Question

I know, according to this article that I should Sign the message, then Encrypt the message.

My program operates like so:

• Get the bytes of the message
• Digitally sign the message, and store the signature in a separate byte array
• Encrypt the message
• Send the signature, then the encrypted message in a packet

Should I do it like so?

• Get the bytes of the message
• Digitally sign the message, and concatenate it with the bytes of the message
• Encrypt the array containing the message and signature
• Send the encrypted data

Appreciate the assistance

Answer 1

Digitally sign the message, and concatenate it with the bytes of the message.

You need to know where one ends and the other starts, but sure. Some APIs just take a key and a message and produce an output of bytes, and then instead of having a separate verify (data)->boolean step, they take a single bunch of bytes and either return the verified message or fail.

So yes, you can send

encrypt(
    concat(
        sign(message, signerPrivateKey), message),
        encryptionKey)

To get a verified message, the receiver has to have received two keys ahead of time: the signers public key and the decrypt key which is the same as the encryptionKey for symmetric crypto and which must be a guarded secret.

If you want to use asymmetric crypto so you only need to exchange public keys, and your message is not always shorter than a key, typically you generate a one-time use symmetric key and only encrypt that asymmetrically since asymmetric algos are typically more expensive than symmetric ones.

oneTimeUseSymmetricCryptoKey := generateKey()
concat(
    encryptAssymetric(
        oneTimeUseSymmetricCryptoKey,
        encrypterPrivateKey),
    encryptSymmetric(
        concat(sign(message, signerPrivateKey), message),
        oneTimeUseSymmetricCryptoKey))

None of this though prevents the message forwarding attack described in the link above. To do that, you need to authenticate the sender, e.g. by choosing a public key to verify the signature AND a key to decrypt based on a sender address which is arrived at independently from the exchange of encrypted bytes.