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

(certificates_chapter)=
# Certificates

OpenPGP fundamentally hinges on the concept of "OpenPGP certificates," often referred to as "OpenPGP keys." These certificates are complex data structures essential for identity verification, data encryption, and digital signatures. Understanding their structure and functionality is pivotal for effective application of the OpenPGP standard.

## Terminology: Understanding "keys"

The term "(cryptographic) keys" is central to grasping the concept of OpenPGP certificates. However, it can refer to different entities, making it a potentially confusing term. Let's clarify those differences.

### Public vs. private keys

The term "key," without additional context, can refer to either public or private asymmetric key material. Additionally, symmetric keys may be used in OpenPGP to encrypt private key material, adding a layer of security and complexity.

### Layers of keys in OpenPGP

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

1. A (bare) ["cryptographic key"](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.

The following section will delve into the OpenPGP-specific layers (2 and 3) to provide a clearer understanding of their roles within OpenPGP certificates.

For detailed insights on structure and handling, refer to our chapters on OpenPGP [certificates](certificates_chapter) and [private keys](private_key_chapter). Additionally, managing certificates, and understanding their authentication and trust models are vital topics. While this document briefly touches upon these aspects, they are integral to working proficiently with OpenPGP.

## Structure of OpenPGP certificates

An OpenPGP certificate (or "OpenPGP key") is a collection of an arbitrary number of elements[^packets]:

[^packets]: In technical terms, the elements of an OpenPGP certificate are a collection "packets". Each component key and identity component is internally represented as one packet. The other common type of element is "signature" packets, which link the components of a certificate together.

- Component OpenPGP keys,
- Identity components,
- Other metadata (this includes connections between the certificate's components).

We sometimes collectively refer to component keys and identity information as "the components of a certificate."

```{figure} diag/OpenPGP_Certificate.png

Typical components in an OpenPGP certificate
```

All elements in an OpenPGP certificate are structured around one central component: the *OpenPGP primary key*. The primary key acts as a personal CA for the certificate's owner: It can make cryptographic statements about subkeys, identities, expiration, revocation, ...

```{note}
OpenPGP certificates are typically long-lived and may be changed (typically by their owner), over time. Components can be added and invalidated, over the lifetime of a certificate
```

### OpenPGP component keys

An OpenPGP certificate usually contains multiple OpenPGP component keys.

OpenPGP component keys consist of an [asymmetric cryptographic keypair](asymmetric_key_pair) and a creation timestamp. These attributes of a component key cannot be changed after creation (in the case of ECDH keys, two additional parameters are part of a component key's constituting data[^ecdh-paramters]).

[^ecdh-paramters]: 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 part of the component key's constituting and immutable properties. Those parameters define a hash function and a symmetric encryption algorithm.

```{figure} diag/Component_Key.svg

An OpenPGP component key
```

Component key representations that include private key material also contain metadata that specifies the password protection scheme for the private key material.

For each OpenPGP component key, an *OpenPGP fingerprint* can be derived from the combination of the public key material and creation timestamp (and ECDH parameters, if applicable).

```{figure} diag/Fingerprint.png

Every OpenPGP component key can be named by a fingerprint
```

The fingerprint of our example component OpenPGP key is `AAA1 8CBB 2546 85C5 8358 3205 63FD 37B6  7F33 00F9 FB0E C457 378C D29F 1026 98B3` [^keyid].

[^keyid]: In OpenPGP version 4, the rightmost 64 bit were sometimes used as a shorter identifier, called "Key ID".
E.g., an OpenPGP version 4 certificate with the fingerprint `B3D2 7B09 FBA4 1235 2B41  8972 C8B8 6AC4 2455 4239` might be referred to by the 64 bit Key ID `C8B8 6AC4 2455 4239` or styled as `0xC8B86AC424554239`.  
Historically, even shorter 32 bit identifiers have sometimes been used, like this: `2455 4239`, or `0x24554239`. You may still see such identifiers in very old documents about PGP. However, 32 bit identifiers have [been unfit for purpose for a long time](https://evil32.com/). At some point, 32 bit identifiers were called "short Key ID", while 64 bit identifiers were called "long Key ID".

Component keys are used in one of two roles: either as "OpenPGP primary key," or as an "OpenPGP subkey".

#### Primary key

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

- Its fingerprint is used as the unique identifier for the full OpenPGP certificate.
- It is used for lifecycle operations, such as adding or invalidating subkeys or identities in a certificate.

```{admonition} Terminology
:class: note

In the RFC, the OpenPGP primary key is also sometimes referred to as "top-level key." It has also sometimes informally been called "master key."
```

#### Subkeys

In addition to the primary key, modern OpenPGP certificates usually contain a number of "subkeys" (however, it's not technically necessary for a certificate to contain subkeys).

Subkeys have the same structure as the primary key, but they are used in a different role. Subkeys are cryptographically linked with the primary key (more on this below).

```{figure} diag/Subkeys.png
:name: Certificate with Subkeys
:alt: Three component keys. The primary key is shown at the top. It can be used for certification. Below it, linked with arrows, are two more component keys, used as subkeys. They are marked as "for encryption" and "for signing", respectively.

OpenPGP certificates can contain a number of subkeys
```

#### Key Flags: defining which operations a component key can perform

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 specify which operations that key can perform.

The commonly used key flags are:

- **C**ertification (issuing third-party certifications)
- **S**igning (signing data)
- **E**ncryption (encrypting data)
- **A**uthentication (commonly used for OpenPGP authentication)

By convention, only the primary key is allowed to perform "certification" operations. All other operations can be configured on either the primary key or a subkey.

```{note}

It is considered good practice to have separate component keys for each type of operation: to allow only *Certification* operations with the primary key, and to use separate *Signing*, *Encryption* and *Authentication* subkeys (independently: with most algorithms, encryption can't be shared with the other capabilities[^key-flag-sharing]).
```

[^key-flag-sharing]: With ECC algorithms, it's actually not possible to share encryption functionality with the signing-based functionalities, e.g.: ed25519 used for signing; cv25519 used for encryption.


### Identity components

#### User IDs

An OpenPGP certificate can contain any number of [User IDs](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-user-id-packet-tag-13). Each User ID associates the certificate with an identity.

Often, identities in a User ID consist of a string that is composed of a name and an email address (this string must be UTF-8 encoded).

```{figure} diag/user_id.png

OpenPGP certificates can contain any number of User IDs
```

#### Primary User ID and its implications

One User ID in a certificate has the special property of being the [Primary User ID](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-primary-user-id).

User IDs are associated with preference settings (such as preferred encryption algorithms, more on this below). The preferences associated with the Primary User ID are used by default.


#### User attributes

[User attributes](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-user-attribute-packet-tag-1) are similar to User IDs, but less commonly used.

The OpenPGP standard currently only defines one format to store in User Attributes: an [image](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-the-image-attribute-subpack), "presumably (but not required to be) that of the key owner".

### Linking the components of an OpenPGP certificate

So far we've looked at the components in an OpenPGP certificate, but certificates actually contain another set of elements, which bind the components together, and add metadata to them.

Internally, an OpenPGP certificate consists of a sequence of OpenPGP packets. These packets are just stringed together, one after the other. When a certificate is stored in a file[^tpk], it's easy to remove some of these packets, or add new ones.

[^tpk]: When stored in a file, OpenPGP certificates are in a format called [transferable public key](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-transferable-public-keys).

However, the owner of a certificate doesn't want a third party to add subkeys (or add identity claims) to their certificate, pretending that the certificate owner put those components there.

To prevent malicious addition of components, OpenPGP uses cryptographic signatures. These signatures show that components have been added by the owner of the OpenPGP certificate (these linking signatures are issued by the primary key of the certificate).

So while anyone can still unilaterally store unrelated subkeys and identity claims in an OpenPGP certificate dataset, OpenPGP implementations that read this file should discard components that don't have a valid cryptographic connection with the certificate.

(Conversely, it's easy for a third party to leave out packets when passing on an OpenPGP certificate. An attacker can, for example, choose to omit revocation packets. The recipient of such a partial copy has no way to notice the omission, without access to a different source for the certificate that contains the revocation packet.)

Note, though, that there are some cases where third parties legitimately add "unbound" packets to certificates (that is: packets that are not signed by the certificate's owner):

- [Third-party certifications](third_party_cert) are traditionally added to the certificate that they make a statement about (this can cause problems in systems that unconditionally accept and include such certifications[^flooding]),
- OpenPGP software may add [unbound identity data](unbound_user_ids), locally.

[^flooding]: Storing third-party identity certifications in the target OpenPGP certificate is convenient for consumers: it is easy to find all relevant certifications in one central location. However, when third parties can unilaterally add certifications, this opens an avenue for denial-of-service attacks by flooding. The SKS network of OpenPGP key servers [allowed and experienced this problem](https://dkg.fifthhorseman.net/blog/openpgp-certificate-flooding.html).

#### Binding subkeys to an OpenPGP certificate

Linking a subkey to an OpenPGP certificate is done with a ["Subkey Binding Signature"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#sigtype-subkey-binding). Such a signature signals that the "primary key wants to be associated with the subkey".

The subkey binding signature also adds metadata.

```{figure} diag/subkey_binding.png

Linking an OpenPGP subkey to the primary key with a binding signature
```

The [Signature packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-packet-tag-2) that binds the subkey to the primary key has the signature type [SubkeyBinding](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-subkey-binding-signature-si).

##### Binding signing subkeys to an OpenPGP certificate

Binding subkeys with the "signing" key flag is a special case:

When binding a signing subkey to a primary key, it is not sufficient that the "primary key wants to be associated with the subkey." In addition, the subkey must signal that it "wants to be associated with that primary key."

Otherwise, Alice could "adopt" Bob's signing subkey and convincingly claim that she made signatures that were in fact issued by Bob.

```{figure} diag/subkey_binding_backsig.png

Linking an OpenPGP signing subkey to the primary key with a binding signature, and an embedded primary key binding signature
```

This additional "Primary Key Binding" Signature is informally called a "back signature" (because the subkey uses the signature to point "back" to the primary key).


#### Binding identities with certifying self-signatures

"User ID" identity components are bound to an OpenPGP certificate by issuing a self-signature ("User Attributes" work analogously).

For example, the User ID `Alice Adams <alice@example.org>` may be associated with Alice's certificate `AAA1 8CBB 2546 85C5 8358 3205 63FD 37B6  7F33 00F9 FB0E C457 378C D29F 1026 98B3`.

Alice can link a User ID to her OpenPGP certificate with a cryptographic signature. To link a User ID, a self-signature is created (usually with the signature type [PositiveCertification](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#sigtype-positive-cert)). This signature is issued by the primary key.

```{figure} diag/user_id_certification.png
---
---
Linking a User ID to an OpenPGP certificate
```

(third_party_cert)=
## Third party (identity) certifications

```{admonition} TODO
:class: warning

This section needs writing
```

## Revocations

```{admonition} TODO
:class: warning

This section only contains notes and still needs to be written
```

Note: certification signatures [can be made irrevocable](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-revocable). 

### Hard vs. soft revocations


## Advanced topics

```{admonition} TODO
:class: warning

This section only contains notes and still needs to be written
```

### Certificate Management / Evolution of a certificate over time

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

### Merging

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

### How to generate "minimized" certificate?

### When are certificates valid?

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

### Best Practices regarding Key Freshness

```{admonition} TODO
:class: warning

- Expiry
- Subkey rotation

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

### Metadata Leak of Social Graph

(unbound_user_ids)=
### Adding unbound User IDs to a certificate

Some OpenPGP subsystems may add User IDs to a certificate, which are not bound to the primary key by the certificate's owner. This can be useful to store local identity information (e.g., Sequoia's public store attaches "pet-names" to certificates, in this way).

## Zooming in: Packet structure

Now that we've established the concepts of the components that OpenPGP certificates consist of, let's look at the internal details of our example certificate.

### A very minimal OpenPGP "Transferrable Secret Key"

We'll start with a very minimal version of [](alice_priv), stored as a "Transferrable Secret Key" (that is, including private key material).

In this section, we use the Sequoia-PGP tool `sq` to handle and transform our example OpenPGP key, and to inspect internal OpenPGP packet data.

One way to produce this minimal version of Alice's key is:

```text
$ sq packet split alice.priv
```

With this command, `sq` generates a set of files, one for each packet in `alice.priv`:

```text
alice.priv-0--SecretKey
alice.priv-1--Signature
alice.priv-2--UserID
alice.priv-3--Signature
alice.priv-4--SecretSubkey
alice.priv-5--Signature
alice.priv-6--SecretSubkey
alice.priv-7--Signature
alice.priv-8--SecretSubkey
alice.priv-9--Signature
```

For our first step, we'll use just the first two of these packets, and join them together as a private key:

```text
$ sq packet join alice.priv-0--SecretKey alice.priv-1--Signature --output alice_minimal.priv
```

This version of Alice's key contains just two packets:

- The [*"Secret-Key Packet"*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-key-packet-formats) for the primary key, and
- A [*"Direct Key Signature"*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#sigtype-direct-key) (a self-signature that binds metadata to the primary key).

In the real world, you won't usually encounter an OpenPGP key that is quite this minimal. However, this is technically a valid OpenPGP key (and we'll add more components to it, later in this section).

In ASCII-armored representation, this very minimal key looks like this:

```text
-----BEGIN PGP PRIVATE KEY BLOCK-----
Comment: AAA1 8CBB 2546 85C5 8358 3205 63FD 37B6  7F33 00F9 FB0E C457 378C D29F 1026 98B3

xUsGZRbqphsAAAAgUyTpQ6+rFfdu1bUSmHlpzRtdEGXr50Liq0f0hrOuZT4A7+GZ
tV8R+6qT6CadO7ItciB9/71C3UvpozaBO6XMz/vCtgYfGwoAAAA9BYJlFuqmBYkF
pI+9AwsJBwMVCggCmwECHgEiIQaqoYy7JUaFxYNYMgVj/Te2fzMA+fsOxFc3jNKf
ECaYswAAAAoJEKqhjLslRoXFZ0cgouNjgeNr0E9W18g4gAIl6FM5SWuQxg12j0S0
7ExCOI5NPRDCrSnAV85mAXOzeIGeiVLPQ40oEal3CX/L+BXIoY2sIEQrLd4TAEEy
0BA8aQZTPEmMdiOCM1QB+V+BQZAO
=f0GN
-----END PGP PRIVATE KEY BLOCK-----
```

We'll now decode this OpenPGP data, and inspect the two packets in detail.

To generate the output, we run the Sequoia-PGP tool `sq`, using the `packet dump` subcommand. The output of `sq` is one block of text, but we'll break the output up into sections here, to discuss the content of each packet:

```text
$ sq packet dump --hex alice_minimal.priv
```

#### Secret-Key Packet

The output starts with the (primary) [Secret-Key Packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-key-packet-formats) (the file `alice.priv-0--SecretKey` contains this packet):

```text
Secret-Key Packet, new CTB, 2 header bytes + 75 bytes
Version: 6
Creation time: 2023-09-29 15:17:58 UTC
Pk algo: Ed25519
Pk size: 256 bits
Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
KeyID: AAA18CBB254685C5

    Secret Key:

        Unencrypted

    00000000  c5                                                 CTB
    00000001     4b                                              length
    00000002        06                                           version
    00000003           65 16 ea a6                               creation_time
    00000007                       1b                            pk_algo
    00000008                           00 00 00 20               public_len
    0000000c                                       53 24 e9 43   ed25519_public
    00000010  af ab 15 f7 6e d5 b5 12  98 79 69 cd 1b 5d 10 65
    00000020  eb e7 42 e2 ab 47 f4 86  b3 ae 65 3e
    0000002c                                       00            s2k_usage
    0000002d                                          ef e1 99   ed25519_secret
    00000030  b5 5f 11 fb aa 93 e8 26  9d 3b b2 2d 72 20 7d ff
    00000040  bd 42 dd 4b e9 a3 36 81  3b a5 cc cf fb
```

The Secret-Key Packet consists in large part of the actual cryptographic key data. Let's look at the packet field by field:

- `CTB: 0xc5`[^CTB]: The [Packet Tag](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-packet-headers) for this packet. The binary representation of the value `0xc5` is `11000101`. Bits 7 and 6 show that the packet is in "OpenPGP packet format" (as opposed to in "Legacy packet format"). The remaining 6 bits encode the Tag's value: "5". This is the value for a Secret-Key Packet, as shown in the list of [Packet Tags](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-packet-tags).
- `length: 0x4b`: The remaining length of this packet.

The packet tag defines the semantics of the remaining data in the packet. We're looking at a Secret-Key Packet, which is a kind of [Key Material Packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-key-material-packets).

- `version: 0x06`: The key material is in version 6 format

This means that the next part of the packet follows the structure of [Version 6 Public Keys](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-version-6-public-keys)

- `creation_time: 0x6516eaa6`: "The time that the key was created" (also see [Time Fields](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-time-fields))
- `pk_algo: 0x1b`: "The public-key algorithm of this key" (decimal value 27, see the list of [Public-Key Algorithms](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-algorithms))
- `public_len: 0x00000020`: "Octet count for the following public key material" (in this case, the length of the following `ed25519_public` field)
- `ed25519_public`: [Algorithm-specific representation](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-part-for-ed2) of the public key material (the format is based on the value of `pk_algo`), in this case 32 bytes of the Ed25519 public key

This concludes the Public Key section of the packet. The remaining data follows the [Secret-Key Packet Format](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-key-packet-formats):

- `s2k_usage: 0x00`: [This "S2K usage" value](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-key-encryption-s2k-u) specifies that the secret-key data is not encrypted
- `ed25519_secret`: [Algorithm-specific representation](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-part-for-ed2) of the secret key data (the format is based on the value of `pk_algo`)

[^CTB]: Sequoia uses the term "CTB" (Cipher Type Byte) to refer to the RFC's ["Packet Tag"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-packet-headers)

```{tip}

The overall structure of OpenPGP packets is described in the [Packet Syntax](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-packet-syntax) chapter of the RFC.
```

Note that the "Secret-Key" Packet contains both the private and the public part of the key.

#### Direct Key Signature

The next packet is a [*"Direct Key Signature"*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#sigtype-direct-key), which is bound to the primary key (the file `alice.priv-1--Signature` contains this packet).

This packet "binds the information in the Signature subpackets to the key". Each entry under "Signature Packet -> Hashed area" is one Signature subpacket, for example, including information about algorithm preferences (*"Symmetric algo preferences"* and *"Hash preferences"*).

```text
Signature Packet, new CTB, 2 header bytes + 182 bytes
Version: 6
Type: DirectKey
Pk algo: Ed25519
Hash algo: SHA512
Hashed area:
Signature creation time: 2023-09-29 15:17:58 UTC (critical)
Key expiration time: P1095DT62781S (critical)
Symmetric algo preferences: AES256, AES128
Hash preferences: SHA512, SHA256
Key flags: C (critical)
Features: MDC
Issuer Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
Unhashed area:
Issuer: AAA18CBB254685C5
Digest prefix: 6747
Level: 0 (signature over data)

    00000000  c2                                                 CTB
    00000001     b6                                              length
    00000002        06                                           version
    00000003           1f                                        type
    00000004              1b                                     pk_algo
    00000005                 0a                                  hash_algo
    00000006                    00 00  00 3d                     hashed_area_len
    0000000a                                 05                  subpacket length
    0000000b                                    82               subpacket tag
    0000000c                                       65 16 ea a6   sig creation time
    00000010  05                                                 subpacket length
    00000011     89                                              subpacket tag
    00000012        05 a4 8f bd                                  key expiry time
    00000016                    03                               subpacket length
    00000017                       0b                            subpacket tag
    00000018                           09 07                     pref sym algos
    0000001a                                 03                  subpacket length
    0000001b                                    15               subpacket tag
    0000001c                                       0a 08         pref hash algos
    0000001e                                             02      subpacket length
    0000001f                                                9b   subpacket tag
    00000020  01                                                 key flags
    00000021     02                                              subpacket length
    00000022        1e                                           subpacket tag
    00000023           01                                        features
    00000024              22                                     subpacket length
    00000025                 21                                  subpacket tag
    00000026                    06                               version
    00000027                       aa  a1 8c bb 25 46 85 c5 83   issuer fp
    00000030  58 32 05 63 fd 37 b6 7f  33 00 f9 fb 0e c4 57 37
    00000040  8c d2 9f 10 26 98 b3
    00000047                       00  00 00 0a                  unhashed_area_len
    0000004b                                    09               subpacket length
    0000004c                                       10            subpacket tag
    0000004d                                          aa a1 8c   issuer
    00000050  bb 25 46 85 c5
    00000055                 67                                  digest_prefix1
    00000056                    47                               digest_prefix2
    00000057                       20                            salt_len
    00000058                           a2 e3 63 81 e3 6b d0 4f   salt
    00000060  56 d7 c8 38 80 02 25 e8  53 39 49 6b 90 c6 0d 76
    00000070  8f 44 b4 ec 4c 42 38 8e
    00000078                           4d 3d 10 c2 ad 29 c0 57   ed25519_sig
    00000080  ce 66 01 73 b3 78 81 9e  89 52 cf 43 8d 28 11 a9
    00000090  77 09 7f cb f8 15 c8 a1  8d ac 20 44 2b 2d de 13
    000000a0  00 41 32 d0 10 3c 69 06  53 3c 49 8c 76 23 82 33
    000000b0  54 01 f9 5f 81 41 90 0e
```

Let’s look at the packet field by field:

- `CTB: 0xc2`: The Packet Tag for this packet. Bits 7 and 6 show that the packet is in “OpenPGP packet format” (as opposed to in “Legacy packet format”). The remaining 6 bits encode the Tag’s value: “2”. This is the value for a Signature Packet.
- `length: 0xb6`: The remaining length of this packet.

The packet tag defines the semantics of the remaining data in the packet. We're looking at a [Signature Packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#signature-packet), so the following data is interpreted accordingly.

- `version: 0x06`: This is a version 6 signature (some of the following packet format is specific to this signature version).
- `type: 0x1f`: The [Signature Type](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-types)
- `pk_algo: 0x1b`: Public-key algorithm (decimal 27 corresponds, to [Ed25519](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-algorithms))
- `hash_algo: 0x0a`: Hash algorithm (decimal 10, corresponds to [SHA2-512](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-hash-algorithms))
- `hashed_area_len: 0x0000003d`: Length of the following hashed subpacket data

The next part of this packet contains "hashed subpacket data." A "subpacket data set" in an OpenPGP Signature contains a list of zero or more "Signature subpackets."

There are two sets of "subpacket data" in a Signature: "hashed," and "unhashed." The difference is that the hashed subpackets are protected by the digital signature of this packet, while the unhashed subpackets are not.

The following subpacket data consists of sets of "subpacket length, subpacket tag, data." We'll show the information for each subpacket as one line, starting with the [subpacket type description](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-subpacket-specifi) (based on the subpacket tag). Note that bit 7 of the subpacket tag signals if that subpacket is ["critical"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#section-5.2.3.7-10)[^critical].

[^critical]: "Critical" here means: the receiver must be able to interpret the subpacket and is expected to fail, otherwise. non-critical subpackets may be ignored by the receiver

- [Signature Creation Time](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#signature-creation-subpacket) (subpacket type 2) *critical*: `0x6516eaa6`
- [Key Expiration Time](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-expiration-subpacket) (subpacket type 9) *critical*: `0x05a48fbd`
- [Preferred Symmetric Ciphers for v1 SEIPD](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#preferred-v1-seipd) (type 11): `0x09 0x07`. (These values [correspond to](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#symmetric-algos): "AES with 256-bit key" and "AES with 128-bit key")
- [Preferred Hash Algorithms](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#preferred-hashes-subpacket) (subpacket type 21): `0x0a 0x08`. (These values [correspond to](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-hash-algorithms): "SHA2-512" and "SHA2-256")
- [Key Flags](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-flags) (subpacket type 27) *critical*: `0x01`. (This value [corresponds](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-key-flags) to the "certifications" key flag)
- [Features](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#features-subpacket) (subpacket type 30): `0x01`. (This value [corresponds](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-features) to: "Symmetrically Encrypted Integrity Protected Data packet version 1")
- [Issuer Fingerprint](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#issuer-fingerprint-subpacket) (subpacket type 33): `aaa18cbb254685c58358320563fd37b67f3300f9fb0ec457378cd29f102698b3` (this is the fingerprint of the component key that issued the signature in this packet. Not that here, the value is the primary key fingerprint of the certificate we're looking at.)

The next part of this packet contains "unhashed subpacket data":

- `unhashed_area_len: 0x0000000a`: Length of the following unhashed subpacket data.

As above, the following subpacket data consists of sets of "length, tag, data." In this case, only subpacket follows:

- [Issuer Key ID](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#issuer-keyid-subpacket) (subpacket type 16): `aaa18cbb254685c5` (this is the shortened version 6 "Key ID" of the fingerprint of this certificate's primary key)

This concludes the unhashed subpacket data.

- `digest_prefix: 0x6747`: "The left 16 bits of the signed hash value"
- `salt_len, salt`: A random [salt value](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-advantages-of-salted-signat) (the size must be [matching for the hash algorithm](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#hash-algorithms-registry))
- `ed25519_sig`: [Algorithm-specific](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-fields-for-ed2) representation of the signature (in this case: 64 bytes of Ed25519 signature)

```{figure} diag/key-minimal.png
:width: 40%

A minimal OpenPGP key, visualized
```

### Seen as an OpenPGP certificate

Let's now look at a "public key" view of the (very minimal) OpenPGP key above. That is, the same data, but without the private key material parts.

An OpenPGP user might give such a certificate to a communication partner, or upload it to a key server:

```text
$ sq key extract-cert alice_minimal.priv
-----BEGIN PGP PUBLIC KEY BLOCK-----
Comment: AAA1 8CBB 2546 85C5 8358 3205 63FD 37B6  7F33 00F9 FB0E C457 378C D29F 1026 98B3

xioGZRbqphsAAAAgUyTpQ6+rFfdu1bUSmHlpzRtdEGXr50Liq0f0hrOuZT7CtgYf
GwoAAAA9BYJlFuqmBYkFpI+9AwsJBwMVCggCmwECHgEiIQaqoYy7JUaFxYNYMgVj
/Te2fzMA+fsOxFc3jNKfECaYswAAAAoJEKqhjLslRoXFZ0cgouNjgeNr0E9W18g4
gAIl6FM5SWuQxg12j0S07ExCOI5NPRDCrSnAV85mAXOzeIGeiVLPQ40oEal3CX/L
+BXIoY2sIEQrLd4TAEEy0BA8aQZTPEmMdiOCM1QB+V+BQZAO
=5nyq
-----END PGP PUBLIC KEY BLOCK-----
``` 

```text
$ sq packet dump --hex alice_minimal.pub
```

The output now starts with a (primary) [Public-Key Packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-packet-formats):

```text
Public-Key Packet, new CTB, 2 header bytes + 42 bytes
    Version: 6
    Creation time: 2023-09-29 15:17:58 UTC
    Pk algo: Ed25519
    Pk size: 256 bits
    Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
    KeyID: AAA18CBB254685C5

    00000000  c6                                                 CTB
    00000001     2a                                              length
    00000002        06                                           version
    00000003           65 16 ea a6                               creation_time
    00000007                       1b                            pk_algo
    00000008                           00 00 00 20               public_len
    0000000c                                       53 24 e9 43   ed25519_public
    00000010  af ab 15 f7 6e d5 b5 12  98 79 69 cd 1b 5d 10 65
    00000020  eb e7 42 e2 ab 47 f4 86  b3 ae 65 3e
```

Note that the packet is almost identical to the Secret Key Packet seen above.

The packet tag (called `CTB` in the output) has changed to the packet type ["Public-Key Packet"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-packet-tag-6) instead of "Secret-Key Packet."

The two packet types are very similar. Compared to the "Secret-Key Packet" packet shown above, this "Public-Key Packet" just leaves out the last section, which contained the private-key related fields `s2k_usage` and `ed25519_secret`.

The second packet in the certificate (the Direct Key Signature) is bit-for-bit identical as in the previous section. So we omit showing it again, here.

```{figure} diag/pubcert-minimal.png
:width: 40%

A minimal OpenPGP public certificate, visualized
```

In the following examples, we will only look at OpenPGP keys that include the private key material. The corresponding "certificate" variants, which only contain the public key material, are easy to imagine: like here, they just leave out the private key material.

### Subkeys

```{admonition} TODO
:class: warning

The following text is unfinished and still needs processing/writing.

**This point marks the end of the material that should be read/edited.**
```

From here on, we'll look at the dumps in shorter format (you can see more detail by copying the certificates into the ["Sequoia OpenPGP Packet dumper"](https://dump.sequoia-pgp.org/) and checking the "HexDump" checkbox).

### User IDs

User IDs are a mechanism for attaching *identities* to an OpenPGP certificate. Typically, a User ID will contain a name and an email address.

To look into these, we'll make a certificate that has one [User ID](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#uid). User IDs are *"intended to represent the name and email address of the key holder"*. A certificate can have multiple User IDs associated with it.

Let's look into the details of this key:

```text
-----BEGIN PGP PRIVATE KEY BLOCK-----
Comment: AAA1 8CBB 2546 85C5 8358 3205 63FD 37B6  7F33 00F9 FB0E C457 378C D29F 1026 98B3
Comment: <alice@example.org>
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=bgQM
-----END PGP PRIVATE KEY BLOCK-----
```

```text
$ sq packet dump --hex alice_userid.priv
Secret-Key Packet, new CTB, 2 header bytes + 75 bytes
    Version: 6
    Creation time: 2023-09-29 15:17:58 UTC
    Pk algo: Ed25519
    Pk size: 256 bits
    Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
    KeyID: AAA18CBB254685C5

    Secret Key:

        Unencrypted

    00000000  c5                                                 CTB
    00000001     4b                                              length
    00000002        06                                           version
    00000003           65 16 ea a6                               creation_time
    00000007                       1b                            pk_algo
    00000008                           00 00 00 20               public_len
    0000000c                                       53 24 e9 43   ed25519_public
    00000010  af ab 15 f7 6e d5 b5 12  98 79 69 cd 1b 5d 10 65
    00000020  eb e7 42 e2 ab 47 f4 86  b3 ae 65 3e
    0000002c                                       00            s2k_usage
    0000002d                                          ef e1 99   ed25519_secret
    00000030  b5 5f 11 fb aa 93 e8 26  9d 3b b2 2d 72 20 7d ff
    00000040  bd 42 dd 4b e9 a3 36 81  3b a5 cc cf fb

Signature Packet, new CTB, 2 header bytes + 182 bytes
    Version: 6
    Type: DirectKey
    Pk algo: Ed25519
    Hash algo: SHA512
    Hashed area:
      Signature creation time: 2023-09-29 15:17:58 UTC (critical)
      Key expiration time: P1095DT62781S (critical)
      Symmetric algo preferences: AES256, AES128
      Hash preferences: SHA512, SHA256
      Key flags: C (critical)
      Features: MDC
      Issuer Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
    Unhashed area:
      Issuer: AAA18CBB254685C5
    Digest prefix: 6747
    Level: 0 (signature over data)

    00000000  c2                                                 CTB
    00000001     b6                                              length
    00000002        06                                           version
    00000003           1f                                        type
    00000004              1b                                     pk_algo
    00000005                 0a                                  hash_algo
    00000006                    00 00  00 3d                     hashed_area_len
    0000000a                                 05                  subpacket length
    0000000b                                    82               subpacket tag
    0000000c                                       65 16 ea a6   sig creation time
    00000010  05                                                 subpacket length
    00000011     89                                              subpacket tag
    00000012        05 a4 8f bd                                  key expiry time
    00000016                    03                               subpacket length
    00000017                       0b                            subpacket tag
    00000018                           09 07                     pref sym algos
    0000001a                                 03                  subpacket length
    0000001b                                    15               subpacket tag
    0000001c                                       0a 08         pref hash algos
    0000001e                                             02      subpacket length
    0000001f                                                9b   subpacket tag
    00000020  01                                                 key flags
    00000021     02                                              subpacket length
    00000022        1e                                           subpacket tag
    00000023           01                                        features
    00000024              22                                     subpacket length
    00000025                 21                                  subpacket tag
    00000026                    06                               version
    00000027                       aa  a1 8c bb 25 46 85 c5 83   issuer fp
    00000030  58 32 05 63 fd 37 b6 7f  33 00 f9 fb 0e c4 57 37
    00000040  8c d2 9f 10 26 98 b3
    00000047                       00  00 00 0a                  unhashed_area_len
    0000004b                                    09               subpacket length
    0000004c                                       10            subpacket tag
    0000004d                                          aa a1 8c   issuer
    00000050  bb 25 46 85 c5
    00000055                 67                                  digest_prefix1
    00000056                    47                               digest_prefix2
    00000057                       20                            salt_len
    00000058                           a2 e3 63 81 e3 6b d0 4f   salt
    00000060  56 d7 c8 38 80 02 25 e8  53 39 49 6b 90 c6 0d 76
    00000070  8f 44 b4 ec 4c 42 38 8e
    00000078                           4d 3d 10 c2 ad 29 c0 57   ed25519_sig
    00000080  ce 66 01 73 b3 78 81 9e  89 52 cf 43 8d 28 11 a9
    00000090  77 09 7f cb f8 15 c8 a1  8d ac 20 44 2b 2d de 13
    000000a0  00 41 32 d0 10 3c 69 06  53 3c 49 8c 76 23 82 33
    000000b0  54 01 f9 5f 81 41 90 0e

User ID Packet, new CTB, 2 header bytes + 19 bytes
    Value: <alice@example.org>

    00000000  cd                                                 CTB
    00000001     13                                              length
    00000002        3c 61 6c 69 63 65  40 65 78 61 6d 70 6c 65   value
    00000010  2e 6f 72 67 3e

Signature Packet, new CTB, 2 header bytes + 185 bytes
    Version: 6
    Type: PositiveCertification
    Pk algo: Ed25519
    Hash algo: SHA512
    Hashed area:
      Signature creation time: 2023-09-29 15:17:58 UTC (critical)
      Key expiration time: P1095DT62781S (critical)
      Symmetric algo preferences: AES256, AES128
      Hash preferences: SHA512, SHA256
      Primary User ID: true (critical)
      Key flags: C (critical)
      Features: MDC
      Issuer Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
    Unhashed area:
      Issuer: AAA18CBB254685C5
    Digest prefix: DBB8
    Level: 0 (signature over data)

    00000000  c2                                                 CTB
    00000001     b9                                              length
    00000002        06                                           version
    00000003           13                                        type
    00000004              1b                                     pk_algo
    00000005                 0a                                  hash_algo
    00000006                    00 00  00 40                     hashed_area_len
    0000000a                                 05                  subpacket length
    0000000b                                    82               subpacket tag
    0000000c                                       65 16 ea a6   sig creation time
    00000010  05                                                 subpacket length
    00000011     89                                              subpacket tag
    00000012        05 a4 8f bd                                  key expiry time
    00000016                    03                               subpacket length
    00000017                       0b                            subpacket tag
    00000018                           09 07                     pref sym algos
    0000001a                                 03                  subpacket length
    0000001b                                    15               subpacket tag
    0000001c                                       0a 08         pref hash algos
    0000001e                                             02      subpacket length
    0000001f                                                99   subpacket tag
    00000020  01                                                 primary user id
    00000021     02                                              subpacket length
    00000022        9b                                           subpacket tag
    00000023           01                                        key flags
    00000024              02                                     subpacket length
    00000025                 1e                                  subpacket tag
    00000026                    01                               features
    00000027                       22                            subpacket length
    00000028                           21                        subpacket tag
    00000029                              06                     version
    0000002a                                 aa a1 8c bb 25 46   issuer fp
    00000030  85 c5 83 58 32 05 63 fd  37 b6 7f 33 00 f9 fb 0e
    00000040  c4 57 37 8c d2 9f 10 26  98 b3
    0000004a                                 00 00 00 0a         unhashed_area_len
    0000004e                                             09      subpacket length
    0000004f                                                10   subpacket tag
    00000050  aa a1 8c bb 25 46 85 c5                            issuer
    00000058                           db                        digest_prefix1
    00000059                              b8                     digest_prefix2
    0000005a                                 20                  salt_len
    0000005b                                    8a 2d 6f da 67   salt
    00000060  35 bc 5d 04 77 b4 9d 67  a8 6e c5 d6 88 53 5f e2
    00000070  ef f9 66 08 bf c2 e0 db  c0 56 0d
    0000007b                                    eb d4 2c a5 19   ed25519_sig
    00000080  01 0f ba 26 d0 82 a2 cf  5c eb 7a a9 72 d9 f3 b2
    00000090  66 07 8b b2 ba 3d b7 89  e4 76 04 6e 35 24 2b 27
    000000a0  29 83 be 91 9c 78 6a cc  b4 d5 69 47 76 2c 29 d6
    000000b0  54 bf 43 19 04 ff 53 98  c0 d5 0b 
```

Instead of two packets, as before, we see four packets in this certificate:

* First, a "Secret-Key Packet,"
* then a "Signature Packet" (these two packets are the same as above).
* Third, a [*"User ID Packet"*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#uid), which contains the name and email address we used
* Finally, a [*"Positive Certification Signature"*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#sigtype-positive-cert) (type 0x13), *"Positive certification of a User ID and Public-Key packet"*. This is a cryptographic artifact that "binds the User ID packet and the Key packet together", i.e. it certifies that the owner of the key wants this User ID associated with their key. (Only the person who controls the private part of this key can create this signature packet. The signature serves as proof that the owner of the key has added this User ID to the certificate)


### Certifications (Third Party Signatures)

### Revocations