What is encryption?

Let’s start with a simple definition.

What is encryption?

Encryption is a process that transforms data to obscure its meaning — in other words, it turns it into something that appears random and nonsensical. The data to be encrypted is often referred to as the plaintext; it becomes the ciphertext when it is encrypted.

The data in question could be a secret message, an image, a voice recording, or even the entire contents of your phone. The original data can be recovered through the process of decryption.

How is encryption used?

Encryption is used to protect the information, for example, the messages that you send to your bank that contain your account details. You might also use it to encrypt your messages to friends, or the contents of your computer. It is used by organisations and governments to protect sensitive data and plans. It can also be used by individuals and organisations who wish to avoid detection, resist oppressive regimes or for nefarious purposes.

Restricting access to data

You might think about encryption as a way of restricting access to data. When data has been encrypted, it is accessible only to those who know how to reverse the encryption process or decrypt the data.

Any ciphertext can be decrypted, and for every example of encryption that you learn about in this course, you will also learn about its decryption process.

The encryption process

The encryption algorithm is the set of instructions that you must follow to modify the plaintext so that it becomes encrypted. This is often referred to as a cipher.

The key

As well as the plaintext, the encryption algorithm takes another input: a key. The key determines the output of the encryption algorithm; different keys will produce different ciphertexts. This component allows you to restrict access to the message, as only those who know the key can decrypt the message.

Keys can take many forms. In the Second World War, British spies used the first lines of poems as keys, which often made them easy to guess. In modern-day encryption, keys are most often very long strings of numbers and letters, which are generated by an algorithm.

The process of creating a key — whether by hand or through an algorithm — is called key generation.

The decryption process

To turn the ciphertext back into plaintext, you must follow the decryption algorithm. The decryption algorithm takes the ciphertext and the key as its inputs and returns the plaintext.

This algorithm might simply be the reverse of the set of instructions that were followed to encrypt the plaintext, or a completely different algorithm entirely.

The algorithms should be designed so that the decryption algorithm always turns the encrypted form of a message back into the original message.

If you make a mistake when following the encryption algorithm, the person decrypting the ciphertext will not be able to determine the plaintext.

A strong encryption scheme

If you have designed a strong encryption scheme, you should be able to let everyone know the encryption and decryption algorithms without making it easier for your enemies to hack your system. As long as the key used to encrypt a message is kept secret, only the people who know the key should be able to decrypt the message correctly.

A summary of encryption

Encryption is the process of transforming data to obscure its meaning. To do this, you need to use an encryption scheme (a cipher), which will consist of:

• An encryption algorithm
• A decryption algorithm
• And a key generation technique

The encryption algorithm takes your plaintext and a key as inputs and will output a ciphertext.

The decryption algorithm takes the key and ciphertext as inputs and will output a plaintext.

The specific key used should be kept secret so that only the intended recipients can open the message.