# 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.
[RFC 6376]: https://www.rfc-editor.org/rfc/rfc6376
[Tokio]: https://tokio.rs
## 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.
[Rust]: https://www.rust-lang.org
[viadkim]: https://crates.io/crates/viadkim
[`Signer`]: https://docs.rs/viadkim/0.2.0/viadkim/signer/struct.Signer.html
[`Verifier`]: https://docs.rs/viadkim/0.2.0/viadkim/verifier/struct.Verifier.html
[API documentation]: https://docs.rs/viadkim
## 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].
[dkimdo]: https://crates.io/crates/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.
[OpenDKIM]: http://opendkim.org
## 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/>.