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Merge pull request 'ch4: heiko writes "zooming in" (and edits everything from 4.2 forward)' (#47) from ch4 into draft

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Reviewed-on: https://codeberg.org/openpgp/notes/pulls/47
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598
book/source/04-certificates.md
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@ -40,7 +40,7 @@ We sometimes collectively refer to component keys and identity information as "t
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, ...
All elements in an OpenPGP certificate are structured around one central component: the *OpenPGP primary key*. The primary key acts as a personal {term}`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
@ -83,6 +83,8 @@ The "OpenPGP primary key" is a component key that serves a central role in an Op
- 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.
The validity of the primary key limits its capacity to confer validity to other components. E.g.: The primary key cannot confer an expiration time beyond its own expiration to a subkey. It can also not confer validity to components after it has been revoked.
```{admonition} Terminology
:class: note
@ -93,7 +95,7 @@ In the RFC, the OpenPGP primary key is also sometimes referred to as "top-level
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).
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 in {numref}`binding_subkeys`).
```{figure} diag/Subkeys.png
:name: Certificate with Subkeys
@ -102,7 +104,7 @@ Subkeys have the same structure as the primary key, but they are used in a diffe
OpenPGP certificates can contain a number of subkeys
```
#### Key Flags: defining which operations a component key can perform
#### 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.
@ -123,8 +125,11 @@ It is considered good practice to have separate component keys for each type of
[^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)=
### Identity components
Identity components in an OpenPGP certificate are used by the certificate holder to state that they are known by a certain identifier (like a name, or an email address).
#### User IDs
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.
@ -140,8 +145,14 @@ OpenPGP certificates can contain any number of User IDs
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 IDs are associated with preference settings (such as preferred encryption algorithms, more on this in {numref}`zooming_in_user_id`). The preferences associated with the Primary User ID are used by default.
```{admonition} TODO
:class: warning
i think crypto-refresh suggests that the direct key signature should hold the default preferences?
we might need to write a more nuanced text here, about how DKS and primary user id interact in v6, and mention the differences to v4?
```
#### User attributes
@ -157,11 +168,11 @@ Internally, an OpenPGP certificate consists of a sequence of OpenPGP packets. Th
[^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.
However, the owner of a certificate doesn't want a third party to add subkeys (or add [identity components](identity_components)) 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.
So while anyone can still unilaterally store unrelated subkeys and [identity components](identity_components) 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.)
@ -172,6 +183,7 @@ Note, though, that there are some cases where third parties legitimately add "un
[^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)=
#### 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".
@ -245,10 +257,20 @@ Note: certification signatures [can be made irrevocable](https://www.ietf.org/ar
This section only contains notes and still needs to be written
```
### Certificate Management / Evolution of a certificate over time
### Certificate management / Evolution of a certificate over time
Minimized versions, merging, effective "append only" semantics, ...
### "Naming" a certificate in user-facing contexts - fingerprints and beyond
```{admonition} TODO
:class: warning
In v4, a 20 byte fingerprint in hex representation was used to name certificates, even in user-facing contexts.
For v6, this type of approach is discouraged, but a replacement mechanism is still pending.
```
### Merging
- How to merge two copies of the same certificate?
@ -262,7 +284,7 @@ Minimized versions, merging, effective "append only" semantics, ...
- Subkey: Revoked/key expired/binding signature expired
- User ID: revoked, binding expired, ...
### Best Practices regarding Key Freshness
### Best practices regarding Key Freshness
```{admonition} TODO
:class: warning
@ -273,30 +295,42 @@ Minimized versions, merging, effective "append only" semantics, ...
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
### Metadata leak of Social Graph
(unbound_user_ids)=
### Adding unbound User IDs to a certificate
```{admonition} TODO
:class: warning
references/links missing
```
Some OpenPGP subsystems may add User IDs to a certificate, which are not bound to the primary key by the certificate's owner. This can be useful to store local identity information (e.g., Sequoia's public store attaches "pet-names" to certificates, in this way).
## 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"
### A very minimal OpenPGP transferable 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).
We'll start with a very minimal version of [](alice_priv), stored as a *transferable secret key* ([RFC 10.2.](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#transferable-secret-keys)) (that is, including private key material).
Note that the secret key material we're using in this chapter is not password protected. To learn more about encrypting private key material with passwords in OpenPGP, see
{numref}`encrypted_secrets`.
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:
(split_alice)=
#### Splitting an OpenPGP key into packets
One way to produce a very minimal version of Alice's key is to split her full key into its component packets, and join only the relevant ones back together into a variant of the key.
```text
$ sq packet split alice.priv
```
With this command, `sq` generates a set of files, one for each packet in `alice.priv`:
With this command, `sq` generates a set of files, each containing an individual OpenPGP packet of the original full key in `alice.priv`:
```text
alice.priv-0--SecretKey
@ -311,16 +345,54 @@ alice.priv-8--SecretSubkey
alice.priv-9--Signature
```
```{admonition} VISUAL
:class: warning
Show a very abstract diagram of the packets of Alice's OpenPGP key (above):
- Secret-Key packet
- Direct Key Signature
- User ID
- Certifying self-signature for User ID
- Secret-Subkey packet
- Subkey binding signature
- Secret-Subkey packet
- Subkey binding signature
- Secret-Subkey packet
- Subkey binding signature
```
#### Joining packets into an OpenPGP key
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
```
#### Inspecting this key
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).
- 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).
This is the shape of the packets we'll be looking at, in the following two sections:
```{figure} diag/key-minimal.png
:width: 40%
A minimal OpenPGP key, visualized
```
```{admonition} VISUAL
:class: warning
This diagram needs adjustments about
- what exactly is signed
- fix naming of fields?
We could show repeat-copies of the individual packet visualization again, below for each packet-related section.
```
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).
@ -342,7 +414,7 @@ ECaYswAAAAoJEKqhjLslRoXFZ0cgouNjgeNr0E9W18g4gAIl6FM5SWuQxg12j0S0
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:
To inspect the internal structure of the OpenPGP data, we run the Sequoia-PGP tool `sq`, using the `packet dump` subcommand. The output of `sq` is one block of text, but to discuss the content of each packet we'll break the output up into sections here:
```text
$ sq packet dump --hex alice_minimal.priv
@ -350,7 +422,7 @@ $ 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):
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
@ -380,12 +452,12 @@ KeyID: AAA18CBB254685C5
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:
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).
- `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).
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
@ -394,27 +466,28 @@ This means that the next part of the packet follows the structure of [Version 6
- `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
- `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 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):
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
- `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)
[^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.
Note that the *Secret-Key packet* contains both the private and the public part of the key.
(zooming_in_dks)=
#### 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).
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"*).
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 algorithm preference* and *hash algorithm preferences*).
```text
Signature Packet, new CTB, 2 header bytes + 182 bytes
@ -486,40 +559,42 @@ Level: 0 (signature over data)
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.
- `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.
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))
- `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."
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.
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].
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" 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
```{note}
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.)
- [Signature creation time](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#signature-creation-subpacket) (subpacket type 2, **critical**): `0x6516eaa6` (also see [Time Fields](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-time-fields))
- [Key expiration time](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-expiration-subpacket) (subpacket type 9, **critical**): `0x05a48fbd` (defined as number of seconds after the key creation time)
- [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.
- `unhashed_area_len: 0x0000000a`: Length of the following unhashed subpacket data (value: 10 bytes).
As above, the following subpacket data consists of sets of "length, tag, data." In this case, only subpacket follows:
As above, the following subpacket data consists of sets of "subpacket length, subpacket 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)
- [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.
@ -527,17 +602,15 @@ This concludes the unhashed subpacket data.
- `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%
The signature is calculated over a hash. The hash, in this case, is calculated over the following data (for details, see [Computing Signatures](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-11.html#name-computing-signatures) in the RFC):
A minimal OpenPGP key, visualized
```
- The signature's salt
- A serialized form of the primary key's public data
- A serialized form of this direct key signature packet (up to, but excluding the unhashed area)
### Seen as an OpenPGP certificate
### Seen as a very minimal 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:
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
@ -557,7 +630,7 @@ gAIl6FM5SWuQxg12j0S07ExCOI5NPRDCrSnAV85mAXOzeIGeiVLPQ40oEal3CX/L
$ 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):
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
@ -579,13 +652,11 @@ Public-Key Packet, new CTB, 2 header bytes + 42 bytes
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.
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 packet tag (called `CTB` in the output) shows the packet type is now [*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*, above. Besides this change, this *Public-Key packet* only leaves out the last section, which contained the private-key related fields `s2k_usage` and `ed25519_secret`.
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.
The following, 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%
@ -593,141 +664,310 @@ The second packet in the certificate (the Direct Key Signature) is bit-for-bit i
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.
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, their packet type is changed from a Secret-Key to a Public-Key variant, and they leave out the private key material.
### Subkeys
(zoom_enc_subkey)=
### Encryption subkey
```{admonition} TODO
:class: warning
Now we'll look at a subkey in Alice's key. An OpenPGP subkey, when it is linked to an OpenPGP certificate (via its primary key), effectively consists of two elements:
The following text is unfinished and still needs processing/writing.
- a key packet that contains the component key itself, and
- a signature packet that links this component key to the primary key (and thus implicitly to the full OpenPGP certificate).
**This point marks the end of the material that should be read/edited.**
```
In this section, we'll use the files that contain individual packets of Alice's key, which we split apart above. In this split representation of Alice's key, the encryption subkey happens to be stored in `alice.priv-4--SecretSubkey`, and the associated binding self-signature for the subkey in `alice.priv-5--Signature`.
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:
````{note}
It's common to look at a packet dump for a full OpenPGP key, 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
Comment: <alice@example.org>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=bgQM
-----END PGP PRIVATE KEY BLOCK-----
$ sq packet dump --hex alice.priv
```
That command shows the details for the full series of packets in an OpenPGP certificate (recall the list of [packets of Alice's key](split_alice)). Finding a particular packet in that list can take a moment.
In the following sections we're making it a bit easier for ourselves, and directly look at individual packets, from the files we created with `sq packet split`, above.
````
#### Secret-Subkey packet
First, we'll look at the *Secret-Subkey packet* that contains the component key data of this subkey:
```text
$ sq packet dump --hex alice_userid.priv
Secret-Key Packet, new CTB, 2 header bytes + 75 bytes
$ sq packet dump --hex alice.priv-4--SecretSubkey
Secret-Subkey Packet, new CTB, 2 header bytes + 75 bytes
Version: 6
Creation time: 2023-09-29 15:17:58 UTC
Pk algo: Ed25519
Pk algo: X25519
Pk size: 256 bits
Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
KeyID: AAA18CBB254685C5
Fingerprint: C0A58384A438E5A14F73712426A4D45DBAEEF4A39E6B30B09D5513F978ACCA94
KeyID: C0A58384A438E5A1
Secret Key:
Unencrypted
00000000 c5 CTB
00000000 c7 CTB
00000001 4b length
00000002 06 version
00000003 65 16 ea a6 creation_time
00000007 1b pk_algo
00000007 19 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
0000000c d1 ae 87 d7 x25519_public
00000010 cc 42 af 99 34 c5 c2 5c ca fa b7 4a c8 43 fc 86
00000020 35 2a 46 01 f3 cc 00 f5 4a 09 3e 3f
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
0000002d 28 7d cd x25519_secret
00000030 da 26 16 37 8d ea 24 c7 ce e7 70 c7 9b e5 6f 0a
00000040 c9 77 fb bd 23 41 73 c9 57 5a bf 7c 4c
```
Signature Packet, new CTB, 2 header bytes + 182 bytes
Notice that the structure of this *Secret-Subkey packet* is the same as the *Secret-Key Packet* of the primary key, above. Only the content of the two packets differs in some points:
- The packet tag (`CTB`) in this packet shows type 7 ([*Secret-Subkey packet*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-subkey-packet-tag-7)).
- The `pk_algo` value is set to `0x19` (decimal 25), which [corresponds to](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-algorithms) X25519. Note that even though both the primary key and this subkey use a cryptographic mechanism based on Curve25519, this encryption key uses Curve 25519 in a different way (X25519 is a Diffie–Hellman function built out of Curve25519).
- Accordingly, both parts of the cryptographic key pair are labeled with the corresponding names `x25519_public` and `x25519_secret` (however, note that this difference only reflects the semantics of the fields, which are implied by the value of `pk_algo`. The actual data in both fields consists of just 32 bytes of cryptographic key material, without any type information.)
#### Subkey binding signature
The subkey packet above by itself is disconnected from the OpenPGP certificate that it is a part of. The link between the subkey and the full OpenPGP key is made with a cryptographic signature, which is issued by the OpenPGP key's primary key.
The type of signature that is used for this is called a *subkey binding signature*, because it "binds" (as in "connects") the subkey to the rest of the key.
```{admonition} VISUAL
:class: warning
Add detailed packet diagram analogous to 4.6.1
```
```{admonition} TODO
:class: warning
david points out: "The information on metadata in binding signatures may also make sense in other contexts (direct key signature)?"
Should this text go elsewhere?
- 4.2.3?
- ch 6?
```
In addition to its core purpose of making the connection, this signature also contains additional metadata about the subkey. One reason why this metadata is in a binding signature (and not in the subkey packet) is that it may change over time. The subkey packet itself may not change over time. So metadata about the subkey that can change is stored in self-signatures: if the key holder wants to change some metadata (for example, the key's expiration time), they can issue a newer version of the same kind of signature. Receiving OpenPGP software will then understand that the newer self-signature supersedes the older signature, and that the metadata in the newer signature reflects the most current intent of the key holder.
Note that this subkey binding signature packet is quite similar to the Direct Key Signature we discussed packet above. Both signatures perform the same function in terms of adding metadata to a component key. In particular, the hashed subpacket data contains many of the same pieces of metadata.
```text
$ sq packet dump --hex alice.priv-5--Signature
Signature Packet, new CTB, 2 header bytes + 171 bytes
Version: 6
Type: DirectKey
Type: SubkeyBinding
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
Key flags: EtEr (critical)
Issuer Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
Unhashed area:
Issuer: AAA18CBB254685C5
Digest prefix: 6747
Digest prefix: 2289
Level: 0 (signature over data)
00000000 c2 CTB
00000001 b6 length
00000001 ab length
00000002 06 version
00000003 1f type
00000003 18 type
00000004 1b pk_algo
00000005 0a hash_algo
00000006 00 00 00 3d hashed_area_len
00000006 00 00 00 32 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
00000016 02 subpacket length
00000017 9b subpacket tag
00000018 0c key flags
00000019 22 subpacket length
0000001a 21 subpacket tag
0000001b 06 version
0000001c aa a1 8c bb issuer fp
00000020 25 46 85 c5 83 58 32 05 63 fd 37 b6 7f 33 00 f9
00000030 fb 0e c4 57 37 8c d2 9f 10 26 98 b3
0000003c 00 00 00 0a unhashed_area_len
00000040 09 subpacket length
00000041 10 subpacket tag
00000042 aa a1 8c bb 25 46 85 c5 issuer
0000004a 22 digest_prefix1
0000004b 89 digest_prefix2
0000004c 20 salt_len
0000004d 0b 0c 89 salt
00000050 b5 ab 15 e3 7f e4 4d b9 a7 ef 71 48 14 3b ab 26
00000060 5f 34 7f 6d 48 2e 9f 78 48 58 6d 9a fb
0000006d 6d b2 db ed25519_sig
00000070 2f 97 8e c8 12 fc 57 7f 85 aa d1 59 bc 80 40 0b
00000080 be 2e f0 e1 23 2d bf 4b 71 7e d0 e4 c0 36 e4 d2
00000090 cf b2 9f b4 a8 4f 3e 2a 21 89 74 c2 33 55 af ac
000000a0 41 36 1b 2b 60 09 f2 d9 19 f4 41 12 0b
```
We'll go over this packet dump in less detail, since its structure mirrors the *Direct Key Signature* (described above) very closely.
The first difference is in the `type` field, showing that this signature is of type `0x18` ([Subkey Binding Signature](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-subkey-binding-signature-si)).
The `pk_algo` of this signature is informed by the algorithm of the primary key (`0x1b`, corresponding to Ed25519). The signature in this packet is issued by the primary key, so by definition it uses the signing algorithm of the primary key (that is: the algorithm used to produce the cryptographic signature in this packet is entire independent of the `pk_algo` of the key material of this subkey itself, which uses the X25519 mechanism).
As shown in the text at the top of this packet dump, the hashed subpacket data contains four pieces of information:
- Signature creation time: `2023-09-29 15:17:58 UTC` (**critical**)
- Key expiration time: `P1095DT62781S` (**critical**)
- Key flags: `EtEr` (**critical**) (encryption for communication, encryption for storage)
- Issuer Fingerprint: `AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3`
The remainder of the packet has the same content as the *Direct Key Signature* above:
- A 16 bit digest prefix
- A salt value
- The cryptographic signature itself
The signature is calculated over a hash. The hash, in this case, is calculated over the following data (for details, see [Computing Signatures](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-11.html#name-computing-signatures) in the RFC):
- The signature's salt
- A serialized form of the primary key's public data
- A serialized form of the subkey's public data
- A serialized form of this subkey binding signature packet (up to, but excluding the unhashed area)
### Signing subkey
```{admonition} TODO
:class: warning
write
```
```text
$ sq packet dump --hex alice.priv-6--SecretSubkey
Secret-Subkey 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: D07B24EC91A14DD240AC2D53E6C8A9E054949A41222EA738576ED19CAEA3DC99
KeyID: D07B24EC91A14DD2
Secret Key:
Unencrypted
00000000 c7 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 33 8c d4 f5 ed25519_public
00000010 1a 73 39 ef ce d6 0f 21 8d a0 58 a2 3c 3d 44 a8
00000020 59 e9 13 1f 12 9c 6f 19 d0 3d 40 a0
0000002c 00 s2k_usage
0000002d 0e cb d1 ed25519_secret
00000030 c9 bc 81 82 aa 77 1f a8 12 a6 2a 74 a4 20 c1 74
00000040 76 f3 86 24 fb a8 25 a5 62 dd d6 a2 91
```
```text
$ sq packet dump --hex alice.priv-7--Signature
Signature Packet, new CTB, 3 header bytes + 325 bytes
Version: 6
Type: SubkeyBinding
Pk algo: Ed25519
Hash algo: SHA512
Hashed area:
Signature creation time: 2023-09-29 15:17:58 UTC (critical)
Key expiration time: P1095DT62781S (critical)
Key flags: S (critical)
Embedded signature: (critical)
Signature Packet
Version: 6
Type: PrimaryKeyBinding
Pk algo: Ed25519
Hash algo: SHA512
Hashed area:
Signature creation time: 2023-09-29 15:17:58 UTC (critical)
Issuer Fingerprint: D07B24EC91A14DD240AC2D53E6C8A9E054949A41222EA738576ED19CAEA3DC99
Digest prefix: 5365
Level: 0 (signature over data)
Issuer Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
Unhashed area:
Issuer: AAA18CBB254685C5
Digest prefix: 841C
Level: 0 (signature over data)
00000000 c2 CTB
00000001 c0 85 length
00000003 06 version
00000004 18 type
00000005 1b pk_algo
00000006 0a hash_algo
00000007 00 00 00 cc hashed_area_len
0000000b 05 subpacket length
0000000c 82 subpacket tag
0000000d 65 16 ea sig creation time
00000010 a6
00000011 05 subpacket length
00000012 89 subpacket tag
00000013 05 a4 8f bd key expiry time
00000017 02 subpacket length
00000018 9b subpacket tag
00000019 02 key flags
0000001a 99 subpacket length
0000001b a0 subpacket tag
0000001c 06 19 1b 0a embedded sig
00000020 00 00 00 29 05 82 65 16 ea a6 22 21 06 d0 7b 24
00000030 ec 91 a1 4d d2 40 ac 2d 53 e6 c8 a9 e0 54 94 9a
00000040 41 22 2e a7 38 57 6e d1 9c ae a3 dc 99 00 00 00
00000050 00 53 65 20 42 03 ad 0c db fc b5 9a 98 a6 15 27
00000060 e4 11 5e f5 f2 a0 3d bc ed 8d 94 27 41 09 f6 3c
00000070 4b f8 8a e5 af 73 e1 7d 54 07 40 3f f3 29 34 c2
00000080 e7 60 56 a5 e1 43 cb 08 ba 66 fe 8b 26 ce e7 cb
00000090 a5 3a 46 bb a5 c8 5d e4 6a de ae 49 e1 3e 07 bf
000000a0 c4 9e 98 14 2f 3e c5 f7 01 3e 3e 4f f6 18 2a ac
000000b0 bd ed 52 0c
000000b4 22 subpacket length
000000b5 21 subpacket tag
000000b6 06 version
000000b7 aa a1 8c bb 25 46 85 c5 83 issuer fp
000000c0 58 32 05 63 fd 37 b6 7f 33 00 f9 fb 0e c4 57 37
000000d0 8c d2 9f 10 26 98 b3
000000d7 00 00 00 0a unhashed_area_len
000000db 09 subpacket length
000000dc 10 subpacket tag
000000dd aa a1 8c issuer
000000e0 bb 25 46 85 c5
000000e5 84 digest_prefix1
000000e6 1c digest_prefix2
000000e7 20 salt_len
000000e8 23 3d b2 49 f3 02 4b 08 salt
000000f0 93 af ba 08 89 f0 e0 91 0f ab 22 26 aa b3 56 57
00000100 30 ea 95 29 06 60 6f 00
00000108 be 44 a1 95 38 a9 6b 3a ed25519_sig
00000110 3e 51 f0 55 09 b1 e2 91 a9 17 86 fa f5 1e 3f d0
00000120 28 46 3c ce 6e 88 14 37 32 ec 3d fa c6 01 ca e5
00000130 a9 4b b7 63 94 c3 0d 92 ab dc fa 23 50 71 60 31
00000140 a6 73 c8 33 5a 9c d9 0a
```
(zooming_in_user_id)=
### Adding an identity component
Now we'll look at an identity that is associated with Alice's certificate.
User IDs are a mechanism for connecting [identities](identity_components) with an OpenPGP certificate. Traditionally, User IDs contain a string that combines a name and an email address.
Like [above](zoom_enc_subkey), to look at the internal packet structure of this identity and its connection the OpenPGP certificate, we'll inspect the two individual packets that constitute the identity component, the [User ID packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-user-id-packet-tag-13), in the file `alice.priv-2--UserID`, and the certifying self-signature a [Positive certification of a User ID and Public-Key packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-positive-certification-of-a) in `alice.priv-3--Signature` (these packets are an excerpt of Alice's full OpenPGP private key).
#### User ID packet
First, let's look at the User ID packet, which encodes an identity that Alice has connected to her OpenPGP certificate:
```text
$ sq packet dump --hex alice.priv-2--UserID
User ID Packet, new CTB, 2 header bytes + 19 bytes
Value: <alice@example.org>
@ -735,7 +975,22 @@ User ID Packet, new CTB, 2 header bytes + 19 bytes
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
```
- `CTB: 0xcd`: 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: “13.” This is the value for a [User ID packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-user-id-packet-tag-13).
- `length: 0x13`: The remaining length of this packet (here: 19 bytes).
- `value`: 19 bytes of data that contain UTF-8 encoded text. The value corresponds to the string `<alice@example.org>`. With this identity component, Alice states that she uses (and has control of) this email address. Note that the email address is enclosed in `<` and `>` characters, following [RFC 2822](https://www.rfc-editor.org/rfc/rfc2822) conventions.
So, a User ID packet is really just a string, marked as a User ID by the packet type id.
#### Linking the User ID with a certification self-signature
As above, when [linking a subkey](zoom_enc_subkey) to the OpenPGP certificate, a self-signature is used to connect this new component to the certificate.
To bind identities to a certificate with a self-signature, one of the signature types `0x10` - `0x13` can be used. Here, the signature type `0x13` (*positive certification*) is used.
```text
$ sq packet dump --hex alice.priv-3--Signature
Signature Packet, new CTB, 2 header bytes + 185 bytes
Version: 6
Type: PositiveCertification
@ -806,13 +1061,52 @@ Signature Packet, new CTB, 2 header bytes + 185 bytes
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)
We'll go over this packet dump in less detail, since its structure closely mirrors the [Direct Key Signature](zooming_in_dks) discussed above.
We're again looking at a Signature packet. Its `type` is `0x13` ([corresponding](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-types) to a *positive certification* signature).
The public key algorithm and hash function used for this signature are Ed25519 and SHA512.
As shown in the text at the top of this packet dump, the hashed subpacket data contains the following metadata:
- 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`
This is a combination of metadata about the User ID itself (including defining this User ID as the *primary User ID* of this certificate), algorithm preferences that are associated with this identity, and settings that apply to the primary key.
````{note}
For historical reasons, the self-signature that binds the primary User ID to the certificate also contains subpackets that apply not to the User ID, but to the primary key itself.
Setting key expiration time and key flags on the primary User ID self-signature is one mechanism to configure the primary key.
The interaction between metadata on direct key signatures and User ID binding self-signatures [is subtle](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-11.html#name-notes-on-self-signatures), and there are changes between version 6 and version 4.
```{admonition} TODO
:class: warning
- link to a section that goes into more depth about "#name-notes-on-self-signatures"?
```
````
Followed, again, by the (informational) unhashed subpacket area.
And finally, a salt value for the signature and the signature itself.
The signature is calculated over a hash. The hash, in this case, is calculated over the following data (for details, see [Computing Signatures](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-11.html#name-computing-signatures) in the RFC):
- The signature's salt
- A serialized form of the primary key's public data
- A serialized form of the User ID
- A serialized form of this self-signature packet (up to, but excluding the unhashed area)
### Certifications (Third Party Signatures)

10
book/source/05-private.md
View file

@ -12,6 +12,16 @@
https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-transferable-secret-keys
(encrypted_secrets)=
## Password protecting secret key material
```{admonition} TODO
:class: warning
S2K, symmetric encryption
```
## Private key operations
The core of private key operations doesn't require access to the whole certificate. A private key subsystem only needs to handle the cryptographic key material.

3
book/source/18-glossary.md
View file

@ -9,6 +9,9 @@ Authentication
Certification
"Third party Signature" on a certificate, making a statement about that certificate, or an identity in the certificate
CA
[Certificate authority](https://en.wikipedia.org/wiki/Certificate_authority) or certification authority. An entity that handles digital certificates, especially by signing or issuing them.
Delegation
See {term}`Trust signature`