Lock and unlock with one key, symmetric encryption
Think of your front door. You lock it with that key on the way out, and unlock it with the same key on the way in. One key does both the locking and the unlocking. Symmetric encryption is exactly this. The key used to lock is the same key that unlocks. It is simple and fast. But a very thorny problem hides here. For a faraway friend to open this locked letter, the same key has to be in your friend's hands too. How do you hand over that key safely?
One key locks and unlocks
Last lesson said encryption is locking.
You lock the content so it cannot be read,
and only the right key can unlock it again.
Symmetric means that key is one and the same
for both the locking and the unlocking.
Lock a letter with that key
and it becomes unreadable to anyone.
Then apply the same key
and it returns to the original content.
The locking key and the unlocking key are one, so it is symmetric.
Lock and unlock with the same key. Locking makes it unreadable, and applying the same key again restores it.
Locking scrambled the letters into nonsense.
Then applying the same key
made it cleanly readable again.
Locking and unlocking shuttle through one key.
It is just like a single front-door key
that both locks and opens the door.
Because the structure is this simple,
an advantage comes along with it.
This method is very fast.
Fast and simple
With just one key,
the computation is simple and light.
So whether a small note
or a very large file,
it locks and unlocks in the blink of an eye.
Even bulky data like a photo or a video
is handled quickly.
Because the speed is high and the load is low,
it is a favorite method in many places.
Simplicity and speed, that is the big appeal of symmetric keys.
Pick small or large data and lock it. Even when big, the same key locks and unlocks it quickly.
Even as the data grew, the work finished quickly.
Thanks to this speed, symmetric keys
are indeed used very widely.
But here let us pause
and think carefully about one thing.
If I am only locking and unlocking my own files,
I just keep the key to myself, and that is that.
But to send a locked letter
to someone far away for them to open?
That person must also have the same key.
How do you hand over that key
Sending only the locked letter, your friend cannot open it.
Because the same key is needed to open it.
So you have to send the key to your friend too.
But if you quietly look at
the path the key travels, it gets worrying.
If someone sits in the middle of that path,
they could secretly copy the key passing by.
The moment the key leaks out,
the lock means nothing at all.
Because whoever copied it can open it just the same.
Send the key to your friend. See for yourself that if someone is on the path, the passing key can be copied.
Since someone was on the path that looked safe,
the key got copied as it was.
The copied key can unlock our lock
just the same.
Yet if you do not send the key at all,
your friend can never open the letter.
Send it and copying is the worry;
do not send it and it cannot be used.
Both of these paths are blocked.
Next, let us set this dead-end side by side and look.
Blocked if you send, blocked if you do not
Set the two forks side by side.
First, the path of sending the key.
Fast and easy, but
if it is copied along the way the lock collapses.
Second, the path of not sending the key.
No worry of copying, but
the other side has no way to open the letter.
One side is blocked by danger,
the other is blocked by uselessness.
Whichever path you take, the end is blocked, a dilemma.
Tap the two options to compare. Send and risk a copy, do not send and it cannot be opened. See both paths blocked side by side.
Whichever side you chose, it was a dead end.
This is the famous key delivery problem.
Even though symmetric keys are fast and good,
the moment a secret is not yours alone
but a secret shared between two,
a way to hand the key over safely becomes necessary.
Right at this point it was stuck for a long time.
But a wholly different idea appears
that cleverly goes around this blocked path.
That is the topic of the next lesson.
Let's wrap up
Gathered on one line, it is this.
A symmetric key uses one and the same key
to lock and to unlock.
So it is simple, and even large data is very fast.
But to share with a distant partner
you must hand over the same key,
and to send it copying is the worry,
while not sending it leaves them unable to open.
This dead-end dilemma is the key delivery problem.
A path that keeps the speed yet solves just this problem, that is the public key, coming next.
Tap the key points in order to review. (same key locks and unlocks -> fast and simple -> how to deliver the key? -> blocked whether you send or not) The next lesson's public key solves this dilemma.
Now you have seen both the light and the shade of symmetric keys.
The clear advantage of locking and unlocking fast
with one and the same key,
along with the hard-to-solve shade
of how to share that key.
The better a tool, the more you must know its weakness
to use it properly.
Next, with the clever idea of splitting
the locking key and the unlocking key into two,
let us see how this shade is lifted.