From cb990c73c478c1bb40d749d0f4e52c10a9ac80fd Mon Sep 17 00:00:00 2001
From: Benedict <benedict@0xb8000.de>
Date: Sat, 13 Aug 2016 17:57:51 +0200
Subject: set3, challenge 24 completed

---
 lib/lib3.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 lib/lib3.h |  6 +++++
 2 files changed, 84 insertions(+)

(limited to 'lib')

diff --git a/lib/lib3.c b/lib/lib3.c
index e976fd1..150c6ea 100644
--- a/lib/lib3.c
+++ b/lib/lib3.c
@@ -203,3 +203,81 @@ int unshift_left_xor(int number, int shifts, unsigned int mask)
 	return restore;
 }
 
+
+int mt_19937_stream_cipher(char *in, int length_in, char *out, int seed)
+{
+	struct mt_19937_state mt_state;
+	char keystream;
+	int i, tmp;
+
+	mt_19937_seed(seed, &mt_state);
+
+	for(i=0;i<length_in;i++) {
+		tmp = mt_19937(&mt_state);
+		keystream = tmp & 0xFF;
+		out[i] = in[i] ^ keystream;
+	}
+}
+
+int mt_19937_stream_cipher_oracle(char *in, int length_in, char *out)
+{
+	int prefix_length = random_number_between(0,50);
+	// only 16 bit seed, 0 would not be a good seed, since the keystream
+	// would allways be 0
+	int seed = random_number_between(1,65536);
+	char *plaintext_full = malloc(length_in + prefix_length);
+
+	printf("use seed: %i\n", seed);
+	// generate random number of random bytes
+	generate_random_bytes(plaintext_full, prefix_length);
+	memcpy(&plaintext_full[prefix_length], in, length_in);
+
+	mt_19937_stream_cipher(plaintext_full, (length_in+prefix_length),
+			out, seed);
+
+	return (length_in + prefix_length);
+}
+
+
+int crack_mt_19937_stream_cipher_16_bit_seed(char *ciphertext, int length_ciphertext,
+		char *plaintext, char *match)
+{
+	int i;
+	int seed;
+	int match_len = strlen(match);
+	for(seed=1;seed<65536;seed++) {
+		mt_19937_stream_cipher(ciphertext, length_ciphertext, plaintext, seed);
+		for(i=1;i<match_len;i++) {
+			if(plaintext[length_ciphertext-i] != match[match_len-i]) {
+				break;
+			}
+			else if (i == (match_len-1))
+				goto out;
+		}
+	}
+out:
+	printf("found seed: %i\n", seed);
+}
+
+unsigned int mt_19937_password_token()
+{
+	int seed = time(NULL);
+	struct mt_19937_state mt_state;
+	printf("password token seed: %i\n", seed);	
+	mt_19937_seed(seed, &mt_state);
+
+	return mt_19937(&mt_state);
+}
+
+int mt_19937_password_token_time_based(unsigned int password_token, int time_window)
+{
+	int start_time = time(NULL);
+	struct mt_19937_state mt_state;
+	int seed;
+	for(seed=start_time-(time_window*60);seed<start_time+(time_window*60);seed++) {
+		mt_19937_seed(seed, &mt_state);
+		if(password_token == mt_19937(&mt_state))
+			return 1;
+	}
+	return 0;
+}
diff --git a/lib/lib3.h b/lib/lib3.h
index b37021d..2c65b97 100644
--- a/lib/lib3.h
+++ b/lib/lib3.h
@@ -33,4 +33,10 @@ unsigned int mt_19937_timestamp_orcale(struct mt_19937_state *mt_state);
 void mt_19937_brute_force_timestamp();
 int unshift_left_xor(int number, int shifts, unsigned int mask);
 int unshift_right_xor(int number, int shifts);
+int mt_19937_stream_cipher(char *in, int length_in, char *out, int seed);
+int mt_19937_stream_cipher_oracle(char *in, int length_in, char *out);
+int crack_mt_19937_stream_cipher_16_bit_seed(char *ciphertext, int length_ciphertext,
+		char *plaintext, char *match);
+unsigned int mt_19937_password_token();
+int mt_19937_password_token_time_based(unsigned int password_token, int time_window);
 #endif
-- 
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