Symmetric Encryption: Definition & Example

Instructor: Martin Gibbs

Martin has 16 years experience in Human Resources Information Systems and has a PhD in Information Technology Management. He is an adjunct professor of computer science and computer programming.

This lesson will cover symmetric encryption, a well-known standard for data encryption. It is a shared-key methodology, meaning the key used to encrypt the data is the same key used to decrypt it. Data Encryption Standard (DES) is also explained.

What Is Symmetric Encryption?

Symmetric encryption is an encryption methodology that uses a single key to encrypt (encode) and decrypt (decode) data. It is the oldest and most well-known technique for encryption. The secret key can be a word, a number, or a string of letters, and it's applied to a message. The message is changed following the rules in the key. Sender and receiver know the key, and can thus code and decode any message that would use that specific key.

There are five main components of a symmetric encryption system: Plaintext, encryption algorithm, secret key, ciphertext, and the decryption algorithm.

Plaintext

The term plaintext refers to the original message that is created and sent into the encryption method; since you're bothering to encrypt it, the plaintext most likely contains sensitive data that should not be seen by prying eyes.

The symmetric encryption example image shows a simple example of how symmetric encryption works.

symmetric encryption example
symmetric encryption example

Encryption Algorithm

The algorithm takes the plaintext and converts it into an unreadable format. A simple example of an encryption algorithm would be changing all Ns to a 3, or all Zs to a 1. The routine may perform several passes and changes, called permutations, on the plaintext. Once it's encrypted, you'll need a key to unlock it.

Key

Think of the key as a decoder ring: the secret of the scrambled text cannot be read without the key. The key holds the information on all the switches and substitutions made to the original plain text.

In symmetric encryption, the key is actually bundled with the algorithm; in this sense, the decoder ring is not universal. The changes and substitutions depend on the key, and vice versa because the sender and recipient share the key.

Ciphertext

The ciphertext is the text that is now scrambled and ready to be sent. It may look like a random stream of data, and is unreadable.

Decryption Algorithm

In the decryption algorithm, the secret key (the decoder ring) is applied to the ciphertext. It converts it back to plaintext, basically performing the encryption in reverse.

Symmetric Encryption in Action: Data Encryption Standard (DES)

The Data Encryption Standard (DES) was the result of work between the US Government and IBM, and adopted as a data encryption standard in 1977.

As a symmetric encryption method, DES takes two inputs: the plaintext and the secret key (the same key is used for decryption). DES is a 64 bit block cipher, because the key works only on 64 bits of data at a time. They key itself is actually 56 bits, with 8 bits used for error checking.

Once the message is received, it is split into 64 bit blocks of data. DES carries out several iterations and substitutions throughout the message in order to make it harder to crack the code.

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