Morse Code
Morse code is a system of communication that uses two different signal durations, called dots and dashes, to encode text characters. It was invented by Samuel F.B. Morse in the 1830s and became widely used for telegraphy, the transmission of messages over long distances by electric wires. Morse code can also be transmitted by other means, such as light, sound or radio waves. Morse code is still used today by amateur radio operators, pilots, sailors and some military forces.
"SOS" is an international distress signal, these three letters are not the abbreviation of any word, just because its code "...---..." (three dots, three long, three dots) is the easiest to send out and identify in telegrams code.
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Caesar Cipher
Caesar Cipher, also known as Shift Cipher, or Caesar Shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet.
Enigma Cipher
The Enigma Cipher was a field cipher used by the Germans during World War II. The Enigma is one of the better known historical encryption machines, and it actually refers to a range of similar cipher machines.
Hill Cipher
Hill Cipher is a polygraphic substitution cipher based on linear algebra. Invented by Lester S. Hill in 1929, it was the first polygraphic cipher in which it was practical (though barely) to operate on more than three symbols at once. It used matrices and matrix multiplication to mix up the plaintext.
Homophonic Substitution Cipher
Homophonic Substitution Cipher is a substitution cipher in which single plaintext letters can be replaced by any of several different ciphertext letters. They are generally much more difficult to break than standard substitution ciphers.