# hackyou 2k14: Crypto - Easy one (100 points)

# cat encryptor.c
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char **argv) {
        if (argc != 3) {
                printf("USAGE: %s INPUT OUTPUT\n", argv[0]);
                return 0;
        }
        FILE* input  = fopen(argv[1], "rb");
        FILE* output = fopen(argv[2], "wb");
        if (!input || !output) {
                printf("Error\n");
                return 0;
        }
        char k[] = "CENSORED";
        char c, p, t = 0;
        int i = 0;
        while ((p = fgetc(input)) != EOF) {
                c = (p + (k[i % strlen(k)] ^ t) + i*i) & 0xff;
                t = p;
                i++;
                fputc(c, output);
        }
        return 0;
}
# cat crypto.py
#!/usr/bin/python

data = open('msg001.enc', 'rb').read()
enc = []
for byte in data[:-1]:
        enc.append(ord(byte))
plain = 'Hi! This is only test message'
i = t = 0
key = ''
for e in enc:
        x = ord(plain[i])
        k = ((e - x - (i * i)) ^ t) & 0xff
        t = x
        key += chr(k)
        i += 1
print key
# ./crypto.py
VeryLongKeyYouWillNeverGuessV
# cat decryptor.c
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char **argv) {
        if (argc != 3) {
                printf("USAGE: %s INPUT OUTPUT\n", argv[0]);
                return 0;
        }
        FILE* input  = fopen(argv[1], "rb");
        FILE* output = fopen(argv[2], "wb");
        if (!input || !output) {
                printf("Error\n");
                return 0;
        }
        char k[] = "VeryLongKeyYouWillNeverGuess";
        char c, p, t = 0;
        int i = 0;
        while ((p = fgetc(input)) != EOF) {
                c = (p - (k[i % strlen(k)] ^ t) - i*i) & 0xff;
                t = c;
                i++;
                fputc(c, output);
        }
        return 0;
}
# gcc -o decryptor decryptor.c
# ./decryptor msg002.enc msg002
# cat msg002
The known-plaintext attack (KPA) is an attack model for cryptanalysis where the attacker has samples of both the plaintext (called a crib), and its encrypted version (ciphertext). These can be used to reveal further secret information such as secret keys and code books. The term "crib" originated at Bletchley Park, the British World War II decryption operation.
The flag is CTF{6d5eba48508efb13dc87220879306619}