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openpgp-notes/book/source/adv/signing_components.md
Heiko Schaefer e913b907d7
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<!--
SPDX-FileCopyrightText: 2023 The "Notes on OpenPGP" project
SPDX-License-Identifier: CC-BY-SA-4.0
-->
# Advanced material: Signatures on components
## Certification recipes
Different {term}`signatures<OpenPGP Signature Packet>` in OpenPGP serve various specific purposes. This section provides practical guidance on creating these {term}`signatures<OpenPGP Signature Packet>`, illustrating each with concrete examples.
(recipe-algorithm-preferences)=
### Change algorithm preferences
To modify the preferred {term}`symmetric<Symmetric Cryptography>`, compression, {term}`hash<Hash Function>`, or {term}`AEAD algorithms<Authenticated Encryption With Associated Data>` for a {term}`key<Transferable Secret Key>`, the {term}`key owner<Certificate Holder>` needs to issue a {term}`direct key signature` ({term}`type ID<Signature Type ID>` `0x1F`) on the {term}`primary key<OpenPGP Primary Key>`.
This {term}`signature<OpenPGP Signature Packet>` should have the following structure:
| {term}`Subpacket<OpenPGP Signature Subpacket>` | Area | {term}`Critical<Criticality Flag>` | Mandatory | Notes |
|---------------------------------------------------------------------------------------------|-----------------------------|------------------------------------------|----------------------|----------------------------------------------------------------------------------------|
| {term}`Signature Creation Time<Signature Creation Time Subpacket>` | {term}`Hashed<Hashed Area>` | True | True | Current time |
| {term}`Issuer Fingerprint<Issuer Fingerprint Subpacket>` | {term}`Hashed<Hashed Area>` | True or False | Strongly Recommended | The {term}`primary key` is the {term}`issuer` |
| {term}`Key Flags<Key Flag>` | {term}`Hashed<Hashed Area>` | True | False | Retain {term}`key flags<Key Flag>` from the previous {term}`self-signature` |
| {term}`Features<Features Subpacket>` | {term}`Hashed<Hashed Area>` | True | False | Retain {term}`features<Features Subpacket>` from the previous {term}`self-signature` |
| {term}`Key Expiration Time<Key Expiration Time Subpacket>` | {term}`Hashed<Hashed Area>` | True | False | Retain {term}`expiration time` from the previous {term}`self-signature`, if applicable |
| {term}`Hash Algorithm Preferences<Preferred Hash Algorithms Subpacket>` | {term}`Hashed<Hashed Area>` | False | False | New {term}`preferences<Algorithm Preferences>` |
| {term}`Compression Algorithm Preferences<Preferred Compression Algorithms Subpacket>` | {term}`Hashed<Hashed Area>` | False | False | New {term}`preferences<Algorithm Preferences>` |
| {term}`Symmetric Algorithm Preferences<Preferred Symmetric Ciphers for v1 SEIPD Subpacket>` | {term}`Hashed<Hashed Area>` | False | False | New {term}`preferences<Algorithm Preferences>` |
| {term}`AEAD Algorithm Preferences<Preferred AEAD Ciphersuites Subpacket>` | {term}`Hashed<Hashed Area>` | False | False | New {term}`preferences<Algorithm Preferences>` |
### Change expiration time
To adjust the {term}`expiration time` of an {term}`OpenPGP certificate`, a new *{term}`DirectKey<Direct Key Signature>`* {term}`signature<OpenPGP Signature Packet>` ({term}`type ID<Signature Type ID>` `0x1F`) with a modified {term}`Key Expiration Time subpacket` must be issued. The structure of this {term}`signature<OpenPGP Signature Packet>` is identical to the one outlined in the previous section on changing {term}`algorithm preferences`.
Additionally, the {term}`expiration time` can be altered for individual {term}`User IDs<User ID>` (detailed below) or separate {term}`subkeys<OpenPGP Subkey>` (see {numref}`bind-subkey`).
### Add User ID
To {term}`bind<Binding>` a {term}`User ID` to an {term}`OpenPGP certificate` a {term}`certification signature<Certification>` ({term}`type ID<Signature Type ID>` `0x10`-`0x13`) is used which should have the following structure:
| {term}`Subpacket<OpenPGP Signature Subpacket>` | Area | {term}`Critical<Criticality Flag>` | Mandatory | Notes |
|------------------------------------------------------------------------|-----------------------------|------------------------------------------|----------------------|-------------------------------------------------------------------------------------|
| {term}`Signature Creation Time<Signature Creation Time Subpacket>` | {term}`Hashed<Hashed Area>` | True | True | Current time |
| {term}`Issuer Fingerprint<Issuer Fingerprint Subpacket>` | {term}`Hashed<Hashed Area>` | True or False | Strongly Recommended | The {term}`primary key<OpenPGP Primary Key>` is the {term}`issuer` |
| {term}`Primary User ID<Primary User ID Subpacket>` | {term}`Hashed<Hashed Area>` | True | False | Optional |
| {term}`Signature Expiration Time<Signature Expiration Time Subpacket>` | {term}`Hashed<Hashed Area>` | True | False | Optional |
In addition to these {term}`subpackets<OpenPGP Signature Subpacket>`, {term}`self-certifications<Self-certification>` for {term}`User IDs<User ID>` can include others – such as {term}`key flags<Key Flag>`, {term}`features<Features Subpacket>`, and {term}`algorithm preferences` – as shown in the previous table. This enables the specification of unique capabilities and {term}`preferences<Algorithm Preferences>` for each {term}`identity` associated with the {term}`certificate<OpenPGP Certificate>`.
### Remove or revoke a User ID
Since {term}`OpenPGP certificates<OpenPGP certificate>` are often distributed by the means of {term}`key servers<Key Server>`, new {term}`signatures<OpenPGP Signature Packet>` on a {term}`certificate<OpenPGP Certificate>` are often "[merged](certificate-merging)" into existing copies of the {term}`certificate<OpenPGP Certificate>` locally by the recipient.
This integration process means it is practically impossible to directly remove {term}`signatures<OpenPGP Signature Packet>` or {term}`User IDs<User ID>` from a {term}`certificate<OpenPGP Certificate>`, as there is no way to communicate the intention of {term}`packet<OpenPGP Signature Packet>` deletion to the recipient.
To effectively mark a {term}`User ID` as invalid, the user can publish a copy of their {term}`certificate<OpenPGP Certificate>` with a {term}`Certification Revocation signature<Certification Revocation Signature Packet>` ({term}`type ID<Signature Type ID>` `0x30`) attached to the invalidated {term}`User ID`. This {term}`signature<OpenPGP Signature Packet>` signals that the specified {term}`User ID` is no longer valid or associated with the {term}`certificate holder`.
The structure of a {term}`Certification Revocation signature<Certification Revocation Signature Packet>` is as follows:
| {term}`Subpacket<OpenPGP Signature Subpacket>` | Area | {term}`Critical<Criticality Flag>` | Mandatory | Notes |
|--------------------------------------------------------------------|-----------------------------|------------------------------------------|----------------------|-------------------------------------------------------------------------------------|
| {term}`Signature Creation Time<Signature Creation Time Subpacket>` | {term}`Hashed<Hashed Area>` | True | True | Current time |
| {term}`Issuer Fingerprint<Issuer Fingerprint Subpacket>` | {term}`Hashed<Hashed Area>` | True or False | Strongly Recommended | The {term}`primary key<OpenPGP Primary Key>` is the {term}`issuer` |
| {term}`Reason for Revocation<Reason for Revocation Subpacket>` | {term}`Hashed<Hashed Area>` | True | False | Determines {term}`soft<Soft Revocation>` or {term}`hard revocation` |
For {term}`User ID` {term}`revocations<Revocation>`, the *{term}`Reason for Revocation<Reason for Revocation Subpacket>`* {term}`subpacket<OpenPGP Signature Subpacket>` is crucial. A value of `0` means no specific reason, leading to a {term}`hard revocation`, while `32` indicates the {term}`User ID` is no longer valid, resulting in a {term}`soft revocation`. Omitting the {term}`reason subpacket<Reason For Revocation Subpacket>` is also equivalent to a {term}`hard revocation`.
It is generally advisable to use reason code `32` for revoking {term}`User IDs<User ID>`.
(recipe-binding-subkeys)=
### Add a subkey
As part of {term}`life-cycle management`, users may need to add a new {term}`subkey<OpenPGP Subkey>` to their {term}`OpenPGP certificate`, often for reasons such as upgrading to a {term}`subkey<OpenPGP Subkey>` with more advanced cryptographic algorithms. The process involves creating a specific {term}`signature<OpenPGP Signature Packet>` structure:
| {term}`Subpacket<OpenPGP Signature Subpacket>` | Area | {term}`Critical<Criticality Flag>` | Mandatory | Notes |
|---------------------------------------------------------------------------------------------|-----------------------------|------------------------------------------|-------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| {term}`Signature Creation Time<Signature Creation Time Subpacket>` | {term}`Hashed<Hashed Area>` | True | True | Current time |
| {term}`Issuer Fingerprint<Issuer Fingerprint Subpacket>` | {term}`Hashed<Hashed Area>` | True or False | Strongly Recommended | The {term}`primary key<OpenPGP Primary Key>` is the {term}`issuer` |
| {term}`Key Flags<Key Flag>` | {term}`Hashed<Hashed Area>` | True | Strongly Recommended | Determine the usage of the {term}`key<OpenPGP Subkey>` |
| {term}`Key Expiration Time<Key Expiration Time Subpacket>` | {term}`Hashed<Hashed Area>` | True | False | Specifies the {term}`expiration time` of the {term}`subkey<OpenPGP Subkey>` |
| {term}`Embedded Signature<Embedded Signature Subpacket>` | {term}`Hashed<Hashed Area>` | True | If {term}`Key Flags<Key Flag>` contains **{term}`S<Signing Key Flag>`** | {term}`Signing subkeys<OpenPGP Signing Subkey>` require embedded *{term}`Primary Key Binding<Primary Key Binding Signature>`* {term}`signature<OpenPGP Signature Packet>` |
| {term}`Hash Algorithm Preferences<Preferred Hash Algorithms Subpacket>` | {term}`Hashed<Hashed Area>` | False | False | Per {term}`key<Component Key>` {term}`preferences<Algorithm Preferences>` |
| {term}`Compression Algorithm Preferences<Preferred Compression Algorithms Subpacket>` | {term}`Hashed<Hashed Area>` | False | False | Per {term}`key<Component Key>` {term}`preferences<Algorithm Preferences>` |
| {term}`Symmetric Algorithm Preferences<Preferred Symmetric Ciphers for v1 SEIPD Subpacket>` | {term}`Hashed<Hashed Area>` | False | False | Per {term}`key<Component Key>` {term}`preferences<Algorithm Preferences>` |
| {term}`AEAD Algorithm Preferences<Preferred AEAD Ciphersuites Subpacket>` | {term}`Hashed<Hashed Area>` | False | False | Per {term}`key<Component Key>` {term}`preferences<Algorithm Preferences>` |
In addition to these {term}`subpackets<OpenPGP Signature Subpacket>`, users can specify {term}`algorithm preferences` for each {term}`subkey<OpenPGP Subkey>`, distinct from those set in the {term}`certificate<OpenPGP Certificate>`'s *{term}`Direct Key<Direct Key Signature>`* {term}`signature<OpenPGP Signature Packet>`.
### Revoke a subkey
{term}`Subkeys<OpenPGP subkey>`, like {term}`User IDs<User ID>`, can be individually revoked in OpenPGP.
This is done by issuing a {term}`Subkey Revocation signature<Subkey Revocation Signature Packet>` ({term}`type ID<Signature Type ID>` `0x28`) using the {term}`primary key<OpenPGP Primary Key>`.
The structure of such a {term}`signature<OpenPGP Signature Packet>` is straightforward:
| {term}`Subpacket<OpenPGP Signature Subpacket>` | Area | {term}`Critical<Criticality Flag>` | Mandatory | Notes |
|--------------------------------------------------------------------|-----------------------------|------------------------------------------|----------------------|-------------------------------------------------------------------------------------|
| {term}`Signature Creation Time<Signature Creation Time Subpacket>` | {term}`Hashed<Hashed Area>` | True | True | Current time |
| {term}`Issuer Fingerprint<Issuer Fingerprint Subpacket>` | {term}`Hashed<Hashed Area>` | True or False | Strongly Recommended | The {term}`primary key<OpenPGP Primary Key>` is the {term}`issuer` |
| {term}`Reason for Revocation<Reason For Revocation Subpacket>` | {term}`Hashed<Hashed Area>` | True | False | Determines {term}`soft<Soft Revocation>` or {term}`hard revocation` |
In {term}`Subkey Revocation signatures<Subkey Revocation Signature Packet>`, the [reason for revocation](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-reason-for-revocation) {term}`subpacket<OpenPGP Signature Subpacket>` can only have values in the range of `0-3`. The values `1` ({term}`key<OpenPGP Subkey>` superseded) and `3` ({term}`key<OpenPGP Subkey>` retired and no longer used) indicate {term}`soft revocations<Soft Revocation>`, whereas values `0` (no reason) and `2` ({term}`key<OpenPGP Subkey>` compromised) indicate {term}`hard revocations<Hard Revocation>`.
Note that a value of `32` is not applicable in these {term}`signatures<OpenPGP Signature Packet>`.
### Revoke a certificate
Users may find themselves needing to revoke their entire {term}`OpenPGP certificate`, rendering it unusable. This could be for various reasons, such as migrating to a new {term}`certificate<OpenPGP certificate>` or in response to a compromise of the {term}`certificate<OpenPGP certificate>`'s {term}`secret key material<Private Key Material>`.
While a {term}`soft-revoked<Soft Revocation>` {term}`certificate<OpenPGP Certificate>` can be re-validated at a later time with a new {term}`certification`, a {term}`hard revocation` is permanent.
The recommended way to {term}`revoke<Revocation>` a {term}`certificate<OpenPGP Certificate>` is by issuing a {term}`Key Revocation signature<Key Revocation Signature Packet>` ({term}`type ID<Signature Type ID>` `0x20`). Its structure is similar to that of a {term}`Certification Revocation signature<Certification Revocation Signature Packet>`.
| {term}`Subpacket<OpenPGP Signature Subpacket>` | Area | {term}`Critical<Criticality Flag>` | Mandatory | Notes |
|--------------------------------------------------------------------|-----------------------------|------------------------------------------|----------------------|-------------------------------------------------------------------------------------|
| {term}`Signature Creation Time<Signature Creation Time Subpacket>` | {term}`Hashed<Hashed Area>` | True | True | Current time |
| {term}`Issuer Fingerprint<Issuer Fingerprint Subpacket>` | {term}`Hashed<Hashed Area>` | True or False | Strongly Recommended | The {term}`primary key<OpenPGP Primary Key>` is the {term}`issuer` |
| {term}`Reason for Revocation<Reason For Revocation Subpacket>` | {term}`Hashed<Hashed Area>` | True | False | Determines {term}`soft<Soft Revocation>` or {term}`hard revocation` |
For {term}`Key Revocation signatures<Key Revocation Signature Packet>`, the guidelines regarding the [*Reason for Revocation* subpacket](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-reason-for-revocation) are the same as those for {term}`Subkey Revocation signatures<Subkey Revocation Signature Packet>`.
### Common subpackets in OpenPGP signatures
In OpenPGP, certain {term}`subpackets<OpenPGP Signature Subpacket>` are universally expected across all types of {term}`signatures<OpenPGP Signature Packet>`, serving fundamental roles in the {term}`signature<OpenPGP Signature Packet>`'s structure, {term}`verification` and {term}`validation`:
* **{term}`Signature Creation Time<Signature Creation Time Subpacket>`**: This is a mandatory {term}`subpacket<OpenPGP Signature Subpacket>` in every {term}`OpenPGP signature<OpenPGP Signature Packet>`. It contains the timestamp of when the {term}`signature<OpenPGP Signature Packet>` was created. For security and integrity, this {term}`subpacket<OpenPGP Signature Subpacket>` must be located in the {term}`hashed area` of the {term}`signature<OpenPGP Signature Packet>` and is recommended to be marked as {term}`critical<Criticality Flag>`.
* **{term}`Issuer Fingerprint<Issuer Fingerprint Subpacket>`**: Essential for {term}`signature<OpenPGP Signature Packet>` {term}`validation`, this {term}`subpacket<OpenPGP Signature Subpacket>` identifies the {term}`key<OpenPGP Primary Key>` (or {term}`subkey<OpenPGP Subkey>`) that was used to create the {term}`signature<OpenPGP Signature Packet>`. OpenPGP v6 {term}`signatures<OpenPGP Signature Packet>` should include the {term}`Issuer Fingerprint subpacket`, containing the 32-byte {term}`fingerprint<OpenPGP Fingerprint>` of the {term}`key<Component Key>`.
```{note}
The {term}`key<Component Key>` used as the {term}`issuer` in the {term}`signature<OpenPGP Signature Packet>` might be a {term}`subkey<OpenPGP Subkey>` of the {term}`certificate<OpenPGP Certificate>`.
```
These {term}`subpackets<OpenPGP Signature Subpacket>` can be placed in either the {term}`hashed<Hashed Area>` or {term}`unhashed area` due to its self-{term}`authenticating<Authentication>` nature. However, we recommend including them in the {term}`signature<OpenPGP Signature Packet>`'s {term}`hashed area`.
## Managing subpacket conflicts and duplication
In {term}`OpenPGP signatures<OpenPGP Signature Packet>`, both the {term}`hashed<Hashed Area>` and {term}`unhashed areas<Unhashed Area>` are composed of lists of {term}`subpackets<OpenPGP Signature Subpacket>`. Inherently, this structure permits the duplication of the same {term}`subpacket<OpenPGP Signature Subpacket>`, which could lead to conflicts. To manage these potential conflicts, the following strategies are used:
- **Precedence of {term}`hashed area`**: {term}`Subpackets<OpenPGP Signature Subpacket>` within the {term}`hashed area` of a {term}`signature<OpenPGP Signature Packet>` take precedence over those in the {term}`unhashed area`. This hierarchy helps resolve conflicts when the same {term}`subpacket<OpenPGP Signature Subpacket>` appears in both areas.
- **Handling conflicts within the same area**: Conflicts can still arise within the same area, such as when two {term}`subpackets<OpenPGP Signature Subpacket>` have different {term}`expiration times<Expiration Time>`. In such cases, the [OpenPGP specification](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-notes-on-subpackets) advises that {term}`implementations<OpenPGP Implementation>` should favor the last occurrence of a conflicting {term}`subpacket<OpenPGP Signature Subpacket>` in the {term}`hashed area`.
In certain scenarios, having duplicate {term}`subpackets<OpenPGP Signature Subpacket>` with conflicting content is logical and even necessary. For example, consider a {term}`signature<OpenPGP Signature Packet>` created by a version 4 {term}`issuer` {term}`key<Component Key>`, which was upgraded from an older OpenPGP version (like v3). Since the {term}`key ID` calculation scheme changed from v3 to v4, the identifiers for the same {term}`key<Component Key>` would differ between these versions. Therefore, a v4 signature might contain two {term}`issuer key ID subpackets<Issuer Fingerprint Subpacket>`, each with different, yet correct values for v3 and v4 {term}`keys<Component Key>`, respectively. This allows for backward compatibility and ensures the {term}`signature<OpenPGP Signature Packet>` can be {term}`validated<Validation>` under both {term}`key ID` calculation schemes.
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