From bd40b9cd4f6436df1b249b0904845a404b903ffd Mon Sep 17 00:00:00 2001
From: Benedict <benedict@0xb8000.de>
Date: Mon, 1 Aug 2016 15:24:33 +0200
Subject: compledted set2, challenge 16

---
 set2/Makefile |  6 ++++--
 set2/task16.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 2 deletions(-)
 create mode 100644 set2/task16.c

(limited to 'set2')

diff --git a/set2/Makefile b/set2/Makefile
index a2d66d4..914471a 100644
--- a/set2/Makefile
+++ b/set2/Makefile
@@ -3,7 +3,7 @@ CC=gcc
 CFLAGS := -g $(CFLAGS) 
 CLIBS=`pkg-config --cflags --libs libcrypto`
 
-all: task9 task10 task11 task12 task13 task15
+all: task9 task10 task11 task12 task13 task15 task16
 
 task9:
 	$(CC) $(CFLAGS) task9.c $(LIB) $(CLIBS) -o task9
@@ -17,5 +17,7 @@ task13:
 	$(CC) $(CFLAGS) task13.c $(LIB) $(CLIBS) -o task13
 task15:
 	$(CC) $(CFLAGS) task15.c $(LIB) $(CLIBS) -o task15
+task16:
+	$(CC) $(CFLAGS) task16.c $(LIB) $(CLIBS) -o task16
 clean:
-	rm task9 task10 task11 task12 task13 task15
+	rm task9 task10 task11 task12 task13 task15 task16
diff --git a/set2/task16.c b/set2/task16.c
new file mode 100644
index 0000000..f60205e
--- /dev/null
+++ b/set2/task16.c
@@ -0,0 +1,48 @@
+#include "../lib/lib2.h"
+#include "../lib/lib.h"
+/**
+  * produces an identical bit error in the following block
+  * this means: we can control the whole content of the following
+  * block, when the counterpart decrpyts it
+  * If there is a 0 in the second block and we want a 1, then flip
+  * the bit in the prior block at the same position
+  * If there is a 0 and we want a null, than do nothing
+  * For one respectivally.
+  *
+  * Since ; and = are escaped we need charecters so that with one flip
+  * we can get ; and =.
+  * for ; use : in the plaintext. in the ciphtertext we need to flip the
+  * last bit of : and the resulting plaintext get ;
+  * for = we use <. we also need to flip the last bit
+  *
+  * so out plaintext we give the orcale function is :admin<true:
+  *
+  *
+  *
+  */
+int main(int argc, char **argv)
+{
+	// initialize key
+	generate_random_bytes(key, 16);
+	memset(iv, 0, 16);
+	//generate_random_bytes(iv, 16);
+	char *encrypted;
+	// one block of our input
+	int length = challenge16_encrypt(":admin<true:1234", &encrypted);
+
+	printf("first the unchanged string:\n");
+	challenge16_decrypt(encrypted, length);
+	/**
+	  * change ciphertext here
+	  * we now that our text start a the third block, because the 
+	  * challenge16_encrypt function prepends a string
+	  * we need to flip three bits, the 8th bit, 56th bit and the
+	  * 96th bit
+	  */
+	encrypted[32-16] ^= 0x01;
+	encrypted[38-16] ^= 0x01;
+	encrypted[43-16] ^= 0x01;
+
+	printf("now the changed string:\n");
+	challenge16_decrypt(encrypted, length);
+}
-- 
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