Kerckhoffs's Principle

Wed, 16 Apr 2025 23:53:08 +0900

Kerckhoffs’s Principle

Hello!

Today, I’d like to talk about something a bit interesting and actually incredibly important called “Kerckhoffs’s Principle”.

Ah, wait, wait. Some of you might be thinking, “I’ve never heard of ‘Kerckhoffs’s Principle,’ and there are already too many katakana words…” Don’t worry. This article is exactly for you.

What does “Secure Cryptography” mean?

For example, imagine someone telling you, “This safe can only be opened by someone who knows the secret way to open it.”

At first glance, that sounds very secure, right? But if you think about it closely, isn’t it a bit unsettling?

Like, “If that secret leaks, isn’t it all over?”

Actually, this is exactly why Kerckhoffs’s Principle comes into play.

What exactly is “Kerckhoffs’s Principle”?

To put it very simply, it’s the idea that:

“A cryptographic system should be secure even if its inner workings become public knowledge.”

In other words, “Security should rely solely on the ‘secret key’, and it’s perfectly fine for the encryption method itself to be public!”

Conversely, “A system that only relies on keeping the cryptographic algorithm (mechanism) a secret is considered to have low reliability.”

“It’s safe because the mechanism is a secret” might be a bit dangerous

A common line of thought goes like this:

“Nobody has seen the inside of this app, so the security is fine.”

I understand the sentiment. But that’s basically the same as saying, “Nobody is looking, so nobody will find any flaws, so it must be fine.”

In reality, the very fact that “nobody is looking” can become a risk in itself.

But why is this principle so important?

The reason is, if someone manages to get their hands on the mechanism and can easily decrypt it, that encryption is compromised.

To use an analogy, it’s like a door lock built with a super complex mechanism, but which can actually be opened with a spare key made at a dollar store.

It’s not “It’s safe because the key is secret”, but rather “Even if you show the entire mechanism, it can’t be opened without the proper key” that is important.

The feeling of “But isn’t that kinda scary?”

This is where many people feel:

“Isn’t it kind of scary to expose the whole mechanism?”

I get it. You can’t help but think, “If I show everything inside, won’t it be copied or misused?”

But that is exactly the core of Kerckhoffs’s Principle. The true security is the “strength that doesn’t collapse even if you show the inside”.

That being said, it takes a little courage at first

Putting yourself in the shoes of a developer, “Making the mechanism public = exposing weaknesses”, so of course, it’s nerve-wracking.

But think about it.

Something that says “Anyone is welcome to verify it” while properly showing the mechanism will ultimately be trusted more.

It’s a lot like human relationships, isn’t it?

“Someone who gets along with you after you’ve shown your true self” is, after all, the most comforting.

So

Kerckhoffs’s Principle might sound a bit theoretical, but its essence is very simple.

It’s just about: “Let’s build a mechanism that won’t break no matter who sees it.”

Proper design over superficial secrets. Code you aren’t ashamed to show, rather than code you want to hide from everyone.

Perhaps this kind of “strength” is what will be increasingly sought after in the coming era.

Thank you for reading!

Security topics can be a bit difficult, but there are parts that connect to “human relationships” and a “sense of everyday security.” Without feeling pressured, learning about it little by little will surely bring something good.

Kerckhoffs's Principle on kenji.blog