transform: Do not look at any bytes of PKCS#5 padding other than the last

This might avoid some timing-related information leaks.  In principle
this is a protocol change: we now no longer use actual PKCS#5 padding;
instead, we use a padding scheme where all but the last byte of the
padding may be sent as anything and are ignored by the receiver.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/transform.c b/transform.c
index 012f618..6618ec5 100644 (file)
--- a/transform.c
+++ b/transform.c
@@ -234,13 +234,7 @@ static uint32_t transform_reverse(void *sst, struct buffer_if *buf,
        return 1;
     }
 
-    padp=buf_unappend(buf,padlen-1);
-    for (i=0; i<padlen-1; i++) {
-       if (*++padp != padlen) {
-           *errmsg="pkcs5: corrupted padding";
-           return 1;
-       }
-    }
+    buf_unappend(buf,padlen-1);
 
     /* Sequence number must be within max_skew of lastrecvseq; lastrecvseq
        is only allowed to increase. */