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I read that encrypted emails are secure, cause only who has your public key can decrypt it.

I also read that it's a common thing to upload your public key on a keyserver.

But if my encrypted email is intercepted, the spy just needs to go on the keyserver to get my public key to be able to decrypt it. Is this right ?

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If you actually read "only who has your public key can decrypt it" somewhere, please provide the reference. –  ruief Nov 15 '13 at 12:55
    
Gilles is right.the entire security is based on your private key. This is why in a choice of encryption service it is important to choose a solution which gives you control over this private key and there is no third party involved like keyserver storage of privatekey for example. –  Marcomad Nov 29 '13 at 16:25
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2 Answers

You misread. Only someone who has your private key can decrypt your encrypted mail. Anyone with your public key can write an email and encrypt it so that only you can read it. The public key is used for encryption and the private key is used for decryption. This is the whole point of public-key cryptography.

If you exchange encrypted emails with someone, each party has his own private key and knows the other guy's public key.

If you sign your emails, it's still you using your private key and the other guy using your public key: the private key is used for signing and the public key is used to verify a signature.

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When you cipher your mail (or any other kind of message or file) using your private key, and distribute you public key, this will guaranty that you are the actual sender of the message and that it's content has not been modified (otherwise the decryption using your public key would fail). This ensure message authenticity.

To ensure privacy, you will need to encrypt your message using your partner's public key. Then, only him can would be able to decrypt it using his private key.

By this mean, by ciphering using your own private key and your partner's public key, you ensure both privacy and authenticity of your message. And, as you said, in order to work, such system needs for you public key and your partner's public key to be freely available.

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Ok, so, to work, both sender and recivier must adopt encryption, right? –  nulll Nov 15 '13 at 11:25
    
Indeed, both the sender and the recipient must share their public keys (operation to be done once, not for every email), then this allow to setup a trusted communication channel between them. –  GZBK Nov 15 '13 at 11:30
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