Public Key Cryptography

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February 28, 2022 (last updated September 16, 2022)

Using a cryptographic algorithm (commonly RSA) two special numbers can be generated. A private key and a public key. The public key can be shared with everyone, including bad actors. The private key is never shared.


Let's say Alice wants to send a private message to Bob. She can encrypt the message with Bob's public key. The only way to successfully decrypt the message is with Bob's private key, which has never been shared. This guarantees the message can only be read by Bob. (Alice can still be impersonated, though.)

Digital Signatures

Let's say Alice wants to send a private message to one or more parties, and wants to ensure that receivers of the message can verify that the message came from her (which means she can't be impersonated). She can "sign" the message with her own private key (this does not reveal the private key). Now, any receiver can decrypt the message with Alice's public key.


Let's say Alice wants to send a private message to Bob and also wants Bob to verify that the message came from her. In this case, Alice can digitally sign the message first, and then encrypt the message. For example, Alice signs a message with her private key, then encrypts the signed message with Bob's public key. Then Bob would decrypt the message with his private key (ensuring only he can read it), and then "confirms the signature" using Alice's public key (ensuring she truly sent it).

Faking Public Keys

Let's say Charles is a bad actor and wants to trick Bob. Charles can't impersonate Alice so long as Alice "signs" all her messages, and Charles can't read Bob's messages so long as Bob has kept his private key safe. But, what Charles can do is give Bob a public key that Charles claims belongs to Alice, when in fact it belongs to Charles. Now, if Bob sends a message intended for Alice, but encrypted with the fake key, only Charles will be able to read it. Even if Bob eventually finds out he has a bad key, he will still send at least one message that only Charles can read. That might be enough for Charles to act badly.

This problem is well known, and is addressed in a few ways. TLS, which puts the s in https, addresses this via certificate authorities. This is why self-signed certificates are dangerous. It lets a potential bad actor authorize/legitimize their own public keys.