Substitution cipher

A substitution cipher is a way of encoding a message to make its content secret. Substitution ciphers are just one of many types of ciphers that can encode messages. Cryptology is the science of sending and receiving secret information, and cryptologists study ciphers, such as substitution ciphers. Encoding data through ciphers and other methods has been important for thousands of years. It is especially important in the modern era because digital information is encoded, or encrypted, so that other people cannot read it. In a substation cipher, the letters that make up a message are substituted for other letters, symbols, or images. However, substitution ciphers can be easy to break.

Background

Substitution ciphers are studied in cryptology, the science that researches the sending and receiving of secret information. Cryptography, which is part of cryptology, is the process of encoding and decoding messages to keep their contents private. People have been using cryptography for thousands of years. One of the earliest known uses of cryptography was from Mesopotamia in 1500 BCE. In modern times, technology has changed when and how cryptography is used. Today, cryptography helps secure digital information. Cryptography allows people to send e-mails and other communications through the Internet securely. In the twenty-first century, more people have access to high-quality encryption than ever before. People can download free encryption software on their computers and mobile devices. The software can help them keep their personal information safe and secure.

In cryptography, a message a person wants to send is called the plaintext. The plaintext is the regular information in the message before it is encoded. This text is then changed to make it readable only to people who understand the code. The changing of the plaintext is called encoding or encrypting. The message in its encrypted form is known as the ciphertext. This text contains the information that has to be decoded to determine what the plaintext says. The ciphertext alphabet is the information that will help decode the ciphertext. It is also sometimes called the key because understanding the alphabet allows one to decode the message. With the key, the ciphertext can be decoded back into plaintext.

Different types of ciphers offer different amounts of protection for messages. People using ciphers have to choose the right type of cipher based on the level of secrecy required. Most people choose complicated ciphers for more sensitive information. They must also choose a cipher that can be used easily to encode and decode messages without being broken, or figured out. When using a cipher, it is vital to keep the key secret. If someone knows the key to the cipher, that person can automatically break the code.

Overview

One common cipher is the substitution cipher. This type of cipher has a ciphertext that includes characters and symbols that are not the characters and symbols that make up the plaintext. For example, a plaintext message may be made up of English-language letters. However, the characters in the substitution ciphertext could be made up of numerals, Chinese characters, or pictures. In the same way, plaintext messages can also come in different forms. For instance, someone may send a plaintext message made up of Chinese characters or numerals. It is important for the receiver of the encoded message to understand what the plaintext message should finally be so that he or she can correctly decode the message. Many substitution ciphers have a one-to-one replacement. That is, one letter of the plaintext alphabet is replaced by one letter or symbol of the ciphertext alphabet. The ciphertext message is written just as it would be in plaintext, but the letter of the plaintext are substituted for the letter or symbol in the ciphertext alphabet. In a simple example of a substitution cipher, a person could create ciphertext alphabet by replacing each letter of the alphabet with its corresponding letter going backward. Look at this example.

Plaintext alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ

Ciphertext alphabet: ZYXWVUTSRQPONMLKJIHGFEDCBA

Plaintext: MEET ME IN THE HALLWAY IN FIVE MINUTES

Ciphertext: NVVG NV RM GSV SZOODZB RM UREV NRMFGVH

A more complicated substitution cipher could be made by including a word in the ciphertext alphabet that the sender and the receiver of the message agreed on. For instance, they could use the word LUCKY. The word is inserted at the beginning of the ciphertext alphabet, and the rest of the alphabet is filled in alphabetically after that word.

Plaintext alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ

Ciphertext alphabet: LUCKYABDEFGHIJMNOPQRSTVWXZ

Plaintext: MEET ME IN THE HALLWAY IN FIVE MINUTES

Ciphertext: IYYR IY EJ RDY DLHHVLX EJ AETY IEJSRYQ

Many substitution ciphers can be easily broken. Because substitution ciphers generally replace symbols or letters on a one-to-one basis, people who read the ciphertext can look for clues about what they are reading. For example, if a person who is reading an encoded message suspects that the plaintext is written in English, that person can look for clues in the ciphertext to help crack the code. A person looking at the ciphertext might look for single letters by themselves. These are often I or a, as these are common one-letter words in English. The person might also look for two-letter words and try to fill in common two-letter words, such as of, on, it, is, in, and at. The person might also look for the most commonly used symbol in the ciphertext. In English, the most commonly used letter is often E, but the most common letter might also be A, O, or T—depending on the message. English words have some common letter groups, such as -ion, -ed, -ing, th-, and sh-. People can also look for repeated symbols, which could be commonly repeated letters in English, such as oo, ll, or ee.

Just as someone reading a ciphertext can use tricks to help break a substitution cipher code, a person creating a substitution cipher can take measures to make the code more difficult to break. For instance, a person might remove space from between the letters in the ciphertext. A person trying to break the cipher will then not have as many tools without the spaces between the words. Furthermore, a person could replace entire letter groups with one symbol in the ciphertext alphabet. For example, the common letter groupings -ion, -ed, -ing, th-, and sh- could be replaced with single letters or single symbols.

Bibliography

Abdel-Wahab, Hussein M. "Introduction to Cryptography." Department of Compute Science, Old Dominion University, www.cs.odu.edu/~cs772/fall10/lectures/introduction‗cryptography.pdf. Accessed 27 June 2017.

Christensen, Chris. "Simple Substitution Ciphers." Northern Kentucky University, www.nku.edu/~christensen/section%201%20substitution%20ciphers.pdf. Accessed 27 June 2017.

Garsia, Adriano. "Breaking a Monoalphabetic Encryption System Using a Known Plaintext Attack." Department of Mathematics, UC San Diego, math.ucsd.edu/~crypto/java/EARLYCIPHERS/Monoalphabetic.html. Accessed 27 June 2017.

Hasinoff, Sam. "Solving Substitution Ciphers." Department of Computer Science, University of Toronto, people.csail.mit.edu/hasinoff/pubs/hasinoff-quipster-2003.pdf. Accessed 27 June 2017.

Hilton, Rod. "Automated Cryptanalysis of Monoalphabetic Substitution Ciphers Using Stochastic Optimization Algorithms." College of Engineering and Applied Science, University of Colorado Denver, cse.ucdenver.edu/~rhilton/docs/Cryptanalysis-Against-Monosub-Ciphers.pdf. Accessed 27 June 2017.

Morris, Steve. "Substitution Ciphers." Department of Mathematics, Cornell University, www.math.cornell.edu/~morris/135/135lec.pdf. Accessed 27 June 2017.

Morrison, Cecily, and Ben Roberts. "Monoalphabetic Substitution Cipher." Student-Run Computing Facility, www.srcf.ucam.org/~bgr25/cipher/monoalph.php. Accessed 27 June 2017.

Riley, David D. "Classic Cryptography." Computer Science Department, University of Wisconsin, 22 May 2010, faculty.cs.uwlax.edu/~riley/CS555Sum10/lectures/2.1‗Crypto‗Classic.pdf. Accessed 27 June 2017.

Waddell, Kaveh. "The Long and Winding History of Encryption." The Atlantic, 13 Jan. 2016, www.theatlantic.com/technology/archive/2016/01/the-long-and-winding-history-of-encryption/423726/. Accessed 27 June 2017.