openpgp-notes/book/source/04-certificates.md

(certificates_chapter)=

Certificates

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.

Terminology: Understanding "keys"

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.

Public vs. private keys

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.

Layers of keys in OpenPGP

In OpenPGP, the term "key" may refer to three distinct layers, each serving a unique purpose:

1. A (bare) "cryptographic key" 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 only touches upon these aspects briefly, they are integral to working proficiently with OpenPGP.

Structure of OpenPGP certificates

An OpenPGP certificate (or "OpenPGP key") is a collection of an arbitrary number of elements¹:

• Component keys
• Identity components
• Additional metadata, including connections between the certificate's components

This documentation collectively refers to component keys and identity components as "the components of a certificate."

:name: fig-openpgp-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.

Typical components in an OpenPGP certificate

Every element in an OpenPGP certificate revolves around a central component: the OpenPGP primary key. The primary key acts as a personal certification authority (CA) 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.

Component keys

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 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 data²).

:name: fig-component-key
: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".

An OpenPGP 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.

Fingerprint

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.

:name: fig-fingerprint
: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](crypto-hash) make it exceedingly difficult, if not practically impossible with current technology, to find keys that share a fingerprint.

The fingerprint of our example OpenPGP component key is C0A5 8384 A438 E5A1 4F73 7124 26A4 D45D BAEE F4A3 9E6B 30B0 9D55 13F9 78AC CA94³.

Primary key

The OpenPGP primary key is a component key that serves a distinct, central role in an OpenPGP certificate:

• Its fingerprint acts as an identifier for the entire OpenPGP certificate.
• It facilitates lifecycle operations, such as adding or invalidating subkeys or identities within a certificate.

:class: note

In the RFC, the OpenPGP primary key is occasionally referred to as "top-level key." Informally, it has also been termed the "master key."

Subkeys

Modern OpenPGP certificates typically include several subkeys in addition to the primary key, although these subkeys are optional.

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.

:name: fig-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.

OpenPGP certificates can contain multiple subkeys.

(identity_components)=

Identity components

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).

User IDs in OpenPGP certificates

OpenPGP certificates can contain multiple User IDs. Each User ID associates the certificate with an identity.

:name: fig-user-ids
: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.

Relationship of User ID to primary component key in an OpenPGP certificate

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 as a name-addr.

For further conventions on User IDs, refer to the document draft-dkg-openpgp-userid-conventions-00, dated 25 August 2023.

Split User IDs

One proposed variant for encoding identities in User ID is to use "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.

(primary_user_id)=

Implications of the Primary User ID

Within a certificate, a specific User ID is designated as the Primary User ID.

Each User ID carries associated preference settings, such as preferred encryption algorithms, which is detailed in {numref}zooming_in_user_id). When a certificate is used in the context of a specific identity, then the preferences associated with that identity component are used. When a certificate is used without reference to a specific identity, the preferences associated with the direct key signature, or the primary User ID take precedence by default.

The primary User ID was historically the main store for preferences that apply to the certificate as a whole. For more on this, see {ref}primary-metadata.

User attributes in OpenPGP

While user attributes 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. Typically, this image represents the key owner, although it is not required.

Linking the components

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.

In very abstract terms, the primary key of a certificate acts as a root of trust or "certification authority." It is responsible for:

• issuing signatures that express the certificate holder's intent to use specific subkeys or identity components;
• conducting other lifecycle operations, including setting expiration dates and marking components as invalidated or "revoked."

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.

Metadata: capabilities, preferences, etc

Much of the metadata for OpenPGP certificates, component keys, and identities, is not actually stored as part of the components that the metadata applies to.

For example, the capabilities of a component key, such as signing or encryption, as well as its expiration time, are not stored as a part of the data of that component key.

Instead, this kind of metadata is stored as part of the signature packets that join components into an OpenPGP certificate:

• For the primary key, its key flags and other metadata can be defined in two ways:
  • With a direct key signature on the primary key (preferred method in OpenPGP version 6),
  • or by associating the metadata with the Primary User ID.
• For subkeys, metadata is defined with the subkey binding signature that links the subkey to the certificate.
• For identity components like User IDs, metadata is associated via the certifying self-signature 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" that delineate the operations a key can perform.

Commonly used key flags include:

• Certification: enables issuing third-party certifications
• Signing: allows the key to sign data
• Encryption: allows the key to encrypt data
• Authentication: primarily used for SSH authentication⁴

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].

Algorithm preferences and feature signaling

OpenPGP has a lot of "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:

• Preferred Hash Algorithms
• Preferred Symmetric Ciphers for v1 SEIPD
• Preferred AEAD Ciphersuites
• Features
• Preferred Compression Algorithms

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.

Revocations

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

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. OpenPGP's decentralized trust model is called the "Web of Trust."

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.

Advanced topics

:class: warning

This section only contains notes and still needs to be written

Certificate management / Evolution of a certificate over time

Minimized versions, merging, effective "append only" semantics, ...

"Naming" a certificate in user-facing contexts - fingerprints and beyond

:class: warning

In v4, a 20 byte fingerprint in hex representation was used to name certificates, even in user-facing contexts.

For v6, this type of approach is discouraged, but a replacement mechanism is still pending.

Merging

• How to merge two copies of the same certificate?
• Canonicalization

How to generate "minimized" certificate?

When are certificates valid?

• Full certificate: Primary revoked/key expired/binding signature expired,
• Subkey: Revoked/key expired/binding signature expired
• User ID: revoked, binding expired, ...

Best practices regarding Key Freshness

:class: warning

- Expiry
- Subkey rotation

Wiktor suggests to check: https://blogs.gentoo.org/mgorny/2018/08/13/openpgp-key-expiration-is-not-a-security-measure/ for important material

Metadata about the primary key: In Direct Key Signature vs. in Primary User ID, in v4 and v6

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write

Metadata leak of Social Graph

(unbound_user_ids)=

Adding unbound User IDs to a certificate

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references/links missing

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).

Third-party certification flooding

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, causing it to shut down operations in 2019.

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1. 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. ↩︎

2. For ECDH 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. ↩︎

3. 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. At one point, 32-bit identifiers were called "short Key ID," while 64-bit identifiers were referred to as "long Key ID." ↩︎

4. Note that the capability offered by the authentication key flag is unrelated to "authentication" in the context of certifying and verifying OpenPGP identities and their connection to certificates. This key flag is about a mechanism that proves control of private key material to a remote system, using cryptographic signatures. ↩︎