Vigenere's Decipher Tool

vigenereDecipher.swift

import Foundation

// MARK: - Main

var cipherText = "vyc iafgrr xl o ozpghf qw qtvitzrn zorj. rwx qascg mvtvyi bg orgcem vyc gxtnvaibcp fd dnf yvyzgsujch. mvgicuhfg, jcrnfkkw xl bqk yqhiv dyzbbi kcrabqcmvr dgidtvh, dlr btykee rngvfktk inkgbthgcw hthkjdxxr ceb rhbhzbtgh. vyc bxhjfbh mvck uxez ofqi xthvaibjgcw bbbkdgox hjv yqbzkkw dy wpkpjwstj rd vcogpdfwuv gcyctdyibcp jcrnfkkw pks efkeksjvlhbjg lqtk htrgcbbi rls xrwtyibcp. rlnpoa, tmczfcksathkfl, nhi ularxsf km sxqkgftk mqlp ubfuk ktlgcxc xg hjzq rhitjc. xy mql udky jrpsxf vycc rcw ngae pg r edhr eiwemoprjnlh. ccqd, rcw ngae pg r edhr uvajkwvp qexqkrjxlh.".lowercased()
var key = "crypto"
let decipheredText = vigenereDecipher(cipher: cipherText, using: key)
print(decipheredText)

// MARK: - Actions

func vigenereDecipher(cipher: String, using key: String) -> String {
    var alphabet = getEnglishAlphabet()
    var plainText = ""
    let cipherText = removePunctuationSymbols(from: (cipher as NSString)) as String
    var shifts = getLetterShiftsArray(from: key)
    var asciiCode: Int = 0
    var j: Int = 0

    for (_, c) in cipherText.enumerated() {
        asciiCode = (Int(c.ascii!) - 97) - shifts[j]

        if asciiCode < 0 {
            asciiCode = asciiCode + 26
        }
        
        asciiCode = asciiCode % 26
        j = j + 1

        if j >= shifts.count {
            j = 0
        }

        plainText.append(alphabet[asciiCode])
    }

    return plainText
}

func removePunctuationSymbols(from text: NSString) -> NSString {
    var resultText = text
    let punctuationSymbols = [" ", ".", ",", "!", "?", "-", "\n"]

    for s in punctuationSymbols {
        resultText = resultText.replacingOccurrences(of: s, with: "") as NSString
    }

    return resultText
}

func getLetterShiftsArray(from key: String) -> [Int] {
    var shifts = [Int]()

    for c in key {
        if let n = c.ascii {
            shifts.append(Int(n) - 97)
        }
    }

    return shifts
}

func getEnglishAlphabet() -> [String] {
    var alphabet: [String] = []
    for i in 97 ... (97+25) {
        alphabet.append(i.asciiCharacter!)
    }

    return alphabet
}

// MARK: - Extensions

extension String {
    var ascii: [UInt32] {
        return unicodeScalars.compactMap{
            $0.isASCII ? $0.value : nil
        }
    }
}

extension Character {
    var ascii: UInt32? {
        return String(self).ascii.first
    }
}

extension Int {
    var asciiCharacter: String? {
        guard let asciiCode = UnicodeScalar(self) else {
            return nil
        }
        return String(asciiCode)
    }
}