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Asked: 2026-05-24T18:36:22+00:00

I was looking for examples how to encode stuff in C using DES cypher

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I was looking for examples how to encode stuff in C using DES cypher and openssl headers and I found this one: http://www.codealias.info/technotes/des_encryption_using_openssl_a_simple_example

The code is almost perfect but I’m not so expert in this stuff and my C knowledge in C is not so big since I use it on PIC and AVR micro controllers…

Anyway in the code:

printf("Clear text\t : %s \n",clear);
memcpy(encrypted,Encrypt(key,clear,sizeof(clear)), sizeof(clear));
printf("Encrypted text\t : %s \n",encrypted);
memcpy(decrypted,Decrypt(key,encrypted,sizeof(clear)), sizeof(clear));
printf("Decrypted text\t : %s \n",decrypted);

As you can see, sizeof(clear) is used as the size of the string… the problem is that on the example we know the size of the text string… but when I’m sending this text over the network the other computer don’t know it…

How can solve this issue… I don’t understand so well why I need to have the size of the original string to decrypt :S

Thanks!!

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1 Answer

  1. Editorial Team

    The world is full of bad security systems designed by people who read Applied Cryptography.

    Don’t send your own ‘encryptyed’ stuff on wire. You’re missing an HMAC, you’re missing a key exchange protocol, you’re missing a wire frame protocol (which is exactly the answer to your question ‘how do I know the size’). Just use an off-the-shelf protocol like TLS/SSL. gnu-tls offers a easy to use API for SSL/TLS, openssl also supports it but is notoriously cumbersome to use. Whatever you do, don’t start writing your own protocol, you’ll come up with yet another broken ‘encryption’ protocol because of a bad key exchange or a ‘optimized nonce’ or a missing frame signature or whatever.

    Here is a simple example using gnu-tls: Simple client example using the C++ API

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