lib: update Pkcs7PaddingUndo

Add validation.

1 files changed, 10 insertions, 1 deletions

diff --git a/lib/blocks.go b/lib/blocks.go
index 76f39d6..9b184d1 100644
--- a/lib/blocks.go
+++ b/lib/blocks.go
@@ -113,5 +113,14 @@ func Pkcs7Padding(in []byte, k int) []byte {
 
 // Removes PKCS#7 Padding from input `in`
 func Pkcs7PaddingUndo(in []byte) []byte {
-	return in[0:(len(in) - int(in[len(in)-1]))]
+	pd := in[len(in)-1] // padding character
+	pl := int(pd)       // padding length
+
+	// Validate
+	for i := len(in) - 1; i >= len(in)-pl; i-- {
+		if in[i] != pd {
+			panic("input is not pkcs#7 padded!")
+		}
+	}
+	return in[0:(len(in) - pl)]
 }