What is the equivalent of readBigUInt64BE in java/kotlin?

Question

I have this snippet of javascript code:

crypto
    .createHash('sha256')
    .update(myString)
    .digest()
    .readBigUInt64BE();

And I am trying to write the same thing in kotlin.

I can get up to the digest part and verify the byte arrays are the same:

val md = MessageDigest.getInstance("SHA-256")
val inputBytes = input.toByteArray()
val bytes = md.digest(inputBytes)

But I cannot seem to find an equivalent of readBigUInt64BE in any utility library (like java.security.MessageDigest )

Answer 1

The Java equivalent of "BigUInt64" is an unsigned long . Java's long is signed , but can be treated as unsigned using the xxxUnsigned() methods of Long .

Since you likely don't care about signed vs unsigned, except when printing the value, using long is fine. If that's not good enough, then you'd need to use BigInteger .

To read the value from the returned bytes from the digest, use a ByteBuffer , which is similar to the Node.js Buffer .

ByteBuffer.wrap(bytes).getLong()

If the JavaScript method name had ended with "LE", you would need to specify that.

ByteBuffer.wrap(bytes).order(ByteOrder.LITTLE_ENDIAN).getLong()

Answer 2

After much searching, and even trying to copy the implementation of readBigUInt64BE , here is the solution I came up with which gives me the same result.

Kotlin:

private fun getHashedValue(input: String): BigInteger {
    val md = MessageDigest.getInstance("SHA-256")
    val inputBytes = input.toByteArray()
    val bytes = md.digest(inputBytes)
    if (bytes.size < 8) { // Not sure if this case works, but I haven't hit it yet
        return BigInteger(1, bytes)
    }
    return BigInteger(1, bytes.sliceArray(0..7))
}

NodeJS Equivalent:

crypto
    .createHash('sha256')
    .update(myString)
    .digest()
    .readBigUInt64BE();