Write compressed OhttpKeys to Uri

Payjoin-cli reads ohttp-keys from a binary file as a consequence.

payjoin-cli/example.config.toml

@@ -54,4 +54,4 @@ ohttp_relay="https://pj.bobspacebkk.com"
 
 # (v2 only, optional) The HPKE keys which need to be fetched ahead of time from the pj_endpoint for the payjoin packets to be encrypted.
 # These can now be fetched and no longer need to be configured.
-ohttp_keys="AQAg3c9qovMZvPzLh8XHgD8q86WG7SmPQvPamCTvEoueKBsABAABAAM"
+ohttp_keys="./path/to/ohttp_keys"

payjoin-cli/src/app/config.rs

@@ -92,8 +92,6 @@ impl AppConfig {
 
                 #[cfg(feature = "v2")]
                 let builder = {
-                    use payjoin::base64::prelude::BASE64_URL_SAFE_NO_PAD;
-                    use payjoin::base64::Engine;
                     builder
                         .set_override_option(
                             "pj_directory",
@@ -102,11 +100,13 @@ impl AppConfig {
                         .set_override_option(
                             "ohttp_keys",
                             matches.get_one::<String>("ohttp_keys").and_then(|s| {
-                                BASE64_URL_SAFE_NO_PAD
-                                    .decode(s)
+                                std::fs::read(s)
                                     .map_err(|e| {
-                                        log::error!("Failed to decode ohttp_keys: {}", e);
-                                        ConfigError::Message(format!("Invalid ohttp_keys: {}", e))
+                                        log::error!("Failed to read ohttp_keys file: {}", e);
+                                        ConfigError::Message(format!(
+                                            "Failed to read ohttp_keys file: {}",
+                                            e
+                                        ))
                                     })
                                     .ok()
                             }),

payjoin-cli/tests/e2e.rs

@@ -243,6 +243,8 @@ mod e2e {
             let ohttp_keys =
                 payjoin::io::fetch_ohttp_keys(ohttp_relay.clone(), directory.clone(), cert.clone())
                     .await?;
+            let ohttp_keys_path = temp_dir.join("ohttp_keys");
+            tokio::fs::write(&ohttp_keys_path, ohttp_keys.encode()?).await?;
 
             let receiver_rpchost = format!("http://{}/wallet/receiver", bitcoind.params.rpc_socket);
             let sender_rpchost = format!("http://{}/wallet/sender", bitcoind.params.rpc_socket);
@@ -268,13 +270,12 @@ mod e2e {
                 .arg("--pj-directory")
                 .arg(&directory)
                 .arg("--ohttp-keys")
-                .arg(&ohttp_keys.to_string())
+                .arg(&ohttp_keys_path)
                 .stdout(Stdio::piped())
                 .stderr(Stdio::inherit())
                 .spawn()
                 .expect("Failed to execute payjoin-cli");
             let bip21 = get_bip21_from_receiver(cli_receive_initiator).await;
-
             let cli_send_initiator = Command::new(payjoin_cli)
                 .arg("--rpchost")
                 .arg(&sender_rpchost)

payjoin-directory/src/lib.rs

@@ -15,7 +15,7 @@ use hyper::{Method, Request, Response, StatusCode, Uri};
 use hyper_util::rt::TokioIo;
 use tokio::net::TcpListener;
 use tokio::sync::Mutex;
-use tracing::{debug, error, info, trace};
+use tracing::{debug, error, trace};
 
 pub const DEFAULT_DIR_PORT: u16 = 8080;
 pub const DEFAULT_DB_HOST: &str = "localhost:6379";
@@ -138,9 +138,6 @@ fn init_ohttp() -> Result<ohttp::Server> {
 
     // create or read from file
     let server_config = ohttp::KeyConfig::new(KEY_ID, KEM, Vec::from(SYMMETRIC))?;
-    let encoded_config = server_config.encode()?;
-    let b64_config = BASE64_URL_SAFE_NO_PAD.encode(encoded_config);
-    info!("ohttp-keys server config base64 UrlSafe: {:?}", b64_config);
     Ok(ohttp::Server::new(server_config)?)
 }
 

payjoin/src/uri/url_ext.rs

@@ -91,12 +91,9 @@ mod tests {
         let mut url = Url::parse("https://example.com").unwrap();
 
         let ohttp_keys =
-            OhttpKeys::from_str("AQAWBG3fkg7fQCN-bafc-BEJOSnDfq8k1M9Cy1kgQZX42GVOvI0bWVAciTaJCy2A_wy7R7VxtU88xej692bv0uXgt98ABAABAAM").unwrap();
+            OhttpKeys::from_str("AQO6SMScPUqSo60A7MY6Ak2hDO0CGAxz7BLYp60syRu0gw").unwrap();
         let _ = url.set_ohttp(Some(ohttp_keys.clone()));
-        assert_eq!(
-            url.fragment(),
-            Some("ohttp=AQAWBG3fkg7fQCN-bafc-BEJOSnDfq8k1M9Cy1kgQZX42GVOvI0bWVAciTaJCy2A_wy7R7VxtU88xej692bv0uXgt98ABAABAAM")
-        );
+        assert_eq!(url.fragment(), Some("ohttp=AQO6SMScPUqSo60A7MY6Ak2hDO0CGAxz7BLYp60syRu0gw"));
 
         assert_eq!(url.ohttp(), Some(ohttp_keys));
 
@@ -124,7 +121,7 @@ mod tests {
         // fragment is not percent encoded so `&ohttp=` is parsed as a query parameter, not a fragment parameter
         let uri = "bitcoin:12c6DSiU4Rq3P4ZxziKxzrL5LmMBrzjrJX?amount=0.01\
                    &pj=https://example.com\
-                   #exp=1720547781&ohttp=AQAWBG3fkg7fQCN-bafc-BEJOSnDfq8k1M9Cy1kgQZX42GVOvI0bWVAciTaJCy2A_wy7R7VxtU88xej692bv0uXgt98ABAABAAM";
+                   #exp=1720547781&ohttp=AQO6SMScPUqSo60A7MY6Ak2hDO0CGAxz7BLYp60syRu0gw";
         let uri = Uri::try_from(uri).unwrap().assume_checked().check_pj_supported().unwrap();
         assert!(uri.extras.endpoint().ohttp().is_none());
     }
@@ -133,7 +130,7 @@ mod tests {
     fn test_valid_v2_url_fragment_on_bip21() {
         let uri = "bitcoin:12c6DSiU4Rq3P4ZxziKxzrL5LmMBrzjrJX?amount=0.01\
                    &pj=https://example.com\
-                   #ohttp%3DAQAWBG3fkg7fQCN-bafc-BEJOSnDfq8k1M9Cy1kgQZX42GVOvI0bWVAciTaJCy2A_wy7R7VxtU88xej692bv0uXgt98ABAABAAM";
+                   #ohttp%3DAQO6SMScPUqSo60A7MY6Ak2hDO0CGAxz7BLYp60syRu0gw";
         let uri = Uri::try_from(uri).unwrap().assume_checked().check_pj_supported().unwrap();
         assert!(uri.extras.endpoint().ohttp().is_some());
     }

payjoin/src/v2.rs

@@ -362,19 +362,48 @@ impl OhttpKeys {
     }
 }
 
+const KEM_ID: &[u8] = b"\x00\x16"; // DHKEM(secp256k1, HKDF-SHA256)
+const SYMMETRIC_LEN: &[u8] = b"\x00\x04"; // 4 bytes
+const SYMMETRIC_KDF_AEAD: &[u8] = b"\x00\x01\x00\x03"; // KDF(HKDF-SHA256), AEAD(ChaCha20Poly1305)
+
 impl fmt::Display for OhttpKeys {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        let encoded = BASE64_URL_SAFE_NO_PAD.encode(self.encode().map_err(|_| fmt::Error)?);
+        let bytes = self.encode().map_err(|_| fmt::Error)?;
+        let key_id = bytes[0];
+        let pubkey = &bytes[3..68];
+
+        let compressed_pubkey =
+            bitcoin::secp256k1::PublicKey::from_slice(pubkey).map_err(|_| fmt::Error)?.serialize();
+
+        let mut buf = vec![key_id];
+        buf.extend_from_slice(&compressed_pubkey);
+
+        let encoded = BASE64_URL_SAFE_NO_PAD.encode(buf);
         write!(f, "{}", encoded)
     }
 }
 
 impl std::str::FromStr for OhttpKeys {
     type Err = ParseOhttpKeysError;
 
+    /// Parses a base64URL-encoded string into OhttpKeys.
+    /// The string format is: key_id || compressed_public_key
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         let bytes = BASE64_URL_SAFE_NO_PAD.decode(s).map_err(ParseOhttpKeysError::DecodeBase64)?;
-        OhttpKeys::decode(&bytes).map_err(ParseOhttpKeysError::DecodeKeyConfig)
+
+        let key_id = *bytes.first().ok_or(ParseOhttpKeysError::InvalidFormat)?;
+        let compressed_pk = bytes.get(1..34).ok_or(ParseOhttpKeysError::InvalidFormat)?;
+
+        let pubkey = bitcoin::secp256k1::PublicKey::from_slice(compressed_pk)
+            .map_err(|_| ParseOhttpKeysError::InvalidPublicKey)?;
+
+        let mut buf = vec![key_id];
+        buf.extend_from_slice(KEM_ID);
+        buf.extend_from_slice(&pubkey.serialize_uncompressed());
+        buf.extend_from_slice(SYMMETRIC_LEN);
+        buf.extend_from_slice(SYMMETRIC_KDF_AEAD);
+
+        ohttp::KeyConfig::decode(&buf).map(Self).map_err(ParseOhttpKeysError::DecodeKeyConfig)
     }
 }
 
@@ -422,13 +451,17 @@ impl serde::Serialize for OhttpKeys {
 
 #[derive(Debug)]
 pub enum ParseOhttpKeysError {
+    InvalidFormat,
+    InvalidPublicKey,
     DecodeBase64(bitcoin::base64::DecodeError),
     DecodeKeyConfig(ohttp::Error),
 }
 
 impl std::fmt::Display for ParseOhttpKeysError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
+            ParseOhttpKeysError::InvalidFormat => write!(f, "Invalid format"),
+            ParseOhttpKeysError::InvalidPublicKey => write!(f, "Invalid public key"),
             ParseOhttpKeysError::DecodeBase64(e) => write!(f, "Failed to decode base64: {}", e),
             ParseOhttpKeysError::DecodeKeyConfig(e) =>
                 write!(f, "Failed to decode KeyConfig: {}", e),
@@ -441,6 +474,7 @@ impl std::error::Error for ParseOhttpKeysError {
         match self {
             ParseOhttpKeysError::DecodeBase64(e) => Some(e),
             ParseOhttpKeysError::DecodeKeyConfig(e) => Some(e),
+            ParseOhttpKeysError::InvalidFormat | ParseOhttpKeysError::InvalidPublicKey => None,
         }
     }
 }

payjoin/tests/integration.rs

@@ -105,7 +105,7 @@ mod integration {
         #[tokio::test]
         async fn test_bad_ohttp_keys() {
             let bad_ohttp_keys =
-                OhttpKeys::from_str("AQAWBG3fkg7fQCN-bafc-BEJOSnDfq8k1M9Cy1kgQZX42GVOvI0bWVAciTaJCy2A_wy7R7VxtU88xej692bv0uXgt98ABAABAAM")
+                OhttpKeys::from_str("AQO6SMScPUqSo60A7MY6Ak2hDO0CGAxz7BLYp60syRu0gw")
                     .expect("Invalid OhttpKeys");
 
             let (cert, key) = local_cert_key();