One time pad, encryption and decryption

cryptography, encryption, java, one-time-pad

Solution

Here you have a full working example:

    // convert secret text to byte array
    final byte[] secret = "secret".getBytes()

    final byte[] encoded = new byte[secret.length];
    final byte[] decoded = new byte[secret.length];

    // Generate random key (has to be exchanged)
    final byte[] key = new byte[secret.length];
    new SecureRandom().nextBytes(key);

    // Encrypt
    for (int i = 0; i < secret.length; i++) {
        encoded[i] = (byte) (secret[i] ^ key[i]);
    }

    // Decrypt
    for (int i = 0; i < encoded.length; i++) {
        decoded[i] = (byte) (encoded[i] ^ key[i]);
    }

    assertTrue(Arrays.equals(secret, decoded));

Problem

I am trying to pick up cryptography and had been trying this exercise Write a program (preferably Java) to generate a one-time pad, which is a relatively large file of all random data (say 1 MB). The program should also be able to encrypt/decrypt files based on the generated one time pad. Tip: use the following test vector to check if your program does encryption correctly. Plaintext (ASCII): Every cloud has a silver lining OTP (HEX): 6dc72fc595e35dcd38c05dca2a0d2dbd8e2df20b129b2cfa29ad17972922a2 ciphertext (HEX): 28b14ab7ecc33ea157b539ea426c5e9def0d81627eed498809c17ef9404cc5 I have tried to generate a one time pad using random number generator as I need to convert them to HEX form. and I am pretty sure I am confused or not tackling it the right way ``` public static void oneTimePad() { Random ran = new Random(); String s = "0123456789ABCDEF"; for(int i = 0; i < 100; i++) { System.out.print(s.charAt(ran.nextInt(s.length()))); } } ``` Above would be my one time pad, and I was wondering how any idea how I could implement the encryption using the one time pad and decrypting it.

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