SubstCrypto.java

package crypt;

import javax.xml.bind.DatatypeConverter;
import java.util.*;

public class SubstCrypto {
    private static char[] createKey(long seed) {
        char[] key = new char[256];

        for(int i = 0;i < key.length;i++) {
            char c = ' ';
            if(i % 27 != 26) {
                c = (char)(i % 27 + 'a');
            }
            key[i] = c;
        }

        shuffle(key, seed);

        return key;
    }

    private static void shuffle(char[] key, long seed) {
        Random random = new Random(seed);

        for(int i = 0;i < key.length;i++) {
            int j = random.nextInt(key.length);

            char tmp = key[i];
            key[i] = key[j];
            key[j] = tmp;
        }
    }

    private static Map<Character, List<Integer>> map(char[] key) {
        Map<Character, List<Integer>> map = new HashMap<>();
        for(int i = 0;i < key.length;i++) {
            if(!map.containsKey(key[i])) {
                map.put(key[i], new ArrayList<>());
            }
            map.get(key[i]).add(i);
        }

        return map;
    }

    public static byte[] encrypt(String plainText, long seed) {
        Random random = new Random();
        plainText = plainText.toLowerCase().replaceAll("[^a-z ]", "");
        Map<Character, List<Integer>> keyMap = map(createKey(seed));
        byte[] out = new byte[plainText.length()];

        for(int i = 0;i < plainText.length();i++) {
            List<Integer> indices = keyMap.get(plainText.charAt(i));
            out[i] = (byte)(int)indices.get(random.nextInt(indices.size()));
        }

        return out;
    }

    public static String decrypt(byte[] cryptText, long seed) {
        StringBuilder plainText = new StringBuilder(cryptText.length);
        char[] key = createKey(seed);

        for(byte b : cryptText) {
            plainText.append(key[b & 0xff]);
        }

        return plainText.toString();
    }

    public static void main(String... argv) {
        byte[] encrypted = encrypt("the quick brown fox jumps over the lazy dog", 1234);
        System.out.println(DatatypeConverter.printHexBinary(encrypted));
        System.out.println(decrypt(encrypted, 1234));
        System.out.println(decrypt(encrypted, 1233));
    }
}