非对称密钥加密技术咨询：公私钥唯一性及消息加密流程疑问

Great question! Let's start with a quick recap to make sure we're aligned on the fundamentals, then dive into your specific questions.

First, the core difference between symmetric and asymmetric cryptography:

• Symmetric cryptography relies on a single shared key for both encrypting and decrypting data. The big flaw here is that if that key gets stolen, an attacker can read or tamper with all messages using it.
• Asymmetric cryptography solves this by using a matched pair of keys: a public key (shared openly with anyone who needs it) and a private key (kept strictly secret by the owner). When you send a message to someone, you encrypt it with their public key, and only their private key can decrypt it—no shared secret needed upfront.

Can two users have the same private key? #

Absolutely not—this would completely destroy the security of the system.

Here's why:

• Your private key is your cryptographic "identity." It's used to decrypt messages sent to you, sign documents to prove they're from you, and authenticate yourself to services.
• If two people share the same private key, either one can decrypt messages meant for the other, forge signatures in each other's name, and fully impersonate the other person. There's no way to tell which user actually performed an action, and all confidentiality and integrity guarantees go out the window.
• Cryptographic algorithms are designed to generate private keys with such a large range of possible values that the odds of two users randomly generating the same key are astronomically low—like winning the global lottery 10 times in a row. It's technically possible in theory, but never something you'll encounter in real life.

Can two users have the same public key? #

Technically possible (if someone copies another's public key), but it's completely useless and defeats the purpose of asymmetric cryptography.

Let's break this down:

• A public key is mathematically derived directly from its paired private key. If two users have the same public key, that means either:
  1. They accidentally generated identical key pairs (again, practically impossible due to the huge key space), or
  2. One user copied the other's public key and is claiming it as their own.
• In the second scenario, if someone sends an encrypted message to the user with the copied public key, that message can only be decrypted by the original owner's private key. The user who copied the public key won't have the matching private key, so they can't read the message at all.
• In real-world systems, public keys are tied to user identities (via things like SSL certificates or digital identity systems). Duplicate public keys would create confusion, break encryption workflows, and offer zero benefits.

内容的提问来源于stack exchange，提问作者Shubham