From 11b2ff584c67cf85ebe2405f6a74ab2799736927 Mon Sep 17 00:00:00 2001
From: Benedict <benedict@0xb8000.de>
Date: Thu, 4 Aug 2016 14:28:15 +0200
Subject: completed set3, task 18

---
 lib/lib3.c    | 83 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 lib/lib3.h    | 16 ++++++++++++
 set3/Makefile | 14 ++++++++++
 set3/task18.c | 20 ++++++++++++++
 4 files changed, 133 insertions(+)
 create mode 100644 lib/lib3.c
 create mode 100644 lib/lib3.h
 create mode 100644 set3/Makefile
 create mode 100644 set3/task18.c

diff --git a/lib/lib3.c b/lib/lib3.c
new file mode 100644
index 0000000..87865ed
--- /dev/null
+++ b/lib/lib3.c
@@ -0,0 +1,83 @@
+#include "lib3.h"
+#include "lib2.h"
+#include "lib.h"
+
+
+#define NR_STIRNGS_CHALLENGE17 10
+
+char *challenge17_encrypt(int *length)
+{
+	int i, t;
+	char **string;
+	char **decoded;
+	string = malloc(sizeof(char *)*NR_STIRNGS_CHALLENGE17);
+	decoded = malloc(sizeof(char *)*NR_STIRNGS_CHALLENGE17);
+	char filename[] = "task17_0";	
+
+	for(i=0;i<10;i++) {
+		filename[strlen(filename)-1] = (char) (i+'0');
+		t = read_base64_file(filename, &string[i]);
+		decoded[i] = malloc(t);
+		length[i] = decode_base64(string[i], decoded[i]);
+		printf("read: %s\n", string[i]);
+	}
+	// choose one randomly
+	int random = rand() % NR_STIRNGS_CHALLENGE17;
+	int padding;
+	printf("plaintext: %s\n", string[random]);
+	*length = strlen(string[random]);
+	char *padded_string = __pkcs7_padding(string[random], *length, 16, &padding);
+	char *encrypted = malloc(strlen(padded_string));
+	*length += padding;
+	aes_cbc(padded_string, strlen(padded_string), encrypted, key, iv, 1);
+	return encrypted;	
+}
+
+int cbc_padding_oracle(char *encrypted, int length)
+{
+	char *decrypted = malloc(length);
+	char *unpadded= malloc(length);
+	
+	aes_cbc(encrypted, length, decrypted, key, iv, 0);
+
+	int valid = valid_pkcs7_padding(decrypted, length, unpadded, 16);
+	free(decrypted);
+	free(unpadded);
+	return valid;
+
+}
+
+int convert_to_little_endian(char *string)
+{
+
+}
+/**
+  * format is: 64 nonce concat with 64 bit counter
+  * calle has to make sure that nonce is at least 8 bytes
+  * its all little endian
+  */
+int aes_ctr(char *in, int length_in, char *out, char *string_key, char *nonce)
+{
+	long counter;
+	unsigned char tmp[16];
+	char keystream[16];
+	long nr_blocks = length_in / BLOCKSIZE;
+	int length_last_block = length_in % BLOCKSIZE;
+	memcpy(tmp, nonce, 16);
+
+	for(counter=0;counter<nr_blocks-1;counter++) {
+		// right now it only works for 256 block :P 
+		tmp[8] = counter % 256;
+		// encrypt nonce and counter and produce keystream
+		aes_ecb(tmp, 16, keystream, string_key, 16, 1);
+		// xor against keystream to encrpy/decrypt
+		xor_string(&in[counter*BLOCKSIZE], keystream, &out[counter*BLOCKSIZE], 16, 16); 
+	}
+
+	// do last block
+	tmp[8] = counter % 256;
+	aes_ecb(tmp, 16, keystream, string_key, 16, 1);
+	xor_string(&in[counter*BLOCKSIZE], keystream, &out[counter*BLOCKSIZE],
+			16, length_last_block);
+
+}
diff --git a/lib/lib3.h b/lib/lib3.h
new file mode 100644
index 0000000..3078cfe
--- /dev/null
+++ b/lib/lib3.h
@@ -0,0 +1,16 @@
+#ifndef __LIB3_H__
+#define __LIB3_H__
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <openssl/aes.h>
+
+#define BLOCKSIZE 16
+
+char *challenge17_encrypt(int *length);
+int cbc_padding_oracle(char *encrypted, int length);
+int aes_ctr(char *in, int length_in, char *out, char *string_key, char *nonce);
+
+
+#endif
diff --git a/set3/Makefile b/set3/Makefile
new file mode 100644
index 0000000..d3b3f03
--- /dev/null
+++ b/set3/Makefile
@@ -0,0 +1,14 @@
+LIB=../lib/lib2.c ../lib/lib.c ../lib/lib3.c
+CC=gcc
+CFLAGS := -g $(CFLAGS) 
+CLIBS=`pkg-config --cflags --libs libcrypto`
+
+all: task17 task18
+
+task17:
+	$(CC) $(CFLAGS) task17.c $(LIB) $(CLIBS) -o task17
+
+task18:
+	$(CC) $(CFLAGS) task18.c $(LIB) $(CLIBS) -o task18
+clean:
+	rm task17 task18
diff --git a/set3/task18.c b/set3/task18.c
new file mode 100644
index 0000000..072156a
--- /dev/null
+++ b/set3/task18.c
@@ -0,0 +1,20 @@
+#include "../lib/lib.h"
+#include "../lib/lib2.h"
+#include "../lib/lib3.h"
+#include <time.h>
+
+int main()
+{
+	char *file;
+	int length_file = read_base64_file("task18.txt", &file);
+	char *ciphertext = malloc(length_file);
+	int length = decode_base64(file, ciphertext);
+
+	char *decrypted = malloc(length);
+	char *key = "YELLOW SUBMARINE";
+	char nonce[16];
+	memset(nonce, 0, 16);
+	aes_ctr(ciphertext, length, decrypted, key, nonce);
+
+	printf("plaintext: %s\n", decrypted);
+}
-- 
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