added example to verify google idtoken

.gitignore

@@ -1,4 +1,4 @@
-/target/
+target/
 **/*.rs.bk
 Cargo.lock
 *~

examples/Cargo.toml

@@ -0,0 +1,4 @@
+[workspace]
+members = ["*"]
+exclude = ["target"]
+resolver = "2"

examples/google-idtoken/Cargo.toml

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+[package]
+name = "google-idtoken"
+version = "0.1.0"
+edition = "2021"
+
+[features]
+default = ["async"]
+async = ["tokio"]
+sync = []
+
+[dependencies]
+openidconnect = { version = "4.0.0-rc.1", features = ["reqwest-blocking"] }
+tokio = { version = "1.41.0", features = ["full"], optional = true }

examples/google-idtoken/src/lib.rs

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+//!
+//! An example using openidconnect to verify a Google Sign-In ID token 
+//! on a backend server ([Authenticate with a backend server]
+//! (https://developers.google.com/identity/sign-in/web/backend-auth)).
+//! To be used with [Android or iOS native app SDKs and apps or 
+//! platforms directly calling Google's OAuth2 or OpenID services]
+//! (https://developers.googleblog.com/2021/08/gsi-jsweb-deprecation.html).
+//!
+//! Before running it, you'll need to generate your own [Google OAuth2 
+//! credentials for Mobile & Desktop Apps]
+//! (https://developers.google.com/identity/protocols/oauth2/native-app).
+//!
+//! Run with:
+//!
+//! ```not_rust
+//! pushd examples && GOOGLE_CLIENT_ID="xxx" GOOGLE_ID_TOKEN="yyy" cargo test -p google-idtoken --all-features -- --show-output && popd
+//! ```
+
+#[cfg(test)]
+mod test {
+    use openidconnect::{core::{CoreClient, CoreIdToken}, reqwest, AuthUrl, ClientId, ClientSecret, IdToken, IssuerUrl, JsonWebKeySet, JsonWebKeySetUrl, Nonce};
+    use std::{env, str::FromStr};
+
+    #[cfg(feature = "sync")]
+    #[test]
+    fn verify_id_token() {
+        let google_client_id = ClientSecret::new(
+            env::var("GOOGLE_CLIENT_ID")
+                .expect("Missing the GOOGLE_CLIENT_ID environment variable."),
+        )
+        .secret()
+        .to_owned();
+
+        let google_id_token = ClientSecret::new(
+            env::var("GOOGLE_ID_TOKEN").expect("Missing the GOOGLE_ID_TOKEN environment variable."),
+        )
+        .secret()
+        .to_owned();
+
+        let client = CoreClient::new(
+            ClientId::new(google_client_id),
+            IssuerUrl::new("https://accounts.google.com".to_string()).unwrap(),
+            JsonWebKeySet::fetch(
+                &JsonWebKeySetUrl::new("https://www.googleapis.com/oauth2/v3/certs".to_string())
+                    .unwrap(),
+                &reqwest::blocking::Client::new(),
+            )
+            .unwrap(),
+        )
+            .set_auth_uri(AuthUrl::new("https://accounts.google.com/o/oauth2/v2/auth".to_string()).unwrap());
+
+        let id_token: CoreIdToken = IdToken::from_str(&google_id_token).unwrap();
+
+        let claims = id_token.claims(&client.id_token_verifier().allow_any_alg(), |_: Option<&Nonce>| Ok(()));
+
+        match claims {
+            Ok(claims) => println!(
+                "name: {}",
+                claims.name().unwrap().get(None).unwrap().as_str()
+            ),
+            Err(err) => println!("{:#?}", err),
+        };
+    }
+
+    #[cfg(feature = "async")]
+    #[tokio::test]
+    async fn verify_id_token_async() {
+        let google_client_id = ClientSecret::new(
+            env::var("GOOGLE_CLIENT_ID")
+                .expect("Missing the GOOGLE_CLIENT_ID environment variable."),
+        )
+        .secret()
+        .to_owned();
+
+        let google_id_token = ClientSecret::new(
+            env::var("GOOGLE_ID_TOKEN").expect("Missing the GOOGLE_ID_TOKEN environment variable."),
+        )
+        .secret()
+        .to_owned();
+
+        let client = CoreClient::new(
+            ClientId::new(google_client_id),
+            IssuerUrl::new("https://accounts.google.com".to_string()).unwrap(),
+            JsonWebKeySet::fetch_async(
+                &JsonWebKeySetUrl::new("https://www.googleapis.com/oauth2/v3/certs".to_string())
+                    .unwrap(),
+                &crate::test::reqwest::Client::new(),
+            )
+            .await
+            .unwrap(),
+        )
+            .set_auth_uri(AuthUrl::new("https://accounts.google.com/o/oauth2/v2/auth".to_string()).unwrap());
+
+        let id_token: CoreIdToken = IdToken::from_str(&google_id_token).unwrap();
+
+        let claims = id_token.claims(&client.id_token_verifier(), |_: Option<&Nonce>| Ok(()));
+
+        match claims {
+            Ok(claims) => println!(
+                "name: {}",
+                claims.name().unwrap().get(None).unwrap().as_str()
+            ),
+            Err(err) => println!("{:#?}", err),
+        };
+    }
+}