viadkim

The viadkim library contains a complete implementation of DomainKeys Identified Mail (DKIM). DKIM is specified in RFC 6376.

This library provides both high-level APIs for signing and verifying email messages, as well as the low-level APIs used to implement this functionality. It is an asynchronous library based on the Tokio async runtime.

This library was developed independently from scratch by following the RFC specification and related documents. The design objectives sketched below are used to guide development.

Design objectives

The goal of viadkim is to provide a free DKIM library suitable for long-lived mail server processes, with strong RFC conformance guarantees.

Of particular importance is that the library should be efficient. Some items of note in this rubric are: doing DNS requests for public key records concurrently; bypass or shortcut message body processing where this is possible, and without the whole message being in memory at once; or sharing message body canonicalisation results among signature evaluation tasks.

Of equal importance is resilience and compatibility in handling inputs. Notably, internationalised email is fully supported in viadkim. But also malformed inputs that do occur in practice, such as stray Latin 1 bytes in headers are handled transparently. Generally, all inputs are handled gracefully, and similarly all outputs should be well-formed.

Furthermore, broad applicability of the library is a goal: extensive configuration options for both the signing and verification process, and ample output data produced by those processes should enable a wide range of DKIM usage patterns.

Finally, care is taken to strictly conform to RFC 6376, including RFC updates and errata. Support for internationalised email was already mentioned, but also, for example, more recent recommendations for supported signature algorithms such as addition of ed25519-sha256 and retirement of rsa-sha1 are adopted.

Usage

This is a Rust library. Include viadkim in Cargo.toml as usual.

Two structs provide the entry points to DKIM processing with viadkim:

• Signer for signing a message
• Verifier for verifying a message’s signatures

See the API documentation for usage instructions.

The minimum supported Rust version is 1.67.0.

Examples

Two simple command-line utilities are included as examples, one for signing a message and one for verifying a message’s signatures.

The program dkimsign produces a DKIM signature for the message provided on standard input. It takes three arguments: a path to a key file containing a signing key in PKCS#8 PEM format, a domain (the d= tag), and a selector (the s= tag). It then prints a DKIM-Signature header that can be prepended to the message.

Example invocation:

cargo run --example dkimsign -- \
  /path/to/key.pem example.com selector < /path/to/msg-to-sign

The program dkimverify verifies the DKIM signatures of a message provided on standard input. It prints each verification result as Rust data structures.

Example invocation:

cargo run --features hickory-resolver \
  --example dkimverify < /path/to/msg-to-verify

In both examples, export the environment variable RUST_LOG=viadkim=trace to enable the library’s trace logging. Edit the examples to experiment with various configuration options.

For interactive use of the viadkim library, please see the command-line utility dkimdo.

Acknowledgements

While this is an independent implementation of DKIM that was created from scratch, the author wishes to give credit to the OpenDKIM project. As a long-time user of OpenDKIM, the author couldn’t help adopting some of its design ideas. One example is the ‘staged’ processing approach, which does not require that the whole message reside in memory at once.

Licence

Copyright © 2022–2024 David Bürgin

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.