1 files changed, 193 insertions, 0 deletions
diff --git a/src/swarm/auth.rs b/src/swarm/auth.rs
new file mode 100644
index 0000000..8b1352c
--- /dev/null
+++ b/src/swarm/auth.rs
@@ -0,0 +1,193 @@
+//! Swarm membership by pre-shared key.
+//!
+//! Mycelium authenticates the sender of every message (`src_pk` can't be forged), so we
+//! always know *who* talks to us. With a swarm key configured, only peers that also know
+//! the key may take part: HELLO and HELLO_REPLY carry a tag proving it, and until a peer
+//! has sent a valid one, everything else it sends is ignored.
+//!
+//! The tag is `HMAC-SHA256(key, label ‖ swarm_id ‖ from_pk ‖ to_pk)`. It needs no
+//! challenge: it is bound to both keys, and only `from_pk` can send as `from_pk`, so a
+//! captured tag is useless to anyone else. Different labels for HELLO and HELLO_REPLY
+//! keep one from being passed off as the other. A HELLO has no swarm ID yet (a new node
+//! doesn't know it), so its tag binds an empty one.
+//!
+//! Limits of a shared key: every member holds the same secret, so a single member can't
+//! be revoked (forgetting it lasts until it says HELLO again), and changing the key
+//! means restarting every node. A member list in the document is the proper successor.
+
+use std::path::Path;
+use std::sync::Mutex;
+use std::time::{Duration, Instant};
+
+use anyhow::{bail, Context, Result};
+use sha2::{Digest, Sha256};
+
+const HELLO_LABEL: &[u8] = b"continuum/hello/1";
+const HELLO_REPLY_LABEL: &[u8] = b"continuum/hello-reply/1";
+
+/// The swarm's pre-shared key.
+pub(crate) struct SwarmKey {
+    key: Vec<u8>,
+    rejections: Mutex<Rejections>,
+}
+
+/// For rate limiting the log of rejected handshakes.
+#[derive(Default)]
+struct Rejections {
+    last_logged: Option<Instant>,
+    suppressed: u64,
+}
+
+/// At most one rejection log line per this interval.
+const REJECTION_LOG_INTERVAL: Duration = Duration::from_secs(10);
+
+impl SwarmKey {
+    /// Reads the key from a file; its bytes are the key, as they are. Generate one with
+    /// e.g. `head -c 32 /dev/urandom > swarm.key`.
+    pub(crate) fn from_file(path: &Path) -> Result<Self> {
+        let key = std::fs::read(path)
+            .with_context(|| format!("Reading the swarm key from {}", path.display()))?;
+        if key.len() < 16 {
+            bail!(
+                "The swarm key in {} has {} bytes; use at least 16 (32 random bytes recommended)",
+                path.display(),
+                key.len()
+            );
+        }
+        Ok(Self::new(key))
+    }
+
+    fn new(key: Vec<u8>) -> Self {
+        Self {
+            key,
+            rejections: Mutex::default(),
+        }
+    }
+
+    /// Records a rejected handshake. Returns how many went unlogged since the last log
+    /// line if this one should be logged, None if it should be suppressed.
+    pub(crate) fn note_rejection(&self) -> Option<u64> {
+        let mut rejections = self
+            .rejections
+            .lock()
+            .unwrap_or_else(|poisoned| poisoned.into_inner());
+        let due = rejections
+            .last_logged
+            .is_none_or(|last| last.elapsed() >= REJECTION_LOG_INTERVAL);
+        if !due {
+            rejections.suppressed += 1;
+            return None;
+        }
+        rejections.last_logged = Some(Instant::now());
+        Some(std::mem::take(&mut rejections.suppressed))
+    }
+
+    /// Tag for a HELLO from `from` to `to`.
+    pub(crate) fn hello_tag(&self, from: &str, to: &str) -> Vec<u8> {
+        self.tag(HELLO_LABEL, "", from, to).to_vec()
+    }
+
+    /// Tag for a HELLO_REPLY from `from` to `to`, naming the swarm.
+    pub(crate) fn hello_reply_tag(&self, swarm_id: &str, from: &str, to: &str) -> Vec<u8> {
+        self.tag(HELLO_REPLY_LABEL, swarm_id, from, to).to_vec()
+    }
+
+    pub(crate) fn verify_hello(&self, tag: &[u8], from: &str, to: &str) -> bool {
+        constant_time_eq(tag, &self.tag(HELLO_LABEL, "", from, to))
+    }
+
+    pub(crate) fn verify_hello_reply(
+        &self,
+        tag: &[u8],
+        swarm_id: &str,
+        from: &str,
+        to: &str,
+    ) -> bool {
+        constant_time_eq(tag, &self.tag(HELLO_REPLY_LABEL, swarm_id, from, to))
+    }
+
+    fn tag(&self, label: &[u8], swarm_id: &str, from: &str, to: &str) -> [u8; 32] {
+        // Length-prefixed fields, so no two different inputs encode the same.
+        let mut message = Vec::new();
+        for field in [label, swarm_id.as_bytes(), from.as_bytes(), to.as_bytes()] {
+            message.extend_from_slice(&(field.len() as u32).to_be_bytes());
+            message.extend_from_slice(field);
+        }
+        hmac_sha256(&self.key, &message)
+    }
+}
+
+/// HMAC-SHA256 (RFC 2104), on top of the `sha2` crate we already use.
+fn hmac_sha256(key: &[u8], message: &[u8]) -> [u8; 32] {
+    const BLOCK: usize = 64;
+
+    let mut block_key = [0u8; BLOCK];
+    if key.len() > BLOCK {
+        block_key[..32].copy_from_slice(&Sha256::digest(key));
+    } else {
+        block_key[..key.len()].copy_from_slice(key);
+    }
+
+    let mut inner = Sha256::new();
+    inner.update(block_key.map(|byte| byte ^ 0x36));
+    inner.update(message);
+    let inner = inner.finalize();
+
+    let mut outer = Sha256::new();
+    outer.update(block_key.map(|byte| byte ^ 0x5c));
+    outer.update(inner);
+
+    let mut tag = [0u8; 32];
+    tag.copy_from_slice(&outer.finalize());
+    tag
+}
+
+fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
+    a.len() == b.len() && a.iter().zip(b).fold(0u8, |diff, (x, y)| diff | (x ^ y)) == 0
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn hex(bytes: &[u8]) -> String {
+        bytes.iter().map(|byte| format!("{byte:02x}")).collect()
+    }
+
+    #[test]
+    fn hmac_matches_rfc_4231() {
+        // Test case 2.
+        assert_eq!(
+            hex(&hmac_sha256(b"Jefe", b"what do ya want for nothing?")),
+            "5bdcc146bf60754e6a042426089575c75a003f089d2739839dec58b964ec3843"
+        );
+        // Test case 6: a key longer than the block size.
+        assert_eq!(
+            hex(&hmac_sha256(
+                &[0xaa; 131],
+                b"Test Using Larger Than Block-Size Key - Hash Key First"
+            )),
+            "60e431591ee0b67f0d8a26aacbf5b77f8e0bc6213728c5140546040f0ee37f54"
+        );
+    }
+
+    #[test]
+    fn tags_bind_key_peers_and_kind() {
+        let key = SwarmKey::new(vec![1; 32]);
+        let other = SwarmKey::new(vec![2; 32]);
+
+        let hello = key.hello_tag("A", "B");
+        assert!(key.verify_hello(&hello, "A", "B"));
+        assert!(!other.verify_hello(&hello, "A", "B"));
+        assert!(!key.verify_hello(&hello, "B", "A"));
+        assert!(!key.verify_hello(&hello, "A", "C"));
+        assert!(!key.verify_hello(&[], "A", "B"));
+
+        let reply = key.hello_reply_tag("swarm", "A", "B");
+        assert!(key.verify_hello_reply(&reply, "swarm", "A", "B"));
+        assert!(!key.verify_hello_reply(&reply, "other-swarm", "A", "B"));
+        // A HELLO tag is no HELLO_REPLY tag, and the other way round.
+        assert!(!key.verify_hello_reply(&hello, "", "A", "B"));
+        assert!(!key.verify_hello(&key.hello_reply_tag("", "A", "B"), "A", "B"));
+    }
+}