feat(api): Add Get Signed Tree Head endpoint

src/api/endpoints.rs

@@ -60,3 +60,21 @@ pub const ADD_CHAIN: Endpoint = (reqwest::Method::POST, "/ct/v1/add-chain");
 /// Outputs are the same as in Section 4.1.
 /// ```
 pub const ADD_PRE_CHAIN: Endpoint = (reqwest::Method::POST, "/ct/v1/add-pre-chain");
+
+/// Reference: https://datatracker.ietf.org/doc/html/rfc6962#section-4.3
+/// ```txt
+/// GET https://<log server>/ct/v1/get-sth
+///
+/// No inputs.
+///
+/// Outputs:
+///
+///    tree_size:  The size of the tree, in entries, in decimal.
+///
+///    timestamp:  The timestamp, in decimal.
+///
+///    sha256_root_hash:  The Merkle Tree Hash of the tree, in base64.
+///
+///    tree_head_signature:  A TreeHeadSignature for the above data.
+/// ```
+pub const GET_STH: Endpoint = (reqwest::Method::GET, "/ct/v1/get-sth");

src/api/mod.rs

@@ -1,5 +1,5 @@
 use reqwest::Url;
-use responses::{AddChainRequest, AddChainResponse};
+use responses::{AddChainRequest, AddChainResponse, GetSthResponse};
 
 pub mod endpoints;
 pub mod responses;
@@ -77,4 +77,19 @@ impl CtApiClient {
 			.json()
 			.await
 	}
+
+	pub async fn get_signed_tree_head(
+		&self
+	) -> reqwest::Result<GetSthResponse> {
+		self.inner_client
+			.request(
+				endpoints::GET_STH.0,
+				self.log_url.to_string() + endpoints::GET_STH.1
+			)
+			.send()
+			.await?
+			.error_for_status()?
+			.json()
+			.await
+	}
 }

src/api/responses.rs

@@ -21,3 +21,14 @@ pub struct AddChainResponse {
 	pub extensions: String,
 	pub signature: String
 }
+
+/// A response given when fetching the Signed Tree Head of a CT log
+/// 
+/// See: [`super::endpoints::GET_STH`]
+#[derive(Debug, Deserialize)]
+pub struct GetSthResponse {
+	pub tree_size: u64,
+	pub timestamp: u64,
+	pub sha256_root_hash: String,
+	pub tree_head_signature: String
+}