Signatures over data

A data signature guarantees the authenticity (and implicitly also the integrity) of a message, e.g., an email or a file.

Note that signatures over data are different from {ref}component_signatures_chapter, which are used to attach metadata or subkeys to a certificate.

One major use case for OpenPGP is to create signatures for software packages or emails. These are examples for signatures over data, with OpenPGP.

When signing data, OpenPGP offers the advantage that it comes with mechanisms for strong authentication, based on bindings between certificates and identities, and the option to certify those bindings.

Other signing solutions, like signify, typically only offer a solution for pure signing, without offering a mechanism for authentication.

Signature types

Two OpenPGP signature types apply to data signatures:

Signature of a binary document (Binary Signature, type ID 0x00): a universal signature type for binary data. This signature type is typically used for files or data streams. Binary signatures are calculated over the data "as is", without performing any transformations.
Signature of a canonical text document (Text Signature, type ID 0x01): used for textual data, such as email bodies. When calculating a text signature, the data is first normalized by converting line endings into a canonical form (<CR><LF>). This normalization mitigates issues caused by platform-specific default text encodings. (This can be useful for detached signatures, when the message file may get re-encoded between signature generation and validation)

Data signatures are always calculated by a component key that carries the signing key flag.

Data signatures are created by hashing the message content and calculating a cryptographic signature over the hash. The resulting cryptographic signature is stored in an OpenPGP signature packet, which can be used in different ways:

Forms of OpenPGP data signatures

OpenPGP signatures over data can be generated and distributed in three forms¹:

Detached: The signature is a standalone artifact, separate from the signed data.
Inline: The original data and the signature over the data are stored in an OpenPGP container.
Cleartext signature: A method to sign text while leaving the original message in a human-readable format.

Detached signatures

A detached signature is produced by calculating an OpenPGP signature over a piece of data. The resulting OpenPGP signature packet can then be distributed alongside or independent of the original data.

This method is especially useful for signing software releases and other files that must not be modified by the signing process.

Inline signatures

This method is usually used with signed and/or encrypted emails.

Most clients that support OpenPGP for encrypted and/or signed messages make use of inline-signatures. To produce a signature, the entirety of the data needs to be processed by the producer. This has the consequence that an application that efficiently emits signed data can only append the signature at the end of the data stream. On the other hand, an application that needs to efficiently verify signed data needs to know the signer's public key and used hash algorithm before processing the data. To solve this issue, so-called One-Pass-Signature packets are prefixed to the signed data. Those are small packets containing the fingerprint of the signing key, as well as the used hash algorithm. This is all the information a receiving application needs to know to initiate the verification process.

To produce an inline-signed message, the original data is first wrapped in a Literal Data packet, which is prefixed with one or more One-Pass-Signature packets, and affixed with the corresponding signature packets. The verifying application can read the One-Pass-Signature packets and initiate the verification process. The literal data can then be processed, such that the signatures at the end of the message can be verified in one pass.

TODO: explain nesting of OPSs.

Cleartext signatures

The Cleartext Signature Framework (CSF) is a mechanism that combines two goals:

It leaves the message in clear text format, so that it can be viewed directly by a human in a program that knows nothing about OpenPGP.
But also adds an OpenPGP signature that allows verification of that message by users whose software supports OpenPGP.

TODO: explain text transforms for cleartext signatures (LF->CRLF etc)

Pitfalls

The RFC points out a number of pitfalls of cleartext signatures, and advises that in many cases, the inline and detached signature forms are preferable.

These signature forms correspond with GnuPG's --detach-sign, --sign and --clear-sign modes. ↩︎

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