diff --git a/library/x509.c b/library/x509.c
index 48c11ef..3e93c3a 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -547,14 +547,39 @@
     return( 0 );
 }
 
-int x509_get_sig_alg( const x509_buf *sig_oid, md_type_t *md_alg,
-                      pk_type_t *pk_alg )
+/*
+ * Get signature algorithm from alg OID and optional parameters
+ */
+int x509_get_sig_alg( const x509_buf *sig_oid, const x509_buf *sig_params,
+                      md_type_t *md_alg, pk_type_t *pk_alg )
 {
-    int ret = oid_get_sig_alg( sig_oid, md_alg, pk_alg );
+    int ret;
 
-    if( ret != 0 )
+    if( ( ret = oid_get_sig_alg( sig_oid, md_alg, pk_alg ) ) != 0 )
         return( POLARSSL_ERR_X509_UNKNOWN_SIG_ALG + ret );
 
+#if defined(POLARSSL_RSASSA_PSS_CERTIFICATES)
+    if( *pk_alg == POLARSSL_PK_RSASSA_PSS )
+    {
+        int salt_len, trailer_field;
+        md_type_t mgf_md;
+
+        /* Make sure params are valid */
+        ret = x509_get_rsassa_pss_params( sig_params,
+                md_alg, &mgf_md, &salt_len, &trailer_field );
+        if( ret != 0 )
+            return( ret );
+
+    }
+    else
+#endif
+    {
+        /* Make sure parameters are absent or NULL */
+        if( ( sig_params->tag != ASN1_NULL && sig_params->tag != 0 ) ||
+              sig_params->len != 0 )
+        return( POLARSSL_ERR_X509_INVALID_ALG );
+    }
+
     return( 0 );
 }