This tutorial will be usefull for you if:

• you don't trust to existing symmetric ciphers (3DES, Rijndale and others);
• you wish to use really secure and fast cipher;
• you think cryptography is like a magic or very complex.

This tutorial will help you to create symmetric cipher in 30 mins.

Symmetric ciphers use the same keys for both encryption and decryption of a message. They are designed to be easily computable and able to process even large messages in real time.

Block symmetric cipher operates on fixed-length groups of bits, called blocks. Block of N bits from the plaintext should be replaced with a block of N bits from the ciphertext.

For process each block we are going to use Feistel network.

So, for create simple symmetric cipher we should:

• Define block size and split plain text into blocks
• Represent each block as positive integer number
• Define Feistel network parameters (Rounds count, Key length)
• Design Round function
• Encrypt each block with Feistel network
• Decryp cipher text

Lets start