Question

I’m struggling with the migration of Java code to Golang for the last few days and I am now stuck. This is the working Java code:

final Key k = new SecretKeySpec(keyString.getBytes(), "AES");
Cipher c = Cipher.getInstance("AES");
c.init(Cipher.DECRYPT_MODE, k);

final InputStream in = new BufferedInputStream(new FileInputStream(fileNameToDecrypt));
final CipherInputStream instream = new CipherInputStream(in, c);

if (instream.read() != 'B') {
    System.out.println("Error");
}

if (instream.read() != 'Z') {
    System.out.println("Error");
}

final CBZip2InputStream zip = new CBZip2InputStream(instream);

My implementation in Golang:

c, _ := aes.NewCipher([]byte(keyString))
// IV must be defined in golang
iv := []byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
             0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}
d := cipher.NewCBCDecrypter(c, iv)

fi, _ := os.Open(fileNameToDecrypt)
stat, _ := fi.Stat()
enc := make([]byte, stat.Size())
dec := make([]byte, stat.Size())
fi.Read(enc)
d.CryptBlocks(dec, enc)
instream := bytes.NewBuffer(dec)
zip := bzip2.NewReader(instream)

What I know so far:

• all error values omitted by _ are nil in this piece of code
• the bzip2 header (“BZ”) must be omitted for CBzip2InputStream, but not for bzip2.NewReader
• the first 16 bytes read from instream in Java and golang are the same, starting with the 17th byte all bytes differ for whatever reason

Answer 1

CBizp2InputStream indeed uses AES ECB. This is a working implementation. I omitted error handling to make the code shorter:

c, _ := aes.NewCipher([]byte(keyString))
bufIn := make([]byte, 16)
bufOut := make([]byte, 16)
dec := bytes.NewBuffer(make([]byte, 0))
var i int

for {
    i, _ = src.Read(bufIn)
    if i == 0 {
        break
    }

    c.Decrypt(bufOut, bufIn)
    dec.Write(bufOut)
}

zip := bzip2.NewReader(dec)
io.Copy(dst, zip)

Additional explanation:

• src is an io.Reader and dst is an io.Writer, both supplied to the decrypt function as arguments
• keyString contains the secret key
• I use i == 0 as breaking condition because err can or cannot be set to io.EOF in the last successful read (see golang io.Reader specification)

Works perfectly. Implementing encryption should now be easy.