Clearsigned

Clearsigned, or cleartext signed, refers to a method of digitally signing a plaintext message so that the content remains human readable. In this approach, the message is signed with the sender’s private key and a separate ASCII signature block is attached, allowing readers to verify the signature without processing a binary or encoded envelope.

Practically, a clearsigned message begins with a header such as BEGIN PGP SIGNED MESSAGE and may include a line indicating the hash algorithm used (for example, Hash: SHA256). The original text appears beneath, followed by a signature block enclosed in BEGIN PGP SIGNATURE. The signature is computed over a canonical form of the signed text to reduce sensitivity to minor formatting changes, such as line breaks or whitespace adjustments.

Clearsigned messages are commonly produced and verified by OpenPGP-compatible tools, including PGP and GnuPG, and have been widely used in email signing workflows. They let recipients copy the text into an editor or mail client and verify the signature using the signer’s public key, without requiring the message to be wrapped in a separate binary or envelope.

Advantages of clearsigning include readability of the signed content and straightforward verification in plain-text channels. Limitations include sensitivity to certain formatting changes, reliance on public-key infrastructure for verification, and limited suitability for binary data or complex attachments. The concept is part of the OpenPGP specification and remains one option among signed-message formats used in digital communications. See also OpenPGP, GnuPG, and digital signatures.