This chapter adds a lot of detail to the material we discussed in the {ref}`certificates_chapter` chapter. Signatures on components are a crucial mechanism for forming OpenPGP certificates (which combine component keys and identities, via signatures on those components).
Additionally, signatures on components play a crucial role for authentication of identities. Mechanisms for decentralized authentication are one of OpenPGP's core strengths, we'll look into how they work.
Finally, signatures on components are also a central mechanism for life-cycle management of OpenPGP certificates and their components. This includes defining or changing expiration dates, or issuing revocations, for certificates or their components.
- A [direct key signature](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-direct-key-signature-type-i) issued as a self-signature can be used to set preferences and advertise features that apply to the whole certificate, while
- A similar [direct key signature](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-direct-key-signature-type-i) issued by a third party, which carries a [trust signature](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-trust-signature) subpacket, acts as a statement by the issuer that they delegate trust to the signed certificate (the issuer thereby uses the remote certificate as a trust root in the *Web of Trust*).
- Certifying self-signatures (type IDs `0x10` - `0x13`) are used to bind a User ID to a certificate, while
- the same signature type IDs issued by a third party are statements by the signer that they endorse the authenticity of the signed User ID to some degree.
This is done by calculating the signature over the User ID and the public primary key.
The resulting User ID certification (typically type 0x13, potentially type 0x10-0x12) can then be inserted into the certificate, right after the User ID packet.
Other examples for self-signatures are binding signatures for subkeys. To add an OpenPGP subkey to a certificate, a subkey binding signature is calculated over the public primary key, followed by the public subkey.
To prevent an attacker from being able to "adopt" a victims signing subkey and then being able to claim to be the origin of signatures in fact made by victim, subkey binding signatures for signing subkeys need to include an embedded "back signature" (formally known as [primary key binding signature](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#sigtype-primary-binding)) made by the signing key itself.
Certifications over User IDs can also be used to certify certificates of third-parties.
If Alice is certain that `Bob Baker <bob@example.com>` controls the key `0xB0B`, she can create a User ID certification signature for that identity and send it to Bob.
Typical use-cases for revocations are marking certificates or individual subkeys as unusable (for example, when the private key has been compromised or superseded), or marking User IDs as no longer used.
A revocation signature can either be hard or soft. A soft revocation of a certificate invalidates it from the revocation signature's creation time onwards. This means signatures issued before the revocation remain intact. A hard revocation, by contrast, invalidates the certificate retroactively, rendering all issued signatures invalid, regardless of creation time. Soft revocations are typically used whenever a key or User ID is retired or superseded gracefully, while hard revocations can, for example, signal compromise of secret key material.
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 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 components](identity_components) in an OpenPGP certificate dataset, OpenPGP implementations that read this certificate 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).
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_signature.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).
In order to specify an expiration time for the subkey, a key expiration time subpacket can be included. Note, that the validity of the subkey is bounded by that of the primary key, meaning an expired primary key causes the subkey to be invalidated, no matter the subkey expiration time.
Note, that a subkey cannot be "older" than the primary key. The value of the subkeys creation date MUST be greater than that of the primary key.
Binding subkeys with the "signing" key flag is a special case. For the most part, it works the same as binding other types of subkeys, but there is an additional requirement:
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.
This is to prevent an attack where the attacker "adopts" the victims signing subkey as their own in order to claim ownership over documents which were in fact signed by the victim.
Contrary to the `SubkeyBinding` signature, which is issued by the certificate's primary key, the `PrimaryKeyBinding` signature is instead created by the subkey.
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) is an embedded `PrimaryKeyBinding` "back signature" (type 0x19).
The *primary key binding signature* is "embedded" as subpacket data in the *subkey binding signature* that connects the signing subkey to the primary key.
"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
```
This signature is calculated over the primary key and User ID.
A revocation signature may contain a subpacket indicating the reason for revocation. This subpacket contains a code which specifies why the revocation was issued. This code determines, whether the revocation is hard or soft.
A soft revocation is typically used for graceful or planned revocations. A soft revocation can be reverted by re-validating the certificate, User ID or subkey with a fresh binding signature.
A soft revocation invalidates the target certificate beginning with the revocations creation time.
Contrary, a hard revocation cannot be re-validated. Furthermore, a hard-revoked certificate is invalidated retroactively.
A missing revocation reason subpacket is equivalent with a hard revocation reason.
As mentioned above, different signatures are used for different purposes.
In this section, we will try to give guidance on how to create certain signatures by example.
#### Change Algorithm Preferences
In order to change what symmetric, compression, hash or AEAD algorithms are preferred by the key owner, they can issue a direct-key signature (type 0x1F) on the primary key.
This signature should have the following structure:
The recommended way to change the expiration time of a certificate is by issuing a new `DirectKey` signature (type 0x1F) with an adjusted Key Expiration Time subpacket.
The structure of such a signature is the same as in the section above.
Self-certifications over User IDs can optionally carry the same subpackets as listed in the previous table (key flags, features, algorithm preferences).
This way, separate capabilities can be assigned to different identities.
Since OpenPGP certificates are often distributed by the means of key servers, new signatures on a certificate are often "merged" into existing copies of the certificate locally by the recipient.
This means, that it is not really possible to remove signatures / User IDs from a certificate, as there is no way to communicate the intention of packet deletion to the recipient.
So to mark a User ID as invalid, the user can publish a copy of their certificate with a `CertificationRevocation` (signature type 0x30) attached to the invalidated User ID.
| Signature Creation Time | Hashed | True | True | Current time |
| Issuer Fingerprint | Hashed | True or false | Strongly Recommended | The primary key is the issuer |
| Reason for Revocation | Hashed | True | False | Decides over soft / hard revocation |
For User ID revocations, the value of the reason subpacket can either be `0` (no reason specified) or `32`, signaling that the User ID is no longer valid.
The latter would result in a soft revocation, while a reason code of `0` is considered a hard revocation.
Omitting the reason packet altogether is also equivalent to a hard revocation.
It is recommended to issue User ID certifications using a reason code `32` and to do certificate revocations using a direct-key signature.
Optional algorithm preference subpackets can be used to signal per-subkey preferences that deviate from those set in the certificates `DirectKey` signature.
| Signature Creation Time | Hashed | True | True | Current time |
| Issuer Fingerprint | Hashed | True or false | Strongly Recommended | The primary key is the issuer |
| Reason for Revocation | Hashed | True | False | Decides over soft / hard revocation |
In `SubkeyRevocation` signatures, the reason subpacket cannot have value `32`, but instead may be from the range of `0-3`.
Values `1` (key superseded) and `3` (key retired and no longer used) are soft reasons, while `0` (no reason) and `2` (key compromised) are considered hard.
A user might want to revoke their whole certificate, rendering it unusable.
Depending on the circumstances, they might either want to revoke it softly, e.g. in case of migration to a new certificate, or they want to issue a hard revocation, e.g. in case of secret key material compromise. A soft-revoked certificate can be re-validated at a later point in time, by issuing a new certification, while a hard revocation is typically permanent.
There are some subpackets that are expected to be included in any type of signature.
* **Signature Creation Time**: Every OpenPGP signature MUST contain a Signature Creation Time subpacket (2) containing the timestamp at which the signature was made. This packet MUST be present in the hashed area of the signature and SHOULD be marked as critical.
* **Issuer Fingerprint**: To be able to verify a signature, the verifier needs to know which (sub-)key was used to issue the signature in the first place. Therefore, every OpenPGP v6 signature SHOULD contain an Issuer Fingerprint subpacket (33) containing the 32 byte fingerprint of the particular component key that was used to create the signature.
Since the issuer fingerprint subpacket is self-authenticating, it can either be included as a hashed or unhashed subpacket, but the authors of this book recommend to place it in the hashed area of the signature.
Since the hashed and unhashed areas of a signature are just lists of subpackets, in principle they allow duplicates of the same subpacket, which might lead to conflicts.
The [specification recommends](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-notes-on-subpackets) that implementations favor the last occurrence of a conflicting packet in the hashed area.
In some cases, duplicate packets with conflicting content even make sense, e.g., if a signature was made by a version 4 issuer key whose key material was migrated from an older OpenPGP version such as v3.
In this case, either the v3 or v4 key could be used to validate the v4 signature, but since the key ID calculation scheme was changed between v3 and v4, these identifiers would differ.
Therefore, the signature could contain two isuer key ID subpackets with conflicting, but correct values.
