Rolling Custom Cryptographic Systems: Part 1

Cryptography– it's always the hot debate topic regarding computers, with society trying to perserve it and ensure ciphers are extremely hard to crack, to aid in the preservation of privacy (thus ensuring free speech). Governments often oppose cryptographic ciphers because of their difficulty to crack, making investigations and research on other people harder.

However, there's no denying that such systems seem very arcane and tough to understand, and this series of posts intends to shed some light on how cryptographers implement systems that are extremely hard to crack.

This post series exists to help educate people on the importance of cryptographic research and how it corresponds to your privacy online, and how you can better protect yourself in a high risk environment. I would like to give credit where it's due, as I learned most of this content from “Applied Cryptography” by Bruce Schneier.

Like a Lightswitch: Boolean Logic

So let's quickly cram an intro to computer science class into a couple paragraphs to preface this all... Boolean Logic is just a fancy term for the ability to do math with nothing more than true or false statements and a few special operations. This is achieved through the use of the binary number system, which behaves very much like the decimal system in the sense that it has a “place” for digits of a certain value. However, instead of having a 1s, 10s, 100s, etc. place, the binary system has a 1s, 2s, 4s, 8s, etc place. A binary digit is called a bit and a number that is 8 binary digits long is called a byte.

Like normal math, we can do addition, subtraction, etc to the binary numbers... But we can do more than that since binary 0 is “False” anything other than 0 is “True”. We can use AND, OR, XOR, NAND, and NOT operations on our numbers now. AND, OR & NOT are all pretty self explanatory in how they work (they take inputs and you perform said operations on them). NAND stands for NOT AND, so you basically perform AND and then invert the output value. XOR will only output true if only one input is true.

Cryptography Basics

So what is cryptography? In the most perfect sense, a cryptographic function is an algorithm that can only be reversed using one method, and is impossible to recover the original contents using any other method. However this is often not the case, and this is why security experts say nothing is 100% secure, because there will always be unknown holes in your cryptographic functions and systems.

When cryptographic functions work through taking a message and a single “key”, performing a series Boolean operations and mathematical operations to use the same key to encrypt and decrypt the message, it is called a symmetric encryption algorithm. Some of the leading symmetric algorithms (in terms of security) are AES-256, CHACHA20 and SALSA20.

If there's 2 keys, one for decryption (called a private key) and one for encryption (called a public key), it is called an asymmetric encryption algorithm. Some of the leading asymmetric algorithms are RSA and EC-Diffie Hellman.

Finally a hashing algorithm is one that takes a message as input, performs a series of operations on it, and outputs a bunch of garbled information- but if you input the same message again, you will get the same output. This is common for storing passwords and login information. Common hashing algorithms are SHA256 and SHA512.

Keys require random numbers to be created, and often times cryptographic systems rely on programs to generate random numbers for keys. The ongoing problem is that computers are incapable of being random, so there is ongoing research to produce Cryptographically Secure Pseudo-Random Number Generator software (CSPRNG). Alternatively, some people opt for Hardware-based Random Number Generators (HRNG) for producing their crypto keys.

Planning our Cryptosystem

Let's say Bob and Alice want to email each other, but they fear Eve- our eavesdropper- might be listening in. How can we securely share secret cryptographic keys in such a manner that it's impossible for Eve to get them?

Using Multiple Systems

Using some code, it's entirely possible to stitch together multiple algorithms. So it's possible that we could send EC-Diffie Hellman encrypted messages, but encrypt our public and private keys with AES-512 encryption and a personal password. So it's not theoretically possible for “Eve” to intercept the encryption and decryption keys without having to trick Bob and Alice.

To do this, we need to understand what EC-Diffie Hellman keys go where. The public key encrypts the message, while the private key decrypts the message. So for this to work, Bob would need to have Alice's public key and his private key encrypted with AES-512, while Alice would need Bob's public key and her private key encrypted also with AES-512.

To simplify this... 1) Bob and Alice generate public and private keypairs 2) Bob and Alice swap public keys. 3) Bob encrypts Alice's public key and his private key. 4) Alice encrypts Bob's public key and her private key. 5) When they wish to email, they unlock their keys. 6) After unlocking their keys, they encrypt their messages. 7) To decrypt the message, Bob or Alice unlocks their keys. 8) They then use their private key to decrypt the message.

This seems rather complex, although most of the process is automated and running behind the scenes. Software like this would manifest itself as a “keychain” or “keyring” in major programs.

The Plan

The first step, which will be shared in the next post, will be to implement a CSPRNG and a hashing algorithm so we can generate keys.

The second step will be to implement a EC-Diffie Hellman cryptographic function, using hashing algorithm and CSPRNG to aid in the generation of keys.

The third step will be to implement AES-512, which will complete the cryptosystem, and allow for encryption of the keys.

The last milestone of this project will be to provide a simple and clean interface so an end-user can encrypt their emails.


Schneier, B. (2015). Applied cryptography: Protocols, algorithms, and source code in C. Indianapolis, IN: Wiley.