Add PQencryptPasswordConn function to libpq, use it in psql and createuser.

The new function supports creating SCRAM verifiers, in addition to md5
hashes. The algorithm is chosen based on password_encryption, by default.

This fixes the issue reported by Jeff Janes, that there was previously
no way to create a SCRAM verifier with "\password".

Michael Paquier and me

Discussion: https://www.postgresql.org/message-id/CAMkU%3D1wfBgFPbfAMYZQE78p%3DVhZX7nN86aWkp0QcCp%3D%2BKxZ%3Dbg%40mail.gmail.com

src/interfaces/libpq/fe-auth-scram.c

@@ -614,6 +614,41 @@ verify_server_signature(fe_scram_state *state)
 	return true;
 }
 
+/*
+ * Build a new SCRAM verifier.
+ */
+char *
+pg_fe_scram_build_verifier(const char *password)
+{
+	char	   *prep_password = NULL;
+	pg_saslprep_rc rc;
+	char		saltbuf[SCRAM_DEFAULT_SALT_LEN];
+	char	   *result;
+
+	/*
+	 * Normalize the password with SASLprep.  If that doesn't work, because
+	 * the password isn't valid UTF-8 or contains prohibited characters, just
+	 * proceed with the original password.  (See comments at top of file.)
+	 */
+	rc = pg_saslprep(password, &prep_password);
+	if (rc == SASLPREP_OOM)
+		return NULL;
+	if (rc == SASLPREP_SUCCESS)
+		password = (const char *) prep_password;
+
+	/* Generate a random salt */
+	if (!pg_frontend_random(saltbuf, SCRAM_DEFAULT_SALT_LEN))
+		return NULL;
+
+	result = scram_build_verifier(saltbuf, SCRAM_DEFAULT_SALT_LEN,
+								  SCRAM_DEFAULT_ITERATIONS, password);
+
+	if (prep_password)
+		free(prep_password);
+
+	return result;
+}
+
 /*
  * Random number generator.
  */