年賀CTF

decrypt.c

#include <stdio.h>
#include <stdlib.h>

unsigned int decode_shl(unsigned int var, unsigned int bit, unsigned int key);
unsigned int decode_shr(unsigned int var, unsigned int bit, unsigned int key);

int main(int argc, char *argv[])
{
	unsigned int var;
	FILE *src_fp, *dest_fp;

	if (argc <= 2) {
		fprintf(stderr, "Usage: %s <InputFile(crypt)> <OutputFile(plain)>\n", argv[0]);
		exit(1);
	}

	if ((src_fp = fopen(argv[1], "r")) == NULL) {
		fprintf(stderr, "File open error: %s", argv[1]);
		exit(1);
	}

	if ((dest_fp = fopen(argv[2], "w")) == NULL) {
		fprintf(stderr, "File open error: %s", argv[2]);
		exit(1);
	}

	while (fread(&var, sizeof(unsigned int), 1, src_fp)) {
		var = decode_shl(var, 0x03, 0xcafebabe);
		var = decode_shr(var, 0x0b, 0xffffffff);
		var = decode_shr(var, 0x13, 0x725c0d30);
		var = decode_shl(var, 0x05, 0xffffffff);
		var = decode_shl(var, 0x0d, 0x63ae69a0);
		var = decode_shr(var, 0x07, 0xdeadbeef);

		fwrite(&var, sizeof(unsigned int), 1, dest_fp);
	}

	fclose(src_fp);
	fclose(dest_fp);
	return 0;
}

unsigned int decode_shl(unsigned int var, unsigned int bit, unsigned int key)
{
	unsigned int i, data;

	unsigned int decoded = 0;
	unsigned int mask    = 0;
	for (i = 0; i < bit; ++i) {
		mask = (mask << 1) | 0x1;
	}

	decoded = var & mask;
	for (; i < sizeof(unsigned int) * 8; i += bit) {
		data = decoded & mask;
		data = (data << bit) & key;
		decoded |= (var ^ data) & (mask <<= bit);
	}

	return decoded;
}

unsigned int decode_shr(unsigned int var, unsigned int bit, unsigned int key)
{
	unsigned int i, data;

	unsigned int decoded = 0;
	unsigned int mask    = 0;
	for (i = 0; i < bit; ++i) {
		mask = (mask >> 1) | 0x80000000;
	}

	decoded = var & mask;
	for (; i < sizeof(unsigned int) * 8; i += bit) {
		data = decoded & mask;
		data = (data >> bit) & key;
		decoded |= (var ^ data) & (mask >>= bit);
	}

	return decoded;
}