Symmetric encryption
Cyber security Lesson 9 2:00 English narration · English + 中文 subtitles burned in
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Transcript
Encryption scrambles a message so that only someone with the key can read it.
加密把一条消息打乱，让只有拿着密钥的人才读得懂。
In symmetric encryption there is one key, and it does both jobs — watch, it appears twice, because it is the same key that locks at one end and unlocks at the other.
在对称加密里只有一把密钥，而且它包办两件事—— 看，它出现了两次，因为在这头上锁、在那头开锁的是同一把钥匙。
Which means sender and receiver must both know it before a single message can move.
这就意味着，在任何一条消息动起来之前， 发送方和接收方必须都已经知道它。
The oldest example is the Caesar cipher: shift every letter forward by a fixed number.
最古老的例子是凯撒密码：把每个字母往后移固定的位数。
With a shift of three, a becomes d, b becomes e, and so on all the way along.
位移量取 3，a 变成 d，b 变成 e，一路都这么推下去。
The key here is the shift itself — the number three.
这里的密钥就是位移量本身——数字 3。
And decrypting is nothing clever: it just shifts back.
解密也没什么玄机：把它移回来就是了。
So why is Caesar useless today?
那凯撒密码今天为什么没用了？
Because there are only twenty-five possible shifts, and a computer can try them all faster than you can read this sentence.
因为可能的位移只有 25 种， 计算机把它们全试一遍，比你读完这句话还快。
Watch: it walks the shifts, and one of them turns the nonsense back into English.
看：它一个个推过去，其中一个把乱码变回了英文。
Trying every key until one works is called a brute-force attack, and it is exactly what real encryption must resist.
把每一把钥匙都试一遍直到试中，这叫暴力破解攻击， 而真正的加密必须扛得住的，恰恰就是它。
Modern symmetric ciphers such as A E S fix that part completely: their keys have billions of billions of possibilities, so brute force is hopeless.
现代的对称密码，比如 AES，把这一半彻底解决了： 它们的密钥有亿亿种可能，暴力破解毫无指望。
But they leave the big problem untouched, and it is a practical one.
但它们把最大的那个问题原样留着，而且这是个很现实的问题。
How do two people share that secret key safely in the first place?
两个人一开始到底怎么把那把密钥安全地交给对方？
The next lesson solves it.
下一课解决它。
Four things to take with you.
带走四点。
One: symmetric means one key encrypts and decrypts.
第一：对称的意思是一把密钥既加密也解密。
Two: Caesar shifts each letter, and the key is the shift.
第二：凯撒密码逐字母位移，密钥就是位移量。
Three: only twenty-five keys, so brute force breaks it instantly.
第三：只有 25 把钥匙，暴力破解瞬间就破。
Four: the real problem left over is sharing the key.
第四：真正留下来的问题是如何共享密钥。
Now write a Caesar cipher in the task below.
现在去下面的题里写一个凯撒密码。