completed set 4, task 31

1 files changed, 28 insertions, 21 deletions

diff --git a/set4/task28.c b/set4/task28.c
index 993d5a3..02ab0f0 100644
--- a/set4/task28.c
+++ b/set4/task28.c
@@ -9,34 +9,46 @@ int main(int argc, char **argv)
 {
 	if(argc  != 2)
 		printf("Please provide ONE key as argument!\n");
-	unsigned int hex[5];
-	unsigned int hex2[5];
+	unsigned int hmac[5];
+	unsigned int hmac2[5];
 	int i;
-	char *text = "comment1=cooking%20MCs;userdata=foo;comment2=%20like%20a%20pound%20of%20bacon";
-	char *append = ";admin=true";
-	char *key = argv[1];
-
+	unsigned char *text = "comment1=cooking%20MCs;userdata=foo;comment2=%20like%20a%20pound%20of%20bacon";
+	unsigned char *append = ";admin=true";
+	unsigned char *key = argv[1];
 	printf("Using secret key: %s\n", key);
 
-	char *padded;
-	int padding_len = sha1_padding(strlen(text), &padded);
+	unsigned char *padded;
+	int padding_len = sha1_padding(strlen(key)+strlen(text), &padded);
 
-	sha1_hmac(hex, text, strlen(text), key, strlen(key));
+	sha1_hmac(hmac, text, strlen(text), key, strlen(key));
 	printf("MAC of original message:\n");
 	for(i=0;i<5;i++)
-		printf("%02x", hex[i]);
+		printf("%02x", hmac[i]);
 
 	printf("\n");
 	
 	/*
 	 * We are appending a text to the original message without knowign the
-	 * key. Actually we don't know the message here, just the length of
-	 * the message.
+	 * key. Actually we don't know the message here, just the hash of the orginal 
+	 * message. We have to append the right padding here, e.g. the size of the
+	 * *complete* message, not only append
 	 */
-	sha1_hmac_forge(hex2, append, strlen(append), hex);
+	unsigned int new_msg_len = strlen(text)+strlen(append)+padding_len;
+	unsigned char *new_msg = malloc(new_msg_len);
+	memcpy(new_msg, text, strlen(text));
+	memcpy(&new_msg[strlen(text)], padded, padding_len);
+	memcpy(&new_msg[strlen(text)+padding_len], append, strlen(append));
+	
+	unsigned char *padding2;
+	// mesage + padding + append + padding
+	int padding2_len = sha1_padding(new_msg_len+strlen(key), &padding2);
+	unsigned char *tmp2 = malloc(strlen(append)+padding2_len);
+	memcpy(tmp2, append, strlen(append));
+	memcpy(&tmp2[strlen(append)], padding2, padding2_len);
+	sha1_hmac_forge(hmac2, tmp2, (strlen(append)+padding2_len), hmac);
 	printf("MAC of forged message:\n");
 	for(i=0;i<5;i++)
-		printf("%02x", hex2[i]);
+		printf("%02x", hmac2[i]);
 
 	printf("\n");
 	
@@ -45,12 +57,7 @@ int main(int argc, char **argv)
 	 * victim. He knows the secret and test and will think that
 	 * this is a message from Alice
 	 */
-	unsigned int new_msg_len = strlen(text)+strlen(append)+padding_len;
-	char *new_msg = malloc(new_msg_len);
-	memcpy(new_msg, text, strlen(text));
-	memcpy(&new_msg[strlen(text)], padded, padding_len);
-	memcpy(&new_msg[strlen(text)+padding_len], append, strlen(append));
-
-	if(!sha1_hmac_verify(hex2, new_msg, new_msg_len, key, strlen(key)))
+	printf("Verifying...\n");
+	if(sha1_hmac_verify(hmac2, new_msg, new_msg_len, key, strlen(key)))
 			printf("Forged MAC got accepted!\n");
 }