgibbons at avaya.com>



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