Urrrgh! Help Plz

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Urrrgh! Help Plz; C programming Advice
Topic Started: Mar 7 2007, 01:36 AM (349 Views)
loki	Mar 7 2007, 01:36 AM Post #1
Advanced Member Posts: 87 Group: Members Member #48 Joined: April 28, 2006	I am not grasping how to implement CBC whatsoever, I have tried and tried and just can't figure out where i am going wrong. all that I am trying to do is implement an xor cipher with cbc, only for the learning purpose of it. But I am at my wits, I have zero idea what I am doing wrong. Could someone have a quick glance at my code and kindly slap me in the face. Or just fix it? I apologise, I should be doing this myself, but I need the help from higher experiance.
	c(x) = 3x3 + x2 + x + 2; Find the inverse


jdege	Mar 7 2007, 01:54 PM Post #2
Elite member Posts: 263 Group: Admin Member #246 Joined: December 7, 2006	loki Mar 7 2007, 01:36 AM I am not grasping how to implement CBC whatsoever, I have tried and tried and just can't figure out where i am going wrong. There's nothing wrong with your C. Well, there's a lot wrong with your C - K&R bracing, screwy indents, use of obsolete modifiers like "register" and "inline", but none of that has anything to do with why your code isn't working. Your problem is your use of the "last" variable in decrypt. Remember your formulas: Code: `C[0] = E(P[0]^IV) P[0] = D(C[0])^IV C[n] = E(P[n]^C[n-1]) P[n] = D(C[n])^C[n-1]` When you're encrypting, you have "last" storing the last output character, C[n-1]. When you're decrypting, you have "last" storing the last output character, P[n-1]. But in decrypt, you're supposed to XOR the previous ciphertext character, not the previous plaintext character. In decrypt, "last" should be set to the "current_byte", after you've decrypted "current_byte" and before you do the next fgetc().
	When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.


loki	Mar 7 2007, 03:56 PM Post #3
Advanced Member Posts: 87 Group: Members Member #48 Joined: April 28, 2006	I removed static and inline, It was a mistake from an earlier project. Opps. I prefer K&R bracing, I like it becuase its clean. Please do explain why thats not good. I made a few adjustments and Its still not working. I find it very depressing, because to me the code is right. Clearly its not, otherwise I would giggling with self achievement. I am really starting to think about making page on how to write a cipher for dummies by a dummy! I appreciate the help, thanks. I think I am going to have to printf every line until I see my error. Included is what I have done.
	c(x) = 3x3 + x2 + x + 2; Find the inverse


jdege	Mar 7 2007, 04:41 PM Post #4
Elite member Posts: 263 Group: Admin Member #246 Joined: December 7, 2006	loki Mar 7 2007, 03:56 PM I removed static and inline, It was a mistake from an earlier project. Opps. I prefer K&R bracing, I like it becuase its clean. Please do explain why thats not good. I made a few adjustments and Its still not working. I find it very depressing, because to me the code is right. Clearly its not, otherwise I would giggling with self achievement. I am really starting to think about making page on how to write a cipher for dummies by a dummy! I appreciate the help, thanks. I think I am going to have to printf every line until I see my error. Included is what I have done. Static has it's place. It's good style to keep scope as tight as possible. Register and inline were never more than hints to the optimizer, and modern optimizers don't need these kind of hints. So register and inline are no more than clutter. As for K&R bracing, some prefer it, others don't. I like being able to see a vertical match between an opening and closing brace - makes it easier for me to identify the scope of a compound statement. It's the oldest, and most pointless, flame war in the history of the C language. But enough of the style quibbles: Code: unsigned int last; unsigned int output; unsigned int current_byte; int mode; int i; case decrypt: i = 0; current_byte = fgetc(plain_text); printf("C[%d] = '%02X'\n", i, current_byte); output = baldr_decrypt(current_byte,user_key); printf("D(C[%d]) = '%02X'\n", i, output); output = output ^ iv; printf("P[%d] = '%02X'\n", i, output); fputc(output, cipher_text); last = current_byte; i += 1; printf("\n"); / while we are still reading data / while ((current_byte = fgetc(plain_text)) != EOF) { / has user key expired ? if so reset it/ if (user_key == '\0') user_key = argv[1]; printf("C[%d] = '%02X'\n", i, current_byte); output = baldr_decrypt(current_byte, user_key); printf("D(C[%d]) = '%02X'\n", i, output); output = output ^ last; printf("P[%d] = '%02X'\n", i, output); last = current_byte; user_key++; / write current byte */ fputc(output, cipher_text); i += 1; printf("\n"); } break;
	When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.


loki	Mar 7 2007, 11:27 PM Post #5
Advanced Member Posts: 87 Group: Members Member #48 Joined: April 28, 2006	Speechless, thank you so much, I can not begin to express the gratitude. I see my error, its becuase I was decrypting current_byte and updating it when I shouldn't have, the new varible output is used so this doesnt happen.
	c(x) = 3x3 + x2 + x + 2; Find the inverse


jdege	Mar 8 2007, 01:07 AM Post #6
Elite member Posts: 263 Group: Admin Member #246 Joined: December 7, 2006	loki Mar 7 2007, 11:27 PM Speechless, thank you so much, I can not begin to express the gratitude. You're welcome. Can I ask for help here, the next time I get stuck?
	When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.


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