postgres_fdw: SCRAM authentication pass-through

This enables SCRAM authentication for postgres_fdw when connecting to
a foreign server without having to store a plain-text password on user
mapping options.

This is done by saving the SCRAM ClientKey and ServeryKey from the
client authentication and using those instead of the plain-text
password for the server-side SCRAM exchange.  The new foreign-server
or user-mapping option "use_scram_passthrough" enables this.

Co-authored-by: Matheus Alcantara <mths.dev@pm.me>
Co-authored-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/27b29a35-9b96-46a9-bc1a-914140869dac@gmail.com

1 files changed, 11 insertions, 3 deletions

diff --git a/src/backend/libpq/auth-scram.c b/src/backend/libpq/auth-scram.c
index 1514133acdc..26dd241efa9 100644
--- a/src/backend/libpq/auth-scram.c
+++ b/src/backend/libpq/auth-scram.c
@@ -101,6 +101,7 @@
 #include "libpq/crypt.h"
 #include "libpq/sasl.h"
 #include "libpq/scram.h"
+#include "miscadmin.h"
 
 static void scram_get_mechanisms(Port *port, StringInfo buf);
 static void *scram_init(Port *port, const char *selected_mech,
@@ -144,6 +145,7 @@ typedef struct
 
 	int			iterations;
 	char	   *salt;			/* base64-encoded */
+	uint8		ClientKey[SCRAM_MAX_KEY_LEN];
 	uint8		StoredKey[SCRAM_MAX_KEY_LEN];
 	uint8		ServerKey[SCRAM_MAX_KEY_LEN];
 
@@ -462,6 +464,13 @@ scram_exchange(void *opaq, const char *input, int inputlen,
 	if (*output)
 		*outputlen = strlen(*output);
 
+	if (result == PG_SASL_EXCHANGE_SUCCESS && state->state == SCRAM_AUTH_FINISHED)
+	{
+		memcpy(MyProcPort->scram_ClientKey, state->ClientKey, sizeof(MyProcPort->scram_ClientKey));
+		memcpy(MyProcPort->scram_ServerKey, state->ServerKey, sizeof(MyProcPort->scram_ServerKey));
+		MyProcPort->has_scram_keys = true;
+	}
+
 	return result;
 }
 
@@ -1140,7 +1149,6 @@ static bool
 verify_client_proof(scram_state *state)
 {
 	uint8		ClientSignature[SCRAM_MAX_KEY_LEN];
-	uint8		ClientKey[SCRAM_MAX_KEY_LEN];
 	uint8		client_StoredKey[SCRAM_MAX_KEY_LEN];
 	pg_hmac_ctx *ctx = pg_hmac_create(state->hash_type);
 	int			i;
@@ -1173,10 +1181,10 @@ verify_client_proof(scram_state *state)
 
 	/* Extract the ClientKey that the client calculated from the proof */
 	for (i = 0; i < state->key_length; i++)
-		ClientKey[i] = state->ClientProof[i] ^ ClientSignature[i];
+		state->ClientKey[i] = state->ClientProof[i] ^ ClientSignature[i];
 
 	/* Hash it one more time, and compare with StoredKey */
-	if (scram_H(ClientKey, state->hash_type, state->key_length,
+	if (scram_H(state->ClientKey, state->hash_type, state->key_length,
 				client_StoredKey, &errstr) < 0)
 		elog(ERROR, "could not hash stored key: %s", errstr);