Metaphors are often used to describe abstract concepts. I used the metaphor of the electric drill in (1.5.1.1) Focus between 'same' and 'different'. The metaphor is a technique to promote understanding by comparing an abstract concept with a concrete thing resembling it.

For example, the public-key cryptography is like a padlock. (*43)

The metaphor is not the same thing to the concept. It is a similar thing. So there are some different parts. It is important to know which part is the same.

For example, padlocks are often made of brass. Turning the key makes the inner disc catch on the notch of the latch. However, this is a part not related to public-key cryptography.

The padlock is similar to the public-key cryptosystem in two viewpoints:

1: Even a person who does not have a key can close the lock.

2: A person who does not have a key cannot open the lock.

For example, the mail has a similar structure. You can send a mail to me even if we do not have the key of my mailbox. Of course, you cannot open my mailbox.

The metaphor helps our understanding if we use it well, but on the contrary, it may cause confusion and misunderstanding. It is important to clarify what is the same and what is different.

---

Footnotes:

*43 I explain the effect of metaphors and analogy in Chapter 6. #TODO link