Signthenencrypt

Sign-then-encrypt is a cryptographic approach in which a sender first signs a message with their private signing key to create a digital signature, and then encrypts the signed data with the recipient’s public key. The recipient can decrypt the ciphertext with their private key and verify the signature using the sender’s public key, confirming both confidentiality and authenticity.

In practice, the process typically involves computing a hash of the message, signing that hash with the sender’s private key, and then forming a package that contains the original message and its signature. This package is encrypted with the recipient’s public key (often as part of a hybrid scheme that uses a symmetric session key for efficiency). Upon receipt, the user decrypts the package, separates the message from the signature, and verifies the signature against the sender’s public key.

The security properties of sign-then-encrypt include confidentiality of the message and authentication of the sender, with integrity ensured by the verification of the signature. Correct implementation relies on a trusted public-key infrastructure to bind identities to keys, and the signature must be verified on the original plaintext rather than on any altered version. Non-repudiation of origin is possible if the sender’s private key remains uncompromised.

Commonly used in secure email systems such as PGP and S/MIME, sign-then-encrypt provides end-to-end protection when properly configured. Advantages include straightforward construction and strong combined guarantees of secrecy and authenticity. Limitations include potential vulnerability to certain implementation weaknesses, higher computational and messaging overhead, and the need for careful key management and certificate validation.