[SNMP4J] 3DES implementation

[Yue Zhang]

Hi Jochen,

Thanks a lot for your reply.

I implemented a temporary patch to Priv3DES for my testing. Could you please 
review it?

  public byte[] extendShortKey(byte[] shortKey, OctetString password,
                               byte[] engineID,
                               AuthenticationProtocol authProtocol) {
    int length = shortKey.length;
    byte[] extendedKey = new byte[getMinKeyLength()];
    System.arraycopy(shortKey, 0, extendedKey, 0, shortKey.length);

    byte[] key = new byte[getMinKeyLength()];
    System.arraycopy(shortKey, 0, key, 0, shortKey.length);
    while (length < getMinKeyLength()) {
      key = authProtocol.passwordToKey(new OctetString(key, 0, length),
                                     engineID);
      int copyBytes = Math.min(getMinKeyLength() - length,
                               authProtocol.getDigestLength());
      System.arraycopy(key, 0, extendedKey, length, copyBytes);
      length += copyBytes;
    }
    return extendedKey;
  }

Best Regards,
Yue