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// Copyright © 2020 rsiddharth <s@ricketyspace.net>
// SPDX-License-Identifier: ISC

package challenge

import (
	"fmt"
	"ricketyspace.net/cryptopals/lib"
)

var cipher string = `HUIfTQsPAh9PE048GmllH0kcDk4TAQsHThsBFkU2AB4BSWQgVB0dQzNTTmVS
BgBHVBwNRU0HBAxTEjwMHghJGgkRTxRMIRpHKwAFHUdZEQQJAGQmB1MANxYG
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Ry1SHgYOVBFIB08XQ0kUCnRvPgwQTgUbGBwAOVREYhAGAQBJEUgETgpPGR8E
LUUGBQgaQRIaHEshGk03AQANR1QdBAkAFwAcUwE9AFxNY2QxGA4LACxSQTZS
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AB0cRSo+AwgKRSANExlJCBQaBAsANU9TKxFJL0dMHRwRTAtPBRwQMAAATQcB
FlRlIkw5QwA2GggaR0YBBg5ZTgIcAAw3SVIaAQcVEU8QTyEaYy0fDE4ITlhI
Jk8DCkkcC3hFMQIEC0EbAVIqCFZBO1IdBgZUVA4QTgUWSR4QJwwRTWM=`

func C6() {
	bs := lib.Base64ToBytes(cipher)

	// Compute keysize.
	ks := lib.KeySizeWithMinDistance(2, 40, bs)
	fmt.Printf("Computed key size: %d\n", ks)

	// Crack key and decrypt data.
	key, text := crack(bs, ks)

	// Barf result.
	fmt.Printf("Key is '%s'\n", key)
	fmt.Printf("Decrypted text:\n%s", text)
}

// 'bs' is the data to decrypt.
// 'ks' is the key size.
func crack(bs []byte, ks int) (string, string) {
	blocks := lib.BreakIntoBlocks(bs, ks)
	blocks = lib.TransposeBlocks(blocks, ks)

	key = ""
	for i := 0; i < len(blocks); i++ {
		k := findKey(blocks[i])

		key += string(k)
	}
	return key, lib.RepeatingXOR(string(bs), key)
}

func findKey(block []byte) byte {
	var k byte
	var s int

	s = 0
	for i := byte(0); i < 255; i++ {
		ks := lib.FillBytes(i, len(block))
		xs := lib.FixedXORBytes(block, ks)

		score := lib.AlphaPunchScore(xs)
		if score > s {
			s = score
			k = i
		}
	}
	return k
}