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

16 files changed, 94 insertions, 10 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 481f93cea1b..71730cc52fe 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1132,6 +1132,26 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><varname>scram_iterations</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>scram_iterations</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        The number of computational iterations to be performed when encrypting
+        a password using SCRAM-SHA-256. The default is <literal>4096</literal>.
+        A higher number of iterations provides additional protection against
+        brute-force attacks on stored passwords, but makes authentication
+        slower. Changing the value has no effect on existing passwords
+        encrypted with SCRAM-SHA-256 as the iteration count is fixed at the
+        time of encryption. In order to make use of a changed value, a new
+        password must be set.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-krb-server-keyfile" xreflabel="krb_server_keyfile">
       <term><varname>krb_server_keyfile</varname> (<type>string</type>)
       <indexterm>
diff --git a/src/backend/libpq/auth-scram.c b/src/backend/libpq/auth-scram.c
index 4441e0d7745..9b286aa4d7f 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 1c0583fe267..a60bd48499e 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 d06074b86f6..fc831565d90 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
diff --git a/src/common/scram-common.c b/src/common/scram-common.c
index bd40d497a96..ef997ef6849 100644
--- a/src/common/scram-common.c
+++ b/src/common/scram-common.c
@@ -214,8 +214,7 @@ scram_build_secret(pg_cryptohash_type hash_type, int key_length,
 	/* Only this hash method is supported currently */
 	Assert(hash_type == PG_SHA256);
 
-	if (iterations <= 0)
-		iterations = SCRAM_DEFAULT_ITERATIONS;
+	Assert(iterations > 0);
 
 	/* Calculate StoredKey and ServerKey */
 	if (scram_SaltedPassword(password, hash_type, key_length,
diff --git a/src/include/common/scram-common.h b/src/include/common/scram-common.h
index 0c75df55597..5ccff96ecee 100644
--- a/src/include/common/scram-common.h
+++ b/src/include/common/scram-common.h
@@ -47,7 +47,7 @@
  * Default number of iterations when generating secret.  Should be at least
  * 4096 per RFC 7677.
  */
-#define SCRAM_DEFAULT_ITERATIONS	4096
+#define SCRAM_SHA_256_DEFAULT_ITERATIONS	4096
 
 extern int	scram_SaltedPassword(const char *password,
 								 pg_cryptohash_type hash_type, int key_length,
diff --git a/src/include/libpq/scram.h b/src/include/libpq/scram.h
index b275e1e87e5..310bc365177 100644
--- a/src/include/libpq/scram.h
+++ b/src/include/libpq/scram.h
@@ -18,6 +18,9 @@
 #include "libpq/libpq-be.h"
 #include "libpq/sasl.h"
 
+/* Number of iterations when generating new secrets */
+extern PGDLLIMPORT int scram_sha_256_iterations;
+
 /* SASL implementation callbacks */
 extern PGDLLIMPORT const pg_be_sasl_mech pg_be_scram_mech;
 
diff --git a/src/interfaces/libpq/fe-auth-scram.c b/src/interfaces/libpq/fe-auth-scram.c
index 277f72b280c..6b779ec7ffd 100644
--- a/src/interfaces/libpq/fe-auth-scram.c
+++ b/src/interfaces/libpq/fe-auth-scram.c
@@ -895,7 +895,7 @@ verify_server_signature(fe_scram_state *state, bool *match,
  * error details.
  */
 char *
-pg_fe_scram_build_secret(const char *password, const char **errstr)
+pg_fe_scram_build_secret(const char *password, int iterations, const char **errstr)
 {
 	char	   *prep_password;
 	pg_saslprep_rc rc;
@@ -927,7 +927,7 @@ pg_fe_scram_build_secret(const char *password, const char **errstr)
 
 	result = scram_build_secret(PG_SHA256, SCRAM_SHA_256_KEY_LEN, saltbuf,
 								SCRAM_DEFAULT_SALT_LEN,
-								SCRAM_DEFAULT_ITERATIONS, password,
+								iterations, password,
 								errstr);
 
 	free(prep_password);
diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index 934e3f4f7ca..b0550e63324 100644
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -1341,7 +1341,9 @@ PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user,
 	{
 		const char *errstr = NULL;
 
-		crypt_pwd = pg_fe_scram_build_secret(passwd, &errstr);
+		crypt_pwd = pg_fe_scram_build_secret(passwd,
+											 conn->scram_sha_256_iterations,
+											 &errstr);
 		if (!crypt_pwd)
 			libpq_append_conn_error(conn, "could not encrypt password: %s", errstr);
 	}
diff --git a/src/interfaces/libpq/fe-auth.h b/src/interfaces/libpq/fe-auth.h
index 1aa556ea2fa..124dd5d0313 100644
--- a/src/interfaces/libpq/fe-auth.h
+++ b/src/interfaces/libpq/fe-auth.h
@@ -26,6 +26,7 @@ extern char *pg_fe_getauthname(PQExpBuffer errorMessage);
 /* Mechanisms in fe-auth-scram.c */
 extern const pg_fe_sasl_mech pg_scram_mech;
 extern char *pg_fe_scram_build_secret(const char *password,
+									  int iterations,
 									  const char **errstr);
 
 #endif							/* FE_AUTH_H */
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 660775e0198..b71378d94c5 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -596,6 +596,7 @@ pqDropServerData(PGconn *conn)
 	conn->std_strings = false;
 	conn->default_transaction_read_only = PG_BOOL_UNKNOWN;
 	conn->in_hot_standby = PG_BOOL_UNKNOWN;
+	conn->scram_sha_256_iterations = SCRAM_SHA_256_DEFAULT_ITERATIONS;
 	conn->sversion = 0;
 
 	/* Drop large-object lookup data */
@@ -4182,6 +4183,7 @@ makeEmptyPGconn(void)
 	conn->std_strings = false;	/* unless server says differently */
 	conn->default_transaction_read_only = PG_BOOL_UNKNOWN;
 	conn->in_hot_standby = PG_BOOL_UNKNOWN;
+	conn->scram_sha_256_iterations = SCRAM_SHA_256_DEFAULT_ITERATIONS;
 	conn->verbosity = PQERRORS_DEFAULT;
 	conn->show_context = PQSHOW_CONTEXT_ERRORS;
 	conn->sock = PGINVALID_SOCKET;
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index ec62550e385..a16bbf32ef5 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -1181,6 +1181,10 @@ pqSaveParameterStatus(PGconn *conn, const char *name, const char *value)
 		conn->in_hot_standby =
 			(strcmp(value, "on") == 0) ? PG_BOOL_YES : PG_BOOL_NO;
 	}
+	else if (strcmp(name, "scram_iterations") == 0)
+	{
+		conn->scram_sha_256_iterations = atoi(value);
+	}
 }
 
 
diff --git a/src/interfaces/libpq/libpq-int.h b/src/interfaces/libpq/libpq-int.h
index 10187c31b9a..88b9838d766 100644
--- a/src/interfaces/libpq/libpq-int.h
+++ b/src/interfaces/libpq/libpq-int.h
@@ -525,6 +525,7 @@ struct pg_conn
 	/* Assorted state for SASL, SSL, GSS, etc */
 	const pg_fe_sasl_mech *sasl;
 	void	   *sasl_state;
+	int			scram_sha_256_iterations;
 
 	/* SSL structures */
 	bool		ssl_in_use;
diff --git a/src/test/authentication/t/001_password.pl b/src/test/authentication/t/001_password.pl
index cba5d7d6487..00857fdae5a 100644
--- a/src/test/authentication/t/001_password.pl
+++ b/src/test/authentication/t/001_password.pl
@@ -86,6 +86,21 @@ $node->safe_psql('postgres',
 	q{SET password_encryption='md5'; CREATE ROLE "md5,role" LOGIN PASSWORD 'pass';}
 );
 
+# Create a role with a non-default iteration count
+$node->safe_psql(
+	'postgres',
+	"SET password_encryption='scram-sha-256';
+	 SET scram_iterations=1024;
+	 CREATE ROLE scram_role_iter LOGIN PASSWORD 'pass';
+	 RESET scram_iterations;"
+);
+
+my $res = $node->safe_psql('postgres',
+	"SELECT substr(rolpassword,1,19)
+	 FROM pg_authid
+	 WHERE rolname = 'scram_role_iter'");
+is($res, 'SCRAM-SHA-256$1024:', 'scram_iterations in server side ROLE');
+
 # Create a database to test regular expression.
 $node->safe_psql('postgres', "CREATE database regex_testdb;");
 
@@ -98,7 +113,7 @@ test_conn($node, 'user=md5_role', 'trust', 0,
 	log_unlike => [qr/connection authenticated:/]);
 
 # SYSTEM_USER is null when not authenticated.
-my $res = $node->safe_psql('postgres', "SELECT SYSTEM_USER IS NULL;");
+$res = $node->safe_psql('postgres', "SELECT SYSTEM_USER IS NULL;");
 is($res, 't', "users with trust authentication use SYSTEM_USER = NULL");
 
 # Test SYSTEM_USER with parallel workers when not authenticated.
@@ -283,6 +298,14 @@ test_conn(
 	log_like => [
 		qr/connection authenticated: identity="scram_role" method=scram-sha-256/
 	]);
+test_conn(
+	$node,
+	'user=scram_role_iter',
+	'scram-sha-256',
+	0,
+	log_like => [
+		qr/connection authenticated: identity="scram_role_iter" method=scram-sha-256/
+	]);
 test_conn($node, 'user=md5_role', 'scram-sha-256', 2,
 	log_unlike => [qr/connection authenticated:/]);
 
diff --git a/src/test/regress/expected/password.out b/src/test/regress/expected/password.out
index 7c84c9da337..84752317356 100644
--- a/src/test/regress/expected/password.out
+++ b/src/test/regress/expected/password.out
@@ -72,6 +72,9 @@ CREATE ROLE regress_passwd6 PASSWORD 'SCRAM-SHA-256$1234';
 CREATE ROLE regress_passwd7 PASSWORD 'md5012345678901234567890123456789zz';
 -- invalid length
 CREATE ROLE regress_passwd8 PASSWORD 'md501234567890123456789012345678901zz';
+-- Changing the SCRAM iteration count
+SET scram_iterations = 1024;
+CREATE ROLE regress_passwd9 PASSWORD 'alterediterationcount';
 SELECT rolname, regexp_replace(rolpassword, '(SCRAM-SHA-256)\$(\d+):([a-zA-Z0-9+/=]+)\$([a-zA-Z0-9+=/]+):([a-zA-Z0-9+/=]+)', '\1$\2:<salt>$<storedkey>:<serverkey>') as rolpassword_masked
     FROM pg_authid
     WHERE rolname LIKE 'regress_passwd%'
@@ -86,7 +89,8 @@ SELECT rolname, regexp_replace(rolpassword, '(SCRAM-SHA-256)\$(\d+):([a-zA-Z0-9+
  regress_passwd6 | SCRAM-SHA-256$4096:<salt>$<storedkey>:<serverkey>
  regress_passwd7 | SCRAM-SHA-256$4096:<salt>$<storedkey>:<serverkey>
  regress_passwd8 | SCRAM-SHA-256$4096:<salt>$<storedkey>:<serverkey>
-(8 rows)
+ regress_passwd9 | SCRAM-SHA-256$1024:<salt>$<storedkey>:<serverkey>
+(9 rows)
 
 -- An empty password is not allowed, in any form
 CREATE ROLE regress_passwd_empty PASSWORD '';
@@ -129,6 +133,7 @@ DROP ROLE regress_passwd5;
 DROP ROLE regress_passwd6;
 DROP ROLE regress_passwd7;
 DROP ROLE regress_passwd8;
+DROP ROLE regress_passwd9;
 DROP ROLE regress_passwd_empty;
 DROP ROLE regress_passwd_sha_len0;
 DROP ROLE regress_passwd_sha_len1;
diff --git a/src/test/regress/sql/password.sql b/src/test/regress/sql/password.sql
index 98f49916e5d..53e86b0b6ce 100644
--- a/src/test/regress/sql/password.sql
+++ b/src/test/regress/sql/password.sql
@@ -63,6 +63,10 @@ CREATE ROLE regress_passwd7 PASSWORD 'md5012345678901234567890123456789zz';
 -- invalid length
 CREATE ROLE regress_passwd8 PASSWORD 'md501234567890123456789012345678901zz';
 
+-- Changing the SCRAM iteration count
+SET scram_iterations = 1024;
+CREATE ROLE regress_passwd9 PASSWORD 'alterediterationcount';
+
 SELECT rolname, regexp_replace(rolpassword, '(SCRAM-SHA-256)\$(\d+):([a-zA-Z0-9+/=]+)\$([a-zA-Z0-9+=/]+):([a-zA-Z0-9+/=]+)', '\1$\2:<salt>$<storedkey>:<serverkey>') as rolpassword_masked
     FROM pg_authid
     WHERE rolname LIKE 'regress_passwd%'
@@ -97,6 +101,7 @@ DROP ROLE regress_passwd5;
 DROP ROLE regress_passwd6;
 DROP ROLE regress_passwd7;
 DROP ROLE regress_passwd8;
+DROP ROLE regress_passwd9;
 DROP ROLE regress_passwd_empty;
 DROP ROLE regress_passwd_sha_len0;
 DROP ROLE regress_passwd_sha_len1;