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Add glossary links for chapter 7

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Signed-off-by: David Runge <dave@sleepmap.de>
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@ -6,67 +6,68 @@ SPDX-License-Identifier: CC-BY-SA-4.0
(signing_data)=
# Signatures over data
In OpenPGP, a *data signature* guarantees the authenticity and, implicitly, the integrity of certain data. Typical use cases of data signatures include the authentication of software packages and emails.
In OpenPGP, a *{term}`data signature`* guarantees the {term}`authenticity<Authentication>` and, implicitly, the integrity of certain data. Typical use cases of {term}`data signatures<Data Signature>` include the {term}`authentication` of software packages and emails.
"Authenticity" in this context means that the data signature was issued by the entity controlling the signing key material. However,
it does not automatically signal if the expected party indeed controls the signer certificate. OpenPGP does offer mechanisms for *strong authentication*, connecting certificates to specific identities. This verifies that the intended communication partner is indeed associated with the cryptographic identity behind the signature[^sign-auth].
"{term}`Authenticity<Authentication>`" in this context means that the {term}`data signature` was issued by {term}`the entity controlling the signing key material<Certificate Holder>`. However,
it does not automatically signal if the expected party indeed controls the {term}`signer` {term}`certificate<OpenPGP Certificate>`. OpenPGP does offer mechanisms for *strong {term}`authentication`*, connecting {term}`certificates<OpenPGP Certificate>` to specific {term}`identities<Identity>`. This verifies that the intended communication partner is indeed associated with the cryptographic {term}`identity` behind the {term}`signature<OpenPGP Signature Packet>`[^sign-auth].
[^sign-auth]: Other signing solutions, like [signify](https://flak.tedunangst.com/post/signify), focus on pure signing without strong authentication of the signer's identity.
[^sign-auth]: Other signing solutions, like [signify](https://flak.tedunangst.com/post/signify), focus on pure signing without strong {term}`authentication` of the {term}`signer`'s {term}`identity`.
Data signatures can only be issued by component keys with the *signing* [key flag](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-key-flags).
{term}`Data signatures<Data Signature>` can only be issued by {term}`component keys<Component Key>` with the *{term}`signing<Signing Key Flag>`* [key flag](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-key-flags).
Note that data signatures are distinct from {ref}`component_signatures_chapter`, which are used to form and maintain certificates, as well as to certify identities on certificates.
Note that {term}`data signatures<Data Signature>` are distinct from {ref}`component_signatures_chapter`, which are used to form and maintain {term}`certificates<OpenPGP Certificate>`, as well as to {term}`certify<Certification>` {term}`identities<Identity>` on {term}`certificates<OpenPGP Certificate>`.
(data_signature_types)=
## Signature types
OpenPGP data signatures use one of two [signature types](signature_types):
{term}`OpenPGP data signatures<Data Signature>` use one of two [signature types](signature_types):
- [**Binary signature**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#sigtype-binary) (type ID `0x00`): This is the standard signature type for binary data and is typically used for files or data streams. Binary signatures are calculated over the data without any modifications or transformations.
- [**Text signature**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-signature-of-a-canonical-te) (type ID `0x01`): Used for textual data, such as email bodies. When calculating a text signature, the data is first normalized by converting line endings into a canonical form (`<CR><LF>`). This approach mitigates issues caused by platform-specific text encodings. This is especially important for detached and cleartext signatures, where the message file might undergo re-encoding between the creation and verification of the signature.
- [**Binary signature**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#sigtype-binary) ({term}`type ID<Signature Type ID>` `0x00`): This is the standard {term}`signature type` for binary data and is typically used for files or data streams. {term}`Binary signatures<Binary Signature>` are calculated over the data without any modifications or transformations.
- [**Text signature**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-signature-of-a-canonical-te) ({term}`type ID<Signature Type ID>` `0x01`): Used for textual data, such as email bodies. When calculating a {term}`text signature`, the data is first normalized by converting line endings into a canonical form (`<CR><LF>`). This approach mitigates issues caused by platform-specific text encodings. This is especially important for detached and {term}`cleartext signatures<Cleartext Signature>`, where the message file might undergo re-encoding between the creation and {term}`verification` of the {term}`signature<OpenPGP Signature Packet>`.
Data signatures are generated by hashing the message content along with the metadata in the OpenPGP signature packet, and calculating a cryptographic signature over that hash. The resulting cryptographic signature is stored in the signature packet.
{term}`Data signatures<Data Signature>` are generated by {term}`hashing<Hash Digest>` the message content along with the {term}`metadata` in the {term}`OpenPGP signature packet`, and calculating a {term}`cryptographic signature` over that {term}`hash<Hash Digest>`. The resulting {term}`cryptographic signature` is stored in the {term}`signature packet<OpenPGP Signature Packet>`.
Data signature packets manifest in three distinct forms, which will be detailed in the subsequent section.
{term}`Data signature packets<Data Signature Packet>` manifest in three distinct forms, which will be detailed in the subsequent section.
(forms_of_openpgp_data_signatures)=
## Forms of OpenPGP data signatures
OpenPGP data signatures can be applied in three distinct forms[^sign-modes-gpg]:
{term}`OpenPGP data signatures<Data Signature>` can be applied in three distinct forms[^sign-modes-gpg]:
- **Detached**: The OpenPGP signature exists as a separate entity, independent of the signed data.
- **Inline**: Both the original data and its corresponding OpenPGP signature are encapsulated within an OpenPGP container.
- **Cleartext signature**: A plaintext message and its OpenPGP signature coexist in a combined text format, preserving the readability of the original message.
- **{term}`Detached<Detached Signature>`**: The OpenPGP signature exists as a separate entity, independent of the signed data.
- **{term}`Inline<Inline Signature>`**: Both the original data and its corresponding {term}`OpenPGP signature<OpenPGP Signature Packet>` are encapsulated within an {term}`OpenPGP message`.
- **{term}`Cleartext signature`**: A plaintext message and its {term}`OpenPGP signature<OpenPGP Signature Packet>` coexist in a combined text format, preserving the readability of the original message.
[^sign-modes-gpg]: These three forms of signature application align with GnuPG's `--detach-sign`, `--sign`, and `--clearsign` command options.
[^sign-modes-gpg]: These three forms of {term}`signature<OpenPGP Signature Packet>` application align with GnuPG's `--detach-sign`, `--sign`, and `--clearsign` command options.
### Detached signatures
A detached signature is produced by calculating an OpenPGP signature over the data intended for signing. The original data remains unchanged, and the OpenPGP signature is stored as a standalone file. A detached signature file can be distributed alongside or independent of the original data. The authenticity and integrity of the original data file can be verified by using the detached signature file.
A {term}`detached signature` is produced by calculating an {term}`OpenPGP signature<OpenPGP Signature Packet>` over the data intended for signing. The original data remains unchanged, and the {term}`OpenPGP signature<OpenPGP Signature Packet>` is stored as a standalone file. A {term}`detached signature` file can be distributed alongside or independent of the original data. The {term}`authenticity<Authentication>` and integrity of the original data file can be {term}`verified<Verification>` by using the {term}`detached signature` file.
This signature format is especially useful for signing software releases and other files where it is imperative that the content remains unaltered during the signing process.
This {term}`signature<OpenPGP Signature Packet>` format is especially useful for signing software releases and other files where it is imperative that the content remains unaltered during the signing process.
(inline_signature)=
### Inline signatures
An inline signature joins the signed data and its corresponding data signature into a single OpenPGP message.
An {term}`inline signature` joins the signed data and its corresponding {term}`data signature` into a single {term}`OpenPGP message`.
This method is commonly used for signing or encrypting emails. Most email software capable of handling OpenPGP communications typically uses inline signatures.
This method is commonly used for signing or encrypting emails. Most email software capable of handling OpenPGP communications typically uses {term}`inline signatures<Inline Signature>`.
#### Structure
An inline-signed OpenPGP message consists of three segments:
An {term}`inline-signed<Inline Signature>` {term}`OpenPGP message` consists of three segments:
1. [**One-pass signature packets**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#one-pass-sig): These one or more packets precede the signed data and enable signature computation in one pass.
1. [**One-pass signature packets**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#one-pass-sig): These one or more {term}`packets<Packet>` precede the signed data and enable {term}`signature<OpenPGP Signature Packet>` computation in one pass.
2. [**Literal data packet**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#lit): This contains the original data (e.g., the body of a message), without additional interpretation or conversion.
3. **Data signature packets**: These contain the cryptographic signature corresponding to the original data.
3. **{term}`Data signature packets<OpenPGP Signature Packet>`**: These contain the {term}`cryptographic signature` corresponding to the original data.
#### Creation
To produce an inline signature, the signer processes the entirety of the data by reading from an input file and writing into an output OpenPGP message file. As the data is processed, the signer simultaneously calculates a cryptographic signature. This procedure results in the appending of a data signature packet to the output OpenPGP message file, where it can be efficiently stored.
To produce an {term}`inline signature`, the {term}`signer` processes the entirety of the data by reading from an input file and writing into an output {term}`OpenPGP message` file. As the data is processed, the {term}`signer` simultaneously calculates a {term}`cryptographic signature`. This procedure results in the appending of a {term}`data signature packet` to the output {term}`OpenPGP message` file, where it can be efficiently stored.
For efficient verification, an application must understand how to handle the literal data prior to its reading. This requirement is addressed by the one-pass signature packets located at the beginning of inline-signed messages. These packets include essential information such as the fingerprint of the signing key and the hash algorithm used for computing the signature's hash digest. This setup enables the verifier to process the data correctly and efficiently.
For efficient {term}`verification`, an application must understand how to handle the {term}`literal data<Literal Data Packet>` prior to its reading. This requirement is addressed by the {term}`one-pass signature packets<One-pass Signature Packet>` located at the beginning of {term}`inline-signed<Inline Signature>` messages. These {term}`packets<Packet>` include essential information such as the {term}`fingerprint<OpenPGP Fingerprint>` of the {term}`signing key<OpenPGP Component Key>` and the {term}`hash<Hash Digest>` algorithm used for computing the {term}`signature<OpenPGP Signature Packet>`'s {term}`hash digest`. This setup enables the verifier to process the data correctly and efficiently.
```{admonition} TODO
:class: warning
@ -78,23 +79,23 @@ Realization: It's probably useful to know the fingerprints right away, to first
#### Verification
Inline-signed messages enable efficient verification in *one pass*, structured as follows:
{term}`Inline-signed<Inline Signature>` messages enable efficient {term}`verification` in *one pass*, structured as follows:
1. **Initiation with one-pass signature packets**: These packets begin the verification process. They include the signer's key ID/fingerprint, essential for identifying the appropriate public key for signature validation.
1. **Initiation with {term}`one-pass signature packets<One-pass Signature Packet>`**: These {term}`packets<Packet>` begin the {term}`verification` process. They include the {term}`signer`'s {term}`key ID`/{term}`fingerprint<OpenPGP Fingerprint>`, essential for identifying the appropriate {term}`public key<OpenPGP Certificate>` for signature {term}`validation`.
2. **Processing the literal data packet**: This step involves hashing the literal data, preparing it for signature verification.
2. **Processing the {term}`literal data packet`**: This step involves {term}`hashing<Hash Digest>` the literal data, preparing it for {term}`signature<OpenPGP Signature Packet>` {term}`verification`.
3. **Verifying signature packets**: Located at the end of the message, these packets are checked against the previously calculated hash digest.
3. **{term}`Verifying<Verification>` {term}`signature packets<OpenPGP Signature Packet>`**: Located at the end of the message, these {term}`packets<Packet>` are checked against the previously calculated {term}`hash digest`.
Important to note, the signer's public key, critical for the final verification step, is not embedded in the message. Verifiers must acquire this key externally (e.g., from a key server) to authenticate the signature successfully.
Important to note, the {term}`signer`'s {term}`public key<OpenPGP Certificate>`, critical for the final {term}`verification` step, is not embedded in the message. Verifiers must acquire this {term}`key` externally (e.g., from a {term}`key server`) to authenticate the {term}`signature<OpenPGP Signature Packet>` successfully.
(cleartext-sig)=
### Cleartext signatures
The *Cleartext Signature Framework* (CSF) in OpenPGP accomplishes two primary objectives:
The *{term}`Cleartext Signature Framework`* (CSF) in OpenPGP accomplishes two primary objectives:
- maintaining the message in a human-readable cleartext format, accessible without OpenPGP-specific software
- incorporating an OpenPGP signature for authentication by users with OpenPGP-compatible software
- incorporating an {term}`OpenPGP signature<OpenPGP Signature Packet>` for {term}`authentication` by users with OpenPGP-compatible software
#### Example
@ -115,31 +116,31 @@ r13/eqMN8kfCDw==
-----END PGP SIGNATURE-----
```
This signature consists of two parts: a message ("hello world") and an ASCII-armored OpenPGP signature. The message is immediately comprehensible to a human reader, while the signature block allows for the message's authenticity verification via OpenPGP software.
This {term}`signature<Cleartext Signature>` consists of two parts: a message ("hello world") and an ASCII-armored {term}`OpenPGP signature<OpenPGP Signature Packet>`. The message is immediately comprehensible to a human reader, while the {term}`signature<OpenPGP Signature Packet>` block allows for the message's {term}`authenticity<Authentication>` {term}`verification` via OpenPGP software.
#### Use case
Clear text signatures combine the advantages of both detached and inline signatures:
{term}`Cleartext signatures<Cleartext Signature>` combine the advantages of both {term}`detached<Detached Signature>` and {term}`inline signatures<Inline Signature>`:
- **Self-contained format**: Cleartext signatures enable the message and its signature to be stored as a single file.
- **Self-contained format**: {term}`Cleartext signatures<Cleartext signature>` enable the message and its {term}`signature<OpenPGP Signature Packet>` to be stored as a single file.
- **Human readability**: The message within a cleartext signature remains accessible in a plain text format. This eliminates the need for specialized software to read the message content.
- **Human readability**: The message within a {term}`cleartext signature` remains accessible in a plain text format. This eliminates the need for specialized software to read the message content.
These features are particularly beneficial in scenarios where signed messages are managed semi-manually and where existing system infrastructure offers limited or no native support for OpenPGP in the workflow[^arch-certifications].
[^arch-certifications]: An illustrative example is the workflow adopted by Arch Linux to certify User IDs of new packagers. This process relies on [cleartext signed statements from existing packagers](https://gitlab.archlinux.org/archlinux/archlinux-keyring/-/blob/master/.gitlab/issue_templates/New%20Packager%20Key.md?ref_type=heads&plain=1#L33-46). These signed statements are stored as attachments in an issue tracking system for later inspection. The advantage of this approach lies in the convenience of having the message and signature in a single file, which simplifies manual handling. Based on the vouches in these cleartext signed messages and an [email confirmation from the new packager](https://gitlab.archlinux.org/archlinux/archlinux-keyring/-/wikis/workflows/verify-a-packager-key), the main key operators can issue OpenPGP third-party certifications.
[^arch-certifications]: An illustrative example is the workflow adopted by Arch Linux to {term}`certify<Certification>` {term}`User IDs<User ID>` of new packagers. This process relies on [cleartext signed statements from existing packagers](https://gitlab.archlinux.org/archlinux/archlinux-keyring/-/blob/master/.gitlab/issue_templates/New%20Packager%20Key.md?ref_type=heads&plain=1#L33-46). These signed statements are stored as attachments in an issue tracking system for later inspection. The advantage of this approach lies in the convenience of having the message and signature in a single file, which simplifies manual handling. Based on the vouches in these {term}`cleartext signed<Cleartext Signature>` messages and an [email confirmation from the new packager](https://gitlab.archlinux.org/archlinux/archlinux-keyring/-/wikis/workflows/verify-a-packager-key), the main key operators can issue {term}`OpenPGP third-party certifications<Third-party Identity Certification>`.
#### Text transformations for cleartext signatures
The cleartext signature framework includes specific text normalization procedures to ensure the integrity and clarity of the message:
The {term}`cleartext signature framework` includes specific text normalization procedures to ensure the integrity and clarity of the message:
- **Escaping dashes**: The framework implements a method of [dash-escaped text](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-dash-escaped-text) within the message. Dash-escaping ensures that the parser correctly distinguishes between the armor headers, which are part of the signature's structure, and any lines in the message that happen to start with a dash.
- **Escaping dashes**: The framework implements a method of [dash-escaped text](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-dash-escaped-text) within the message. Dash-escaping ensures that the parser correctly distinguishes between the armor headers, which are part of the {term}`signature<OpenPGP Signature Packet>`'s structure, and any lines in the message that happen to start with a dash.
- **Normalization of line endings**: Consistent with the approach for any other [text signature](data_signature_types), a cleartext signature is calculated on the text with normalized line endings (`<CR><LF>`). This ensures that the signature remains valid regardless of the text format of the receiving implementation.
- **Normalization of line endings**: Consistent with the approach for any other [text signature](data_signature_types), a {term}`cleartext signature` is calculated on the text with normalized line endings (`<CR><LF>`). This ensures that the {term}`signature<OpenPGP Signature Packet>` remains valid regardless of the text format of the receiving {term}`implementation<OpenPGP Implementation>`.
#### Pitfalls
Despite their widespread adoption, cleartext signatures have their limitations and are sometimes viewed as a "legacy method"[^csf-gnupg]. The RFC details the [pitfalls of cleartext signatures](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-issues-with-the-cleartext-s), such as incompatibility with semantically meaningful whitespace, challenges with large messages, and security vulnerabilities related to misleading Hash header manipulations. Given these issues, safer alternatives like inline and detached signature forms are advised.
Despite their widespread adoption, {term}`cleartext signatures<Cleartext Signature>` have their limitations and are sometimes viewed as a "legacy method"[^csf-gnupg]. The {term}`RFC` details the [pitfalls of cleartext signatures](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-issues-with-the-cleartext-s), such as incompatibility with semantically meaningful whitespace, challenges with large messages, and security vulnerabilities related to misleading Hash header manipulations. Given these issues, safer alternatives like {term}`inline<Inline Signature>` and {term}`detached signature` forms are advised.
[^csf-gnupg]: https://lists.gnupg.org/pipermail/gnupg-devel/2023-November/035428.html
@ -151,4 +152,4 @@ Despite their widespread adoption, cleartext signatures have their limitations a
:class: warning
Write
```
```