RSAEncryption.java

import java.math.BigInteger;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class RSAEncryption {

    private static final List<Integer> primes = List.of(2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757, 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863, 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997, 1009, 1013, 1019, 1021, 1031, 1033, 1039, 1049, 1051, 1061, 1063, 1069, 1087, 1091, 1093, 1097, 1103, 1109, 1117, 1123, 1129, 1151, 1153, 1163, 1171, 1181, 1187, 1193, 1201, 1213, 1217, 1229, 1231, 1237, 1249, 1259, 1277, 1279, 1289, 1291, 1297, 1301, 1319, 1321, 1327, 1361, 1367, 1373, 1381, 1399, 1409, 1423, 1427, 1429, 1433, 1439, 1447, 1451, 1453, 1459, 1471, 1481, 1487, 1489, 1493, 1499, 1511, 1523, 1531, 1543, 1549, 1553, 1559, 1567, 1571, 1579, 1583, 1597, 1601, 1607, 1609, 1613, 1619, 1627, 1637, 1657, 1663, 1667, 1669, 1693, 1697, 1699, 1721, 1723, 1733, 1741, 1747, 1753, 1759, 1777, 1783, 1787, 1789, 1801, 1811, 1823, 1831, 1847, 1861, 1867, 1871, 1873, 1877, 1879, 1889, 1901, 1907, 1913, 1931, 1933, 1949, 1951, 1973, 1979, 1987, 1993, 1997, 1999, 2003, 2011, 2017, 2027, 2029, 2039, 2053, 2063, 2069, 2081, 2083, 2087, 2089, 2099, 2111, 2113, 2129, 2131, 2137, 2141, 2143, 2153, 2161, 2179, 2203, 2207, 2213, 2221, 2237, 2239, 2243, 2251, 2267, 2269, 2273, 2281, 2287, 2293, 2297, 2309, 2311, 2339, 2341, 2347, 2351, 2371, 2377, 2381, 2383, 2387, 2393, 2399, 2411, 2417, 2423, 2437, 2441, 2447, 2459, 2467, 2473, 2477, 2503, 2521, 2531, 2539, 2549, 2551, 2557, 2579, 2591, 2593, 2609, 2617, 2621, 2633, 2647, 2657, 2659, 2663, 2671, 2677, 2683, 2687, 2689, 2693, 2699, 2707, 2711, 2729, 2731, 2741, 2749, 2753, 2767, 2777, 2789, 2791, 2801, 2803, 2819, 2833, 2837, 2843, 2851, 2857, 2861, 2879, 2887, 2897, 2903, 2909, 2917, 2927, 2939, 2953, 2957, 2969, 2971, 2999, 3001, 3011, 3019, 3023, 3037, 3041, 3049, 3061, 3067, 3079, 3083, 3087, 3109, 3119, 3121, 3137, 3163, 3167, 3169, 3181, 3191, 3203, 3209, 3217, 3221, 3229, 3251, 3257, 3259, 3271, 3299, 3313, 3319, 3329, 3331, 3343, 3359, 3371, 3373, 3389, 3391, 3407, 3413, 3433, 3449, 3457, 3461, 3463, 3467, 3471, 3473, 3487, 3491, 3499, 3511, 3517, 3527, 3539, 3541, 3547, 3557, 3571, 3581, 3583, 3593, 3607, 3617, 3623, 3631, 3643, 3659, 3671, 3673, 3677, 3691, 3697, 3701, 3709, 3719, 3727, 3733, 3739, 3761, 3767, 3779, 3787, 3793, 3797, 3803, 3821, 3827, 3833, 3847, 3851, 3853, 3863, 3877, 3881, 3889, 3907, 3911, 3917, 3919, 3923, 3929, 3931, 3943, 3953, 3989, 4001, 4007, 4009, 4013, 4019, 4027, 4049, 4057, 4073, 4079, 4091, 4093, 4099, 4111, 4127, 4129, 4133, 4139, 4153, 4157, 4159, 4177, 4181, 4187, 4193, 4201, 4211, 4217, 4229, 4231, 4241, 4251, 4253, 4259, 4261, 4271, 4273, 4289, 4297, 4327, 4337, 4349, 4357, 4363, 4373, 4391, 4397, 4441, 4447, 4451, 4457, 4463, 4481, 4487, 4493, 4507, 4519, 4523, 4547, 4549, 4557, 4567, 4583, 4597, 4603, 4621, 4637, 4643, 4649, 4651, 4657, 4663, 4673, 4679, 4681, 4691, 4703, 4721, 4729, 4733, 4751, 4759, 4783, 4787, 4793, 4799, 4813, 4817, 4831, 4861, 4871, 4877, 4889, 4903, 4909, 4919, 4931, 4933, 4943, 4951, 4957, 4969, 4973, 4987, 4993, 4999, 5003, 5009, 5021, 5027, 5039, 5051, 5059, 5077, 5081, 5087, 5099, 5101, 5113, 5119, 5147, 5153, 5167, 5171, 5179, 5189, 5197, 5209, 5227, 5231, 5233, 5261, 5273, 5279, 5281, 5297, 5309, 5323, 5339, 5347, 5351, 5369, 5377, 5381, 5393, 5399, 5407, 5413, 5417, 5419, 5431, 5437, 5449, 5471, 5477, 5479, 5483, 5501, 5521, 5527, 5531, 5557, 5563, 5569, 5573, 5581, 5591, 5623, 5639, 5651, 5653, 5657, 5683, 5687, 5693, 5701, 5711, 5717, 5737, 5741, 5743, 5749, 5777, 5783, 5791, 5801, 5807, 5821, 5827, 5839, 5849, 5857, 5861, 5867, 5881, 5897, 5903, 5923, 5939, 5953, 5987, 6007, 6011, 6029, 6037, 6043, 6047, 6053, 6067, 6073, 6079, 6089, 6091, 6101, 6113, 6121, 6131, 6133, 6143, 6151, 6163, 6173, 6197, 6199, 6203, 6217, 6221, 6229, 6247, 6257, 6269, 6271, 6277, 6281, 6287, 6299, 6301, 6311, 6323, 6329, 6337, 6343, 6353, 6361, 6367, 6373, 6389, 6397, 6427, 6449, 6451, 6469, 6471, 6481, 6491, 6521, 6529, 6547, 6551, 6557, 6563, 6571, 6577, 6581, 6599, 6607, 6619, 6637, 6653, 6659, 6661, 6673, 6689, 6691, 6701, 6703, 6709, 6719, 6733, 6737, 6761, 6769, 6779, 6781, 6791, 6793, 6803, 6827, 6833, 6841, 6857, 6863, 6871, 6883, 6899, 6907, 6911, 6917, 6947, 6949, 6959, 6967, 6971, 6977, 6983, 6997, 7001, 7013, 7019, 7027, 7039, 7043, 7057, 7069, 7079, 7103, 7109, 7121, 7127, 7129, 7151, 7159, 7177, 7187, 7193, 7207, 7211, 7213, 7229, 7237, 7247, 7253, 7283, 7297, 7307, 7311, 7331, 7349, 7351, 7369, 7433, 7441, 7459, 7477, 7481, 7487, 7489, 7499, 7507, 7517, 7523, 7537, 7541, 7549, 7561, 7573, 7577, 7583, 7589, 7591, 7603, 7619, 7621, 7639, 7643, 7649, 7669, 7673, 7687, 7691, 7699, 7703, 7717, 7723, 7739, 7753, 7757, 7789, 7793, 7817, 7829, 7841, 7853, 7867, 7873, 7877, 7879, 7883, 7901, 7907, 7919, 7927, 7937, 7949, 7957, 7963, 7987, 7993, 8009, 8011, 8017, 8039, 8053, 8059, 8081, 8087, 8089, 8093, 8101, 8111, 8117, 8123, 8147, 8161, 8167, 8171, 8179, 8187, 8191, 8209, 8219, 8221, 8231, 8233, 8257, 8263, 8269, 8273, 8287, 8291, 8297, 8311, 8329, 8353, 8363, 8369, 8377, 8387, 8419, 8423, 8429, 8441, 8449, 8453, 8467, 8473, 8489, 8491, 8497, 8513, 8521, 8537, 8539, 8543, 8563, 8579, 8597, 8599, 8609, 8627, 8633, 8647, 8663, 8669, 8677, 8689, 8693, 8699, 8707, 8713, 8719, 8731, 8737, 8741, 8747, 8753, 8761, 8779, 8783, 8787, 8807, 8819, 8821, 8831, 8837, 8849, 8861, 8879, 8887, 8893, 8903, 8923, 8929, 8933, 8941, 8951, 8969, 8971, 8999, 9001, 9011, 9013, 9029, 9041, 9049, 9059, 9067, 9091, 9103, 9109, 9127, 9133, 9137, 9151, 9157, 9161, 9173, 9187, 9199, 9209, 9221, 9227, 9239, 9241, 9257, 9277, 9281, 9283, 9293, 9311, 9319, 9323, 9337, 9341, 9349, 9371, 9377, 9391, 9397, 9413, 9419, 9421, 9431, 9433, 9437, 9461, 9467, 9473, 9477, 9479, 9491, 9497, 9521, 9539, 9547, 9551, 9587, 9607, 9613, 9619, 9623, 9629, 9643, 9661, 9677, 9689, 9697, 9719, 9721, 9733, 9739, 9757, 9769, 9787, 9791, 9803, 9817, 9829, 9839, 9851, 9857, 9859, 9871, 9887, 9901, 9907, 9923, 9929, 9937, 9941, 9949, 9967, 9973, 9999);

    private BigInteger p = BigInteger.valueOf(primes.get(new Random().nextInt(primes.size())));
    private BigInteger q = BigInteger.valueOf(primes.get(new Random().nextInt(primes.size())));
    private BigInteger n = p.multiply(q);
    private BigInteger phiN = (p.subtract(BigInteger.ONE)).multiply(q.subtract(BigInteger.ONE));
    private BigInteger e = calculateE(phiN);
    private BigInteger d = calculateD(e, phiN);

    public List<BigInteger> getPublicKey() {
        return List.of(e, n);
    }

    public List<BigInteger> encryptMessage(String message) {
        List<BigInteger> encryptedMessage = new ArrayList<>();

        for (char ch : message.toCharArray()) {
            BigInteger m = BigInteger.valueOf(ch);
            BigInteger cipher = m.modPow(e, n);
            encryptedMessage.add(cipher);
        }
        return encryptedMessage;
    }

    public String decryptMessage(List<BigInteger> encryptedMessage) {
        StringBuilder decryptedMessage = new StringBuilder();

        for (BigInteger cipher : encryptedMessage) {
            BigInteger m = cipher.modPow(d, n);
            decryptedMessage.append((char) m.intValue());
        }

        return decryptedMessage.toString();
    }

    private BigInteger calculateE(BigInteger phi) {
        List<BigInteger> coPrimeList = new ArrayList<>();
        BigInteger i = BigInteger.ONE;

        while (i.compareTo(phi) < 0) {
            if (phi.gcd(i).equals(BigInteger.ONE)) {
                coPrimeList.add(i);
            }
            i = i.add(BigInteger.ONE);
        }

        return coPrimeList.get(new Random().nextInt(coPrimeList.size()));
    }

    private BigInteger calculateD(BigInteger e, BigInteger phi) {
        BigInteger t = BigInteger.ZERO;
        BigInteger newT = BigInteger.ONE;
        BigInteger r = phi;
        BigInteger newR = e;

        while (!newR.equals(BigInteger.ZERO)) {
            BigInteger quotient = r.divide(newR);

            BigInteger tempT = t;
            t = newT;
            newT = tempT.subtract(quotient.multiply(newT));

            BigInteger tempR = r;
            r = newR;
            newR = tempR.subtract(quotient.multiply(newR));
        }

        if (r.compareTo(BigInteger.ONE) > 0) {
            throw new ArithmeticException("e is not invertible");
        }
        if (t.compareTo(BigInteger.ZERO) < 0) {
            t = t.add(phi);
        }

        return t;
    }

}