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.
[^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.
OpenPGP component keys consist of an [asymmetric cryptographic keypair](asymmetric_key_pair) and a creation timestamp. These attributes of a component key cannot be changed after creation (in the case of ECDH keys, two additional parameters are part of a component key's constituting data[^ecdh-paramters]).
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.
Component key representations that include private key material also contain metadata that specifies the password protection scheme for the private key material.
Component keys containing private key material also contain metadata that specifies the password protection scheme for the private key material. However, in this chapter, we're looking at *OpenPGP certificates*, which *don't* contain private key information. Each component key of such a certificate contains only the public part of its cryptographic key data. To read more about private keys in OpenPGP, see {numref}`private_key_chapter`.
For each OpenPGP component key, an *OpenPGP fingerprint* can be generated. This fingerprint is derived from the combination of the public key material and creation timestamp (and ECDH parameters, if applicable).
[^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."
Subkeys have the same structure as the primary key, but they are used in a different role. Subkeys are cryptographically linked with the primary key (more on this below).
Subkeys have the same structural attributes as the primary key but fulfill a different role. Subkeys are cryptographically linked with the primary key (more on this in {numref}`binding_subkeys`).
:alt: Three component keys depicted. The primary key is positioned at the top, designated for certification. Below it, linked by arrows, are two more component keys, used as subkeys. They are 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.
By convention, only the primary key is allowed to perform "certification" operations. All other operations can be configured on either the primary key or a subkey.
It is considered good practice to have separate component keys for each type of operation: to allow only *Certification* operations with the primary key, and to use separate *Signing*, *Encryption* and *Authentication* subkeys (independently: with most algorithms, encryption can't be shared with the other capabilities[^key-flag-sharing]).
[^key-flag-sharing]: With ECC algorithms, it's actually not possible to share encryption functionality with the signing-based functionalities, e.g.: ed25519 used for signing; cv25519 used for encryption.
Instead, key flags, together with other metadata about that component key (such as the key expiration time), are stored using mechanisms that join components together as an OpenPGP certificate:
- For the primary key, two different mechanisms can be used to define its key flags (as well as other metadata): That configuration can be associated with the [Primary User ID](primary_user_id), or via a [direct key signature](direct_key_signature).
- For subkeys, their key flags (and other metadata) are defined with the mechanism that connects the subkey with the certificate (via the primary key). More on that [below](binding_subkeys).
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).
An OpenPGP certificate can contain any number of [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.
Often, identities in a User ID consist of a UTF-8 encoded string that is composed of a name and an email address. By convention, User IDs typically consist of an [RFC2822](https://www.rfc-editor.org/rfc/rfc2822) *name-addr*.
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).
One User ID in a certificate has the special property of being the [Primary User ID](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-primary-user-id).
User IDs are associated with preference settings (such as preferred encryption algorithms, more on this in {numref}`zooming_in_user_id`). The preferences associated with the Primary User ID are used 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, but less commonly used.
The OpenPGP standard currently only defines one format to store in User Attributes: an [image](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-the-image-attribute-subpack), "presumably (but not required to be) that of the key owner".
To form an OpenPGP certificate out of a collection of components, the certificate holder links these components together (using their OpenPGP software).
The OpenPGP term for linking components is "binding," as in: "a subkey is bound to the primary key." The bindings are realized using cryptographic signatures (much more details about this are in {ref}`certifications_chapter`).
The primary key issues signatures that express the certificate holder's intent to use subkeys or identity components. It also performs other lifecycle operations, such as setting expiration times, or marking components as invalidated ("revoked").
Binding components together with digital signatures means that recipients of an OpenPGP certificate only need to verify that the primary key is the correct one to use for their communication partner (traditionally, this has often been done by manually verifying the *fingerprint* of the primary key). Once the validity of the primary key is established, the validity of all other components can be automatically determined by the user's OpenPGP software. To a first estimation, components are valid parts of a certificate if there is a statement signed with the certificate's primary key that expresses this validity.
In the past, the SKS keyserver network has accepted third party signatures and added them to certificates without any limitations. This has caused problems: anyone can add a large number of certifications to some certificates, which opens the door to a type of "vandalism", by growing certificates unreasonably, and making them annoying to use[^flooding].
[^flooding]: Storing third-party identity certifications in the target OpenPGP certificate is convenient for consumers: it is easy to find all relevant certifications in one central location. However, when third parties can unilaterally add certifications, this opens an avenue for denial-of-service attacks by flooding. The SKS network of OpenPGP key servers [allowed and experienced this problem](https://dkg.fifthhorseman.net/blog/openpgp-certificate-flooding.html).
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).
