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@ -6,224 +6,232 @@ SPDX-License-Identifier: CC-BY-SA-4.0
(certificates_chapter)=
# Certificates
OpenPGP fundamentally hinges on the concept of "OpenPGP certificates," also known as "OpenPGP public 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.
OpenPGP fundamentally hinges on the concept of "{term}`OpenPGP certificates<OpenPGP Certificate>`," also known as "{term}`OpenPGP public keys<OpenPGP Public Key>`." These {term}`certificates<OpenPGP Certificate>` are complex data structures essential for {term}`identity verification`, data encryption, and {term}`digital signatures<OpenPGP Signature Packet>`. Understanding their structure and function is pivotal to effectively applying the OpenPGP standard.
An OpenPGP certificate, by definition, does not contain private key material.
An {term}`OpenPGP certificate`, by definition, does not contain {term}`private key material`.
Fundamentally, the effective management of certificates and a thorough grasp of their authentication and trust models are crucial for proficient OpenPGP usage. Although this document offers just a brief overview of these aspects, they form a fundamental part of the broader OpenPGP framework and warrant further study.
Fundamentally, the effective management of {term}`certificates<OpenPGP Certificate>` and a thorough grasp of their {term}`authentication` and {term}`trust models<Trust Model>` are crucial for proficient OpenPGP usage. Although this document offers just a brief overview of these aspects, they form a fundamental part of the broader OpenPGP framework and warrant further study.
- For an in-depth exploration of OpenPGP's private key material, refer to {ref}`private_key_chapter`. This chapter provides essential insights into private key management and security practices.
- For an in-depth exploration of OpenPGP's {term}`private key material`, refer to {ref}`private_key_chapter`. This chapter provides essential insights into {term}`private key<Transferable Secret Key>` management and security practices.
- The bindings that link the components of a certificate are comprehensively discussed in {ref}`component_signatures_chapter`, offering a deeper understanding of certificate structure and integrity.
- The bindings that link the {term}`components<Component>` of a {term}`certificate<OpenPGP Certificate>` are comprehensively discussed in {ref}`component_signatures_chapter`, offering a deeper understanding of {term}`certificate<OpenPGP Certificate>` structure and integrity.
- Finally, our chapter {ref}`zoom_certificates` discusses the internal structure of certificates in detail.
- Finally, our chapter {ref}`zoom_certificates` discusses the internal structure of {term}`certificates<OpenPGP Certificate>` in detail.
## 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.
The term "{term}`(cryptographic) keys<Cryptographic Key>`" is central to grasping the concept of {term}`OpenPGP certificates<OpenPGP certificate>`. 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.
The term "{term}`key`," without additional context, can refer to either public or private {term}`asymmetric<Asymmetric Cryptography>` key material. Additionally, {term}`symmetric<Symmetric Cryptography>` keys may be used in OpenPGP to encrypt {term}`private key material`, adding a layer of security and complexity.
(layers_of_keys_in_openpgp)=
### Layers of keys in OpenPGP
In OpenPGP, the term "key" may refer to three distinct layers, each serving a unique purpose:
In OpenPGP, the term "{term}`key`" may refer to three distinct layers, each serving a unique purpose:
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.
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 {term}`private key<Transferable Secret Key>`, the key consists of the exponent `d` along with the prime numbers `p` and `q`.
2. An OpenPGP *{term}`component key<OpenPGP Component Key>`* includes either an "{term}`OpenPGP primary key`" or an "{term}`OpenPGP subkey`." It is a building block of an {term}`OpenPGP certificate`, consisting of a cryptographic keypair coupled with some invariant {term}`metadata`, such as key {term}`creation time`.
3. An "{term}`OpenPGP certificate`" (or "OpenPGP key") consists of several {term}`component keys<OpenPGP Component Key>`, {term}`identity components<Identity Component>`, and other elements. These {term}`certificates<Certificate>` are dynamic, evolving over time as {term}`components<Component>` are added, {term}`expire<Expiration>`, or are marked as {term}`invalid<Validation>`.
The following section will delve into the OpenPGP-specific layers (2 and 3) to provide a clearer understanding of their roles within OpenPGP certificates.
The following section will delve into the OpenPGP-specific layers (2 and 3) to provide a clearer understanding of their roles within {term}`OpenPGP certificates<OpenPGP Certificate>`.
## Structure of OpenPGP certificates
An OpenPGP certificate (or "OpenPGP key") is a collection of an arbitrary number of elements[^packets]:
An {term}`OpenPGP certificate` (or "{term}`OpenPGP key`") is a collection of an arbitrary number of elements[^packets]:
[^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.
[^packets]: In technical terms, the elements of an {term}`OpenPGP certificate` are a collection of "{term}`packets<Packet>`." Each {term}`component key<OpenPGP Component Key>` and {term}`identity component` is internally represented as a {term}`packet`. Another common type of {term}`packet` is the "{term}`signature`" {term}`packet`, which connect the {term}`components<Component>` of a {term}`certificate<OpenPGP Certificate>`.
- Component keys
- Identity components
- Additional metadata, including connections between the certificate's components
- {term}`Component keys<OpenPGP Component Key>`
- {term}`Identity components<Identity Component>`
- Additional {term}`metadata`, including connections between the {term}`certificate<OpenPGP Certificate>`'s {term}`components<Component>`
This documentation collectively refers to component keys and identity components as "the components of a certificate."
This documentation collectively refers to {term}`component keys<OpenPGP Component Key>` and {term}`identity components<Identity Component>` as "the {term}`components<Component>` of a {term}`certificate<OpenPGP Certificate>`."
```{figure} diag/Components_of_an_OpenPGP_Certificate.svg
:name: fig-openpgp-certificate-components
: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
Typical {term}`components<Component>` in an {term}`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.
Every element in an {term}`OpenPGP certificate` revolves around a central {term}`component`: the *{term}`OpenPGP primary key`*. The primary key acts as a personal *{term}`certification authority`* ({term}`CA<Certification Authority>`) for the {term}`certificate<OpenPGP Certificate>`'s owner, enabling cryptographic statements regarding {term}`subkeys<OpenPGP Subkey>`, {term}`identities<Identity>`, {term}`expiration`, {term}`revocation`, and more.
```{note}
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. However, once published, components [cannot be removed](append-only) from certificates.
{term}`OpenPGP certificates<OpenPGP Certificate>` tend to have a long lifespan, with the potential for modifications (typically by their owner) over time. {term}`Components<Component>` may be added or {term}`invalidated<Validation>` throughout a {term}`certificate<OpenPGP Certificate>`'s lifetime. However, once published, {term}`components<Component>` [cannot be removed](append-only) from {term}`certificates<OpenPGP Certificate>`.
```
(component_keys)=
## 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."
An {term}`OpenPGP certificate` usually contains multiple {term}`component keys<OpenPGP Component Key>`. {term}`Component keys<OpenPGP Component Key>` serve in one of two roles: either as an "{term}`OpenPGP primary key`" or as an "{term}`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]).
{term}`OpenPGP component keys<OpenPGP Component Key>` logically consist of an [asymmetric cryptographic keypair](asymmetric_key_pair) and a creation timestamp. Once created, these attributes of a {term}`component key<OpenPGP Component Key>` remain fixed (for ECDH keys, two additional parameters are part of a {term}`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.
[^ecdh-parameters]: For [ECDH](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-part-for-ecd) {term}`component keys<OpenPGP Component Key>`, two additional algorithm parameters are integral to the {term}`component key<OpenPGP Component Key>`'s constitutive and immutable properties. Those parameters specify a hash function and a {term}`symmetric<Symmetric Cryptography>` encryption algorithm.
```{figure} diag/Component_Key.png
: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".
: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
An {term}`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`.
{term}`Component keys<OpenPGP Component Key>` containing {term}`private key material` also include {term}`metadata` specifying the password protection scheme. This is another facet of {term}`metadata`, akin to the aforementioned creation timestamp and additional parameters for certain algorithms. However, this discussion focuses on {term}`OpenPGP certificates<OpenPGP Certificate>`, in which the {term}`component keys<OpenPGP Component Key>` contain only the public part of its cryptographic key data. For information on {term}`private keys<Transferable Secret Key>` in OpenPGP, see {numref}`private_key_chapter`.
(fingerprint)=
### 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.
Each {term}`OpenPGP component key` possesses an *{term}`OpenPGP fingerprint`*. This {term}`fingerprint<OpenPGP Fingerprint>` is derived from the {term}`public key material<OpenPGP Certificate>`, the {term}`creation timestamp<Creation Time>`, and, when relevant, the ECDH parameters.
```{figure} diag/Fingerprint.svg
: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.
: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.
Every {term}`OpenPGP component key` is identifiable by a {term}`fingerprint<OpenPGP 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 CA94`[^keyid].
The {term}`fingerprint<OpenPGP Fingerprint>` of our example {term}`OpenPGP component key` is `C0A5 8384 A438 E5A1 4F73 7124 26A4 D45D BAEE F4A3 9E6B 30B0 9D55 13F9 78AC CA94`[^keyid].
[^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."
[^keyid]: In OpenPGP version 4, the rightmost 64 bits were sometimes used as a shorter identifier, called "{term}`Key ID`."
For example, an OpenPGP version 4 {term}`certificate<OpenPGP Certificate>` with the {term}`fingerprint<OpenPGP Fingerprint>` `B3D2 7B09 FBA4 1235 2B41 8972 C8B8 6AC4 2455 4239` might be referenced by the 64-bit {term}`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 {term}`Key ID`," while 64-bit identifiers were referred to as "long Key ID."
```{note}
In practice, the fingerprint of a component key, while not theoretically unique, functions effectively as a unique identifier. The use of a [cryptographic hash algorithm](crypto-hash) in generating fingerprints makes the occurrence of two different component keys with the same fingerprint extremely unlikely[^finger-unique].
In practice, the {term}`fingerprint<OpenPGP Fingerprint>` of a {term}`component key<OpenPGP Component Key>`, while not theoretically unique, functions effectively as a unique identifier. The use of a [cryptographic hash algorithm](crypto-hash) in generating {term}`fingerprints<OpenPGP Fingerprint>` makes the occurrence of two different {term}`component keys<OpenPGP Component Key>` with the same {term}`fingerprint<OpenPGP Fingerprint>` extremely unlikely[^finger-unique].
```
[^finger-unique]: For both OpenPGP version 6 and version 4, the likelihood of accidental occurrence of duplicate fingerprints is negligible when key material is generated based on an acceptable source of entropy. A separate question is if an attacker can purposely craft a second key with the same fingerprint as a given pre-existing component key. With the current state of the art, this is not possible for OpenPGP version 6 and version 4 keys. However, at the time of this writing, the SHA-1-based fingerprints of OpenPGP version 4 are considered insufficiently strong at protecting against the generation of pairs of key material with the same fingerprint.
[^finger-unique]: For both {term}`OpenPGP version 6<RFC>` and version 4, the likelihood of accidental occurrence of duplicate {term}`fingerprints<OpenPGP Fingerprint>` is negligible when {term}`key material` is generated based on an acceptable source of entropy. A separate question is if an attacker can purposely craft a second key with the same {term}`fingerprint<OpenPGP Fingerprint>` as a given pre-existing {term}`component key<OpenPGP Component Key>`. With the current state of the art, this is not possible for OpenPGP version 6 and version 4 keys. However, at the time of this writing, the SHA-1-based {term}`fingerprints<OpenPGP Fingerprint>` of OpenPGP version 4 are considered insufficiently strong at protecting against the generation of pairs of {term}`key material` with the same {term}`fingerprint<OpenPGP Fingerprint>`.
(primary_key)=
### Primary key
The OpenPGP primary key is a component key that serves a distinct, central role in an OpenPGP certificate:
The {term}`OpenPGP primary key` is a {term}`component key<OpenPGP Component Key>` that serves a distinct, central role in an {term}`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.
- Its {term}`fingerprint<OpenPGP Fingerprint>` acts as an identifier for the entire {term}`OpenPGP certificate`.
- It facilitates lifecycle operations, such as adding or {term}`invalidating<Validation>` {term}`subkeys<OpenPGP Subkey>` or {term}`identities<Identity>` within a {term}`certificate<OpenPGP Certificate>`.
```{admonition} Terminology
: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."
In the {term}`RFC`, the {term}`OpenPGP primary key` is occasionally referred to as "top-level key." Informally, it has also been termed the "{term}`master key<OpenPGP Primary Key>`."
```
(subkeys)=
### Subkeys
Modern OpenPGP certificates typically include several subkeys in addition to the primary key, although these subkeys are optional.
Modern {term}`OpenPGP certificates<OpenPGP Certificate>` typically include several {term}`subkeys<OpenPGP Subkey>` in addition to the {term}`primary key<OpenPGP Primary Key>`, although these {term}`subkeys<OpenPGP Subkey>` 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`.
While {term}`subkeys<OpenPGP Subkey>` have the same structural attributes as the {term}`primary key<OpenPGP Primary Key>`, they fulfill different roles. {term}`Subkeys<OpenPGP Subkey>` are cryptographically linked with the {term}`primary key<OpenPGP Primary Key>`, a relationship further discussed in {numref}`binding_subkeys`.
```{figure} diag/Binding_Subkeys.png
: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.
{term}`OpenPGP certificates<OpenPGP Certificate>` can contain multiple {term}`subkeys<OpenPGP Subkey>`.
```
(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).
{term}`Identity components<Identity Component>` in an {term}`OpenPGP certificate` are used by the {term}`certificate holder` to state that they are known by a certain identifier (like a name, or an email address).
(user_ids_in_openpgp_certificates)=
### User IDs in OpenPGP certificates
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.
{term}`OpenPGP certificates<OpenPGP Certificate>` 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 {term}`User ID` associates the {term}`certificate<OpenPGP Certificate>` with an {term}`identity`.
```{figure} diag/Binding_a_UserID.png
: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
Relationship of {term}`User ID` to primary {term}`component key` in an {term}`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](https://www.rfc-editor.org/rfc/rfc2822) as a *name-addr*.
A typical {term}`User ID` {term}`identity` is a UTF-8-encoded string composed of a name and an email address. By convention, {term}`User IDs<User ID>` 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.
For further conventions on {term}`User IDs<User ID>`, refer to the document [draft-dkg-openpgp-userid-conventions-00](https://datatracker.ietf.org/doc/draft-dkg-openpgp-userid-conventions/), dated 25 August 2023.
**Split User IDs**
One proposed variant for encoding identities in User IDs is to use ["split User IDs"](https://dkg.fifthhorseman.net/blog/2021-dkg-openpgp-transition.html#split-user-ids). Although uncommon, there are currently no significant technical barriers to implementing this format[^dkg-split].
One proposed variant for encoding {term}`identities<Identity>` in {term}`User IDs<User ID>` is to use ["split User IDs"](https://dkg.fifthhorseman.net/blog/2021-dkg-openpgp-transition.html#split-user-ids). Although uncommon, there are currently no significant technical barriers to implementing this format[^dkg-split].
[^dkg-split]: Historically, the OpenPGP ecosystem faced challenges in this context. For further details, refer to Daniel Kahn Gillmor's January 2019 article, ["What were Separated User IDs"](https://dkg.fifthhorseman.net/blog/2019-dkg-openpgp-transition.html#what-were-separated-user-ids).
The rationale for split User IDs lies in the distinction between a name and an email address, which represent two separate facets of an individual's identity. Separating these elements simplifies the process for third parties tasked with certifying that an identity is legitimately connected to a certificate.
The rationale for split {term}`User IDs<User ID>` lies in the distinction between a name and an email address, which represent two separate facets of an individual's {term}`identity`. Separating these elements simplifies the process for third parties tasked with certifying that an {term}`identity` is legitimately connected to a {term}`certificate<OpenPGP Certificate>`.
Consider this scenario: A third party is confident about the email-based identity of an individual (e.g.,`<alice@example.org>`) and is willing to certify it. However, they might not have sufficient knowledge about the person's name-based identity (e.g., `Alice Adams`), so are unwilling to extend the same level of certification. Split User IDs address this dichotomy by allowing distinct certification processes for each type of identity.
Consider this scenario: A third party is confident about the email-based {term}`identity` of an individual (e.g.,`<alice@example.org>`) and is willing to certify it. However, they might not have sufficient knowledge about the person's name-based {term}`identity` (e.g., `Alice Adams`), so are unwilling to extend the same level of {term}`certification`. Split {term}`User IDs<User ID>` address this dichotomy by allowing distinct {term}`certification` processes for each type of {term}`identity`.
(primary_user_id)=
### Implications of the Primary User ID
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).
Within a {term}`certificate<OpenPGP Certificate>`, a specific {term}`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`). 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.
Each {term}`User ID` carries associated preference settings, such as preferred encryption algorithms, which is detailed in {numref}`zooming_in_user_id`). When a {term}`certificate<OpenPGP Certificate>` is used in the context of a specific {term}`identity`, then the preferences associated with that {term}`identity component` are used. When a {term}`certificate<OpenPGP Certificate>` is used without reference to a specific {term}`identity`, the preferences associated with the {term}`direct key signature`, or the {term}`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`.
The {term}`primary User ID` was historically the main store for preferences that apply to the {term}`certificate<OpenPGP Certificate>` as a whole. For more on this, see {ref}`primary-metadata`.
### User attributes in OpenPGP
While
[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.
[user attributes](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-user-attribute-packet-tag-1) are similar to {term}`User IDs<User ID>`, 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) in JPEG format. 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`.
To form an {term}`OpenPGP certificate`, individual {term}`components<Component>` are interconnected by the {term}`certificate holder` using their OpenPGP software. Within OpenPGP, this process is termed "binding", as in "a {term}`subkey<OpenPGP Subkey>` is bound to the {term}`primary key<OpenPGP Primary Key>`." These bindings are realized using cryptographic {term}`signatures<OpenPGP Signature Packet>`. 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:
In very abstract terms, the {term}`primary key<OpenPGP Primary Key>` of a {term}`certificate<OpenPGP Certificate>` acts as a root of trust or "{term}`certification authority<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."
- issuing {term}`signatures<OpenPGP Signature Packet>` that express the {term}`certificate holder`'s intent to use specific {term}`subkeys<OpenPGP Subkey>` or {term}`identity components<Identity Component>`;
- conducting other lifecycle operations, including setting {term}`expiration` dates and marking {term}`components<Component>` as {term}`invalidated<Validation>` or "`revoked<Revocation>`."
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.
By binding {term}`components<Component>` using digital {term}`signatures<OpenPGP Signature Packet>`, recipients of an {term}`OpenPGP certificate` need only {term}`validate<Validation>` the {term}`authenticity<Authentication>` of the {term}`primary key` to use for their communication partner. Traditionally, this is done by manually verifying the *{term}`fingerprint<OpenPGP Fingerprint>`* of the {term}`primary key<OpenPGP Primary Key>`. Once the {term}`validity<Validation>` of the {term}`primary key<OpenPGP Primary Key>` is confirmed, the {term}`validity<Validation>` of the remaining {term}`components<Component>` can be automatically assessed by the user's OpenPGP software. Generally, {term}`components<Component>` are {term}`valid<Validation>` parts of a {term}`certificate<OpenPGP Certificate>` if there is a statement signed by the {term}`certificate<OpenPGP Certificate>`'s {term}`primary key<OpenPGP Primary Key>` endorsing this {term}`validity<Validation>`.
(metadata_in_certificates)=
## Metadata in certificates
OpenPGP certificates, their component keys, and identities possess metadata that is not stored within the components it pertains to. Instead, this metadata is stored within signature packets, which are integral to the structure of an OpenPGP certificate.
{term}`OpenPGP certificates<OpenPGP Certificate>`, their {term}`component keys<Component Key>`, and {term}`identities<Identity>` possess {term}`metadata` that is not stored within the {term}`components<Component>` it pertains to. Instead, this {term}`metadata` is stored within signature packets, which are integral to the structure of an OpenPGP certificate.
Key attributes, such as capabilities (like *signing* or *encryption*) and expiration times, are examples of metadata not stored in the component key data. How this metadata is stored depends on the component:
Key attributes, such as {term}`capabilities<Capability>` (like *signing* or *encryption*) and {term}`expiration times<Expiration Time>`, are examples of {term}`metadata` not stored in the {term}`component key` data. How this {term}`metadata` is stored depends on the {term}`component`:
- **Primary key metadata** is defined either through a direct key signature on the primary key (preferred in OpenPGP version 6), or by associating the metadata with the [Primary User ID](primary_user_id).
- **{term}`Primary key<OpenPGP Primary Key>` {term}`metadata`** is defined either through a {term}`direct key signature` on the {term}`primary key<OpenPGP Primary Key>` (preferred in OpenPGP version 6), or by associating the {term}`metadata` with the [Primary User ID](primary_user_id).
- **Subkey metadata** is defined within the [subkey binding signature](binding_subkeys) that links the subkey to the certificate.
- **{term}`Subkey<OpenPGP Subkey>` {term}`metadata`** is defined within the [subkey binding signature](binding_subkeys) that links the {term}`subkey<OpenPGP Subkey>` to the {term}`certificate<OpenPGP Certificate>`.
- **Identity component metadata** is associated via the [certifying self-signature](bind_ident) that links the identity (usually in the form of a User ID) to the certificate.
- **{term}`Identity component` {term}`metadata`** is associated via the [certifying self-signature](bind_ident) that links the {term}`identity` (usually in the form of a {term}`User ID`) to the {term}`certificate<OpenPGP Certificate>`.
It is crucial to note that the components of an OpenPGP certificate remain static after their creation. The use of signatures to store metadata allows for subsequent modifications without altering the original components. For instance, a certificate holder can update the expiration time of a component by issuing a new, superseding signature.
It is crucial to note that the {term}`components<Component>` of an {term}`OpenPGP certificate` remain static after their creation. The use of {term}`signatures<OpenPGP Signature Packet>` to store {term}`metadata` allows for subsequent modifications without altering the original {term}`component<Component>`. For instance, a {term}`certificate holder` can update the {term}`expiration time` of a {term}`component` by issuing a new, superseding {term}`signature<OpenPGP Signature Packet>`.
```{figure} diag/Primary_key_metadata.png
:name: fig-primary-metadata
:alt: Depicts a direct key signature, associated with a primary component key.
Metadata can be associated with the primary key using a *direct key signature*.
{term}`Metadata` can be associated with the {term}`primary key<OpenPGP Primary Key>` using a *{term}`direct key signature`*.
```
(capabilities_key_flags)=
### 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.
Each {term}`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.
Commonly used key flags include:
Commonly used {term}`key flags<Key Flag>` 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 authentication[^auth-flag]
- **{term}`Certification<Certification Key Flag>`**: enables issuing third-party {term}`certifications<Certification>`
- **{term}`Signing<Signing Key Flag>`**: allows the key to sign data
- **{term}`Encryption<Encryption Key Flag>`**: allows the key to encrypt data
- **{term}`Authentication<Authentication Key Flag>`**: primarily used for SSH authentication[^auth-flag]
[^auth-flag]: It's important to note that the function of the [authentication](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-authentication-via-digital-) key flag is unrelated to the authentication process used in certifying OpenPGP identities and linking them to certificates. Rather, this flag indicates a mechanism that uses cryptographic signatures to confirm control of private key material with a remote system.
[^auth-flag]: It's important to note that the function of the [authentication](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-authentication-via-digital-) {term}`key flag` is unrelated to the {term}`authentication` process used in certifying OpenPGP {term}`identities<Identity>` and linking them to {term}`certificate<OpenPGP Certificate>`. Rather, this flag indicates a mechanism that uses {term}`cryptographic signatures<OpenPGP Signature Packet>` to confirm control of {term}`private key material` with a remote system.
```{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. A component key can technically have multiple capabilities. 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).
Distinct {term}`component keys<Component Key>` handle specific operations. Only the {term}`primary key<OpenPGP Primary Key>` can be used for {term}`certification`, although it can have additional {term}`capabilities<Capability>`. {term}`Subkeys<OpenPGP Subkey>` can be used for signing, encryption, and authentication but cannot have the {term}`certification` {term}`capability`. A {term}`component key` can technically have multiple {term}`capabilities<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].
Notably, in many algorithms, encryption and signing-related functionalities (i.e., {term}`certification<Certification Key Flag>`, {term}`signing<Signing Key Flag>`, {term}`authentication<Authentication Key Flag>`) 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.
[^key-flag-sharing]: With ECC algorithms, it's impossible to combine {term}`encryption<Encryption Key Flag>` functions with those intended for {term}`signing<Signing Key Flag>`. For example, ed25519 is specifically used for {term}`signing<Signing Key Flag>`; cv25519 is designated for {term}`encryption<Encryption Key Flag>`.
### Algorithm preferences and feature signaling
@ -231,7 +239,7 @@ OpenPGP incorporates significant ["cryptographic agility"](https://en.wikipedia.
This agility facilitates the easy adoption of new cryptographic primitives into the standard, allowing for a seamless transition. Users can gradually migrate to new cryptographic mechanisms without disruption.
However, this approach requires that OpenPGP software determine the cryptographic mechanisms that a set of communication partners can handle and prefer. OpenPGP employs several mechanisms for this purpose, which allow negotiation between sender and recipient. It's important to note that OpenPGP is not an online scheme; thus, this negotiation is effectively one-way. The active party interprets the preferences expressed in the certificate of the passive party.
However, this approach requires that OpenPGP software determine the cryptographic mechanisms that a set of communication partners can handle and prefer. OpenPGP employs several mechanisms for this purpose, which allow negotiation between sender and recipient. It's important to note that OpenPGP is not an online scheme; thus, this negotiation is effectively one-way. The active party interprets the preferences expressed in the {term}`certificate<OpenPGP Certificate>` of the passive party.
Negotiation mechanisms in OpenPGP include:
@ -241,55 +249,57 @@ Negotiation mechanisms in OpenPGP include:
- [Features subpacket](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#features-subpacket)
- [Preferred compression algorithms](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#preferred-compression-subpacket)
Beyond these explicitly expressed preferences, implementations also deduce capabilities of communication partners based on the version of the OpenPGP certificate they possess.
Beyond these explicitly expressed preferences, implementations also deduce {term}`capabilities<Capability>` of communication partners based on the version of the {term}`OpenPGP certificate` they possess.
#### 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.
As a starting point, a {term}`certificate<OpenPGP certificate>` has a set of preferences that apply generally. These are defined either in a {term}`direct key signature`, or via the {term}`primary User ID` of the {term}`certificate<OpenPGP 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.
Additionally, OpenPGP allows modeling {term}`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 {term}`identity`-specific preferences can be expressed on the certifying {term}`signatures<OpenPGP Signature Packet>` that bind {term}`User IDs<User ID>` to a {term}`certificate<OpenPGP certificate>`.
## A typical OpenPGP certificate, revisited
Following our review of how keys and identity components are linked, let's reexamine the OpenPGP certificate from {numref}`fig-openpgp-certificate-components`. Our focus now extends to all of its binding signatures and the direct key signature that contains metadata for the full certificate:
Following our review of how {term}`keys<Component Key>` and {term}`identity components<Identity Component>` are linked, let's reexamine the {term}`OpenPGP certificate` from {numref}`fig-openpgp-certificate-components`. Our focus now extends to all of its binding signatures and the {term}`direct key signature` that contains {term}`metadata` for the full {term}`certificate<OpenPGP certificate>`:
```{figure} diag/OpenPGP_Certificate.png
:name: fig-openpgp-certificate
:alt: Depicts an OpenPGP certificate, including a set of components, binding signatures, and a direct key signature on the primary key.
This shows a typical OpenPGP certificate, including binding signatures for all of its components, and a signature that associates metadata with the primary key.
This shows a typical {term}`OpenPGP certificate`, including binding {term}`signatures<OpenPGP Signature Packet>` for all of its {term}`components<Component>`, and a {term}`signature<OpenPGP Signature Packet>` that associates {term}`metadata` with the {term}`primary key<OpenPGP Primary Key>`.
```
(revocations)=
## Revocations
When a certificate owner needs to invalidate certain components of their certificate, or even the entire certificate, they accomplish this through "revocation." Revoking the primary key renders the entire certificate invalid.
When a {term}`certificate holder` needs to {term}`invalidate<Validation>` certain {term}`components<Component>` of their {term}`certificate<OpenPGP Certificate>`, or even the entire {term}`certificate<OpenPGP Certificate>`, they accomplish this through "{term}`revocation`." {term}`Revoking<Revocation>` the {term}`primary key<OpenPGP Primary Key>` renders the entire {term}`certificate<OpenPGP Certificate>` {term}`invalid<Validation>`.
Notably, revocations are not the only means by which components can become invalid. Other factors, such as the passing of a component's expiration time, can also render components invalid.
Notably, {term}`revocations<Revocation>` are not the only means by which {term}`components<Component>` can become {term}`invalid<Validation>`. Other factors, such as the passing of a {term}`component`'s {term}`expiration time`, can also render {term}`components<Component>` {term}`invalid<Validation>`.
For more detailed information on revoking specific components of a certificate, see the section on {ref}`self-revocations`.
For more detailed information on {term}`revoking<Revocation>` specific {term}`components<Component>` of a {term}`certificate<OpenPGP Certificate>`, see the section on {ref}`self-revocations`.
(third_party_identity_certifications)=
## Third-party (identity) certifications
Since its inception, third-party identity certifications have been a cornerstone of the OpenPGP ecosystem. The original PGP designers, starting with Phil Zimmermann, advocated for decentralized trust models over reliance on centralized authorities. This decentralized approach in OpenPGP is known as the ["Web of Trust."](wot)
Since its inception, {term}`third-party identity certifications<Third-party Identity Certification>` have been a cornerstone of the OpenPGP ecosystem. The original PGP designers, starting with Phil Zimmermann, advocated for decentralized {term}`trust models<Trust Model>` over reliance on centralized authorities. This decentralized approach in OpenPGP is known as the ["Web of Trust."](wot)
Third-party certifications are statements by OpenPGP users confirming that a user with a specific identity is the owner of a particular OpenPGP certificate.
Third-party {term}`certifications<Certification>` are statements by OpenPGP users confirming that a user with a specific {term}`identity` is the owner of a particular {term}`OpenPGP certificate`.
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.
For example, Bob's OpenPGP software may issue a {term}`certification` that Bob has checked that the {term}`User ID` `Alice Adams <alice@example.org>` and the {term}`certificate<OpenPGP Certificate>` with the {term}`fingerprint<OpenPGP Fingerprint>` `AAA1 8CBB 2546 85C5 8358 3205 63FD 37B6 7F33 00F9 FB0E C457 378C D29F 1026 98B3` are legitimately linked.
Take, for instance, a scenario where Bob's OpenPGP software issues a certification confirming as legitimate the link between the User ID `Alice Adams <alice@example.org>` and the certificate bearing the fingerprint `AAA1 8CBB 2546 85C5 8358 3205 63FD 37B6 7F33 00F9 FB0E C457 378C D29F 1026 98B3`.
Take, for instance, a scenario where Bob's OpenPGP software issues a {term}`certification` confirming as legitimate the link between the {term}`User ID` `Alice Adams <alice@example.org>` and the {term}`certificate<OpenPGP Certificate>` bearing the {term}`fingerprint<OpenPGP Fingerprint>` `AAA1 8CBB 2546 85C5 8358 3205 63FD 37B6 7F33 00F9 FB0E C457 378C D29F 1026 98B3`.
This process assumes that Bob knows the person known as `Alice Adams` and is confident that `alice@example.org` is indeed Alice's email address. Bob also verifies that the certificate his OpenPGP software associates with Alice matches the one Alice uses. In essence, both users must have a certificate for Alice with an identical fingerprint. In OpenPGP version 6, manual fingerprint comparison by end-users is discouraged, with a replacement verification mechanism still under development. The verification process must occur over a sufficiently secure channel, such as an end-to-end encrypted video call or a face-to-face meeting.
This process assumes that Bob knows the person known as `Alice Adams` and is confident that `alice@example.org` is indeed Alice's email address. Bob also verifies that the {term}`certificate<OpenPGP Certificate>` his OpenPGP software associates with Alice matches the one Alice uses. In essence, both users must have a {term}`certificate<OpenPGP Certificate>` for Alice with an identical {term}`fingerprint<OpenPGP Fingerprint>`. In OpenPGP version 6, manual {term}`fingerprint<OpenPGP Fingerprint>` comparison by end-users is discouraged, with a replacement {term}`verification` mechanism still under development. The {term}`verification` process must occur over a sufficiently secure channel, such as an end-to-end encrypted video call or a face-to-face meeting.
For more on third-party certifications, see {ref}`third_party_cert`.
For more on third-party {term}`certifications<Certification>`, see {ref}`third_party_cert`.
(cert-flooding)=
### Security considerations
While a convenience for consumers, indiscriminately accepting and integrating third-party identity certifications comes with significant risks.
While a convenience for consumers, indiscriminately accepting and integrating {term}`third-party identity certifications<Third-party Identity Certification>` 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.
Without any restrictions in place, malicious entities can flood a {term}`certificate<OpenPGP Certificate>` with excessive {term}`certifications<Certification>`. Called "certificate flooding," this form of digital vandalism grossly expands the {term}`certificate<OpenPGP Certificate>` size, making the {term}`certificate<OpenPGP 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.
It also opens the door to potential denial-of-service attacks, rendering the {term}`certificate<OpenPGP 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.