goto err;
}
- ret = BN_bn2bin(tmp, key);
+ /* return the padded key, i.e. same number of bytes as the modulus */
+ ret = BN_bn2binpad(tmp, key, BN_num_bytes(dh->params.p));
err:
BN_CTX_end(ctx);
BN_CTX_free(ctx);
return ret;
}
+/*-
+ * NB: This function is inherently not constant time due to the
+ * RFC 5246 (8.1.2) padding style that strips leading zero bytes.
+ */
int DH_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)
{
+ int ret = 0, i;
+ volatile size_t npad = 0, mask = 1;
+
+ /* compute the key; ret is constant unless compute_key is external */
#ifdef FIPS_MODULE
- return compute_key(key, pub_key, dh);
+ ret = compute_key(key, pub_key, dh);
#else
- return dh->meth->compute_key(key, pub_key, dh);
+ ret = dh->meth->compute_key(key, pub_key, dh);
#endif
+ if (ret <= 0)
+ return ret;
+
+ /* count leading zero bytes, yet still touch all bytes */
+ for (i = 0; i < ret; i++) {
+ mask &= !key[i];
+ npad += mask;
+ }
+
+ /* unpad key */
+ ret -= npad;
+ /* key-dependent memory access, potentially leaking npad / ret */
+ memmove(key, key + npad, ret);
+ /* key-dependent memory access, potentially leaking npad / ret */
+ memset(key + ret, 0, npad);
+
+ return ret;
}
int DH_compute_key_padded(unsigned char *key, const BIGNUM *pub_key, DH *dh)
{
int rv, pad;
+ /* rv is constant unless compute_key is external */
#ifdef FIPS_MODULE
rv = compute_key(key, pub_key, dh);
#else
if (rv <= 0)
return rv;
pad = BN_num_bytes(dh->params.p) - rv;
+ /* pad is constant (zero) unless compute_key is external */
if (pad > 0) {
memmove(key + pad, key, rv);
memset(key, 0, pad);
=head1 NAME
-DH_generate_key, DH_compute_key - perform Diffie-Hellman key exchange
+DH_generate_key, DH_compute_key, DH_compute_key_padded - perform
+Diffie-Hellman key exchange
=head1 SYNOPSIS
int DH_generate_key(DH *dh);
- int DH_compute_key(unsigned char *key, BIGNUM *pub_key, DH *dh);
+ int DH_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh);
+
+ int DH_compute_key_padded(unsigned char *key, const BIGNUM *pub_key, DH *dh);
=head1 DESCRIPTION
-Both of the functions described on this page are deprecated.
+All of the functions described on this page are deprecated.
Applications should instead use L<EVP_PKEY_derive_init(3)>
and L<EVP_PKEY_derive(3)>.
DH_generate_key() performs the first step of a Diffie-Hellman key
exchange by generating private and public DH values. By calling
-DH_compute_key(), these are combined with the other party's public
-value to compute the shared key.
+DH_compute_key() or DH_compute_key_padded(), these are combined with
+the other party's public value to compute the shared key.
DH_generate_key() expects B<dh> to contain the shared parameters
B<dh-E<gt>p> and B<dh-E<gt>g>. It generates a random private DH value
DH_compute_key() computes the shared secret from the private DH value
in B<dh> and the other party's public value in B<pub_key> and stores
it in B<key>. B<key> must point to B<DH_size(dh)> bytes of memory.
+The padding style is RFC 5246 (8.1.2) that strips leading zero bytes.
+It is not constant time due to the leading zero bytes being stripped.
+The return value should be considered public.
+
+DH_compute_key_padded() is similar but stores a fixed number of bytes.
+The padding style is NIST SP 800-56A (C.1) that retains leading zero bytes.
+It is constant time due to the leading zero bytes being retained.
+The return value should be considered public.
=head1 RETURN VALUES
DH_compute_key() returns the size of the shared secret on success, -1
on error.
+DH_compute_key_padded() returns B<DH_size(dh)> on success, -1 on error.
+
The error codes can be obtained by L<ERR_get_error(3)>.
=head1 SEE ALSO
=head1 HISTORY
-Both of these functions were deprecated in OpenSSL 3.0.
+DH_compute_key_padded() was added in OpenSSL 1.0.2.
+
+All of these functions were deprecated in OpenSSL 3.0.
=head1 COPYRIGHT