Hmac should blank preparedKey after deriving iKey and oKey

[05nelsonm]

Current implementation of Hmac.Engine is that the provided key ByteArray is sized to the digest.blockSize() by either hashing it once (if larger than), padding it with 0's (if smaller), or using the provided key. It then uses the prepared key to derive iKey and oKey.

After those are derived, the preparedKey should be blanked. As such, the provided key should always be copied so we are not modifying the provided ByteArray, so that the copy can be blanked before dereferencing.

val preparedKey = if (key.size > digest.blockSize()) {
    digest.digest(key).copyOf(digest.blockSize())
} else {
    key.copyOf(digest.blockSize())
}

// ... derive iKey and oKey

preparedKey.fill(0)