feat(bdk_electrum): add use-openssl as a feature

[oleonardolima]

partially addresses #1598

Description

It's a simple PR to expose the use-openssl from electrum-client to bdk_electrum. It partially addresses #1598, allowing the user to use openssl as an alternative to rustls, as there are some problems with it when handling some types of TLS certificates.

Do we need to add some sort of bdk_electrum tests using the new exposed use-openssl feature ?

Notes to the reviewers

Changelog notice

• Adds use-openssl as feature to bdk_electrum, exposing electrum-client use-openssl feature.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

tACK 19849bd

I tested with example_electrum and confirmed that if I use the default bdk_electrum features ("use-rustls") then I'm unable to sync to "ssl://electrum.blockstream.info:60002". But if I disable default features and enable the new "use-openssl" feature then I am able to sync to "ssl://electrum.blockstream.info:60002".

[notmandatory]

I just have one question, should we make use-openssl the default instead of use-rustls since it seems to be more compatible with self-signed certs?

[oleonardolima]

I just have one question, should we make use-openssl the default instead of use-rustls since it seems to be more compatible with self-signed certs?

I'm fine with it, but I'm not sure about other impacts. Do bindings and other users rely on the default feature being rustls? Does OpenSSL work fine on bindings/mobile stuff? cc @thunderbiscuit @reez

[tnull]

Does OpenSSL work fine on bindings/mobile stuff?

I can only speak to this part, and the answer is: no, unfortunately it doesn't work on mobile as nobody has figured out a way to have it link in the right libraries when cross-compiling for Android. It would be great if we could (which would allow dropping a bunch of dependencies and would be more secure), but so far we seem to be stuck on rustls.

[thunderbiscuit]

Yes I second what tnull said: open-ssl is not currently a solution for the Android libraries as we built them. It's not impossible to get it to work and would be great to figure it out one day, but as is rustls is our best option.

@oleonardolima to answer your question "Do bindings and other users rely on the default feature being rustls?" I think the answer is no (at least for bdk-ffi). We currently disable the default features and simply enable use-rustls-ring, so changing the default for the bdk client would not impact us. If you think it's a better default to use open-ssl, I'm open to it.

[notmandatory]

I had to rebase this one due to a small merge conflict.

[oleonardolima]

I had to rebase it again to solve the CI issue (although I think a re-run would've solved it 😅 ).

[oleonardolima]

Thanks for the comment @tnull and @thunderbiscuit, I wasn't aware of it.