Question

In the similar question “Conversion of byte[] into a String and then back to a byte[]” is said to not to do the byte[] to String and back conversion, what looks like apply to most cases, mainly when you don’t know the encoding used.

But, in my case I’m trying to save to a DB the javax.crypto.SecretKey data, and recoverd it after.

The interface provide a method getEncoded() which returns the key data encoded as byte[], and with another class I can use this byte[] to recover the key.

So, the question is, how do I write the key bytes as String, and later get back the byte[] to regenerate the key?

Answer 1

javax.crypto.SecretKey is binary data, so you can’t convert it directly to a String. You can encode it as a hex string or in Base64.

See Apache Commons Codec.

Update: If you dont want to depend on third-party libraries (and can’t/don’t want to store plain binary data, as Jon suggests) you can do some ad-hoc encoding, for example, following erickson’s suggestion:

public static String bytesToString(byte[] b) {
    byte[] b2 = new byte[b.length + 1];
    b2[0] = 1;
    System.arraycopy(b, 0, b2, 1, b.length);
    return new BigInteger(b2).toString(36);
}

public static byte[] stringToBytes(String s) {
    byte[] b2 = new BigInteger(s, 36).toByteArray();
    return Arrays.copyOfRange(b2, 1, b2.length);
}

It’s rather, ugly, non-standard and not optimal (in output size). But it’s also very small, correct and it has no dependencies; it can be practical, specially if your data is small sized.

Updated: I replaced the Character.MAX_RADIX by the literal value (36), following GregS’s comment. It might seem less elegant but it’s actually more secure. (You can also use 32 or 16).