How to Generate a key (for SecretKeySpec) out of a string literal.

Question

Hi,
I have a string literal representation of a HEX value key which I use to send encrypted data from C++ to Java (on Sockets).
I haven't managed to convert the string representation to a value that I can use with SecretKeySpec() constructor to create a working AES 128b key.
The last attempt was this:
String strKey = new String("E5E6E7E9EA392A2B2D256489012145E5");
byte[] byteKey = new BigInteger(strKey,16).toByteArray();
SecretKey skey = new SecretKeySpec(byteKey, "AES");It didn't work because the byte array was of size 17 byte. (java.security.InvalidKeyException: Invalid AES key length: 17 bytes)
Is there an obvious straight forward way to this which I'm missing? (How would you transfer a key in HEX representation?)
Thank you.

Answer 1

Because of the way negative BigIntegers are represented, when the high-order bit of the high-order byte of a positive BigInteger is 1, the toByteArray() emits an "extra" high order byte of zero hence your length of 17. Another problem is that, if your Hex string happens to begin "00X..." where X is less than 8, then toByteArray() will not emit the superflous leading zero byte and the size will be 15 or less.
java.math.BigInteger b = new java.math.BigInteger("ECBA1127878", 16);
byte [] keyBytes = new byte[16];
byte [] bigBytes = b.toByteArray();
System.arraycopy(bigBytes, 0, keyBytes, 0, Math.min(keyBytes.length, bigBytes.length));as an example is only a 4 lines of code.