openpgp-notes/book/source/06-signatures.md

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# OpenPGP Signatures

Signatures make up the magic of OpenPGP. They act as the syntax that allows forming and interpreting rich statements about certificates and their components, as well as data.

Without signatures, there would only be loose keys, impossible to associate with a certificate, or their owner. Signatures are the glue that allows for components (component keys and identity components) to be assembled into hierarchical certificates, and for messages to gain authenticity.

## Terminology

The term *signature* can have multiple meanings in the context of OpenPGP:

- Cryptographic keys create raw signatures which are byte sequences calculated according to some signature scheme.
- [*OpenPGP signature packets*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-signature-packet-type-id-2), which combine a *type* setting, additional metadata, and a raw cryptographic signature.

For the purpose of this document, the term signature will refer to OpenPGP signature packets.

```{admonition} VISUAL
:class: warning

show our visuals for these two layers of meaning?

- "sig-circle", vs
- box with yellow tag-thing, including sig-circle
```

## Types of signatures in OpenPGP

The OpenPGP standard defines a set of [Signature Types](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-signature-types), each identified by a numerical *signature type ID*. Signature types define the intent of a signature, and how it needs to be interpreted.

```{figure} mermaid/06-terminology.png

An overview of signature types in OpenPGP
```

Most OpenPGP signature types can be classified as either:

- *Signatures over data*, or
- *Signatures on components* (that is: signatures that apply to component keys or identity components).

In this chapter, we discuss the general principles of OpenPGP signatures, which apply to all types of OpenPGP signatures.

For more detail about specific types of signatures, see the chapters {ref}`signing_data` and {ref}`component_signatures_chapter`, respectively.

## Structure of an OpenPGP signature

As outlined above, an OpenPGP signature is a composite data structure, which combines:

- A *signature type ID* (see above), which specifies the intended meaning of the signature,
- Metadata (which is variable and depends in part on the type ID),
  - Most of this metadata is encoded as so-called "subpackets," see {ref}`signature_subpackets`,
- A raw cryptographic signature.


```{admonition} VISUAL
:class: warning

show a version of our "yellow tag-thing with sig-circle" visual?
```

The cryptographic signature is calculated by its issuer. It certifies a hash digest, which in turn combines a set of input data. The exact input data depends on the signature type. Roughly: the hash digest is over the elements that the OpenPGP signature makes a statement about, combined with the metadata in the OpenPGP signature packet itself. More on this later.

(signature_subpackets)=
## Signature subpackets

Just a cryptographic signature, combined with a signature type identifier, is often not sufficiently expressive. For this reason, the OpenPGP protocol introduced signature subpackets (in [RFC 2440](https://datatracker.ietf.org/doc/html/rfc2440)).

Subpackets are well-defined data structures that can be placed into signature packets as subelements. They give additional context and meaning to a signature. Subpackets encode data in a key-value format. All possible keys are defined in the RFC as [subpacket type IDs](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-signature-subpacket-types-r), and the value format (and meaning) are defined in the RFC for each subpacket type ID.

Typical examples are:
- the *issuer fingerprint* subpacket, which contains the fingerprint of the issuer key, or
- the *key flags* subpacket which defines what purpose a component key is used for, in a certificate.

Signature subpackets can reside in two different areas of a signature packet:

- Subpackets in the *hashed area* are incorporated in the digest calculation that is done during signature calculation and are therefore covered by the cryptographic signature. In other words; hashed subpackets are *authenticated*.
- If a subpacket is placed in the *unhashed area* instead, it is not included in the signature calculation procedure and is therefore not protected against tampering. The unhashed area can be used to retroactively add, change or remove subpackets from a signature without invalidating it.  Since the unhashed area doesn't provide any cryptographic guarantees, it is only intended for advisory packets, or packets that self-authenticate (e.g. the issuer fingerprint subpacket, whose "correctness" can be proven by successfully verifying the signature using the referenced issuer key).
 
In most cases, signature subpackets are stored in the hashed area.

### Criticality of subpackets

Each signature subpacket has a flag that indicates whether the subpacket is *critical*.

Since different OpenPGP implementations might support subsets of the standard, it would be fatal if, for example, an implementation did not understand the concept of signature expiration. Such an implementation would potentially accept an already expired signature.
By marking the expiration date subpacket as critical, the user can indicate that implementations that do not understand this type of subpacket are supposed to reject the signature as invalid.

Sections 5.2.3.11 - 5.2.3.36 give guidance on which subpackets are usually marked as critical.

## Advanced topics

### Notations

### "Negotiating" signature hash algorithm based on recipients preference subpackets

### Explore viability of having multiple signatures, e.g. v4+v6?

```{admonition} TODO
:class: warning

C-R 5.2. says: An implementation MUST generate a version 6 signature when signing with a version 6 key. An implementation MUST generate a version 4 signature when signing with a version 4 key.
```