Struct sequoia_openpgp::packet::signature::Signature4

pub struct Signature4 { /* private fields */ }

Holds a v4 Signature packet.

This holds a version 4 Signature packet. Normally, you won’t directly work with this data structure, but with the Signature enum, which is version agnostic. An exception is when you need to do version-specific operations. But currently, there aren’t any version-specific methods.

Implementations

impl Signature4

Hashing-related functionality.

pub fn hash_for_confirmation(&self, hash: &mut dyn Digest)

Hashes this signature for use in a Third-Party Confirmation signature.

impl Signature4

pub fn new( typ: SignatureType, pk_algo: PublicKeyAlgorithm, hash_algo: HashAlgorithm, hashed_area: SubpacketArea, unhashed_area: SubpacketArea, digest_prefix: [u8; 2], mpis: Signature, ) -> Self

Creates a new signature packet.

If you want to sign something, consider using the SignatureBuilder interface.

pub fn pk_algo(&self) -> PublicKeyAlgorithm

Gets the public key algorithm.

pub fn digest_prefix(&self) -> &[u8; 2]

Gets the hash prefix.

pub fn mpis(&self) -> &Signature

Gets the signature packet’s MPIs.

pub fn computed_digest(&self) -> Option<&[u8]>

Gets the computed hash value.

This is set by the PacketParser when parsing the message, during successful verification of signatures, and on signatures during certificate canonicalization.

pub fn level(&self) -> usize

Gets the signature level.

A level of 0 indicates that the signature is directly over the data, a level of 1 means that the signature is a notarization over all level 0 signatures and the data, and so on.

pub fn exportable(&self) -> Result<()>

Returns whether or not this signature should be exported.

This checks whether the Exportable Certification subpacket is absent or present and 1, and that the signature does not include any sensitive Revocation Key (designated revokers) subpackets.

Methods from Deref<Target = SignatureFields>

pub fn hash_standalone(&self, hash: &mut dyn Digest)

Hashes this standalone signature.

pub fn hash_timestamp(&self, hash: &mut dyn Digest)

Hashes this timestamp signature.

pub fn hash_direct_key<P>( &self, hash: &mut dyn Digest, key: &Key<P, PrimaryRole>, )

where P: KeyParts,

Hashes this direct key signature over the specified primary key, and the primary key.

pub fn hash_subkey_binding<P, Q>( &self, hash: &mut dyn Digest, key: &Key<P, PrimaryRole>, subkey: &Key<Q, SubordinateRole>, )

where P: KeyParts, Q: KeyParts,

Hashes this subkey binding over the specified primary key and subkey, the primary key, and the subkey.

pub fn hash_primary_key_binding<P, Q>( &self, hash: &mut dyn Digest, key: &Key<P, PrimaryRole>, subkey: &Key<Q, SubordinateRole>, )

where P: KeyParts, Q: KeyParts,

Hashes this primary key binding over the specified primary key and subkey, the primary key, and the subkey.

pub fn hash_userid_binding<P>( &self, hash: &mut dyn Digest, key: &Key<P, PrimaryRole>, userid: &UserID, )

where P: KeyParts,

Hashes this user ID binding over the specified primary key and user ID, the primary key, and the userid.

pub fn hash_user_attribute_binding<P>( &self, hash: &mut dyn Digest, key: &Key<P, PrimaryRole>, ua: &UserAttribute, )

where P: KeyParts,

Hashes this user attribute binding over the specified primary key and user attribute, the primary key, and the user attribute.

pub fn version(&self) -> u8

Gets the version.

pub fn typ(&self) -> SignatureType

Gets the signature type.

This function is called typ and not type, because type is a reserved word.

pub fn hash_algo(&self) -> HashAlgorithm

Gets the hash algorithm.

Methods from Deref<Target = SubpacketAreas>

pub fn hashed_area(&self) -> &SubpacketArea

Gets a reference to the hashed area.

pub fn hashed_area_mut(&mut self) -> &mut SubpacketArea

Gets a mutable reference to the hashed area.

Note: if you modify the hashed area of a Signature4, this will invalidate the signature. Instead, you should normally convert the Signature4 into a signature::SignatureBuilder, modify that, and then create a new signature.

pub fn unhashed_area(&self) -> &SubpacketArea

Gets a reference to the unhashed area.

pub fn unhashed_area_mut(&mut self) -> &mut SubpacketArea

Gets a mutable reference to the unhashed area.

pub fn sort(&mut self)

Sorts the subpacket areas.

See SubpacketArea::sort().

pub fn subpacket(&self, tag: SubpacketTag) -> Option<&Subpacket>

Returns a reference to the last instance of the specified subpacket, if any.

This function returns the last instance of the specified subpacket in the subpacket areas in which it can occur. Thus, when looking for the Signature Creation Time subpacket, this function only considers the hashed subpacket area. But, when looking for the Embedded Signature subpacket, this function considers both subpacket areas.

Unknown subpackets are assumed to only safely occur in the hashed subpacket area. Thus, any instances of them in the unhashed area are ignored.

For subpackets that can safely occur in both subpacket areas, this function prefers instances in the hashed subpacket area.

pub fn subpacket_mut(&mut self, tag: SubpacketTag) -> Option<&mut Subpacket>

Returns a mutable reference to the last instance of the specified subpacket, if any.

This function returns the last instance of the specified subpacket in the subpacket areas in which it can occur. Thus, when looking for the Signature Creation Time subpacket, this function only considers the hashed subpacket area. But, when looking for the Embedded Signature subpacket, this function considers both subpacket areas.

Unknown subpackets are assumed to only safely occur in the hashed subpacket area. Thus, any instances of them in the unhashed area are ignored.

For subpackets that can safely occur in both subpacket areas, this function prefers instances in the hashed subpacket area.

pub fn subpackets( &self, tag: SubpacketTag, ) -> impl Iterator<Item = &Subpacket> + Send + Sync

Returns an iterator over all instances of the specified subpacket.

This function returns an iterator over all instances of the specified subpacket in the subpacket areas in which it can occur. Thus, when looking for the Issuer subpacket, the iterator includes instances of the subpacket from both the hashed subpacket area and the unhashed subpacket area, but when looking for the Signature Creation Time subpacket, the iterator only includes instances of the subpacket from the hashed subpacket area; any instances of the subpacket in the unhashed subpacket area are ignored.

Unknown subpackets are assumed to only safely occur in the hashed subpacket area. Thus, any instances of them in the unhashed area are ignored.

pub fn signature_creation_time(&self) -> Option<SystemTime>

Returns the value of the Signature Creation Time subpacket.

The Signature Creation Time subpacket specifies when the signature was created. According to the standard, all signatures must include a Signature Creation Time subpacket in the signature’s hashed area. This doesn’t mean that the time stamp is correct: the issuer can always forge it.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn signature_validity_period(&self) -> Option<Duration>

Returns the value of the Signature Expiration Time subpacket.

This function is called signature_validity_period and not signature_expiration_time, which would be more consistent with the subpacket’s name, because the latter suggests an absolute time, but the time is actually relative to the signature’s creation time, which is stored in the signature’s Signature Creation Time subpacket.

A Signature Expiration Time subpacket specifies when the signature expires. This is different from the Key Expiration Time subpacket, which is accessed using SubpacketAreas::key_validity_period, and used to specify when an associated key expires. The difference is that in the former case, the signature itself expires, but in the latter case, only the associated key expires. This difference is critical: if a binding signature expires, then an OpenPGP implementation will still consider the associated key to be valid if there is another valid binding signature, even if it is older than the expired signature; if the active binding signature indicates that the key has expired, then OpenPGP implementations will not fallback to an older binding signature.

There are several cases where having a signature expire is useful. Say Alice certifies Bob’s certificate for bob@example.org. She can limit the lifetime of the certification to force her to reevaluate the certification shortly before it expires. For instance, is Bob still associated with example.org? Does she have reason to believe that his key has been compromised? Using an expiration is common in the X.509 ecosystem. For instance, Let’s Encrypt issues certificates with 90-day lifetimes.

Having signatures expire can also be useful when deploying software. For instance, you might have a service that installs an update if it has been signed by a trusted certificate. To prevent an adversary from coercing the service to install an older version, you could limit the signature’s lifetime to just a few minutes.

If the subpacket is not present in the hashed subpacket area, this returns None. If this function returns None, or the returned period is 0, the signature does not expire.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn signature_expiration_time(&self) -> Option<SystemTime>

Returns the value of the Signature Expiration Time subpacket as an absolute time.

A Signature Expiration Time subpacket specifies when the signature expires. The value stored is not an absolute time, but a duration, which is relative to the Signature’s creation time. To better reflect the subpacket’s name, this method returns the absolute expiry time, and the SubpacketAreas::signature_validity_period method returns the subpacket’s raw value.

The Signature Expiration Time subpacket is different from the Key Expiration Time subpacket, which is accessed using SubpacketAreas::key_validity_period, and used specifies when an associated key expires. The difference is that in the former case, the signature itself expires, but in the latter case, only the associated key expires. This difference is critical: if a binding signature expires, then an OpenPGP implementation will still consider the associated key to be valid if there is another valid binding signature, even if it is older than the expired signature; if the active binding signature indicates that the key has expired, then OpenPGP implementations will not fallback to an older binding signature.

There are several cases where having a signature expire is useful. Say Alice certifies Bob’s certificate for bob@example.org. She can limit the lifetime of the certification to force her to reevaluate the certification shortly before it expires. For instance, is Bob still associated with example.org? Does she have reason to believe that his key has been compromised? Using an expiration is common in the X.509 ecosystem. For instance, Let’s Encrypt issues certificates with 90-day lifetimes.

Having signatures expire can also be useful when deploying software. For instance, you might have a service that installs an update if it has been signed by a trusted certificate. To prevent an adversary from coercing the service to install an older version, you could limit the signature’s lifetime to just a few minutes.

If the subpacket is not present in the hashed subpacket area, this returns None. If this function returns None, the signature does not expire.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn signature_alive<T, U>( &self, time: T, clock_skew_tolerance: U, ) -> Result<()>

where T: Into<Option<SystemTime>>, U: Into<Option<Duration>>,

Returns whether or not the signature is alive at the specified time.

A signature is considered to be alive if creation time - tolerance <= time and time < expiration time.

This function does not check whether the key is revoked.

If time is None, then this function uses the current time for time.

If time is None, and clock_skew_tolerance is None, then this function uses CLOCK_SKEW_TOLERANCE for the tolerance. If time is not None and clock_skew_tolerance is None, it uses no tolerance. The intuition here is that we only need a tolerance when checking if a signature is alive right now; if we are checking at a specific time, we don’t want to use a tolerance.

A small amount of tolerance for clock skew is necessary, because although most computers synchronize their clocks with a time server, up to a few seconds of clock skew are not unusual in practice. And, even worse, several minutes of clock skew appear to be not uncommon on virtual machines.

Not accounting for clock skew can result in signatures being unexpectedly considered invalid. Consider: computer A sends a message to computer B at 9:00, but computer B, whose clock says the current time is 8:59, rejects it, because the signature appears to have been made in the future. This is particularly problematic for low-latency protocols built on top of OpenPGP, e.g., when two MUAs synchronize their state via a shared IMAP folder.

Being tolerant to potential clock skew is not always appropriate. For instance, when determining a User ID’s current self signature at time t, we don’t ever want to consider a self-signature made after t to be valid, even if it was made just a few moments after t. This goes doubly so for soft revocation certificates: the user might send a message that she is retiring, and then immediately create a soft revocation. The soft revocation should not invalidate the message.

Unfortunately, in many cases, whether we should account for clock skew or not depends on application-specific context. As a rule of thumb, if the time and the timestamp come from different clocks, you probably want to account for clock skew.

Errors

Section 5.2.3.4 of RFC 4880 states that a Signature Creation Time subpacket “MUST be present in the hashed area.” Consequently, if such a packet does not exist, this function returns Error::MalformedPacket.

Examples

Alice’s desktop computer and laptop exchange messages in real time via a shared IMAP folder. Unfortunately, the clocks are not perfectly synchronized: the desktop computer’s clock is a few seconds ahead of the laptop’s clock. When there is little or no propagation delay, this means that the laptop will consider the signatures to be invalid, because they appear to have been created in the future. Using a tolerance prevents this from happening.

use std::time::{SystemTime, Duration};
use sequoia_openpgp as openpgp;
use openpgp::cert::prelude::*;
use openpgp::packet::signature::SignatureBuilder;
use openpgp::types::SignatureType;

let (alice, _) =
    CertBuilder::general_purpose(None, Some("alice@example.org"))
        .generate()?;

// Alice's Desktop computer signs a message.  Its clock is a
// few seconds fast.
let now = SystemTime::now() + Duration::new(5, 0);

let mut alices_signer = alice.primary_key().key().clone()
    .parts_into_secret()?.into_keypair()?;
let msg = "START PROTOCOL";
let mut sig = SignatureBuilder::new(SignatureType::Binary)
    .set_signature_creation_time(now)?
    .sign_message(&mut alices_signer, msg)?;

// The desktop computer transfers the message to the laptop
// via the shared IMAP folder.  Because the laptop receives a
// push notification, it immediately processes it.
// Unfortunately, it is considered to be invalid: the message
// appears to be from the future!
assert!(sig.signature_alive(None, Duration::new(0, 0)).is_err());

// But, using the small default tolerance causes the laptop
// to consider the signature to be alive.
assert!(sig.signature_alive(None, None).is_ok());

pub fn key_validity_period(&self) -> Option<Duration>

Returns the value of the Key Expiration Time subpacket.

This function is called key_validity_period and not key_expiration_time, which would be more consistent with the subpacket’s name, because the latter suggests an absolute time, but the time is actually relative to the associated key’s (not the signature’s) creation time, which is stored in the Key.

A Key Expiration Time subpacket specifies when the associated key expires. This is different from the Signature Expiration Time subpacket (accessed using SubpacketAreas::signature_validity_period), which is used to specify when the signature expires. That is, in the former case, the associated key expires, but in the latter case, the signature itself expires. This difference is critical: if a binding signature expires, then an OpenPGP implementation will still consider the associated key to be valid if there is another valid binding signature, even if it is older than the expired signature; if the active binding signature indicates that the key has expired, then OpenPGP implementations will not fallback to an older binding signature.

If the subpacket is not present in the hashed subpacket area, this returns None. If this function returns None, or the returned period is 0, the key does not expire.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn key_expiration_time<P, R>(&self, key: &Key<P, R>) -> Option<SystemTime>

where P: KeyParts, R: KeyRole,

Returns the value of the Key Expiration Time subpacket as an absolute time.

A Key Expiration Time subpacket specifies when a key expires. The value stored is not an absolute time, but a duration, which is relative to the associated Key’s creation time, which is stored in the Key packet, not the binding signature. As such, the Key Expiration Time subpacket is only meaningful on a key’s binding signature. To better reflect the subpacket’s name, this method returns the absolute expiry time, and the SubpacketAreas::key_validity_period method returns the subpacket’s raw value.

The Key Expiration Time subpacket is different from the Signature Expiration Time subpacket, which is accessed using SubpacketAreas::signature_validity_period, and specifies when a signature expires. The difference is that in the former case, only the associated key expires, but in the latter case, the signature itself expires. This difference is critical: if a binding signature expires, then an OpenPGP implementation will still consider the associated key to be valid if there is another valid binding signature, even if it is older than the expired signature; if the active binding signature indicates that the key has expired, then OpenPGP implementations will not fallback to an older binding signature.

Because the absolute time is relative to the key’s creation time, which is stored in the key itself, this function needs the associated key. Since there is no way to get the associated key from a signature, the key must be passed to this function. This function does not check that the key is in fact associated with this signature.

If the subpacket is not present in the hashed subpacket area, this returns None. If this function returns None, the signature does not expire.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn key_alive<P, R, T>(&self, key: &Key<P, R>, t: T) -> Result<()>

where P: KeyParts, R: KeyRole, T: Into<Option<SystemTime>>,

Returns whether or not a key is alive at the specified time.

A Key is considered to be alive if creation time - tolerance <= time and time < expiration time.

This function does not check whether the signature is alive (cf. SubpacketAreas::signature_alive), or whether the key is revoked (cf. ValidKeyAmalgamation::revoked).

If time is None, then this function uses the current time for time.

Whereas a Key’s expiration time is stored in the Key’s active binding signature in the Key Expiration Time subpacket, its creation time is stored in the Key packet. As such, the associated Key must be passed to this function. This function, however, has no way to check that the signature is actually a binding signature for the specified Key.

Examples

Even keys that don’t expire may not be considered alive. This is the case if they were created after the specified time.

use std::time::{SystemTime, Duration};
use sequoia_openpgp as openpgp;
use openpgp::cert::prelude::*;
use openpgp::policy::StandardPolicy;

let p = &StandardPolicy::new();

let (cert, _) = CertBuilder::new().generate()?;

let mut pk = cert.primary_key().key();
let sig = cert.primary_key().with_policy(p, None)?.binding_signature();

assert!(sig.key_alive(pk, None).is_ok());
// A key is not considered alive prior to its creation time.
let the_past = SystemTime::now() - Duration::new(300, 0);
assert!(sig.key_alive(pk, the_past).is_err());

pub fn exportable_certification(&self) -> Option<bool>

Returns the value of the Exportable Certification subpacket.

The Exportable Certification subpacket indicates whether the signature should be exported (e.g., published on a public key server) or not. When using Serialize::export to export a certificate, signatures that have this subpacket present and set to false are not serialized.

Normally, you’ll want to use Signature4::exportable to check if a signature should be exported. That function also checks whether the signature includes any sensitive Revocation Key subpackets, which also shouldn’t be exported.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn trust_signature(&self) -> Option<(u8, u8)>

Returns the value of the Trust Signature subpacket.

The Trust Signature subpacket indicates the degree to which a certificate holder is trusted to certify other keys.

A level of 0 means that the certificate holder is not trusted to certificate other keys, a level of 1 means that the certificate holder is a trusted introducer (a certificate authority) and any certifications that they make should be considered valid. A level of 2 means the certificate holder can designate level 1 trusted introducers, etc.

The trust indicates the degree of confidence. A value of 120 means that a certification should be considered valid. A value of 60 means that a certification should only be considered partially valid. In the latter case, typically three such certifications are required for a binding to be considered authenticated.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn regular_expressions(&self) -> impl Iterator<Item = &[u8]> + Send + Sync

Returns the values of all Regular Expression subpackets.

The Regular Expression subpacket is used in conjunction with a Trust Signature subpacket, which is accessed using SubpacketAreas::trust_signature, to limit the scope of a trusted introducer. This is useful, for instance, when a company has a CA and you only want to trust them to certify their own employees.

Note: The serialized form includes a trailing NUL byte. Sequoia strips the NUL when parsing the subpacket.

This returns all instances of the Regular Expression subpacket in the hashed subpacket area.

pub fn revocable(&self) -> Option<bool>

Returns the value of the Revocable subpacket.

The Revocable subpacket indicates whether a certification may be later revoked by creating a Certification revocation signature (0x30) that targets the signature using the Signature Target subpacket (accessed using the SubpacketAreas::signature_target method).

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn revocation_keys( &self, ) -> impl Iterator<Item = &RevocationKey> + Send + Sync

Returns the values of all Revocation Key subpackets.

A Revocation Key subpacket indicates certificates (so-called designated revokers) that are allowed to revoke the signer’s certificate. For instance, if Alice trusts Bob, she can set him as a designated revoker. This is useful if Alice loses access to her key, and therefore is unable to generate a revocation certificate on her own. In this case, she can still Bob to generate one on her behalf.

When getting a certificate’s revocation keys, all valid self-signatures should be checked, not only the active self-signature. This prevents an attacker who has gained access to the private key material from invalidating a third-party revocation by publishing a new self signature that doesn’t include any revocation keys.

Due to the complexity of verifying such signatures, many OpenPGP implementations do not support this feature.

This returns all instance of the Revocation Key subpacket in the hashed subpacket area.

pub fn issuers(&self) -> impl Iterator<Item = &KeyID> + Send + Sync

Returns the values of all Issuer subpackets.

The Issuer subpacket is used when processing a signature to identify which certificate created the signature. Since this information is self-authenticating (the act of validating the signature authenticates the subpacket), it may be stored in the unhashed subpacket area.

This returns all instances of the Issuer subpacket in both the hashed subpacket area and the unhashed subpacket area.

pub fn issuer_fingerprints( &self, ) -> impl Iterator<Item = &Fingerprint> + Send + Sync

Returns the values of all Issuer Fingerprint subpackets.

The Issuer Fingerprint subpacket is used when processing a signature to identify which certificate created the signature. Since this information is self-authenticating (the act of validating the signature authenticates the subpacket), it is normally stored in the unhashed subpacket area.

This returns all instances of the Issuer Fingerprint subpacket in both the hashed subpacket area and the unhashed subpacket area.

pub fn notation_data(&self) -> impl Iterator<Item = &NotationData> + Send + Sync

Returns all Notation Data subpackets.

Notation Data subpackets are key-value pairs. They can be used by applications to annotate signatures in a structured way. For instance, they can define additional, application-specific security requirements. Because they are functionally equivalent to subpackets, they can also be used for OpenPGP extensions. This is how the Intended Recipient subpacket started life.

Notation names are structured, and are divided into two namespaces: the user namespace and the IETF namespace. Names in the user namespace have the form name@example.org and their meaning is defined by the owner of the domain. The meaning of the notation name@example.org, for instance, is defined by whoever controls example.org. Names in the IETF namespace do not contain an @ and are managed by IANA. See Section 5.2.3.16 of RFC 4880 for details.

This returns all instances of the Notation Data subpacket in the hashed subpacket area.

pub fn notation<'a, N>( &'a self, name: N, ) -> impl Iterator<Item = &'a [u8]> + Send + Sync

where N: 'a + AsRef<str> + Send + Sync,

Returns the values of all Notation Data subpackets with the given name.

Notation Data subpackets are key-value pairs. They can be used by applications to annotate signatures in a structured way. For instance, they can define additional, application-specific security requirements. Because they are functionally equivalent to subpackets, they can also be used for OpenPGP extensions. This is how the Intended Recipient subpacket started life.

Notation names are structured, and are divided into two namespaces: the user namespace and the IETF namespace. Names in the user namespace have the form name@example.org and their meaning is defined by the owner of the domain. The meaning of the notation name@example.org, for instance, is defined by whoever controls example.org. Names in the IETF namespace do not contain an @ and are managed by IANA. See Section 5.2.3.16 of RFC 4880 for details.

This returns the values of all instances of the Notation Data subpacket with the specified name in the hashed subpacket area.

pub fn preferred_symmetric_algorithms(&self) -> Option<&[SymmetricAlgorithm]>

Returns the value of the Preferred Symmetric Algorithms subpacket.

A Preferred Symmetric Algorithms subpacket lists what symmetric algorithms the user prefers. When encrypting a message for a recipient, the OpenPGP implementation should not use an algorithm that is not on this list.

This subpacket is a type of preference. When looking up a preference, an OpenPGP implementation should first look for the subpacket on the binding signature of the User ID or the User Attribute used to locate the certificate (or the primary User ID, if it was addressed by Key ID or fingerprint). If the binding signature doesn’t contain the subpacket, then the direct key signature should be checked. See the Preferences trait for details.

Unless addressing different User IDs really should result in different behavior, it is best to only set this preference on the direct key signature. This guarantees that even if some or all User IDs are stripped, the behavior remains consistent.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn preferred_hash_algorithms(&self) -> Option<&[HashAlgorithm]>

Returns the value of the Preferred Hash Algorithms subpacket.

A Preferred Hash Algorithms subpacket lists what hash algorithms the user prefers. When signing a message that should be verified by a particular recipient, the OpenPGP implementation should not use an algorithm that is not on this list.

This subpacket is a type of preference. When looking up a preference, an OpenPGP implementation should first look for the subpacket on the binding signature of the User ID or the User Attribute used to locate the certificate (or the primary User ID, if it was addressed by Key ID or fingerprint). If the binding signature doesn’t contain the subpacket, then the direct key signature should be checked. See the Preferences trait for details.

Unless addressing different User IDs really should result in different behavior, it is best to only set this preference on the direct key signature. This guarantees that even if some or all User IDs are stripped, the behavior remains consistent.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn preferred_compression_algorithms( &self, ) -> Option<&[CompressionAlgorithm]>

Returns the value of the Preferred Compression Algorithms subpacket.

A Preferred Compression Algorithms subpacket lists what compression algorithms the user prefers. When compressing a message for a recipient, the OpenPGP implementation should not use an algorithm that is not on the list.

This subpacket is a type of preference. When looking up a preference, an OpenPGP implementation should first for the subpacket on the binding signature of the User ID or the User Attribute used to locate the certificate (or the primary User ID, if it was addressed by Key ID or fingerprint). If the binding signature doesn’t contain the subpacket, then the direct key signature should be checked. See the Preferences trait for details.

Unless addressing different User IDs really should result in different behavior, it is best to only set this preference on the direct key signature. This guarantees that even if some or all User IDs are stripped, the behavior remains consistent.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn preferred_aead_algorithms(&self) -> Option<&[AEADAlgorithm]>

👎Deprecated

Returns the value of the Preferred AEAD Algorithms subpacket.

The Preferred AEAD Algorithms subpacket indicates what AEAD algorithms the key holder prefers ordered by preference. If this is set, then the AEAD feature flag should in the Features subpacket should also be set.

Note: because support for AEAD has not yet been standardized, we recommend not yet advertising support for it.

This subpacket is a type of preference. When looking up a preference, an OpenPGP implementation should first look for the subpacket on the binding signature of the User ID or the User Attribute used to locate the certificate (or the primary User ID, if it was addressed by Key ID or fingerprint). If the binding signature doesn’t contain the subpacket, then the direct key signature should be checked. See the Preferences trait for details.

Unless addressing different User IDs really should result in different behavior, it is best to only set this preference on the direct key signature. This guarantees that even if some or all User IDs are stripped, the behavior remains consistent.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn key_server_preferences(&self) -> Option<KeyServerPreferences>

Returns the value of the Key Server Preferences subpacket.

The Key Server Preferences subpacket indicates to key servers how they should handle the certificate.

This subpacket is a type of preference. When looking up a preference, an OpenPGP implementation should first for the subpacket on the binding signature of the User ID or the User Attribute used to locate the certificate (or the primary User ID, if it was addressed by Key ID or fingerprint). If the binding signature doesn’t contain the subpacket, then the direct key signature should be checked. See the Preferences trait for details.

Unless addressing different User IDs really should result in different behavior, it is best to only set this preference on the direct key signature. This guarantees that even if some or all User IDs are stripped, the behavior remains consistent.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn preferred_key_server(&self) -> Option<&[u8]>

Returns the value of the Preferred Key Server subpacket.

The Preferred Key Server subpacket contains a link to a key server where the certificate holder plans to publish updates to their certificate (e.g., extensions to the expiration time, new subkeys, revocation certificates).

The Preferred Key Server subpacket should be handled cautiously, because it can be used by a certificate holder to track communication partners.

This subpacket is a type of preference. When looking up a preference, an OpenPGP implementation should first look for the subpacket on the binding signature of the User ID or the User Attribute used to locate the certificate (or the primary User ID, if it was addressed by Key ID or fingerprint). If the binding signature doesn’t contain the subpacket, then the direct key signature should be checked. See the Preferences trait for details.

Unless addressing different User IDs really should result in different behavior, it is best to only set this preference on the direct key signature. This guarantees that even if some or all User IDs are stripped, the behavior remains consistent.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn policy_uri(&self) -> Option<&[u8]>

Returns the value of the Policy URI subpacket.

The Policy URI subpacket contains a link to a policy document, which contains information about the conditions under which the signature was made.

This subpacket is a type of preference. When looking up a preference, an OpenPGP implementation should first look for the subpacket on the binding signature of the User ID or the User Attribute used to locate the certificate (or the primary User ID, if it was addressed by Key ID or fingerprint). If the binding signature doesn’t contain the subpacket, then the direct key signature should be checked. See the Preferences trait for details.

Unless addressing different User IDs really should result in different behavior, it is best to only set this preference on the direct key signature. This guarantees that even if some or all User IDs are stripped, the behavior remains consistent.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn primary_userid(&self) -> Option<bool>

Returns the value of the Primary UserID subpacket.

The Primary User ID subpacket indicates whether the associated User ID or User Attribute should be considered the primary User ID. It is possible that this is set on multiple User IDs. See the documentation for ValidCert::primary_userid for an explanation of how Sequoia resolves this ambiguity.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn key_flags(&self) -> Option<KeyFlags>

Returns the value of the Key Flags subpacket.

The Key Flags subpacket describes a key’s capabilities (certification capable, signing capable, etc.). In the case of subkeys, the Key Flags are located on the subkey’s binding signature. For primary keys, locating the correct Key Flags subpacket is more complex: First, the primary User ID is consulted. If the primary User ID contains a Key Flags subpacket, that is used. Otherwise, any direct key signature is considered. If that still doesn’t contain a Key Flags packet, then the primary key should be assumed to be certification capable.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn signers_user_id(&self) -> Option<&[u8]>

Returns the value of the Signer’s UserID subpacket.

The Signer’s User ID subpacket indicates, which User ID made the signature. This is useful when a key has multiple User IDs, which correspond to different roles. For instance, it is not uncommon to use the same certificate in private as well as for a club.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn reason_for_revocation(&self) -> Option<(ReasonForRevocation, &[u8])>

Returns the value of the Reason for Revocation subpacket.

The Reason For Revocation subpacket indicates why a key, User ID, or User Attribute is being revoked. It includes both a machine readable code, and a human-readable string. The code is essential as it indicates to the OpenPGP implementation that reads the certificate whether the key was compromised (a hard revocation), or is no longer used (a soft revocation). In the former case, the OpenPGP implementation must conservatively consider all past signatures as suspect whereas in the latter case, past signatures can still be considered valid.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn features(&self) -> Option<Features>

Returns the value of the Features subpacket.

A Features subpacket lists what OpenPGP features the user wants to use. When creating a message, features that the intended recipients do not support should not be used. However, because this information is rarely held up to date in practice, this information is only advisory, and implementations are allowed to infer what features the recipients support from contextual clues, e.g., their past behavior.

This subpacket is a type of preference. When looking up a preference, an OpenPGP implementation should first look for the subpacket on the binding signature of the User ID or the User Attribute used to locate the certificate (or the primary User ID, if it was addressed by Key ID or fingerprint). If the binding signature doesn’t contain the subpacket, then the direct key signature should be checked. See the Preferences trait for details.

Unless addressing different User IDs really should result in different behavior, it is best to only set this preference on the direct key signature. This guarantees that even if some or all User IDs are stripped, the behavior remains consistent.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn signature_target( &self, ) -> Option<(PublicKeyAlgorithm, HashAlgorithm, &[u8])>

Returns the value of the Signature Target subpacket.

The Signature Target subpacket is used to identify the target of a signature. This is used when revoking a signature, and by timestamp signatures. It contains a hash of the target signature.

If the subpacket is not present in the hashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned.

pub fn embedded_signatures( &self, ) -> impl Iterator<Item = &Signature> + Send + Sync

Returns references to all Embedded Signature subpackets.

The Embedded Signature subpacket is normally used to hold a Primary Key Binding signature, which binds a signing-capable, authentication-capable, or certification-capable subkey to the primary key. Since this information is self-authenticating, it is usually stored in the unhashed subpacket area.

If the subpacket is not present in the hashed subpacket area or in the unhashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned. Otherwise, the last one is returned from the unhashed subpacket area.

pub fn embedded_signatures_mut( &mut self, ) -> impl Iterator<Item = &mut Signature> + Send + Sync

Returns mutable references to all Embedded Signature subpackets.

The Embedded Signature subpacket is normally used to hold a Primary Key Binding signature, which binds a signing-capable, authentication-capable, or certification-capable subkey to the primary key. Since this information is self-authenticating, it is usually stored in the unhashed subpacket area.

If the subpacket is not present in the hashed subpacket area or in the unhashed subpacket area, this returns None.

Note: if the signature contains multiple instances of this subpacket in the hashed subpacket area, the last one is returned. Otherwise, the last one is returned from the unhashed subpacket area.

pub fn intended_recipients( &self, ) -> impl Iterator<Item = &Fingerprint> + Send + Sync

Returns the intended recipients.

The Intended Recipient subpacket holds the fingerprint of a certificate.

When signing a message, the message should include one such subpacket for each intended recipient. Note: not all messages have intended recipients. For instance, when signing an open letter, or a software release, the message is intended for anyone.

When processing a signature, the application should ensure that if there are any such subpackets, then one of the subpackets identifies the recipient’s certificate (or user signed the message). If this is not the case, then an attacker may have taken the message out of its original context. For instance, if Alice sends a signed email to Bob, with the content: “I agree to the contract”, and Bob forwards that message to Carol, then Carol may think that Alice agreed to a contract with her if the signature appears to be valid! By adding an intended recipient, it is possible for Carol’s mail client to warn her that although Alice signed the message, the content was intended for Bob and not for her.

This returns all instances of the Intended Recipient subpacket in the hashed subpacket area.

pub fn attested_certifications( &self, ) -> Result<impl Iterator<Item = &[u8]> + Send + Sync>

Returns the digests of attested certifications.

This feature is experimental.

Allows the certificate holder to attest to third party certifications, allowing them to be distributed with the certificate. This can be used to address certificate flooding concerns.

Note: The maximum size of the hashed signature subpacket area constrains the number of attestations that can be stored in a signature. If the certificate holder attested to more certifications, the digests are split across multiple attested key signatures with the same creation time.

The standard strongly suggests that the digests should be sorted. However, this function returns the digests in the order they are stored in the subpacket, which may not be sorted.

To address both issues, collect all digests from all attested key signatures with the most recent creation time into a data structure that allows efficient lookups, such as HashSet or BTreeSet.

See Section 5.2.3.30 of RFC 4880bis for details.

Trait Implementations

impl Clone for Signature4

fn clone(&self) -> Signature4

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Signature4

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl DerefMut for Signature4

fn deref_mut(&mut self) -> &mut Self::Target

Mutably dereferences the value.

impl From<Signature4> for Packet

fn from(s: Signature4) -> Self

Converts to this type from the input type.

impl From<Signature4> for Signature

fn from(s: Signature4) -> Self

Converts to this type from the input type.

impl From<Signature4> for SignatureBuilder

fn from(sig: Signature4) -> Self

Converts to this type from the input type.

impl Hash for Signature4

fn hash<H: Hasher>(&self, state: &mut H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl Hash for Signature4

fn hash(&self, hash: &mut dyn Digest)

Updates the given hash with this object.

impl Marshal for Signature4

fn serialize(&self, o: &mut dyn Write) -> Result<()>

Writes a serialized version of the specified Signature packet to o.

Errors

Returns Error::InvalidArgument if either the hashed-area or the unhashed-area exceeds the size limit of 2^16.

fn export(&self, o: &mut dyn Write) -> Result<()>

Exports a serialized version of the object to o.

impl MarshalInto for Signature4

fn serialized_len(&self) -> usize

Computes the maximal length of the serialized representation.

fn serialize_into(&self, buf: &mut [u8]) -> Result<usize>

Serializes into the given buffer.

fn export_into(&self, buf: &mut [u8]) -> Result<usize>

Exports into the given buffer.

fn export_to_vec(&self) -> Result<Vec<u8>>

Exports to a vector.

fn to_vec(&self) -> Result<Vec<u8>>

Serializes the packet to a vector.

impl Ord for Signature4

fn cmp(&self, other: &Signature4) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized + PartialOrd,

Restrict a value to a certain interval.

impl PartialEq for Signature4

fn eq(&self, other: &Signature4) -> bool

This method tests for self and other values to be equal, and is used by ==.

This method compares the serialized version of the two packets. Thus, the computed values are ignored (level, computed_digest).

Note: because this function also compares the unhashed subpacket area, it is possible for a malicious party to take valid signatures, add subpackets to the unhashed area, yielding valid but distinct signatures. If you want to ignore the unhashed area, you should instead use the Signature::normalized_eq method.

fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl PartialOrd for Signature4

fn partial_cmp(&self, other: &Signature4) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl TryFrom<Signature> for Signature4

type Error = Error

The type returned in the event of a conversion error.

fn try_from(sig: Signature) -> Result<Self>

Performs the conversion.

impl Deref for Signature4

type Target = SignatureFields

The resulting type after dereferencing.

fn deref(&self) -> &Self::Target

Dereferences the value.

impl Eq for Signature4