9

Suppose I have the public key of a recipient I'd like to send an email to, and I've never sent this person an email before. Does it make sense to encrypt my first email to this recipient and also attach my public key? Can they decrypt my email?

(This is somewhat an "etiquette" sort of question.)

thanks

14

They don't need your public key at all to decrypt.

You need their public key to encrypt the message, which they decrypt with their private key. The only thing they need your public key for is to verify that the message comes from you. In that sense it doesn't make sense to attach your public key, unless, for instance, you plan to confirm fingerprints over the phone soon after.

2

In the asymmetric encryption scheme you can achieve two things:

  • message confidentiality : this is done by using someone's public key to encrypt a message. Only the owner of the private key can decrypt the message
  • message authentication : this is done by using your own private key to sign a message. Anyone with the public key can verify the signature of the message.

However, this problem is hindered by the fact that getting the right public key is somehow difficult. If someone was to attack the communication channel through which you pass the key, you could be fooled into thinking that the attacker own key is your recipient key. So in order to achieve authentication, you need to verify the public key first.

This is usually done through the "web of trust". If you cannot get the key from hand to hand, you look for someone you already trust (and have a trusted key) that can vouch for your recipient. If that third person already have a key for your recipient that he trusts, he can sign it for you. This way, you can assess that the key is the good one by verifying your trusted third person signature. By transitivity you have know a trusted key for your new recipient.

In your particular case: if you send your public key in the email, the recipient will have no way to verify that you are who you claim to be. For what he knows, you could be an attacker trying to impersonate someone else and forging key pairs.

1

The public key can be used to send an email securely to the holder of the corresponding private key, however the holder of the public key cannot ascertain that the message actually comes from you. Attaching your public key in the email though can problematic because he cannot verify that the public key attached actually belongs to you. If all he cares about is that subsequent emails come from the same person, then this may be fine, but if he need to care that the message actually comes from you specifically, then he would still need to consider your public key as unverified until you have another more reliable verification.

  • Yes, I realize there are identity/trust issues, but the recipient can decrypt my email, though? Or is my attached public key also encrypted?… – Geremia Mar 3 '15 at 3:38
  • Yes, only the private key holder can decrypt message encrypted with a public key. – Lie Ryan Mar 3 '15 at 3:42
  • Oh, yes, duh. Only the recipient can decrypt my message I encrypted with his public key, so my attached public key is most definitely encrypted, too. (I looked at the source of the email, and everything is one encrypted block.) – Geremia Mar 3 '15 at 18:10
1

I had the same question and after some thought I think its correct to send your public key as long as you encrypt as well.

So encrypt the following with your receivers's public key:

  • your message
  • your public key at the bottom

That way when they decrypt it, they will see your message, and your public key. This was the receiver of your message can also reply to you securely, using your public key to encrypt their reply to you.

  • why encrypt something that is meant to be public? – schroeder May 7 '17 at 10:08

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