Am I performing this encrypting scheme correctly?

Question

I'm trying out some encryption and I'm wondering if I'm doing it correctly.

The idea is a two layer encryption ensuring the sender is the sender, while having the proxy not knowing what the content of the message is from the sender (since the proxy knows the id of the sender). Then remove the ID of the sender and send the data to the LAST receiver which only can access the message and not the id of the sender.

I have a client that:

1. Encrypts a message with a symmetric key, and only the message content.
2. Encrypts the symmetric key with the LAST receiver's public key.
3. Encrypts the encrypted message again with a Proxy public key.

Proxy receives the encrypted data and should just verify that the sender is the actual sender. Then remove the sender's id and send it to the LAST receiver only having an encrypted message with no id of the sender which it can decrypt using the symmetric key.

What should I sign, and how should I encrypt the encrypted message and private key? In one or two separate?

Answer 1

You are performing a lot of unnecessary steps.

What you appear to want is to maintain encryption between Alice and Bob, but to use a 3rd party to verify that Alice is the sender.

You could accomplish this by:

1. Alice encrypts message with Bob's public key
2. Alice signs the message with Alice's private key (or send authentication token encrypted with Proxy's public key)
3. The Proxy receives and verifies the signature (or auth token) with Alice's public key
4. The Proxy strips the signature then passes it on to Bob.

That covers all your requirements more simply. One encryption. Sender verified. Sender remains anonymous to receiver.