Steganography

Description
Steganography is defined as "the practice of concealing a message within another message". In computing/electronic contexts [which is what we are using], it refers to a computer file, message, image, or video that is concealed within another file, message, image, or video. This can manifest in many ways, including hidden messages in the Spectrogram of an audio file, high contrast text included in the background of an image, or words hidden in the code of different files - just to name a few possible examples.

Coagula
"Coagula is an image synth. This means that it is both a simple image editor, and a program for making sound from those images." In laymans terms, it can be used to write messages or draw images with a paintbrush tool, and then covert that image into an audio file. Afterwards, you can hide this audio file inside of a video or other medium.

Sonic Visualizer
A free, open-source application for Windows, Linux, and Mac, designed for anyone searching for a way to look at what lies inside the audio file.

Audacity
Audacity is a free, open source software. It's an easy-to-use, multi-track audio editor and recorder for Windows, macOS, GNU/Linux and other operating systems. The main useage in this context, however, is to edit audio files or look at spectrograms of audio files to see hidden messages.

SSTV Encoder [Android Only ]
An app that can be used to convert images into SSTV signals (a type of audio file).

Robot36 [Android Only ]
An app that can be used to decode SSTV signals/files.

paint.net
A free image editing program that can be used to edit images in a similar way to (albeit far more limited than) Photoshop.

Gimp
A free image editing program that can be used to edit images in a similar way to (albeit far more limited than) Photoshop. More advanced than paint.net though.

Photoshop
A top tier image editing program that is used commonly in ARG design & solving - however, it is highly expensive.

Morse Code
Morse code assigns a unique combination of signal made of short and long impulsions (usually represented with a dot " ." and a dash " -" ) to each letter, digit, or punctuation sign.

Vigenere Cipher
In the Vigenère cipher, a message is encrypted using a secret key, as well as an encryption table (called a Vigenere square, Vigenere table. The tablw recta typically contains the 26 letters of the Latin alphabet from A to Z along the top of each column, and repeated along the left side at the beginning of each row. Each row of the square has the 26 letters of the Latin alphabet, shifted one position to the right in a cyclic way as the rows progress downwards. Once B moves to the front, A moves down to the end. This continues for the entire square.

Caesar Cipher
Consists of each letter being replaced by another letter located a little further in the alphabet (therefore shifted but always the same for given cipher message). The shift distance is chosen by a number called the offset. For example, a shift of 3 would turn the letter "A" into the letter "D".

A1Z26
Consists of replacing each letter by its position in the alphabet, for example A=1, B=2, Z=26, hence its over name A1Z26.

Enigma Machine
A cipher originally invented in the 1920's and cracked during WW2. Uses a series of rotating letters (rotors) in conjunction with reflectors and a plugboard that swaps letters around. Incredibly complex, as the settings for all of these elements must be known in order to decrypt it.

Polybius Cipher
Polybius square uses a 5x5 grid filled with letters for encryption. Since the alphabet has 26 letters and the grid has 25 cells, a letter to remove is chosen, usually it's J, V , W or Z which are deleted. A key word is typically written in the first boxes in the grid, and the rest of the alphabet (minus the letters used in the key word) take up the following boxes.

Monoalphabetic Substitution Cipher
The monoalphabetical substitution consists in using a mixed alphabet (with the letters in an unusual order) and replacing the letters of the alphabet normal by it. To use this cipher, you will need to create your own scrambled alphabet (called a deranged alphabet) in order to tell the cipher how to shuffle the letters in your message. You can generate a deranged alphabet here.

Atbash Cipher
Atbash cipher (also called mirror cipher or backwards alphabet or reverse alphabet) uses a substitution alphabet and its reciprocal [reverse], a combination of the normal alphabet (ABCDEFGHIJKLMNOPQRSTUVWXYZ) and its reverse alphabet (ZYXWVUTSRQPONMLKJIHGFEDCBA).

Base64
Base 64 encoding requires a binary input. For a text, the values depend on its coding (often ASCII or Unicode). This cipher is commonly thought of as overused, unfun, and too easy by the ARG community.

Binary Code
Associates combinations of the numbers '1' and '0' with each letter of the alphabet a number. This will replace each letter by a number that can then be converted to binary. This is a type of computer language which consists of an arithmetic base conversion from base 10 to base 2. This cipher is commonly thought of as overused, unfun, and too easy by the ARG community.