Implement signing and verification using *ring*

[djc]

Took some time to figure out the remaining issues, but this seems to be in pretty good shape now!

Also fixed the test that broke on main after 25abc977799d76dd3af4d2b71c4c95748c1e285e.

[mdecimus]

Merged, thank you very much for the contribution! I was thinking that ring should be the default crypto backend since trust-dns is already using it. I'll do some tests and make that change shortly.

[mdecimus]

@djc I found something weird, the ARC validation is failing with CryptoError("ring::error::Unspecified") on a message that can be successfully verified with rust-crypto. This message is available under resources/arc/002.txt and here is the output of the test:

$ cargo test arc_verify
    Finished test [unoptimized + debuginfo] target(s) in 0.08s
     Running unittests src/lib.rs (target/debug/deps/mail_auth-6af072c951a7fde6)

running 3 tests
test dmarc::verify::test::dmarc_verify_report_address ... ok
test dmarc::verify::test::dmarc_verify ... ok
test arc::verify::test::arc_verify ... FAILED

failures:

---- arc::verify::test::arc_verify stdout ----
file /home/vagrant/code/mail-auth/resources/arc/002.txt
thread 'arc::verify::test::arc_verify' panicked at 'assertion failed: `(left == right)`
  left: `Fail(CryptoError("ring::error::Unspecified"))`,
 right: `Pass`', src/arc/verify.rs:211:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
    arc::verify::test::arc_verify

test result: FAILED. 2 passed; 1 failed; 0 ignored; 0 measured; 29 filtered out; finished in 0.01s

error: test failed, to rerun pass `--lib`

[djc]

Ah, but you added this after the PR was merged, right? I'll have a look at it tomorrow.

[mdecimus]

That's correct, I added the 002.txt test to help you track the problem. The error is triggered when the Ed25519 seal is verified. Thanks.