Prove membership of a set in an RSA accumulator

Prove.kt

fun proveMembershipOrNull(x: BigInteger): RSAProof? {
    return if (!data.containsKey(x)) {
        null
    } else {
        var product = BigInteger.ONE
        for ((k, v) in data) {
            if (k != x) {
                product *= hashToPrime(k, ACCUMULATED_PRIME_SIZE, v).first
            }
        }
        RSAProof(A0.modPow(product, n), hashToPrime(x, ACCUMULATED_PRIME_SIZE, getNonce(x)).first)
    }
}