rsa.kt

fun Int.charsRepeat(c: Char): String = if (this == 0) "" else (1..this).map { c.toString() }.reduce { s1, s2 -> "$s1$s2" }

fun <T> List<T>.reduceToString() = if (isEmpty()) "" else this.map { it.toString() }.reduce { c1, c2 -> "$c1$c2" }

fun powMod(n: Long, p: Long, mod: Long): Long {
    var p = p
    var res: Long = 1
    var pow = n
    while (p > 0) {
        if (p and 1 == 1L) res = res * pow % mod
        pow = pow * pow % mod
        p /= 2
    }
    return res
}

infix fun Long.multiReverse(mod: Long) = (1..mod).first { (it * this) % mod == 1L }

fun encode(m: Long, e: Long, N: Long) = powMod(m, e, N)

fun decode(c: Long, d: Long, N: Long) = powMod(c, d, N)

fun String.charPairs(): List<Pair<Char, Char>> {
    val res = mutableListOf<Pair<Char, Char>>()
    var char: Char? = null
    this.forEach {
        char?.apply { res.add(Pair(this, it)) }
        char = if (char != null) null else it
    }
    char?.apply { res.add(Pair(this, 0.toChar())) }
    return res
}

fun String.divide(size: Int) = (0..(length + size - 1) / size - 1).map { substring(it * size, minOf((it + 1) * size, length)) }

fun main(args: Array<String>) {
    val p: Long = 8627
    val q: Long = 9929

    val N = p * q
    val E: Long = 257

    val f = (q - 1) * (p - 1)

    val d = E multiReverse f
    println("d = $d")

    val mess = "message"
    println(mess.charPairs())

    val c = mess.charPairs()
            .map { encode(it.first.toLong().shl(8) + it.second.toLong(), E, N) }
            .map { it.toString(16) }
            .map { "${(8 - it.length).charsRepeat('0')}$it" }
            .reduceToString()
    println("c = $c")

    val m = c
            .divide(8)
            .map { it.toLong(16) }
            .map { decode(it, d, N) }
            .map { listOf(it.shr(8), it.and(0xff)) }
            .flatten()
            .map(Long::toChar)
            .filter { it != 0.toChar() }
            .reduceToString()
    println("m = $m")
}