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What is a Cipher?

Mary Elizabeth
Updated May 17, 2024
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The American English term cipher and the chiefly British English variant, cipher, are used loosely to refer to any coded or encrypted message and more particularly to refer to messages encrypted using a secret key. The word came into English through Old French and Medieval Latin, but originates in the Arabic word which is transliterated in English as sifr, which is a variant of a word meaning “to be empty,” and referring to the zero, which comes from the same Arabic root. In the realm of cryptography—hidden writing, cipher can be a verb meaning “to encrypt a text” or a noun, referring either to the particular cryptographic system used or to the key used in that system or to the text transmitted by that system. In other words, a cipher is three different things.

When the noun cipher is used to refer to a message that has been encrypted, it is referring to what is more specifically called ciphertext. This stands in contrast to plaintext, which refers to the regular, untreated material. Plaintext is what one has both before encryption and after decryption, with ciphertext temporarily replacing the plaintext during the time that it is being protected.

In the case in which the noun cipher is used to refer to the secret key, it is referring a value that is or can be used to encrypt a plaintext message. A symmetric key is used both to encrypt and decrypt the ciphertext. When asymmetric keys are used, there is a key to encrypt the data and a separate key to decrypt the data.

This brings us to the meaning of cipher in which the system of encryption and decryption is referred to. The system that uses a symmetric key is called Private-Key Encryption. The system that uses asymmetric keys is called Public-Key Encryption. Other categorizations of cipher systems include transposition ciphers and substitution ciphers, which describe different ways of treating the plaintext. The famous cipher used by Julius Caesar and often alluded to was a simple substitution cipher. Ciphers are also categorized as block ciphers or streaming ciphers.

Ciphers are used to facilitate private communications of many types. Ciphers may be used by a government, a spy, a business, or a terrorist. Ciphers are used on the Internet for email and credit card transactions, for example. In addition to making messages unreadable by those for whom they were not intended, ciphers also assist in the authentication of messages, assuring the recipient that the message is from the sender it purports to be from.

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Mary Elizabeth
By Mary Elizabeth
Passionate about reading, writing, and research, Mary Elizabeth is dedicated to correcting misinformation on the Internet. In addition to writing articles on art, literature, and music for WiseGeek, Mary works as a teacher, composer, and author who has written books, study guides, and teaching materials. Mary has also created music composition content for Sibelius Software. She earned her B.A. from University of Chicago's writing program and an M.A. from the University of Vermont.
Discussion Comments
By ysmina — On Dec 23, 2014

When I first heard about Caesar's cipher, I was shocked about how such a great military man could use such a simple cipher for his correspondence. But then I learned that most people were illiterate at the time and ciphers were mostly unknown. So it would not have occurred to most that the correspondence was a cipher even if they saw it.

By discographer — On Dec 23, 2014

I didn't know much about ciphers until I started watching detective shows. Ciphers are a topic of discussion for a few of these shows where they show different kinds of ciphers used by criminals or terrorists.

One I learned about recently is the "skip code." It's a cipher where one must read the first word and then, every third word. To someone who doesn't know what they're looking for, sentences with this cipher sound odd and confusing. But when one reads the selected words, there is a hidden message.

By bear78 — On Dec 22, 2014

Most online stores use complex encryption for credit card information, but this does not mean that they're completely secure. A few years ago, I was the victim of fraud. My credit card information was stolen and used to make purchases. Ironically, I had only used that card to shop from well established and popular online stores. So the information had to be stolen from one of those cites.

I had my credit card canceled and I was lucky that my bank has a fraud protection system, so they paid me back. But I've been weary of giving out my credit card information online ever since then. It's possible that encryption systems may have gotten better since this incident. But I don't believe that any of these are ever 100% secure.

By Soulfox — On Dec 15, 2014

@Logicfest -- It is difficult to break computer generated ciphers, but not impossible. We hear regularly about hackers that have found their way around them and have swiped a bunch of information.

How can they do it? Computers are great at creating complex keys needed for cipher decoding, but a computer can also be used to break a cipher key. It takes some skill to develop the techniques to break those ciphers, but it can be done and a computer can be used as a tool for evil in that way.

By Logicfest — On Dec 14, 2014

@Vincenzo -- You are correct about that. We probably saw the height of "analog" cipher technology in World War II. Think of the complex machines used at the time (the Germans were notorious for using detailed ciphers that were hard to crack without the right key), but consider that the United States was still able to intercept Japanese ciphers and win the battle of Midway (the turning point of the war in the Pacific).

The point is they had some complex ciphers back then and people were still able to break them. Breaking those ciphers is a lot tougher with digital ciphers that have long, complex keys that take a computer to figure out.

By Vincenzo — On Dec 14, 2014

I'm glad to see the author pointed out how computers are now used in ciphers. Without computers to write complex keys, ciphers could get so complex that it would be close to impossible to decode them.

In fact, thanks to computers, we're probably using more ciphers than ever as they are necessary to keep financial data and such safe on the Internet. In fact, we have all benefited from advances in cipher technology and few people realize it.

Mary Elizabeth
Mary Elizabeth
Passionate about reading, writing, and research, Mary Elizabeth is dedicated to correcting misinformation on the...
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