From ef5d50369232debe4cb10c84748a9b3966a78915 Mon Sep 17 00:00:00 2001
From: Georgio Nicolas <me@georgio.xyz>
Date: Wed, 25 Feb 2026 15:55:46 +0100
Subject: [PATCH] add lp peer config

---
 common/nym-lp/src/lib.rs         |   1 +
 common/nym-lp/src/peer_config.rs | 129 +++++++++++++++++++++++++++++++
 2 files changed, 130 insertions(+)
 create mode 100644 common/nym-lp/src/peer_config.rs

diff --git a/common/nym-lp/src/lib.rs b/common/nym-lp/src/lib.rs
index 8e4c5de707..8015d521e8 100644
--- a/common/nym-lp/src/lib.rs
+++ b/common/nym-lp/src/lib.rs
@@ -5,6 +5,7 @@ pub mod codec;
 pub mod error;
 pub mod packet;
 pub mod peer;
+pub mod peer_config;
 pub mod psq;
 pub mod replay;
 pub mod session;
diff --git a/common/nym-lp/src/peer_config.rs b/common/nym-lp/src/peer_config.rs
new file mode 100644
index 0000000000..264f440371
--- /dev/null
+++ b/common/nym-lp/src/peer_config.rs
@@ -0,0 +1,129 @@
+use std::char::MAX;
+
+use rand09::{self, CryptoRng, Rng};
+
+use crate::LpError;
+
+pub const MAX_HOPS: u8 = 16;
+
+pub const LP_PEER_CONFIG_SIZE: usize = 20;
+
+// 20 bytes
+pub struct LpPeerConfig {
+    // these 3 fields will be packed in one u8
+    // with 2 bits left at the end
+    hop_id: u8,
+    is_exit: bool,
+    censorship_resistance: bool,
+
+
+    // if we add config params later, we can use this
+    filler: [u8; 3],
+
+    seed: [u8; 16],
+}
+
+impl LpPeerConfig {
+   pub fn new<R>(
+        rng: &mut R,
+        hop_id: u8,
+        is_exit: bool,
+        censorship_resistance: bool,
+    ) -> Result<Self, LpError>
+    where
+        R: Rng + CryptoRng,
+    {
+        let seed: [u8; 16] = rng.random();
+        let filler: [u8; 3] = rng.random();
+        Self::build(hop_id, is_exit, censorship_resistance, seed, filler)
+    }
+
+    fn build(
+        hop_id: u8,
+        is_exit: bool,
+        censorship_resistance: bool,
+        seed: [u8; 16],
+        filler: [u8; 3],
+    ) -> Result<Self, LpError> {
+        if hop_id >= MAX_HOPS {
+            Err(LpError::Internal(format!(
+                "Requested hop index ({}) is greater than the allowed maximum {}.",
+                hop_id,
+                MAX_HOPS - 1
+            )))
+        } else if is_exit && hop_id == 0 {
+            Err(LpError::Internal(
+                "Hop index for exit node cannot be zero".into(),
+            ))
+        } else {
+            Ok(Self {
+                hop_id,
+                is_exit,
+                censorship_resistance,
+
+                seed,
+                filler,
+            })
+        }
+    }
+
+    pub fn serialize(&self) -> [u8; LP_PEER_CONFIG_SIZE] {
+        let mut output_bytes: [u8; LP_PEER_CONFIG_SIZE] = [0u8; LP_PEER_CONFIG_SIZE];
+        output_bytes[0..4].copy_from_slice(self.pack_config().as_slice());
+        output_bytes[4..].copy_from_slice(&self.seed);
+        output_bytes
+    }
+   pub fn deserialize(bytes: &[u8]) -> Result<Self, LpError> {
+        if bytes.len() != LP_PEER_CONFIG_SIZE {
+            Err(LpError::DeserializationError(format!(
+                "Invalid Lp Config Length ({}), expected ({})",
+                bytes.len(),
+                LP_PEER_CONFIG_SIZE
+            )))
+        } else {
+            let (hop_id, is_exit, censorship_resistance) = Self::unpack_first_byte(bytes[0]);
+
+            let filler: [u8; 3] = [bytes[1], bytes[2], bytes[3]];
+            let mut seed: [u8; 16] = [0u8; 16];
+            seed.copy_from_slice(&bytes[4..LP_PEER_CONFIG_SIZE]);
+            Self::build(hop_id, is_exit, censorship_resistance, seed, filler)
+        }
+    }
+
+    fn pack_config(&self) -> [u8; 4] {
+        [
+            self.pack_first_byte(),
+            self.filler[0],
+            self.filler[1],
+            self.filler[2],
+        ]
+    }
+
+    fn pack_first_byte(&self) -> u8 {
+        let mut byte = self.hop_id;
+
+        if self.is_exit {
+            byte |= 0b0001_0000;
+        }
+        if self.censorship_resistance {
+            byte |= 0b0010_0000;
+        }
+
+        // there will be 2 free bits at the end
+
+        byte
+    }
+    fn unpack_first_byte(byte: u8) -> (u8, bool, bool) {
+        // extract 4 bits
+        let hop_id = byte & 0b0000_1111;
+
+        // extract 5th bit
+        let is_exit = (byte & 0b0001_0000) >> 4 == 1;
+        // extract 6th bit
+        let censorship_resistance = (byte & 0b0010_0000) >> 5 == 1;
+
+        // if we need to use the last 2 bits, we can add something here
+        (hop_id, is_exit, censorship_resistance)
+    }
+}
+