OpenPGP fundamentally hinges on the concept of "OpenPGP certificates," also known as "OpenPGP keys." These certificates are complex data structures essential for identity verification, data encryption, and digital signatures. Understanding their structure and function is pivotal to effectively applying the OpenPGP standard.
The term "(cryptographic) keys" is central to grasping the concept of OpenPGP certificates. However, it can refer to different entities, making it a potentially confusing term. Let's clarify those differences.
The term "key," without additional context, can refer to either public or private asymmetric key material. Additionally, symmetric keys may be used in OpenPGP to encrypt private key material, adding a layer of security and complexity.
1. A (bare) ["cryptographic key"](asymmetric_key_pair) comprises the private and/or public parameters forming a key. For instance, in the case of an RSA private key, the key consists of the exponent `d` along with the prime numbers `p` and `q`.
2. An OpenPGP *component key* includes either an "OpenPGP primary key" or an "OpenPGP subkey." It is a building block of an OpenPGP certificate, consisting of a cryptographic keypair coupled with some invariant metadata, such as key creation time.
3. An "OpenPGP certificate" (or "OpenPGP key") consists of several component keys, identity components, and other elements. These certificates are dynamic, evolving over time as components are added, expire, or are marked as invalid.
The following section will delve into the OpenPGP-specific layers (2 and 3) to provide a clearer understanding of their roles within OpenPGP certificates.
For a discussion of private key material in OpenPGP, see the chapter {ref}`private_key_chapter`. Bindings that connect the components of a certificate are discussed in our chapter {ref}`component_signatures_chapter`. For much more detail on the internal (packet) structure of certificates and keys refer to our chapter {ref}`zoom_certificates`. Additionally, managing certificates, and understanding their authentication and trust models are vital topics. While this document briefly touches upon these aspects, they are integral to working proficiently with OpenPGP.
[^packets]: In technical terms, the elements of an OpenPGP certificate are a collection of "packets." Each component key and identity component is internally represented as a packet. Another common type of packet is the "signature" packet, which connect the components of a certificate.
:alt: Depicts a box with white background and the title "OpenPGP certificate". In the box several other boxes and accompanying texts, representing component keys and User IDs, are shown. There are three component keys boxes with a green frame, each with a dotted lower-left section, that shows the text "key creation time" and the green public key symbol in the lower right area. All three have a title, a unique fingerprint below the box and a unique capability keyword, perpendicular to the box on the right side. The top-most component key box has a light-green background, with the title "Component Key (primary)" and capability keyword "certification". The second-to-top component key box has a white background, with the title "Component Key" and capability keyword "encryption". The lowest component key box has a white background, with the title "Component Key" and capability keyword "signing". There are two User ID boxes, each with a black frame, open to top left and lower right corner. Both boxes have a user icon on the top left side, the title "User ID" on the top right side and a User ID string at the bottom. The top box has "Alice Adams <alice@example.org>" and the lower box has "Alice" as User ID string.
Every element in an OpenPGP certificate revolves around a central component: the *OpenPGP primary key*. The primary key acts as a personal CA (Certification Authority) for the certificate's owner, enabling cryptographic statements regarding subkeys, identities, expiration, revocation, and more.
OpenPGP certificates tend to have a long lifespan, with the potential for modifications (typically by their owner) over time. Components may be added or invalidated throughout a certificate's lifetime.
An OpenPGP certificate usually contains multiple component keys. Component keys serve in one of two roles: either as an "OpenPGP primary key" or as an "OpenPGP subkey."
OpenPGP component keys logically consist of an [asymmetric cryptographic keypair](asymmetric_key_pair) and a creation timestamp. Once created, these attributes of a component key remain fixed (for ECDH keys, two additional parameters are part of a component key's constitutive data[^ecdh-parameters]).
[^ecdh-parameters]: For [ECDH](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-part-for-ecd) component keys, two additional algorithm parameters are integral to the component key's constitutive and immutable properties. Those parameters specify a hash function and a symmetric encryption algorithm.
:alt: Depicts a box with white background and no title. In the box one other box is shown. The inner box has a green frame, with a dotted lower-left section, that shows the text "key creation time" and the green public key symbol, as well as the red-dotted private key symbol in the lower right area. In the top left of the inner box the text reads "Component Key".
Component keys containing private key material also include metadata specifying the password protection scheme. This is another facet of metadata, akin to the aforementioned creation timestamp and additional parameters for certain algorithms. However, this discussion focuses on OpenPGP certificates, in which the component keys contain only the public part of its cryptographic key data. For information on private keys in OpenPGP, see {numref}`private_key_chapter`.
Each OpenPGP component key possesses an *OpenPGP fingerprint*. This fingerprint is derived from the public key material, the creation timestamp, and, when relevant, the ECDH parameters.
:alt: Depicts a box with white background and the title "Fingerprint of an OpenPGP component key". Inside, another box with a green frame, the title "Component Key", the text "key creation time" on the lower left and a the green public key symbol on the lower right is shown. Below the component key box a fingerprint in a box with a light-yellow background and a yellow dotted line is depicted. The word "Fingerprint" is shown left of the box with the fingerprint and both are connected with a yellow dotted line.
Every OpenPGP component key is identifiable by a fingerprint. Although it's technically possible for different keys to share a fingerprint, cryptographic mechanisms make it exceedingly difficult, if not practically impossible with current technology, to find keys that share a fingerprint.
[^keyid]: In OpenPGP version 4, the rightmost 64 bits were sometimes used as a shorter identifier, called "Key ID."
For example, an OpenPGP version 4 certificate with the fingerprint `B3D2 7B09 FBA4 1235 2B41 8972 C8B8 6AC4 2455 4239` might be referenced by the 64-bit Key ID `C8B8 6AC4 2455 4239` or formatted as `0xC8B86AC424554239`.
Historically, even shorter 32-bit identifiers were used, like this: `2455 4239`, or `0x24554239`. Such identifiers still appear in very old documents about PGP. However, [32-bit identifiers have been long deemed unfit for purpose](https://evil32.com/). At one point, 32-bit identifiers were called "short Key ID," while 64-bit identifiers were referred to as "long Key ID."
While subkeys have the same structural attributes as the primary key, they fulfill different roles. Subkeys are cryptographically linked with the primary key, a relationship further discussed in {numref}`binding_subkeys`.
:alt: Diagram depicting three component keys. The primary key is positioned at the top, designated for certification. Below it, connected by arrows, are two subkeys labeled as "for encryption" and "for signing," respectively.
Identity components in an OpenPGP certificate are used by the certificate holder to state that they are known by a certain identifier (like a name, or an email address).
OpenPGP certificates can contain multiple [User IDs](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-user-id-packet-tag-13). Each User ID associates the certificate with an identity.
:alt: Depicts a diagram with white background and the title "User IDs". Inside, a public primary component key for certification and a User ID is shown. A green arrow points from component key to User ID and is annotated with a signature.
A typical User ID identity is a UTF-8-encoded string composed of a name and an email address. By convention, User IDs align with the format described in [RFC2822](https://www.rfc-editor.org/rfc/rfc2822) as a *name-addr*.
For further conventions on User IDs, refer to the document [draft-dkg-openpgp-userid-conventions-00](https://datatracker.ietf.org/doc/draft-dkg-openpgp-userid-conventions/), dated 25 August 2023.
One proposed variant for encoding identities in User ID is to use ["split User IDs"](https://dkg.fifthhorseman.net/blog/2021-dkg-openpgp-transition.html#split-user-ids). This style of User IDs is currently uncommon, but there is no technical impediment to using this format right now.
An argument for split User IDs is that a name and an email address are two distinct identities, which are easier to reason about separately. This is particularly relevant when third parties consider certifying that an identity is legitimately connected to a certificate.
For example, some third party may be sure about the email identity of a contact, and happy to issue a certification for an email-based identity (such as `<alice@example.org>`). But they may not have any insight into a name based identity (such as `Alice Adams`), and thus not willing to certify such a name-based identity.
Within a certificate, a specific User ID is designated as the [Primary User ID](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-primary-user-id).
Each User ID carries associated preference settings, such as preferred encryption algorithms, which is detailed in {numref}`zooming_in_user_id`). The preferences associated with the Primary User ID take precedence by default.
[user attributes](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-user-attribute-packet-tag-1) are similar to User IDs, they are less commonly used.
Currently, the OpenPGP standard prescribes only one format to be stored in user attributes: an [image](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-the-image-attribute-subpack). Typically, this image represents the key owner, although it is not required.
To form an OpenPGP certificate, individual components are interconnected by the certificate holder using their OpenPGP software. Within OpenPGP, this process is termed "binding," as in "a subkey is bound to the primary key." These bindings are realized using cryptographic signatures. An in-depth discussion of this topic can be found in {ref}`component_signatures_chapter`).
By binding components using digital signatures, recipients of an OpenPGP certificate need only validate the authenticity of the primary key to use for their communication partner. Traditionally, this is done by manually verifying the *fingerprint* of the primary key. Once the validity of the primary key is confirmed, the validity of the remaining components can be automatically assessed by the user's OpenPGP software. Generally, components are valid parts of a certificate if there is a statement signed by the certificate's primary key endorsing this validity.
Much of the metadata in OpenPGP certificates is actually not stored inside the components that the metadata applies to. Instead, much of the metadata for certificates, component keys and identities is defined as a part of the signatures that join components into a certificate.
For example, the capabilities of a component key (such as *signing* or *encryption*), or the expiration time of a component key, are not encoded as a part of the *packet* that encodes the data of that component key.
Instead, are stored using mechanisms that bind components into an OpenPGP certificate:
- For the primary key, its key flags and other metadata can be defined in two ways: they can be linked with the [Primary User ID](primary_user_id) or through a [direct key signature](direct_key_signature).
- For subkeys, the key flags and other metadata are set using the mechanism that binds the subkey to the certificate, specifically through the primary key. Further details on [binding subkeys](binding_subkeys) are below.
- For identity components, like User IDs, metadata is associated via the [certifying self-signature](bind_ident) that links the identity to the certificate.
### Defining operational capabilities of component keys with key flags
Each component key has a set of ["key flags"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-flags) that delineate the operations a key can perform.
- **Authentication**: primarily used for OpenPGP authentication
```{note}
Distinct component keys handle specific operations. Only the primary key can be used for certification, although it can have additional capabilities. Subkeys can be used for signing, encryption, and authentication but cannot have the certification capability. It is considered good practice, however, to [use separate keys for each capability](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#section-10.1.5-7).
Notably, in many algorithms, encryption and signing-related functionalities (i.e., certification, signing, authentication) are mutually exclusive, because the algorithms only support one of those two families of operations[^key-flag-sharing]).
```
[^key-flag-sharing]: With ECC algorithms, it's impossible to combine encryption functions with those intended for signing. For example, ed25519 is specifically used for signing; cv25519 is designated for encryption.
OpenPGP has a lot of ["cryptographic agility"](https://en.wikipedia.org/wiki/Cryptographic_agility). That is, OpenPGP doesn't just use one fixed set of algorithms, but defines a suite of cryptographic primitives that users (or their applications) can pick from.
This agility has the advantage that adoption of new cryptographic primitives into the standard is relatively easy, and can be done without disruption. Users can gradually migrate to using new cryptographic mechanisms.
However, it also means that OpenPGP software needs to figure out which mechanisms a set of communication partners can handle and prefers. To do this, there are a number of mechanisms in OpenPGP that can be negotiated between sender and recipient. The sender interprets the preferences of the recipient:
In addition to these explicitly expressed preferences, implementations also deduce capabilities of communication partners based on the OpenPGP version of the certificate that they write to.
#### User ID-specific preferences
As a starting point, a certificate has a set of preferences that apply generally. These are defined either in a direct key signature, or via the primary User ID of the certificate.
Additionally, OpenPGP allows modeling User ID-specific preferences. The idea is that a user may prefer a different suite of algorithms on their private email account compared to their work email account. Such identity-specific preferences can be expressed on the certifying signatures that bind User IDs to a certificate.
When the owner of a certificate wants to invalidate some components of that certificate, or the entire certificate, they can do so by "revoking" the component in question. Revoking the primary key renders the entire certificate invalid.
More on revoking components of a certificate in {ref}`self-revocations`.
Note that there are other ways besides revocations in which components can become invalid. For example, the component's expiration time may have passed.
Third-party identity certifications have been a pivotal mechanism in the OpenPGP ecosystem since the beginning. The designers of PGP, beginning with Phil Zimmermann, have favored decentralized trust models, which don't hinge on centralized authorities.
Third-party certifications are statements by OpenPGP users who attest that they have confirmed that a particular OpenPGP certificate belongs to a user with a particular identity.
For example, Bob's OpenPGP software may issue a certification that Bob has checked that the User ID `Alice Adams <alice@example.org>` and the certificate with the fingerprint `AAA1 8CBB 2546 85C5 8358 3205 63FD 37B6 7F33 00F9 FB0E C457 378C D29F 1026 98B3` are legitimately linked.
This presupposes that Bob knows this person who goes by "Alice Adams", and is satisfied that Alice uses the email address `alice@example.org`. Further, that Bob has verified that the certificate his OpenPGP software uses for Alice matches the certificate that Alice is using. Effectively this verification must ensure that both users have a certificate for Alice with the same fingerprint. In OpenPGP version 6, manual comparison of the fingerprint by end users is discouraged. A replacement mechanism is still pending. The verification must use a sufficiently secure channel, for example an end-to-end encrypted video call, or an in-person meeting.
For more on third-party certifications, see {ref}`third_party_cert`.
Some OpenPGP subsystems may add User IDs to a certificate, which are not bound to the primary key by the certificate's owner. This can be useful to store local identity information (e.g., Sequoia's public store attaches "pet-names" to certificates, in this way).
While a convenience for consumers, indiscriminately accepting and integrating third-party identity certifications comes with significant risks.
Without any restrictions in place, malicious entities can flood a certificate with excessive certifications. Called "certificate flooding," this form of digital vandalism grossly expands the certificate size, making the certificate cumbersome and impractical for users.
It also opens the door to potential denial-of-service attacks, rendering the certificate non-functional or significantly impeding its operation.
The popular [SKS keyserver network experienced certificate flooding firsthand](https://dkg.fifthhorseman.net/blog/openpgp-certificate-flooding.html), causing it to shut down operations in 2019.
