Pad ohttp req/res messages to consistent 8192 bytes (#395)

- Padded messages (POST, GET, Response 202, Response 200) are no longer
distinguishable

Rationale: 7kb x 2x/min / 1h = ~1MB/hr which is not too much overhead.

- allocate fixed length arrays instead of `resize`ing vectors.

payjoin-cli/src/app/v2.rs

@@ -141,7 +141,7 @@ impl App {
         println!("Got a request from the sender. Responding with a Payjoin proposal.");
         let res = post_request(req).await?;
         payjoin_proposal
-            .process_res(res.bytes().await?.to_vec(), ohttp_ctx)
+            .process_res(&res.bytes().await?, ohttp_ctx)
             .map_err(|e| anyhow!("Failed to deserialize response {}", e))?;
         let payjoin_psbt = payjoin_proposal.psbt().clone();
         println!(
@@ -198,16 +198,11 @@ impl App {
                 println!("Posting Original PSBT Payload request...");
                 let response = post_request(req).await?;
                 println!("Sent fallback transaction");
-                let v2_ctx = Arc::new(
-                    ctx.process_response(&mut response.bytes().await?.to_vec().as_slice())?,
-                );
+                let v2_ctx = Arc::new(ctx.process_response(&response.bytes().await?)?);
                 loop {
                     let (req, ohttp_ctx) = v2_ctx.extract_req(self.config.ohttp_relay.clone())?;
                     let response = post_request(req).await?;
-                    match v2_ctx.process_response(
-                        &mut response.bytes().await?.to_vec().as_slice(),
-                        ohttp_ctx,
-                    ) {
+                    match v2_ctx.process_response(&response.bytes().await?, ohttp_ctx) {
                         Ok(Some(psbt)) => return Ok(psbt),
                         Ok(None) => {
                             println!("No response yet.");

payjoin-directory/src/lib.rs

@@ -23,7 +23,12 @@ pub const DEFAULT_DIR_PORT: u16 = 8080;
 pub const DEFAULT_DB_HOST: &str = "localhost:6379";
 pub const DEFAULT_TIMEOUT_SECS: u64 = 30;
 
-const MAX_BUFFER_SIZE: usize = 65536;
+const ENCAPSULATED_MESSAGE_BYTES: usize = 8192;
+const CHACHA20_POLY1305_NONCE_LEN: usize = 32; // chacha20poly1305 n_k
+const POLY1305_TAG_SIZE: usize = 16;
+pub const BHTTP_REQ_BYTES: usize =
+    ENCAPSULATED_MESSAGE_BYTES - (CHACHA20_POLY1305_NONCE_LEN + POLY1305_TAG_SIZE);
+const V1_MAX_BUFFER_SIZE: usize = 65536;
 
 const V1_REJECT_RES_JSON: &str =
     r#"{{"errorCode": "original-psbt-rejected ", "message": "Body is not a string"}}"#;
@@ -208,9 +213,11 @@ async fn handle_ohttp_gateway(
     bhttp_res
         .write_bhttp(bhttp::Mode::KnownLength, &mut bhttp_bytes)
         .map_err(|e| HandlerError::InternalServerError(e.into()))?;
+    bhttp_bytes.resize(BHTTP_REQ_BYTES, 0);
     let ohttp_res = res_ctx
         .encapsulate(&bhttp_bytes)
         .map_err(|e| HandlerError::InternalServerError(e.into()))?;
+    assert!(ohttp_res.len() == ENCAPSULATED_MESSAGE_BYTES, "Unexpected OHTTP response size");
     Ok(Response::new(full(ohttp_res)))
 }
 
@@ -323,7 +330,7 @@ async fn put_payjoin_v1(
     let id = decode_short_id(id)?;
     let req =
         body.collect().await.map_err(|e| HandlerError::InternalServerError(e.into()))?.to_bytes();
-    if req.len() > MAX_BUFFER_SIZE {
+    if req.len() > V1_MAX_BUFFER_SIZE {
         return Err(HandlerError::PayloadTooLarge);
     }
 
@@ -344,7 +351,7 @@ async fn post_subdir(
     let id = decode_short_id(id)?;
     let req =
         body.collect().await.map_err(|e| HandlerError::InternalServerError(e.into()))?.to_bytes();
-    if req.len() > MAX_BUFFER_SIZE {
+    if req.len() > V1_MAX_BUFFER_SIZE {
         return Err(HandlerError::PayloadTooLarge);
     }
 

payjoin/src/ohttp.rs

@@ -3,13 +3,21 @@ use std::{error, fmt};
 
 use bitcoin::base64::prelude::BASE64_URL_SAFE_NO_PAD;
 use bitcoin::base64::Engine;
+use bitcoin::key::constants::UNCOMPRESSED_PUBLIC_KEY_SIZE;
+
+pub const ENCAPSULATED_MESSAGE_BYTES: usize = 8192;
+const N_ENC: usize = UNCOMPRESSED_PUBLIC_KEY_SIZE;
+const N_T: usize = crate::hpke::POLY1305_TAG_SIZE;
+const OHTTP_REQ_HEADER_BYTES: usize = 7;
+pub const PADDED_BHTTP_REQ_BYTES: usize =
+    ENCAPSULATED_MESSAGE_BYTES - (N_ENC + N_T + OHTTP_REQ_HEADER_BYTES);
 
 pub fn ohttp_encapsulate(
     ohttp_keys: &mut ohttp::KeyConfig,
     method: &str,
     target_resource: &str,
     body: Option<&[u8]>,
-) -> Result<(Vec<u8>, ohttp::ClientResponse), OhttpEncapsulationError> {
+) -> Result<([u8; ENCAPSULATED_MESSAGE_BYTES], ohttp::ClientResponse), OhttpEncapsulationError> {
     use std::fmt::Write;
 
     let ctx = ohttp::ClientRequest::from_config(ohttp_keys)?;
@@ -31,16 +39,21 @@ pub fn ohttp_encapsulate(
     if let Some(body) = body {
         bhttp_message.write_content(body);
     }
-    let mut bhttp_req = Vec::new();
-    let _ = bhttp_message.write_bhttp(bhttp::Mode::KnownLength, &mut bhttp_req);
-    let encapsulated = ctx.encapsulate(&bhttp_req)?;
-    Ok(encapsulated)
+
+    let mut bhttp_req = [0u8; PADDED_BHTTP_REQ_BYTES];
+    let _ = bhttp_message.write_bhttp(bhttp::Mode::KnownLength, &mut bhttp_req.as_mut_slice());
+    let (encapsulated, ohttp_ctx) = ctx.encapsulate(&bhttp_req)?;
+
+    let mut buffer = [0u8; ENCAPSULATED_MESSAGE_BYTES];
+    let len = encapsulated.len().min(ENCAPSULATED_MESSAGE_BYTES);
+    buffer[..len].copy_from_slice(&encapsulated[..len]);
+    Ok((buffer, ohttp_ctx))
 }
 
 /// decapsulate ohttp, bhttp response and return http response body and status code
 pub fn ohttp_decapsulate(
     res_ctx: ohttp::ClientResponse,
-    ohttp_body: &[u8],
+    ohttp_body: &[u8; ENCAPSULATED_MESSAGE_BYTES],
 ) -> Result<http::Response<Vec<u8>>, OhttpEncapsulationError> {
     let bhttp_body = res_ctx.decapsulate(ohttp_body)?;
     let mut r = std::io::Cursor::new(bhttp_body);

payjoin/src/receive/v2/error.rs

@@ -12,6 +12,8 @@ pub(crate) enum InternalSessionError {
     Expired(std::time::SystemTime),
     /// OHTTP Encapsulation failed
     OhttpEncapsulation(OhttpEncapsulationError),
+    /// Unexpected response size
+    UnexpectedResponseSize(usize),
 }
 
 impl fmt::Display for SessionError {
@@ -20,6 +22,12 @@ impl fmt::Display for SessionError {
             InternalSessionError::Expired(expiry) => write!(f, "Session expired at {:?}", expiry),
             InternalSessionError::OhttpEncapsulation(e) =>
                 write!(f, "OHTTP Encapsulation Error: {}", e),
+            InternalSessionError::UnexpectedResponseSize(size) => write!(
+                f,
+                "Unexpected response size {}, expected {} bytes",
+                size,
+                crate::ohttp::ENCAPSULATED_MESSAGE_BYTES
+            ),
         }
     }
 }
@@ -29,6 +37,7 @@ impl error::Error for SessionError {
         match &self.0 {
             InternalSessionError::Expired(_) => None,
             InternalSessionError::OhttpEncapsulation(e) => Some(e),
+            InternalSessionError::UnexpectedResponseSize(_) => None,
         }
     }
 }

payjoin/src/receive/v2/mod.rs

@@ -113,13 +113,17 @@ impl Receiver {
     /// indicating no UncheckedProposal is available yet.
     pub fn process_res(
         &mut self,
-        mut body: impl std::io::Read,
+        body: &[u8],
         context: ohttp::ClientResponse,
     ) -> Result<Option<UncheckedProposal>, Error> {
-        let mut buf = Vec::new();
-        let _ = body.read_to_end(&mut buf);
+        let response_array: &[u8; crate::ohttp::ENCAPSULATED_MESSAGE_BYTES] =
+            body.try_into().map_err(|_| {
+                Error::Server(Box::new(SessionError::from(
+                    InternalSessionError::UnexpectedResponseSize(body.len()),
+                )))
+            })?;
         log::trace!("decapsulating directory response");
-        let response = ohttp_decapsulate(context, &buf)?;
+        let response = ohttp_decapsulate(context, response_array)?;
         if response.body().is_empty() {
             log::debug!("response is empty");
             return Ok(None);
@@ -134,7 +138,10 @@ impl Receiver {
 
     fn fallback_req_body(
         &mut self,
-    ) -> Result<(Vec<u8>, ohttp::ClientResponse), OhttpEncapsulationError> {
+    ) -> Result<
+        ([u8; crate::ohttp::ENCAPSULATED_MESSAGE_BYTES], ohttp::ClientResponse),
+        OhttpEncapsulationError,
+    > {
         let fallback_target = self.pj_url();
         ohttp_encapsulate(&mut self.context.ohttp_keys, "GET", fallback_target.as_str(), None)
     }
@@ -509,10 +516,16 @@ impl PayjoinProposal {
     /// choose to broadcast the original PSBT.
     pub fn process_res(
         &self,
-        res: Vec<u8>,
+        res: &[u8],
         ohttp_context: ohttp::ClientResponse,
     ) -> Result<(), Error> {
-        let res = ohttp_decapsulate(ohttp_context, &res)?;
+        let response_array: &[u8; crate::ohttp::ENCAPSULATED_MESSAGE_BYTES] =
+            res.try_into().map_err(|_| {
+                Error::Server(Box::new(SessionError::from(
+                    InternalSessionError::UnexpectedResponseSize(res.len()),
+                )))
+            })?;
+        let res = ohttp_decapsulate(ohttp_context, response_array)?;
         if res.status().is_success() {
             Ok(())
         } else {

payjoin/src/request.rs

@@ -32,7 +32,7 @@ impl Request {
     }
 
     #[cfg(feature = "v2")]
-    pub fn new_v2(url: Url, body: Vec<u8>) -> Self {
-        Self { url, content_type: V2_REQ_CONTENT_TYPE, body }
+    pub fn new_v2(url: Url, body: [u8; crate::ohttp::ENCAPSULATED_MESSAGE_BYTES]) -> Self {
+        Self { url, content_type: V2_REQ_CONTENT_TYPE, body: body.to_vec() }
     }
 }

payjoin/src/send/error.rs

@@ -66,6 +66,8 @@ pub(crate) enum InternalValidationError {
     OhttpEncapsulation(crate::ohttp::OhttpEncapsulationError),
     #[cfg(feature = "v2")]
     UnexpectedStatusCode,
+    #[cfg(feature = "v2")]
+    UnexpectedResponseSize(usize),
 }
 
 impl From<InternalValidationError> for ValidationError {
@@ -119,6 +121,8 @@ impl fmt::Display for ValidationError {
             OhttpEncapsulation(e) => write!(f, "Ohttp encapsulation error: {}", e),
             #[cfg(feature = "v2")]
             UnexpectedStatusCode => write!(f, "unexpected status code"),
+            #[cfg(feature = "v2")]
+            UnexpectedResponseSize(size) => write!(f, "unexpected response size {}, expected {} bytes", size, crate::ohttp::ENCAPSULATED_MESSAGE_BYTES),
         }
     }
 }
@@ -164,6 +168,8 @@ impl std::error::Error for ValidationError {
             OhttpEncapsulation(error) => Some(error),
             #[cfg(feature = "v2")]
             UnexpectedStatusCode => None,
+            #[cfg(feature = "v2")]
+            UnexpectedResponseSize(_) => None,
         }
     }
 }

payjoin/src/send/mod.rs

@@ -364,13 +364,12 @@ pub struct V2PostContext {
 
 #[cfg(feature = "v2")]
 impl V2PostContext {
-    pub fn process_response(
-        self,
-        response: &mut impl std::io::Read,
-    ) -> Result<V2GetContext, ResponseError> {
-        let mut res_buf = Vec::new();
-        response.read_to_end(&mut res_buf).map_err(InternalValidationError::Io)?;
-        let response = ohttp_decapsulate(self.ohttp_ctx, &res_buf)
+    pub fn process_response(self, response: &[u8]) -> Result<V2GetContext, ResponseError> {
+        let response_array: &[u8; crate::ohttp::ENCAPSULATED_MESSAGE_BYTES] =
+            response
+                .try_into()
+                .map_err(|_| InternalValidationError::UnexpectedResponseSize(response.len()))?;
+        let response = ohttp_decapsulate(self.ohttp_ctx, response_array)
             .map_err(InternalValidationError::OhttpEncapsulation)?;
         match response.status() {
             http::StatusCode::OK => {
@@ -424,12 +423,15 @@ impl V2GetContext {
 
     pub fn process_response(
         &self,
-        response: &mut impl std::io::Read,
+        response: &[u8],
         ohttp_ctx: ohttp::ClientResponse,
     ) -> Result<Option<Psbt>, ResponseError> {
-        let mut res_buf = Vec::new();
-        response.read_to_end(&mut res_buf).map_err(InternalValidationError::Io)?;
-        let response = ohttp_decapsulate(ohttp_ctx, &res_buf)
+        let response_array: &[u8; crate::ohttp::ENCAPSULATED_MESSAGE_BYTES] =
+            response
+                .try_into()
+                .map_err(|_| InternalValidationError::UnexpectedResponseSize(response.len()))?;
+
+        let response = ohttp_decapsulate(ohttp_ctx, response_array)
             .map_err(InternalValidationError::OhttpEncapsulation)?;
         let body = match response.status() {
             http::StatusCode::OK => response.body().to_vec(),

payjoin/tests/integration.rs

@@ -368,8 +368,7 @@ mod integration {
                     .unwrap();
                 log::info!("Response: {:#?}", &response);
                 assert!(response.status().is_success());
-                let send_ctx =
-                    send_ctx.process_response(&mut response.bytes().await?.to_vec().as_slice())?;
+                let send_ctx = send_ctx.process_response(&response.bytes().await?)?;
                 // POST Original PSBT
 
                 // **********************
@@ -390,8 +389,7 @@ mod integration {
                     .body(req.body)
                     .send()
                     .await?;
-                let res = response.bytes().await?.to_vec();
-                payjoin_proposal.process_res(res, ctx)?;
+                payjoin_proposal.process_res(&response.bytes().await?, ctx)?;
 
                 // **********************
                 // Inside the Sender:
@@ -407,9 +405,8 @@ mod integration {
                     .await
                     .unwrap();
                 log::info!("Response: {:#?}", &response);
-                let checked_payjoin_proposal_psbt = send_ctx
-                    .process_response(&mut response.bytes().await?.to_vec().as_slice(), ohttp_ctx)?
-                    .unwrap();
+                let checked_payjoin_proposal_psbt =
+                    send_ctx.process_response(&response.bytes().await?, ohttp_ctx)?.unwrap();
                 let payjoin_tx = extract_pj_tx(&sender, checked_payjoin_proposal_psbt)?;
                 sender.send_raw_transaction(&payjoin_tx)?;
                 log::info!("sent");
@@ -503,8 +500,7 @@ mod integration {
                 let (req, ctx) = session.extract_req()?;
                 let response = agent.post(req.url).body(req.body).send().await?;
                 assert!(response.status().is_success());
-                let response_body =
-                    session.process_res(response.bytes().await?.to_vec().as_slice(), ctx).unwrap();
+                let response_body = session.process_res(&response.bytes().await?, ctx).unwrap();
                 // No proposal yet since sender has not responded
                 assert!(response_body.is_none());
 
@@ -530,8 +526,7 @@ mod integration {
                     .unwrap();
                 log::info!("Response: {:#?}", &response);
                 assert!(response.status().is_success());
-                let get_ctx =
-                    post_ctx.process_response(&mut response.bytes().await?.to_vec().as_slice())?;
+                let get_ctx = post_ctx.process_response(&response.bytes().await?)?;
                 let (Request { url, body, content_type, .. }, ohttp_ctx) =
                     get_ctx.extract_req(directory.to_owned())?;
                 let response = agent
@@ -541,9 +536,7 @@ mod integration {
                     .send()
                     .await?;
                 // No response body yet since we are async and pushed fallback_psbt to the buffer
-                assert!(get_ctx
-                    .process_response(&mut response.bytes().await?.to_vec().as_slice(), ohttp_ctx)?
-                    .is_none());
+                assert!(get_ctx.process_response(&response.bytes().await?, ohttp_ctx)?.is_none());
 
                 // **********************
                 // Inside the Receiver:
@@ -560,8 +553,7 @@ mod integration {
                 assert!(!payjoin_proposal.is_output_substitution_disabled());
                 let (req, ctx) = payjoin_proposal.extract_v2_req()?;
                 let response = agent.post(req.url).body(req.body).send().await?;
-                let res = response.bytes().await?.to_vec();
-                payjoin_proposal.process_res(res, ctx)?;
+                payjoin_proposal.process_res(&response.bytes().await?, ctx)?;
 
                 // **********************
                 // Inside the Sender:
@@ -575,9 +567,8 @@ mod integration {
                     .body(body.clone())
                     .send()
                     .await?;
-                let checked_payjoin_proposal_psbt = get_ctx
-                    .process_response(&mut response.bytes().await?.to_vec().as_slice(), ohttp_ctx)?
-                    .unwrap();
+                let checked_payjoin_proposal_psbt =
+                    get_ctx.process_response(&response.bytes().await?, ohttp_ctx)?.unwrap();
                 let payjoin_tx = extract_pj_tx(&sender, checked_payjoin_proposal_psbt)?;
                 sender.send_raw_transaction(&payjoin_tx)?;
                 log::info!("sent");
@@ -739,7 +730,7 @@ mod integration {
                     let (req, ctx) = payjoin_proposal.extract_v2_req().unwrap();
                     let response = agent_clone.post(req.url).body(req.body).send().await?;
                     payjoin_proposal
-                        .process_res(response.bytes().await?.to_vec(), ctx)
+                        .process_res(&response.bytes().await?, ctx)
                         .map_err(|e| e.to_string())?;
                     Ok::<_, Box<dyn std::error::Error + Send + Sync>>(())
                 });