密钥是一组按特定顺序排列的随机字符。加密协议使用密钥来更改数据，以便对数据进行加密，这样没有密钥的任何人都无法解码信息。

阅读本文后，您将能够：

- 了解密码学中的"密钥"是什么
- 了解密钥如何在加密过程中打乱数据
- 阐释交换公钥如何使HTTPS成为可能

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In cryptography, a key is a string of characters used within an encryption algorithm for altering data so that it appears random. Like a physical key, it locks (encrypts) data so that only someone with the right key can unlock (decrypt) it.

The original data is known as the *plaintext*, and the data after the key encrypts it is known as the *ciphertext*.

The formula:

plaintext

+ key

= ciphertext

Before the advent of computers, ciphertext was often created by simply replacing one letter with another letter in the plaintext, a method known as a "substitution cipher." For instance, suppose that someone sends a message reading "Hello" to another person, and each letter is replaced with the one after it in the alphabet: "Hello" becomes "Ifmmp."

H | E | L | L | O |

+1 | +1 | +1 | +1 | +1 |

I | F | M | M | P |

"Ifmmp" looks like a nonsensical string of letters, but if someone knows the key, they can substitute the proper letters and decrypt the message as "Hello." For this example, the key is (*letter*) - 1, moving each letter down one spot in the alphabet to arrive at the real letter.

I | F | M | M | P |

-1 | -1 | -1 | -1 | -1 |

H | E | L | L | O |

Such ciphers are relatively easy to break with simple statistical analysis, since certain letters will appear more often than others in any given text (for instance E is the most common letter in the English language). To combat this, cryptographers developed a system called the one-time pad.

A one-time pad is a single-use-only key that has at least as many values as the plaintext has characters. In other words, each letter will be replaced by a letter that's a unique number of letters removed from it in the alphabet.

For example, suppose someone has to encrypt the message "Hello," and they use a one-time pad with the values 7, 17, 24, 9, 11.

H | E | L | L | O |

7 | 17 | 24 | 9 | 11 |

Whereas before we simply moved up one position for each letter (letter + 1), now we move a different number of places in the alphabet for each letter. We add 7 to the first letter, H; we add 17 to the second letter; and so on. For any calculations that take us past Z, we simply go back to the beginning of the alphabet and keep adding.

H | E | L | L | O |

7 | 17 | 24 | 9 | 11 |

O | V | J | U | Z |

Starting from the plaintext "Hello," we now have the ciphertext "Ovjuz," using the key "7, 17, 24, 9, 11."

For communication via a one-time pad to work, both sides of the conversation have to use the same key for each individual message (symmetric encryption), although a different key is used every time there's a new message. Although to any third parties "Ovjuz" looks like random nonsense, the person who receives the ciphertext "Ovjuz" will know to use the key "7, 17, 24, 9, 11" to decrypt it (subtracting instead of adding):

O | V | J | U | Z |

7 | 17 | 24 | 9 | 11 |

H | E | L | L | O |

Thus, a simple message can be altered by a string of random data, a key, in order to be encrypted or decrypted.

Although the above examples of early cryptography illustrate how using a random string of information to turn plaintext into ciphertext works, cryptographic keys today are far more complex.

For instance, the public key for cloudflare.com is:

04 CE D7 61 49 49 FD 4B 35 8B 1B 86 BC A3 C5 BC D8 20 6E 31 17 2D 92 8A B7 34 F4 DB 11 70 4E 49 16 61 FC AE FA 7F BA 6F 0C 05 53 74 C6 79 7F 81 12 8A F7 E2 5E 6C F5 FA 10 69 6B 67 D9 D5 96 51 B0 (*Copied from the cloudflare.com SSL certificate*)

This is much more complex than the "7 17 24 9 11" key we used above to encrypt "Hello."

Instead of simply adding or subtracting, modern encryption uses complex mathematical formulas known as *algorithms*. And instead of a simple string of random numbers for a key, modern keys are typically randomized even further.

This is the case for several reasons:

- Computers are capable of far more complicated calculations in a shorter amount of time than human cryptographers, making more complex encryption not only possible, but necessary.
- Computers can alter information at the binary level, the 1s and 0s that make up data, not just at the level of individual letters and numbers.
- If encrypted data is not randomized enough, a computer program will be able to decrypt it. True randomness is extremely important for truly secure encryption. Although it is not possible to write a program that produces truly random data 100% of the time, computers are much better at it than humans are on their own.

Combined with an encryption algorithm, a cryptographic key will scramble a text beyond human recognition.

As an example, let's encrypt the message "Hello" with the cloudflare.com public key. Using an encryption algorithm with that key, we get "KZ0KVey8l1c=" as the ciphertext. (For the technically inclined: Blowfish algorithm, CBC mode, base64 encoding.)

There are two kinds of encryption: symmetric encryption and asymmetric encryption, also known as public key encryption. In symmetric encryption, both sides of a conversation use the same key for turning plaintext into ciphertext and vice versa.

In asymmetric or public key encryption, the two sides of the conversation each use a different key. One key is called the public key, and one key is called the private key – thusly named because one of the parties keeps it secret and never shares it with anyone. When plaintext is encrypted with the public key, only the private key can decrypt it, not the public key. It works the other way, too: When plaintext is encrypted with the private key, only the public key can decrypt it.

SSL (or TLS, as it is called today), is an encryption protocol used to keep Internet communications secure, and a website that is served over HTTPS instead of HTTP uses this kind of encryption. In TLS/SSL, a website or web application will have both a public key and a private key. The public key is shared publicly in the website's SSL certificate for anyone to see. The private key is installed on the origin server and never shared.

TLS/SSL communication sessions begin with a TLS handshake, during which the website and the client use the public key and the private key in order to generate new keys, which are called session keys. These session keys are then used by both sides to encrypt their messages back and forth.

Thus, TLS starts with asymmetric encryption (with two keys) and moves to symmetric encryption (with one key). Both sides use the same keys during the communication session, but when they start a new session, they will generate new keys together.

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