Fast Recovery

[bhgomes]

After reviewing the design offline, we can use something like the following schema:

struct Note {
    ephemeral_public_key: PublicKey,
    ciphertext: Ciphertext,
    tag: u8,
}

fn create_note(ephemeral_secret_key: SecretKey, public_view_key: &PublicKey, asset: Asset) -> Note {
    let ephemeral_public_key = derive(&ephemeral_secret_key);
    Note {
        ciphertext: encrypt(agree(&ephemeral_secret_key, public_view_key), asset),
        tag: HASH(public_view_key, &ephemeral_public_key),
        ephemeral_public_key,
    }
}

fn try_decrypt(note: &Note, secret_view_key: &SecretKey, public_view_key: &PublicKey) -> Option<Asset> {
    if note.tag == HASH(public_view_key, &note.ephemeral_public_key) {
        decrypt(agree(secret_view_key, &note.ephemeral_public_key), &note.ciphertext)
    } else {
        None
    }
}

where HASH : PublicKey -> PublicKey -> u8 is some hash function that is faster than decryption.

Originally posted by @bhgomes in Manta-Network/spec#4 (comment)