Add checksum verification for chunk data

cryptpng.go

@@ -109,14 +109,14 @@ func DecryptDataPng(f *os.File, fout *os.File) {
 		salt = append(salt, saltChunk.data...)
 	}
 	var data []byte
-	for _, cryptChunk := range png.GetChunksByName(chunkName) {
+	for i, cryptChunk := range png.GetChunksByName(chunkName) {
+		if !cryptChunk.Verify() {
+			log.Fatalf("Corrupted chunk data, chunk #%d", i)
+		}
 		data = append(data, cryptChunk.data...)
 	}
 	if len(data) > 0 {
 		data, err = decryptData(data, salt)
-		if err != nil {
-			log.Println("\nThe provided password is probably incorrect.")
-		}
 		check(err)
 		_, err = fout.Write(data)
 		check(err)

pngUtils.go

@@ -30,6 +30,15 @@ func (c *ChunkData) GetRaw() []byte {
 	return raw
 }
 
+// verifies the integrity of the chunks data using crc
+func (c *ChunkData) Verify() bool {
+	var data []byte
+	data = append(data, []byte(c.name)...)
+	data = append(data, c.data...)
+	crc := crc32.ChecksumIEEE(data)
+	return c.crc == crc
+}
+
 type PngData struct {
 	header []byte
 	chunks []ChunkData