commit 8804f69d46ef5cb5fad403f4df8e14725966443d
author Paul Bakker <p.j.bakker@polarssl.org> Thu Feb 28 18:06:26 2013 +0100
committer Paul Bakker <p.j.bakker@polarssl.org> Wed Mar 06 18:01:03 2013 +0100
tree a043915ea04358ebdd14b46976d7fdcd7c4d1a0a
parent a43231c5a53622c19a314e7966e43cc60bb5aed1

Removed timing differences due to bad padding from RSA decrypt for
PKCS#1 v1.5 operations

library/rsa.c

diff --git a/library/rsa.c b/library/rsa.c
index cc14d8e..e53d9a2 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -623,9 +623,9 @@
                                  unsigned char *output,
                                  size_t output_max_len)
 {
-    int ret;
-    size_t ilen;
-    unsigned char *p;
+    int ret, correct = 1;
+    size_t ilen, pad_count = 0;
+    unsigned char *p, *q;
     unsigned char bt;
     unsigned char buf[POLARSSL_MPI_MAX_SIZE];
 
@@ -647,36 +647,57 @@
     p = buf;
 
     if( *p++ != 0 )
-        return( POLARSSL_ERR_RSA_INVALID_PADDING );
+        correct = 0;
 
     bt = *p++;
     if( ( bt != RSA_CRYPT && mode == RSA_PRIVATE ) ||
         ( bt != RSA_SIGN && mode == RSA_PUBLIC ) )
     {
-        return( POLARSSL_ERR_RSA_INVALID_PADDING );
+        correct = 0;
     }
 
     if( bt == RSA_CRYPT )
     {
         while( *p != 0 && p < buf + ilen - 1 )
-            p++;
+            pad_count += ( *p++ != 0 );
 
-        if( *p != 0 || p >= buf + ilen - 1 )
-            return( POLARSSL_ERR_RSA_INVALID_PADDING );
+        correct &= ( *p == 0 && p < buf + ilen - 1 );
 
+        q = p;
+
+        // Also pass over all other bytes to reduce timing differences
+        //
+        while ( q < buf + ilen - 1 )
+            pad_count += ( *q++ != 0 );
+
+        // Prevent compiler optimization of pad_count
+        //
+        correct |= pad_count & 0x100000; /* Always 0 unless 1M bit keys */
         p++;
     }
     else
     {
         while( *p == 0xFF && p < buf + ilen - 1 )
-            p++;
+            pad_count += ( *p++ == 0xFF );
 
-        if( *p != 0 || p >= buf + ilen - 1 )
-            return( POLARSSL_ERR_RSA_INVALID_PADDING );
+        correct &= ( *p == 0 && p < buf + ilen - 1 );
 
+        q = p;
+
+        // Also pass over all other bytes to reduce timing differences
+        //
+        while ( q < buf + ilen - 1 )
+            pad_count += ( *q++ != 0 );
+
+        // Prevent compiler optimization of pad_count
+        //
+        correct |= pad_count & 0x100000; /* Always 0 unless 1M bit keys */
         p++;
     }
 
+    if( correct == 0 )
+        return( POLARSSL_ERR_RSA_INVALID_PADDING );
+
     if (ilen - (p - buf) > output_max_len)
         return( POLARSSL_ERR_RSA_OUTPUT_TOO_LARGE );