Add a module to authenticate by proving private key knowledge

This adds an authentication module to match the service side logic
at lightningdevkit/vss-server#79

Author: TheBlueMatt

Cargo.toml

@@ -14,8 +14,9 @@ categories = ["web-programming::http-client", "cryptography::cryptocurrencies"]
 build = "build.rs"
 
 [features]
-default = ["lnurl-auth"]
+default = ["lnurl-auth", "sigs-auth"]
 lnurl-auth = ["dep:bitcoin", "dep:url", "dep:serde", "dep:serde_json", "reqwest/json"]
+sigs-auth = ["dep:bitcoin"]
 
 [dependencies]
 prost = "0.11.6"

src/headers/mod.rs

@@ -12,6 +12,9 @@ mod lnurl_auth_jwt;
 #[cfg(feature = "lnurl-auth")]
 pub use lnurl_auth_jwt::LnurlAuthToJwtProvider;
 
+#[cfg(feature = "sigs-auth")]
+pub mod sigs_auth;
+
 /// Defines a trait around how headers are provided for each VSS request.
 #[async_trait]
 pub trait VssHeaderProvider: Send + Sync {

src/headers/sigs_auth.rs

@@ -0,0 +1,78 @@
+//! Provides the [`SigsAuthProvider`].
+
+use crate::headers::{VssHeaderProvider, VssHeaderProviderError};
+use async_trait::async_trait;
+use bitcoin::hashes::sha256::Hash as Sha256;
+use bitcoin::hashes::Hash as _;
+use bitcoin::secp256k1::{Message, Secp256k1, SecretKey, SignOnly};
+use std::collections::HashMap;
+use std::fmt::Write as _;
+use std::io::Write as _;
+use std::time::SystemTime;
+
+/// A 64-byte constant which, after appending the public key, is signed in order to prove knowledge
+/// of the corresponding private key.
+pub const SIGNING_CONSTANT: &'static [u8] =
+	b"VSS Signature Authorizer Signing Salt Constant..................";
+
+fn build_token(secret_key: &SecretKey, secp_ctx: &Secp256k1<SignOnly>) -> String {
+	let pubkey = secret_key.public_key(secp_ctx);
+	let old_time = "System time must be at least Jan 1, 1970";
+	let now = SystemTime::now().duration_since(SystemTime::UNIX_EPOCH).expect(old_time).as_secs();
+
+	// 2^64 serialized as a string is 20 bytes.
+	let mut buffer = [0u8; SIGNING_CONSTANT.len() + 33 + 20];
+	let mut stream = &mut buffer[..];
+	stream.write_all(SIGNING_CONSTANT).unwrap();
+	stream.write_all(&pubkey.serialize()).unwrap();
+	write!(stream, "{now}").unwrap();
+	let bytes_remaining = stream.len();
+	let bytes_to_sign = &buffer[..buffer.len() - bytes_remaining];
+
+	let hash = Sha256::hash(&bytes_to_sign);
+	let sig = secp_ctx.sign_ecdsa(&Message::from_digest(hash.to_byte_array()), secret_key);
+	let mut out = String::with_capacity((33 + 64 + 20) * 2);
+	write!(&mut out, "{pubkey:x}").unwrap();
+	for c in sig.serialize_compact() {
+		write!(&mut out, "{:02x}", c).unwrap();
+	}
+	write!(&mut out, "{now}").unwrap();
+	out
+}
+
+/// A simple auth provider which simply proves knowledge of a private key.
+///
+/// It provides a good default authentication mechanism for testing, or in the case that
+/// denial-of-service protection against new-account-flooding is mitigated at another layer
+/// (e.g. via Apple DeviceCheck or similar remote attestation technologies).
+pub struct SigsAuthProvider {
+	key: SecretKey,
+	secp_ctx: Secp256k1<SignOnly>,
+	default_headers: HashMap<String, String>,
+}
+
+impl SigsAuthProvider {
+	/// Creates a new auth provider which simply proves knowledge of a private key.
+	///
+	/// This provides an incredibly simple authentication scheme and allows the server to ensure
+	/// data for separate clients is kept separate, without any application-specific logic.
+	///
+	/// In addition to the automatically-added `Authentication` header, any headers provided in
+	/// `default_headers` (except an `Authentication` header) will be added to the headers list.
+	pub fn new(key: SecretKey, default_headers: HashMap<String, String>) -> Self {
+		SigsAuthProvider { secp_ctx: Secp256k1::signing_only(), key, default_headers }
+	}
+}
+
+#[async_trait]
+impl VssHeaderProvider for SigsAuthProvider {
+	async fn get_headers(
+		&self, _request: &[u8],
+	) -> Result<HashMap<String, String>, VssHeaderProviderError> {
+		// TODO: We might consider not re-signing on every request, but its cheap enough that it
+		// doesn't really matter
+		let mut headers = self.default_headers.clone();
+		headers.insert("Authorization".to_owned(), build_token(&self.key, &self.secp_ctx));
+		Ok(headers)
+	}
+}