danilovlad / openssl-aes

A simple OpenSSL example of using the EVP interface to encrypt and decrypt data with aes256 CBC mode.

OpenSSL-AES

An example of using OpenSSL EVP Interface for Advanced Encryption Standard (AES) in cipher block chaining mode (CBC) with 256 bit keys. For more information visit the OpenSSL docs

Usage

Compile the code with:

root@server:~$ make
gcc main.c -g -Wall -lcrypto aes.c -o main

Reason

I saw loads of questions on stackoverflow on how to implement a simple aes256 example. So here it is! AES-256 is just a subset of the Rijndael block ciphers. Block ciphers operate on fixed sized matrices called "blocks". For AES these blocks are 4x4 matrices and each element is 1 byte (Hence 16 byte "block size"). Any message not a multiple of the block size will be extended to fill the space. This is the default behavoir for the EVP_ENCRYPTFINAL_ex functions. ie: 12 chars becomes 16 chars, 22 chars becomes 32 chars.

int enc_length = *(plaintext -> length) + (AES_BLOCK_SIZE - *(plaintext -> length) % AES_BLOCK_SIZE);

Sample Output

root@server:~$ make
gcc main.c -g -Wall -lcrypto aes.c -o main
root@server:~$ ./main
Enter a message up to 1024 chars: 
Hello World! My name is Danilo and I love you!
Key:

9A 24 EB 27 
BB FE 4B 78 
66 10 0A 26 
9F 41 A8 F1 
AC 00 F0 1C 
40 4E BB 2C 
B9 A7 18 4E 
2B 18 E4 C7 

IV:

2F 2F 66 EC 
F0 EE B0 FB 
1E 80 77 06 
94 FB A2 BB 
35 82 A9 C0 
E5 3F 59 19 
B5 F5 EE ED 
E4 A6 16 A8 

User Message:

48 65 6C 6C 
6F 20 57 6F 
72 6C 64 21 
20 4D 79 20 
6E 61 6D 65 
20 69 73 20 
44 61 6E 69 
6C 6F 20 61 
6E 64 20 49 
20 6C 6F 76 
65 20 79 6F 
75 21 0A 

Hello World! My name is Danilo and I love you!

Sending message to be encrypted...
Encrypted Message:

EF 6C 0F E6 
13 1C E0 CC 
E7 5B 46 FE 
A5 20 F4 F9 
C2 06 61 4C 
A6 43 81 3F 
CE 81 01 C1 
8C 50 60 DC 
E3 B2 89 CF 
46 A7 C8 FE 
03 4F DD E1 
11 C2 1C 58 

Sending message to be decrypted...
Decrypted Message:

48 65 6C 6C 
6F 20 57 6F 
72 6C 64 21 
20 4D 79 20 
6E 61 6D 65 
20 69 73 20 
44 61 6E 69 
6C 6F 20 61 
6E 64 20 49 
20 6C 6F 76 
65 20 79 6F 
75 21 0A 

Hello World! My name is Danilo and I love you!