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}`certifications_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.
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.
In OpenPGP, 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.
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.
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.
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.
Instead, key flags, along with other metadata about that component key, such as the key expiration time, 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.
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.
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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).
Within a certificate, a specific User ID is desginated 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.
Third-party identity certifications have historically played a pivotal role in the OpenPGP ecosystem.
### Security considerations
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.
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).
