After completing the video lectures of the Security Tube Linux 64 bit Assembler Expert course (SLAE64), a series of assessments must be completed to gain certification. This is the seventh and final assignment; build a payload encrypter/decrypter.

We have used payload encoders in previous assignments, but this time we will build a hidden payload that requires a key to decrypt.

The choice of encryption method is left to the student and I spent a good while looking at the various encryption methods:

• AES - A modern and widely used block cipher scheme. Very complex to implement and would probably require a 3rd party library, making the assignment pretty pointless.
• RC4 - A fast and relatively easy to implement stream cipher. Unfortunately, Vivek used RC4 in his demo, I didn’t want to repeat his work.
• FISH, Scream, MUGI, etc - Complex, limited implementation documentation (my maths isn’t up to scratch).

While researching these schemes I stumbled onto some of the more classical cryptography schemes.

While modern crypto schemes produce streams or blocks of pseudo-random noise which is generally XORed against the cleartext, classical ciphers tended to use relocation or shifting of characters. Simple rotational schemes such as Caesar’s cipher or ROT13 barely qualify as encryption, the encoding operation is either fixed or easily brute forced. However, a Substitution Cipher requires a mapping table and this can be varied, forming a kind of key, although a large and cumbersome one.

The Vigenère cipher is a hybrid of substitution and rotation using a table built on an ascending rotation factor. However, it also makes use of a variable length key and so it is suitable for our needs.