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I'm interested in encrypting e-mails. My very, very basic understanding is that I am to generate a private key, and share my public key with the message recipient (who will share his public key with me). Then, I can encrypt my messages utilizing his public key so that only he will be able to decrypt it.

If my understanding is correct - that's great for 1-to-1 communication. But I would like to encrypt e-mails between a group of users (specifically five users). All five users should be able to encrypt/send an email to all of the other users.

What's the best way to do this?

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    See superuser.com/a/554518/291741. The comment to the answer explains it well. – Steffen Ullrich Dec 7 '14 at 12:34
  • Your very basic understanding is wrong. Your private key is used to show other people that the email is really from you. You use the recipient's public key to encrypt a message that only he should read. Because only he has the private key required to decrypt it. Public keys are public, as the name suggests, so everyone may know that key. – Alexander Dec 7 '14 at 15:58
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PGP uses hybrid encryption. That means that the public/private key system is not used to encrypt the whole message. It is only used to encrypt a symmetric key which is used to encrypt the actual message.

When you address a PGP message to multiple recipients, you only encrypt the message once with a newly created symmetric key, and then encrypt that symmetric key multiple times, once for each intended recipient. That way each recipient can decrypt their personal copy of the symmetric key and then use it to decrypt the message.

Your message would grossly oversimplified look like this:

Symmetric key encrypted with public key of user A
Symmetric key encrypted with public key of user B
Symmetric key encrypted with public key of user C
Symmetric key encrypted with public key of user D
Symmetric key encrypted with public key of user E
Message encrypted with symmetric key

By the way: Hybrid encryption is also used when you send a message to only one recipient. The reasons is that using a different symmetric key for each transmission prevents an eavesdropper from noticing when you write the same message to the same recipient twice. It is also better for performance, because most symmetric encryption algorithms are much faster than most asymmetric ones.

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