1 files changed, 129 insertions, 0 deletions
diff --git a/set1/task6.c b/set1/task6.c
new file mode 100644
index 0000000..7055044
--- /dev/null
+++ b/set1/task6.c
@@ -0,0 +1,129 @@
+#include "../lib/lib.h"
+
+/**
+ * split ciphertext in blocks of size blocksize
+ * @return returns an array of string of size with blocksize elements
+ */
+
+
+char **transpose_blocks(char *ciphertext, int blocksize, int length)
+{
+	char **blocks;
+	int i, j;
+	int number_blocks = length / blocksize;
+
+	blocks = malloc(blocksize*(sizeof(char*)));
+
+	for(i=0;i<blocksize;i++) {
+		blocks[i] = malloc(number_blocks+1);
+	}
+
+	for(j=0;j<blocksize;j++) {
+		for(i=0;i<number_blocks;i++) {
+			blocks[j][i] = ciphertext[(i*blocksize)+j];
+		}
+	}
+
+	return blocks;
+}
+
+int main()
+{
+	FILE *fp;
+	int keysize;
+	char ch;
+	char *file_content, *new_file_content, *chiphertext, *block1, *block2;
+	int file_size = 60, file_pos = 0;
+	int i; 
+	double min_hamming_distance, tmp;
+	
+	fp = fopen("6.txt", "r");
+
+	if (fp == NULL) {
+		printf("Error open file\n");
+		exit(1);
+	}
+	
+	file_content = malloc(file_size);
+
+	if (file_content == NULL) {
+		perror("out of memory");
+		exit(1);
+	}
+
+	// read data and decode it from base64
+	// result is not a hex strin or?
+
+	while ( (ch = fgetc(fp)) != EOF) {
+		// ignore new lines as this is part of base64
+		if (ch == '\n' || ch == '\r')
+			continue;
+
+		if (file_pos+1 >= file_size) {
+			new_file_content = realloc(file_content, (file_size+60));
+			if (new_file_content != NULL) {
+				file_content = new_file_content;
+				file_size += 60;
+			}
+			else {
+				printf("error allocating memory\n");
+				exit(1);
+			}
+		}
+		file_content[file_pos++] = ch;
+	}
+	file_content[file_pos] = '\0';
+
+	int ciphertext_len = 0;
+
+	chiphertext = malloc(file_pos+1);
+	ciphertext_len = decode_base64(file_content, chiphertext);
+
+	block1 = malloc(41);
+	block2 = malloc(41);
+
+	// max. hamming distacne is 100.
+	min_hamming_distance = 101;
+
+	int j=0;
+	// split ciphertext in 4 blocks of size 2 to 40
+	// and compute hamming distance of these blocks
+	for(i=2; i <= 40; i++) {
+		for(j=0;j< ciphertext_len/i;j++) {
+			memcpy(block1, &chiphertext[j], i);
+			block1[i+1] = '\0';
+			memcpy(block2, &chiphertext[j+i], i);
+			block2[i+1] = '\0';
+		
+			tmp += (double) hamming_distance_equal_length(block1, block2, i);
+		}
+		tmp = ((double)tmp / (double) (ciphertext_len/i)/ (double)i);
+		if (tmp <= min_hamming_distance) {
+			min_hamming_distance = tmp;
+			keysize = i;
+		}
+	}
+	
+	int number_blocks = ciphertext_len/keysize;
+	printf("use keysize: %i with hammind_distance: %f, number of blocks:%i\n", keysize, min_hamming_distance, number_blocks);
+	// split into keysize blcoks and transpose them 
+	char **transposed_blocks = transpose_blocks(chiphertext, keysize, ciphertext_len);
+
+	char key[keysize+1];
+	struct key_and_freq tmp_NOT_USED_HERE;
+
+	for(i=0;i<keysize;i++) {
+		key[i] = brute_force_single_byte_xor(transposed_blocks[i],
+					number_blocks, &tmp_NOT_USED_HERE);
+	}
+
+	key[keysize+1] = '\0';	
+	char *cleartext = malloc(ciphertext_len+1);
+	// xor with all single byte keys on the whole data
+	cleartext[ciphertext_len] = '\0';
+	xor_string(chiphertext, key, cleartext, keysize, ciphertext_len);
+
+	printf("%s\n", cleartext);
+	printf("used key: %s\n", key);
+	return 0;
+}