Make SCRAM iteration count configurable

Replace the hardcoded value with a GUC such that the iteration
count can be raised in order to increase protection against
brute-force attacks.  The hardcoded value for SCRAM iteration
count was defined to be 4096, which is taken from RFC 7677, so
set the default for the GUC to 4096 to match.  In RFC 7677 the
recommendation is at least 15000 iterations but 4096 is listed
as a SHOULD requirement given that it's estimated to yield a
0.5s processing time on a mobile handset of the time of RFC
writing (late 2015).

Raising the iteration count of SCRAM will make stored passwords
more resilient to brute-force attacks at a higher computational
cost during connection establishment.  Lowering the count will
reduce computational overhead during connections at the tradeoff
of reducing strength against brute-force attacks.

There are however platforms where even a modest iteration count
yields a too high computational overhead, with weaker password
encryption schemes chosen as a result.  In these situations,
SCRAM with a very low iteration count still gives benefits over
weaker schemes like md5, so we allow the iteration count to be
set to one at the low end.

The new GUC is intentionally generically named such that it can
be made to support future SCRAM standards should they emerge.
At that point the value can be made into key:value pairs with
an undefined key as a default which will be backwards compatible
with this.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Jonathan S. Katz <jkatz@postgresql.org>
Discussion: https://postgr.es/m/F72E7BC7-189F-4B17-BF47-9735EB72C364@yesql.se

3 files changed, 21 insertions, 2 deletions

diff --git a/src/backend/libpq/auth-scram.c b/src/backend/libpq/auth-scram.c
index 4441e0d774..9b286aa4d7 100644
--- a/src/backend/libpq/auth-scram.c
+++ b/src/backend/libpq/auth-scram.c
@@ -192,6 +192,11 @@ static char *scram_mock_salt(const char *username,
 							 int key_length);
 
 /*
+ * The number of iterations to use when generating new secrets.
+ */
+int			scram_sha_256_iterations = SCRAM_SHA_256_DEFAULT_ITERATIONS;
+
+/*
  * Get a list of SASL mechanisms that this module supports.
  *
  * For the convenience of building the FE/BE packet that lists the
@@ -496,7 +501,7 @@ pg_be_scram_build_secret(const char *password)
 
 	result = scram_build_secret(PG_SHA256, SCRAM_SHA_256_KEY_LEN,
 								saltbuf, SCRAM_DEFAULT_SALT_LEN,
-								SCRAM_DEFAULT_ITERATIONS, password,
+								scram_sha_256_iterations, password,
 								&errstr);
 
 	if (prep_password)
@@ -717,7 +722,7 @@ mock_scram_secret(const char *username, pg_cryptohash_type *hash_type,
 	encoded_salt[encoded_len] = '\0';
 
 	*salt = encoded_salt;
-	*iterations = SCRAM_DEFAULT_ITERATIONS;
+	*iterations = SCRAM_SHA_256_DEFAULT_ITERATIONS;
 
 	/* StoredKey and ServerKey are not used in a doomed authentication */
 	memset(stored_key, 0, SCRAM_MAX_KEY_LEN);
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 1c0583fe26..a60bd48499 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -41,9 +41,11 @@
 #include "commands/trigger.h"
 #include "commands/user.h"
 #include "commands/vacuum.h"
+#include "common/scram-common.h"
 #include "jit/jit.h"
 #include "libpq/auth.h"
 #include "libpq/libpq.h"
+#include "libpq/scram.h"
 #include "nodes/queryjumble.h"
 #include "optimizer/cost.h"
 #include "optimizer/geqo.h"
@@ -3468,6 +3470,17 @@ struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"scram_iterations", PGC_USERSET, CONN_AUTH_AUTH,
+			gettext_noop("Sets the iteration count for SCRAM secret generation."),
+			NULL,
+			GUC_REPORT
+		},
+		&scram_sha_256_iterations,
+		SCRAM_SHA_256_DEFAULT_ITERATIONS, 1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, 0, 0, 0, NULL, NULL, NULL
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index d06074b86f..fc831565d9 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -95,6 +95,7 @@
 
 #authentication_timeout = 1min		# 1s-600s
 #password_encryption = scram-sha-256	# scram-sha-256 or md5
+#scram_iterations = 4096
 #db_user_namespace = off
 
 # GSSAPI using Kerberos