package com.peachpasswords.sync

import java.math.BigInteger

object SASDerivation {
    fun derive(handshakeHash: ByteArray): String {
        require(handshakeHash.size >= 8) { "Handshake hash too short for SAS" }
        // Use 64 PRF-output bits so modulo bias at the 1e8 boundary is
        // negligible. Browser pins the identical big-endian rule.
        val value = BigInteger(1, handshakeHash.copyOfRange(0, 8))
        return value.mod(BigInteger.valueOf(100_000_000L))
            .toString()
            .padStart(8, '0')
    }
}