Shorten subdirectory IDs (#386)

The message for the last commit contains a rationale for 64 bit values
and hashing.

payjoin-directory/src/db.rs

@@ -7,6 +7,19 @@ use tracing::debug;
 const DEFAULT_COLUMN: &str = "";
 const PJ_V1_COLUMN: &str = "pjv1";
 
+// TODO move to payjoin crate as pub?
+// TODO impl From<HpkePublicKey> for ShortId
+// TODO impl Display for ShortId (Base64)
+// TODO impl TryFrom<&str> for ShortId (Base64)
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct ShortId(pub [u8; 8]);
+
+impl ShortId {
+    pub fn column_key(&self, column: &str) -> Vec<u8> {
+        self.0.iter().chain(column.as_bytes()).copied().collect()
+    }
+}
+
 #[derive(Debug, Clone)]
 pub(crate) struct DbPool {
     client: Client,
@@ -19,23 +32,28 @@ impl DbPool {
         Ok(Self { client, timeout })
     }
 
-    pub async fn push_default(&self, pubkey_id: &str, data: Vec<u8>) -> RedisResult<()> {
+    pub async fn push_default(&self, pubkey_id: &ShortId, data: Vec<u8>) -> RedisResult<()> {
         self.push(pubkey_id, DEFAULT_COLUMN, data).await
     }
 
-    pub async fn peek_default(&self, pubkey_id: &str) -> Option<RedisResult<Vec<u8>>> {
+    pub async fn peek_default(&self, pubkey_id: &ShortId) -> Option<RedisResult<Vec<u8>>> {
         self.peek_with_timeout(pubkey_id, DEFAULT_COLUMN).await
     }
 
-    pub async fn push_v1(&self, pubkey_id: &str, data: Vec<u8>) -> RedisResult<()> {
+    pub async fn push_v1(&self, pubkey_id: &ShortId, data: Vec<u8>) -> RedisResult<()> {
         self.push(pubkey_id, PJ_V1_COLUMN, data).await
     }
 
-    pub async fn peek_v1(&self, pubkey_id: &str) -> Option<RedisResult<Vec<u8>>> {
+    pub async fn peek_v1(&self, pubkey_id: &ShortId) -> Option<RedisResult<Vec<u8>>> {
         self.peek_with_timeout(pubkey_id, PJ_V1_COLUMN).await
     }
 
-    async fn push(&self, pubkey_id: &str, channel_type: &str, data: Vec<u8>) -> RedisResult<()> {
+    async fn push(
+        &self,
+        pubkey_id: &ShortId,
+        channel_type: &str,
+        data: Vec<u8>,
+    ) -> RedisResult<()> {
         let mut conn = self.client.get_async_connection().await?;
         let key = channel_name(pubkey_id, channel_type);
         () = conn.set(&key, data.clone()).await?;
@@ -45,13 +63,13 @@ impl DbPool {
 
     async fn peek_with_timeout(
         &self,
-        pubkey_id: &str,
+        pubkey_id: &ShortId,
         channel_type: &str,
     ) -> Option<RedisResult<Vec<u8>>> {
         tokio::time::timeout(self.timeout, self.peek(pubkey_id, channel_type)).await.ok()
     }
 
-    async fn peek(&self, pubkey_id: &str, channel_type: &str) -> RedisResult<Vec<u8>> {
+    async fn peek(&self, pubkey_id: &ShortId, channel_type: &str) -> RedisResult<Vec<u8>> {
         let mut conn = self.client.get_async_connection().await?;
         let key = channel_name(pubkey_id, channel_type);
 
@@ -99,6 +117,6 @@ impl DbPool {
     }
 }
 
-fn channel_name(pubkey_id: &str, channel_type: &str) -> String {
-    format!("{}:{}", pubkey_id, channel_type)
+fn channel_name(pubkey_id: &ShortId, channel_type: &str) -> Vec<u8> {
+    pubkey_id.column_key(channel_type)
 }

payjoin-directory/src/lib.rs

@@ -3,6 +3,8 @@ use std::sync::Arc;
 use std::time::Duration;
 
 use anyhow::Result;
+use bitcoin::base64::prelude::BASE64_URL_SAFE_NO_PAD;
+use bitcoin::base64::Engine;
 use http_body_util::combinators::BoxBody;
 use http_body_util::{BodyExt, Empty, Full};
 use hyper::body::{Body, Bytes, Incoming};
@@ -15,6 +17,8 @@ use tokio::net::TcpListener;
 use tokio::sync::Mutex;
 use tracing::{debug, error, info, trace};
 
+use crate::db::ShortId;
+
 pub const DEFAULT_DIR_PORT: u16 = 8080;
 pub const DEFAULT_DB_HOST: &str = "localhost:6379";
 pub const DEFAULT_TIMEOUT_SECS: u64 = 30;
@@ -295,7 +299,7 @@ async fn post_fallback_v1(
     };
 
     let v2_compat_body = format!("{}\n{}", body_str, query);
-    let id = shorten_string(id);
+    let id = decode_short_id(id)?;
     pool.push_default(&id, v2_compat_body.into())
         .await
         .map_err(|e| HandlerError::BadRequest(e.into()))?;
@@ -316,7 +320,7 @@ async fn put_payjoin_v1(
     trace!("Put_payjoin_v1");
     let ok_response = Response::builder().status(StatusCode::OK).body(empty())?;
 
-    let id = shorten_string(id);
+    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 {
@@ -337,7 +341,7 @@ async fn post_subdir(
     let none_response = Response::builder().status(StatusCode::OK).body(empty())?;
     trace!("post_subdir");
 
-    let id = shorten_string(id);
+    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 {
@@ -355,7 +359,7 @@ async fn get_subdir(
     pool: DbPool,
 ) -> Result<Response<BoxBody<Bytes, hyper::Error>>, HandlerError> {
     trace!("get_subdir");
-    let id = shorten_string(id);
+    let id = decode_short_id(id)?;
     match pool.peek_default(&id).await {
         Some(result) => match result {
             Ok(buffered_req) => Ok(Response::new(full(buffered_req))),
@@ -385,7 +389,15 @@ async fn get_ohttp_keys(
     Ok(res)
 }
 
-fn shorten_string(input: &str) -> String { input.chars().take(8).collect() }
+fn decode_short_id(input: &str) -> Result<ShortId, HandlerError> {
+    let decoded =
+        BASE64_URL_SAFE_NO_PAD.decode(input).map_err(|e| HandlerError::BadRequest(e.into()))?;
+
+    decoded[..8]
+        .try_into()
+        .map_err(|_| HandlerError::BadRequest(anyhow::anyhow!("Invalid subdirectory ID")))
+        .map(ShortId)
+}
 
 fn empty() -> BoxBody<Bytes, hyper::Error> {
     Empty::<Bytes>::new().map_err(|never| match never {}).boxed()

payjoin/src/lib.rs

@@ -30,6 +30,8 @@ pub mod send;
 #[cfg(feature = "v2")]
 pub(crate) mod hpke;
 #[cfg(feature = "v2")]
+pub use crate::hpke::{HpkeKeyPair, HpkePublicKey};
+#[cfg(feature = "v2")]
 pub(crate) mod ohttp;
 #[cfg(feature = "v2")]
 pub use crate::ohttp::OhttpKeys;

payjoin/src/receive/v2/mod.rs

@@ -3,6 +3,7 @@ use std::time::{Duration, SystemTime};
 
 use bitcoin::base64::prelude::BASE64_URL_SAFE_NO_PAD;
 use bitcoin::base64::Engine;
+use bitcoin::hashes::{sha256, Hash};
 use bitcoin::psbt::Psbt;
 use bitcoin::{Address, FeeRate, OutPoint, Script, TxOut};
 use serde::de::Deserializer;
@@ -48,7 +49,8 @@ where
 }
 
 fn subdir_path_from_pubkey(pubkey: &HpkePublicKey) -> String {
-    BASE64_URL_SAFE_NO_PAD.encode(pubkey.to_compressed_bytes())
+    let hash = sha256::Hash::hash(&pubkey.to_compressed_bytes());
+    BASE64_URL_SAFE_NO_PAD.encode(&hash.as_byte_array()[..8])
 }
 
 /// A payjoin V2 receiver, allowing for polled requests to the
@@ -182,27 +184,32 @@ impl Receiver {
         PjUriBuilder::new(
             self.context.address.clone(),
             self.pj_url(),
+            Some(self.context.s.public_key().clone()),
             Some(self.context.ohttp_keys.clone()),
             Some(self.context.expiry),
         )
     }
 
-    // The contents of the `&pj=` query parameter including the base64url-encoded public key receiver subdirectory.
+    // The contents of the `&pj=` query parameter.
     // This identifies a session at the payjoin directory server.
     pub fn pj_url(&self) -> Url {
-        let pubkey = &self.id();
-        let pubkey_base64 = BASE64_URL_SAFE_NO_PAD.encode(pubkey);
+        let id_base64 = BASE64_URL_SAFE_NO_PAD.encode(self.id());
         let mut url = self.context.directory.clone();
         {
             let mut path_segments =
                 url.path_segments_mut().expect("Payjoin Directory URL cannot be a base");
-            path_segments.push(&pubkey_base64);
+            path_segments.push(&id_base64);
         }
         url
     }
 
-    /// The per-session public key to use as an identifier
-    pub fn id(&self) -> [u8; 33] { self.context.s.public_key().to_compressed_bytes() }
+    /// The per-session identifier
+    pub fn id(&self) -> [u8; 8] {
+        let hash = sha256::Hash::hash(&self.context.s.public_key().to_compressed_bytes());
+        hash.as_byte_array()[..8]
+            .try_into()
+            .expect("truncating SHA256 to 8 bytes should always succeed")
+    }
 }
 
 /// The sender's original PSBT and optional parameters

payjoin/src/send/error.rs

@@ -4,6 +4,9 @@ use bitcoin::locktime::absolute::LockTime;
 use bitcoin::transaction::Version;
 use bitcoin::{AddressType, Sequence};
 
+#[cfg(feature = "v2")]
+use crate::uri::error::ParseReceiverPubkeyError;
+
 /// Error that may occur when the response from receiver is malformed.
 ///
 /// This is currently opaque type because we aren't sure which variants will stay.
@@ -194,7 +197,7 @@ pub(crate) enum InternalCreateRequestError {
     #[cfg(feature = "v2")]
     OhttpEncapsulation(crate::ohttp::OhttpEncapsulationError),
     #[cfg(feature = "v2")]
-    ParseSubdirectory(ParseSubdirectoryError),
+    ParseReceiverPubkey(ParseReceiverPubkeyError),
     #[cfg(feature = "v2")]
     MissingOhttpConfig,
     #[cfg(feature = "v2")]
@@ -225,7 +228,7 @@ impl fmt::Display for CreateRequestError {
             #[cfg(feature = "v2")]
             OhttpEncapsulation(e) => write!(f, "v2 error: {}", e),
             #[cfg(feature = "v2")]
-            ParseSubdirectory(e) => write!(f, "cannot parse subdirectory: {}", e),
+            ParseReceiverPubkey(e) => write!(f, "cannot parse receiver public key: {}", e),
             #[cfg(feature = "v2")]
             MissingOhttpConfig => write!(f, "no ohttp configuration with which to make a v2 request available"),
             #[cfg(feature = "v2")]
@@ -258,7 +261,7 @@ impl std::error::Error for CreateRequestError {
             #[cfg(feature = "v2")]
             OhttpEncapsulation(error) => Some(error),
             #[cfg(feature = "v2")]
-            ParseSubdirectory(error) => Some(error),
+            ParseReceiverPubkey(error) => Some(error),
             #[cfg(feature = "v2")]
             MissingOhttpConfig => None,
             #[cfg(feature = "v2")]
@@ -278,44 +281,9 @@ impl From<crate::psbt::AddressTypeError> for CreateRequestError {
 }
 
 #[cfg(feature = "v2")]
-impl From<ParseSubdirectoryError> for CreateRequestError {
-    fn from(value: ParseSubdirectoryError) -> Self {
-        CreateRequestError(InternalCreateRequestError::ParseSubdirectory(value))
-    }
-}
-
-#[cfg(feature = "v2")]
-#[derive(Debug)]
-pub(crate) enum ParseSubdirectoryError {
-    MissingSubdirectory,
-    SubdirectoryNotBase64(bitcoin::base64::DecodeError),
-    SubdirectoryInvalidPubkey(crate::hpke::HpkeError),
-}
-
-#[cfg(feature = "v2")]
-impl std::fmt::Display for ParseSubdirectoryError {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        use ParseSubdirectoryError::*;
-
-        match &self {
-            MissingSubdirectory => write!(f, "subdirectory is missing"),
-            SubdirectoryNotBase64(e) => write!(f, "subdirectory is not valid base64: {}", e),
-            SubdirectoryInvalidPubkey(e) =>
-                write!(f, "subdirectory does not represent a valid pubkey: {}", e),
-        }
-    }
-}
-
-#[cfg(feature = "v2")]
-impl std::error::Error for ParseSubdirectoryError {
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
-        use ParseSubdirectoryError::*;
-
-        match &self {
-            MissingSubdirectory => None,
-            SubdirectoryNotBase64(error) => Some(error),
-            SubdirectoryInvalidPubkey(error) => Some(error),
-        }
+impl From<ParseReceiverPubkeyError> for CreateRequestError {
+    fn from(value: ParseReceiverPubkeyError) -> Self {
+        CreateRequestError(InternalCreateRequestError::ParseReceiverPubkey(value))
     }
 }
 

payjoin/src/send/mod.rs

@@ -25,6 +25,8 @@ use std::str::FromStr;
 
 #[cfg(feature = "v2")]
 use bitcoin::base64::{prelude::BASE64_URL_SAFE_NO_PAD, Engine};
+#[cfg(feature = "v2")]
+use bitcoin::hashes::{sha256, Hash};
 use bitcoin::psbt::Psbt;
 use bitcoin::{Amount, FeeRate, Script, ScriptBuf, TxOut, Weight};
 pub use error::{CreateRequestError, ResponseError, ValidationError};
@@ -322,21 +324,11 @@ impl Sender {
     }
 
     #[cfg(feature = "v2")]
-    fn extract_rs_pubkey(&self) -> Result<HpkePublicKey, error::ParseSubdirectoryError> {
-        use error::ParseSubdirectoryError;
-
-        let subdirectory = self
-            .endpoint
-            .path_segments()
-            .and_then(|mut segments| segments.next())
-            .ok_or(ParseSubdirectoryError::MissingSubdirectory)?;
-
-        let pubkey_bytes = BASE64_URL_SAFE_NO_PAD
-            .decode(subdirectory)
-            .map_err(ParseSubdirectoryError::SubdirectoryNotBase64)?;
-
-        HpkePublicKey::from_compressed_bytes(&pubkey_bytes)
-            .map_err(ParseSubdirectoryError::SubdirectoryInvalidPubkey)
+    fn extract_rs_pubkey(
+        &self,
+    ) -> Result<HpkePublicKey, crate::uri::error::ParseReceiverPubkeyError> {
+        use crate::uri::UrlExt;
+        self.endpoint.receiver_pubkey()
     }
 
     pub fn endpoint(&self) -> &Url { &self.endpoint }
@@ -404,8 +396,10 @@ impl V2GetContext {
     ) -> Result<(Request, ohttp::ClientResponse), CreateRequestError> {
         use crate::uri::UrlExt;
         let mut url = self.endpoint.clone();
-        let subdir = BASE64_URL_SAFE_NO_PAD
-            .encode(self.hpke_ctx.reply_pair.public_key().to_compressed_bytes());
+
+        // TODO unify with receiver's fn subdir_path_from_pubkey
+        let hash = sha256::Hash::hash(&self.hpke_ctx.reply_pair.public_key().to_compressed_bytes());
+        let subdir = BASE64_URL_SAFE_NO_PAD.encode(&hash.as_byte_array()[..8]);
         url.set_path(&subdir);
         let body = encrypt_message_a(
             Vec::new(),

payjoin/src/uri/error.rs

@@ -11,6 +11,41 @@ pub(crate) enum InternalPjParseError {
     UnsecureEndpoint,
 }
 
+#[cfg(feature = "v2")]
+#[derive(Debug)]
+pub(crate) enum ParseReceiverPubkeyError {
+    MissingPubkey,
+    PubkeyNotBase64(bitcoin::base64::DecodeError),
+    InvalidPubkey(crate::hpke::HpkeError),
+}
+
+#[cfg(feature = "v2")]
+impl std::fmt::Display for ParseReceiverPubkeyError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        use ParseReceiverPubkeyError::*;
+
+        match &self {
+            MissingPubkey => write!(f, "receiver public key is missing"),
+            PubkeyNotBase64(e) => write!(f, "receiver public is not valid base64: {}", e),
+            InvalidPubkey(e) =>
+                write!(f, "receiver public key does not represent a valid pubkey: {}", e),
+        }
+    }
+}
+
+#[cfg(feature = "v2")]
+impl std::error::Error for ParseReceiverPubkeyError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use ParseReceiverPubkeyError::*;
+
+        match &self {
+            MissingPubkey => None,
+            PubkeyNotBase64(error) => Some(error),
+            InvalidPubkey(error) => Some(error),
+        }
+    }
+}
+
 impl From<InternalPjParseError> for PjParseError {
     fn from(value: InternalPjParseError) -> Self { PjParseError(value) }
 }