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#14 in Cryptography

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Used in 68 crates (58 directly)

LGPL-2.0-or-later

5MB
68K SLoC

This crate aims to provide a complete implementation of OpenPGP as defined by RFC 4880 as well as some extensions (e.g., RFC 6637, which describes ECC cryptography for OpenPGP). This includes support for unbuffered message processing.

A few features that the OpenPGP community considers to be deprecated (e.g., version 3 compatibility) have been left out. We have also updated some OpenPGP defaults to avoid foot guns (e.g., we selected modern algorithm defaults). If some functionality is missing, please file a bug report.

A non-goal of this crate is support for any sort of high-level, bolted-on functionality. For instance, RFC 4880 does not define trust models, such as the web of trust, direct trust, or TOFU. Neither does this crate. RFC 4880 does provide some mechanisms for creating trust models (specifically, UserID certifications), and this crate does expose those mechanisms.

We also try hard to avoid dictating how OpenPGP should be used. This doesn't mean that we don't have opinions about how OpenPGP should be used in a number of common scenarios (for instance, message validation). But, in this crate, we refrain from expressing those opinions; we will expose an opinionated, high-level interface in the future. In order to figure out the most appropriate high-level interfaces, we look at existing users. If you are using Sequoia, please get in contact so that we can learn from your use cases, discuss your opinions, and develop a high-level interface based on these experiences in the future.

Despite —or maybe because of— its unopinionated nature we found it easy to develop opinionated OpenPGP software based on Sequoia.

Experimental Features

This crate implements functionality from RFC 4880bis, notably AEAD encryption containers. As of this writing, this RFC is still a draft and the syntax or semantic defined in it may change or go away. Therefore, all related functionality may change and artifacts created using this functionality may not be usable in the future. Do not use it for things other than experiments.

This crate aims to provide a complete implementation of OpenPGP as defined by RFC 4880 as well as several extensions (e.g., RFC 6637, which describes ECC cryptography for OpenPGP, and RFC 4880bis, the draft of the next OpenPGP standard). This includes support for unbuffered message processing.

Feature flags

This crate uses features to enable or disable optional functionality. You can tweak the features in your Cargo.toml file, like so:

sequoia-openpgp = { version = "*", default-features = false, features = ["compression", ...] }

By default, Sequoia is built using Nettle as cryptographic backend with all compression algorithms enabled. Using the default features is only appropriate for leaf crates, see this section.

Note that if you use default-features = false, you need to explicitly enable a crypto backend, and also enable compression features.

Crypto backends

Sequoia supports multiple cryptographic libraries that can be selected at compile time. Currently, these libraries are available:

  • The Nettle cryptographic library. This is the default backend, and is selected by the default feature set. If you use default-features = false, you need to explicitly include the crypto-nettle feature to enable it.

  • The OpenSSL backend. To select this backend, use default-features = false, and explicitly include the crypto-openssl feature to enable it.

  • The Botan backend. To select this backend, use default-features = false, and explicitly include the crypto-botan feature to enable it. crypto-botan defaults to Botan v3, which was release in April 2023. Use crypto-botan2 to use v2.

  • The Windows Cryptography API: Next Generation (CNG). To select this backend, use default-features = false, and explicitly include the crypto-cng feature to enable it. Currently, the CNG backend requires at least Windows 10.

  • The RustCrypto crates. To select this backend, use default-features = false, and explicitly include the crypto-rust feature to enable it. As of this writing, the RustCrypto crates are not recommended for general use as they cannot offer the same security guarantees as more mature cryptographic libraries.

Experimental and variable-time cryptographic backends

Some cryptographic backends are not yet considered mature enough for general consumption. The use of such backends requires explicit opt-in using the feature flag allow-experimental-crypto.

Some cryptographic backends can not guarantee that cryptographic operations require a constant amount of time. This may leak secret keys in some settings. The use of such backends requires explicit opt-in using the feature flag allow-variable-time-crypto.

How to select crypto backends in crates

In Rust, features are unified, and consequently features should be additive, i.e. it should be safe to enable any combination of features. But, this does not hold for crypto backends, because exactly one cryptographic backend has to be selected in order to compile Sequoia.

To accommodate this, we came up with the following rule: in any project using Sequoia, exactly one crate may select the cryptographic backend, and that crate is the leaf crate (i.e. the binary or cdylib crate). Any non-leaf, library crate must refrain from selecting a crypto backend, including the default one, by disabling the default features.

To recap, follow these rules depending on what kind of crate you are developing:

Leaf crate

The leaf crate should pick a default backend (you may defer to Sequoia to pick the default one), but should allow your downstream users to switch backends:

# Cargo.toml
[dependencies]
sequoia-openpgp = { version = "*", default-features = false }
# If you need compression features, enable them here:
# sequoia-openpgp = { version = "*", default-features = false, features = ["compression"] }

[features]
# Pick a crypto backend enabled by default (here we defer to Sequoia
# to pick the default):
default = ["sequoia-openpgp/default"]

# .. but allow others to select a different backend, as well
crypto-nettle = ["sequoia-openpgp/crypto-nettle"]
crypto-openssl = ["sequoia-openpgp/crypto-openssl"]
crypto-botan = ["sequoia-openpgp/crypto-botan"]
crypto-botan2 = ["sequoia-openpgp/crypto-botan2"]
crypto-rust = ["sequoia-openpgp/crypto-rust"]
crypto-cng = ["sequoia-openpgp/crypto-cng"]

# Experimental and variable-time cryptographic backend opt-ins
allow-experimental-crypto = ["sequoia-openpgp/allow-experimental-crypto"]
allow-variable-time-crypto = ["sequoia-openpgp/allow-variable-time-crypto"]

Intermediate crate

Non-leaf crates must not select a cryptographic backend, and must disable the default features. Additionally, to make cargo test work without having to select a crypto backend, and to enable docs.rs to build your documentation, do selectively enable crypto backends for those cases:

# Cargo.toml
[dependencies]
sequoia-openpgp = { version = "*", default-features = false }
# If you need compression features, enable them here:
# sequoia-openpgp = { version = "*", default-features = false, features = ["compression"] }

# Enables a crypto backend for the tests:
[target.'cfg(not(windows))'.dev-dependencies]
sequoia-openpgp = { version = "1", default-features = false, features = ["crypto-nettle", "__implicit-crypto-backend-for-tests"]  }

# Enables a crypto backend for the tests:
[target.'cfg(windows)'.dev-dependencies]
sequoia-openpgp = { version = "1", default-features = false, features = ["crypto-cng", "__implicit-crypto-backend-for-tests"] }

# Enables a crypto backend for the docs.rs generation:
[package.metadata.docs.rs]
features = ["sequoia-openpgp/default"]

Compression algorithms

Use the compression flag to enable support for all compression algorithms, compression-deflate to enable DEFLATE and zlib compression support, and compression-bzip2 to enable bzip2 support.

Compiling to WASM

With the right feature flags, Sequoia can be compiled to WASM. To do that, enable the RustCrypto backend, and make sure not to enable bzip2 compression support:

sequoia-openpgp = { version = "*", default-features = false, features = ["crypto-rust", "allow-experimental-crypto", "allow-variable-time-crypto"] }

Or, with compression-deflate support:

sequoia-openpgp = { version = "*", default-features = false, features = ["crypto-rust", "allow-experimental-crypto", "allow-variable-time-crypto", "compression-deflate"] }

Minimum Supported Rust Version (MSRV)

sequoia-openpgp requires Rust 1.67.

Dependencies

~7–19MB
~234K SLoC