-The EVP_PKEY_derive_init() function initializes a public key algorithm
-context using key B<pkey> for shared secret derivation.
+The EVP_PKEY_derive_init_ex() function initializes a public key algorithm
+context for shared secret derivation using the key exchange algorithm
+B<exchange>.
+The key exchange algorithm B<exchange> should be fetched using a call to
+L<EVP_KEYEXCH_fetch(3)>.
+The EVP_PKEY object associated with B<ctx> must be compatible with that
+algorithm.
+B<exchange> may be NULL in which case the EVP_KEYEXCH algorithm is fetched
+implicitly based on the type of EVP_PKEY associated with B<ctx>.
+See L<provider(7)/Implicit fetch> for more information about implict fetches.
+
+The EVP_PKEY_derive_init() function is the same as EVP_PKEY_derive_init_ex()
+except that the EVP_KEYEXCH algorithm is always implicitly fetched.