- Alice and Bob both generate separate EC keypairs
- Alice obtains Bob's public key, and together with her private key creates a shared secret key
- Alice encrypts a message using the shared key and some mode of authenticated encryption (AES-GCM, for example)
- Bob generates the same shared secret key using his private key and Alice's public key
- Bob decrypts the message, paying attention to whether or not the auth-tag is valid
My question is; if Bob is confident that Alice's public key really belongs to Alice, then can he be confident that it was Alice who encrypted the message? Or to put it another way, in this scenario does the auth-tag effectively serve as a signature created with a public-key? Intuitively I want to say yes, but I just want to make sure I'm not overlooking anything.