[uniffi] Make mls-rs-crypto-openssl optional

For Android, we will be using a crypto provider based on BoringSSL. It
is therefore important that mls-rs-uniffi becomes agnostic to the
crypto provider.

This PR handles this very simply: by requiring two things:

- the user sets a Cargo feature to depend on the right crypto provider
  implementation,
- that the crypto provider has a suitable `new()` method that can be
  used to instantiate it.

A better alternative would be to make the choice of crypto provider
dynamic via a trait. Unfortunately, I don’t think this is supported
with UniFFI because it doesn’t support associated types in exported
traits and because it also doesn’t support generic exported types.

Making a single non-generic type which uses

```rust
struct CryptoProviderAdapter {
    crypto_provider: Box<dyn CryptoProvider>
}
```

fail because the `CryptoProvider` trait has an associated type, which
I cannot specify at this point.

mls-rs-uniffi/Cargo.toml

@@ -14,15 +14,20 @@ rust-version = "1.68.2"
 crate-type = ["lib", "cdylib"]
 name = "mls_rs_uniffi"
 
+[features]
+openssl = ["mls-rs-crypto-openssl"]
+default = ["openssl"]
+
 [dependencies]
 async-trait = "0.1.77"
 maybe-async = "0.2.10"
 mls-rs = { version = "0.39.0", path = "../mls-rs" }
 mls-rs-core = { version = "0.18.0", path = "../mls-rs-core" }
-mls-rs-crypto-openssl = { version = "0.9.0", path = "../mls-rs-crypto-openssl" }
 thiserror = "1.0.57"
 uniffi = { git = "https://github.com/mozilla/uniffi-rs/", rev = "eeb785c", version = "0.27.0" }
 
+mls-rs-crypto-openssl = { version = "0.9.0", path = "../mls-rs-crypto-openssl", optional = true }
+
 [target.'cfg(mls_build_async)'.dependencies]
 tokio = { version = "1.36.0", features = ["sync"] }
 

mls-rs-uniffi/src/config.rs

@@ -6,9 +6,8 @@ use mls_rs::{
     identity::basic,
     storage_provider::in_memory::InMemoryGroupStateStorage,
 };
-use mls_rs_crypto_openssl::OpensslCryptoProvider;
 
-use self::group_state::{GroupStateStorage, GroupStateStorageAdapter};
+use crate::config::group_state::{GroupStateStorage, GroupStateStorageAdapter};
 use crate::Error;
 
 pub mod group_state;
@@ -56,10 +55,19 @@ impl mls_rs_core::group::GroupStateStorage for ClientGroupStorage {
     }
 }
 
+#[cfg(not(any(feature = "openssl")))]
+compile_error!(
+    "The crypto provider must be set by enabling exactly one of the \
+     following Cargo features: [\"openssl\"]."
+);
+
+#[cfg(feature = "openssl")]
+pub type UniFFICryptoProvider = mls_rs_crypto_openssl::OpensslCryptoProvider;
+
 pub type UniFFIConfig = client_builder::WithIdentityProvider<
     basic::BasicIdentityProvider,
     client_builder::WithCryptoProvider<
-        OpensslCryptoProvider,
+        UniFFICryptoProvider,
         WithGroupStateStorage<ClientGroupStorage, client_builder::BaseConfig>,
     >,
 >;

mls-rs-uniffi/src/lib.rs

@@ -21,8 +21,8 @@ mod config;
 
 use std::sync::Arc;
 
-pub use config::ClientConfig;
-use config::UniFFIConfig;
+pub use config::{client_config_default, ClientConfig};
+use config::{UniFFIConfig, UniFFICryptoProvider};
 
 #[cfg(not(mls_build_async))]
 use std::sync::Mutex;
@@ -36,7 +36,6 @@ use mls_rs::mls_rules;
 use mls_rs::{CipherSuiteProvider, CryptoProvider};
 use mls_rs_core::identity;
 use mls_rs_core::identity::{BasicCredential, IdentityProvider};
-use mls_rs_crypto_openssl::OpensslCryptoProvider;
 
 uniffi::setup_scaffolding!();
 
@@ -320,7 +319,7 @@ impl TryFrom<mls_rs::CipherSuite> for CipherSuite {
 pub async fn generate_signature_keypair(
     cipher_suite: CipherSuite,
 ) -> Result<SignatureKeypair, Error> {
-    let crypto_provider = mls_rs_crypto_openssl::OpensslCryptoProvider::default();
+    let crypto_provider = UniFFICryptoProvider::new();
     let cipher_suite_provider = crypto_provider
         .cipher_suite_provider(cipher_suite.into())
         .ok_or(MlsError::UnsupportedCipherSuite(cipher_suite.into()))?;
@@ -364,7 +363,6 @@ impl Client {
         let cipher_suite = signature_keypair.cipher_suite;
         let public_key = signature_keypair.public_key;
         let secret_key = signature_keypair.secret_key;
-        let crypto_provider = OpensslCryptoProvider::new();
         let basic_credential = BasicCredential::new(id);
         let signing_identity =
             identity::SigningIdentity::new(basic_credential.into_credential(), public_key.into());
@@ -373,7 +371,7 @@ impl Client {
             .with_single_welcome_message(true);
         let mls_rules = mls_rules::DefaultMlsRules::new().with_commit_options(commit_options);
         let client = mls_rs::Client::builder()
-            .crypto_provider(crypto_provider)
+            .crypto_provider(UniFFICryptoProvider::new())
             .identity_provider(basic::BasicIdentityProvider::new())
             .signing_identity(signing_identity, secret_key.into(), cipher_suite.into())
             .group_state_storage(client_config.group_state_storage.into())