openpgp-notes/book/source/07-signing_data.md

(signing_data)=

Signatures over data

In OpenPGP, a data signature guarantees the authenticity and, implicitly, the integrity of certain data. Typical use cases of data signatures include the authentication of software packages and emails.

"Authenticity" in this context means that the data signature was issued by the entity controlling the signing key material. However, it does not automatically signal if the expected party indeed controls the signer certificate. OpenPGP does offer mechanisms for strong authentication, connecting certificates to specific identities. This verifies that the intended communication partner is indeed associated with the cryptographic identity behind the signature¹.

Data signatures can only be issued by component keys with the signing key flag.

Note that data signatures are distinct from {ref}component_signatures_chapter, which are used to form and maintain certificates, as well as to certify identities on certificates.

(data_signature_types)=

Signature types

OpenPGP data signatures use one of two signature types:

• Binary signature (type ID 0x00): This is the standard signature type for binary data and is typically used for files or data streams. Binary signatures are calculated over the data without any modifications or transformations.
• 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 approach mitigates issues caused by platform-specific text encodings. This is especially important for detached and cleartext signatures, where the message file might undergo re-encoding between the creation and verification of the signature.

Data signatures are generated by hashing the message content along with the metadata in the OpenPGP signature packet, and calculating a cryptographic signature over that hash. The resulting cryptographic signature is stored in the signature packet.

Data signature packets manifest in three distinct forms, which will be detailed in the subsequent section.

Forms of OpenPGP data signatures

OpenPGP data signatures can be applied in three distinct forms²:

• Detached: The OpenPGP signature exists as a separate entity, independent of the signed data.
• Inline: Both the original data and its corresponding OpenPGP signature are encapsulated within an OpenPGP container.
• Cleartext signature: A plaintext message and its OpenPGP signature coexist in a combined text format, preserving the readability of the original message.

Detached signatures

A detached signature is produced by calculating an OpenPGP signature over the data intended for signing. The original data remains unchanged, and the OpenPGP signature is stored as a standalone file. A detached signature file can be distributed alongside or independent of the original data. The authenticity and integrity of the original data file can be verified by using the detached signature file.

This signature format is especially useful for signing software releases and other files where it is imperative that the content remains unaltered during the signing process.

Inline signatures

An inline signature joins the signed data and its corresponding data signature into a single OpenPGP message.

This method is commonly used for signing or encrypting emails. Most email software capable of handling OpenPGP communications typically uses inline signatures.

Structure

An inline-signed OpenPGP message consists of three segments:

1. One-pass signature packets: These one or more packets precede the signed data and enable signature computation in one pass.

2. Literal data packet: This contains the original data (e.g., the body of a message), without additional interpretation or conversion.

3. Data signature packets: These contain the cryptographic signature corresponding to the original data.

Creation

To produce an inline signature, the signer processes the entirety of the data by reading from an input file and writing into an output OpenPGP message file. As the data is processed, the signer simultaneously calculates a cryptographic signature. This procedure results in the appending of a data signature packet to the output OpenPGP message file, where it can be efficiently stored.

For efficient verification, an application must understand how to handle the literal data prior to its reading. This requirement is addressed by the one-pass signature packets located at the beginning of inline-signed messages. These packets include essential information such as the fingerprint of the signing key and the hash algorithm used for computing the signature's hash digest. This setup enables the verifier to process the data correctly and efficiently.

:class: warning

Is the signer keyid/fingerprint in the OPS important for the verifier to be able to verify the signature efficiently? Or is it (only?) there to be hashed and signed, along with the literal data?

Realization: It's probably useful to know the fingerprints right away, to first go find the public key material, before calculating the hash of a huge file.

Verification

Inline-signed messages enable efficient verification in one pass, structured as follows:

1. Initiation with one-pass signature packets: These packets begin the verification process. They include the signer's key ID/fingerprint, essential for identifying the appropriate public key for signature validation.

2. Processing the literal data packet: This step involves hashing the literal data, preparing it for signature verification.

3. Verifying signature packets: Located at the end of the message, these packets are checked against the previously calculated hash digest.

Important to note, the signer's public key, critical for the final verification step, is not embedded in the message. Verifiers must acquire this key externally (e.g., from a key server) to authenticate the signature successfully.

Cleartext signatures

The Cleartext Signature Framework (CSF) in OpenPGP accomplishes two primary objectives:

• maintaining the message in a human-readable cleartext format, accessible without OpenPGP-specific software
• incorporating an OpenPGP signature for authentication by users with OpenPGP-compatible software

Example

The following is a detailed example of a {numref}cleartext signature:

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

hello world
-----BEGIN PGP SIGNATURE-----

wpgGARsKAAAAKQWCZT0vBCIhBtB7JOyRoU3SQKwtU+bIqeBUlJpBIi6nOFdu0Zyu
o9yZAAAAANqgIHAzoRTzu/7Zuxc8Izf4r3/qSCmBfDqWzTXqmVtsSBSHACka3qbN
eehqu8H6S0UK8V7yHbpVhExu9Hu72jWEzU/B0h9MR5gDhJPoWurx8YfyXBDsRS4y
r13/eqMN8kfCDw==
=Ks9w
-----END PGP SIGNATURE-----

This signature consists of two parts: a message ("hello world") and an ASCII-armored OpenPGP signature. The message is immediately comprehensible to a human reader, while the signature block allows for the message's authenticity verification via OpenPGP software.

Use case

Clear text signatures combine the advantages of both detached and inline signatures:

• Self-contained format: Cleartext signatures enable the message and its signature to be stored as a single file.

• Human readability: The message within a cleartext signature remains accessible in a plain text format. This eliminates the need for specialized software to read the message content.

These features are particularly beneficial in scenarios where signed messages are managed semi-manually and where existing system infrastructure offers limited or no native support for OpenPGP in the workflow³.

Text transformations for cleartext signatures

The cleartext signature framework includes specific text normalization procedures to ensure the integrity and clarity of the message:

• Escaping dashes: The framework implements a method of dash-escaped text within the message. Dash-escaping ensures that the parser correctly distinguishes between the armor headers, which are part of the signature's structure, and any lines in the message that happen to start with a dash.

• Normalization of line endings: Consistent with the approach for any other text signature, a cleartext signature is calculated on the text with normalized line endings (<CR><LF>). This ensures that the signature remains valid regardless of the text format of the receiving implementation.

Pitfalls

Despite their widespread adoption, cleartext signatures have their limitations and are sometimes viewed as a "legacy method"⁴. The RFC details the pitfalls of cleartext signatures, such as incompatibility with semantically meaningful whitespace, challenges with large messages, and security vulnerabilities related to misleading Hash header manipulations. Given these issues, safer alternatives like inline and detached signature forms are advised.

Advanced topics

Nesting of one-pass signatures

:class: warning

Write

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1. Other signing solutions, like signify, focus on pure signing without strong authentication of the signer's identity. ↩︎

2. These three forms of signature application align with GnuPG's --detach-sign, --sign, and --clearsign command options. ↩︎

3. An illustrative example is the workflow adopted by Arch Linux to certify User IDs of new packagers. This process relies on cleartext signed statements from existing packagers. These signed statements are stored as attachments in an issue tracking system for later inspection. The advantage of this approach lies in the convenience of having the message and signature in a single file, which simplifies manual handling. Based on the vouches in these cleartext signed messages and an email confirmation from the new packager, the main key operators can issue OpenPGP third-party certifications. ↩︎

4. https://lists.gnupg.org/pipermail/gnupg-devel/2023-November/035428.html ↩︎