How to determine whether a byte array contains hashed data

Question

Is it possible to determine whether a byte array contains data hashed with PBKDF2WithHmacSHA1? Is there a pattern that could help?

Answer 1

Here below is how I solved the issue in Scala:

class Password(value: String, salt: Option[String]) {

  private final val IterationCount = 2048
  private final val KeyLength = 256
  private final val SaltLength = KeyLength / 8

  ...

  def hash = {
    val zalt = if (salt.isDefined)
      salt.get.getBytes(DefaultCharset)
    else 
      SecureRandom.getInstance("SHA1PRNG").generateSeed(SaltLength)

    val secretKeyFactory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1")
    val secretKey = secretKeyFactory.generateSecret(
      new PBEKeySpec(value.toCharArray, zalt, IterationCount, KeyLength)
    )

    val byteBuffer = ByteBuffer.allocate(2 + KeyLength)
    byteBuffer.putShort(KeyLength)
    byteBuffer.put(secretKey.getEncoded)

    new Password(
      Base64.encodeBase64String(byteBuffer.array),
      Some(new String(zalt, DefaultCharset))
    )
  }

  def isHashed = Base64.decodeBase64(value).length > KeyLength
}

The length of the key is prepended to the encoded hash... and to determine whether or not the current Password instance is hashed I just check the length of the whole buffer – the complete source code is available here.