1.1 Plaintext and ciphertext

This section is part of the amber and green pathways.

Figure 2

As in previous weeks, there is some terminology we need to introduce:

• plaintext – information that can be directly read by humans or a machine (this article is an example of plaintext). Plaintext is a historic term pre-dating computers, when encryption was only used for hardcopy text, nowadays it is associated with many formats including music, movies and computer programs
• ciphertext – the encrypted data
• a cipher – the mathematics (or algorithm) responsible for turning plaintext into ciphertext and reverting ciphertext to plaintext (you might also see the word ‘code’ used – there is a technical difference between the two but it need not concern us now)
• encryption – the process of converting plaintext to ciphertext (occasionally you may see it called ‘encipherment’)
• decryption – the process of reverting ciphertext to plaintext (occasionally ‘decipherment’).

Figure 3 The functional components of encryption

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This diagram shows the information from Table 1 in visual form. It shows an upside down T-shaped funnel, where the ciphertext is inputted at one end of the T. The key and cipher are combined and fed into the funnel from above, meeting the ciphertext in the middle of the T. The encrypted text is then seen coming out of the other end.

Figure 3 The functional components of encryption