rust-protocol: Add first part of packet encoding.

common/rust-protocol/src/lib.rs

@@ -5,7 +5,7 @@
 use core::convert::TryInto;
 
 use crc16::{State, KERMIT};
-use xxtea_nostd::decrypt;
+use xxtea_nostd::{decrypt, encrypt};
 
 #[derive(Debug, Clone)]
 pub enum Packet {
@@ -27,6 +27,17 @@ impl Packet {
         Self::decode(&checksummed[..22])
     }
 
+    pub fn encode_and_encrypt(&self, key: &[u8], salt: u64, data: &mut [u8]) -> bool {
+        data[0..8].copy_from_slice(&salt.to_le_bytes());
+        if !self.encode(&mut data[8..30]) {
+            return false;
+        }
+        let checksum = State::<KERMIT>::calculate(&data[0..30]);
+        data[30..].copy_from_slice(&checksum.to_le_bytes());
+        encrypt_cbc(key, data);
+        true
+    }
+
     fn decode(data: &[u8]) -> Option<Packet> {
         let type_ = data[0] & 0x1f;
         // count can be at most 7, so we do not need any checks when indexing 8-element arrays
@@ -44,6 +55,32 @@ impl Packet {
             _ => None,
         }
     }
+
+    fn encode(&self, data: &mut [u8]) -> bool {
+        match self {
+            Self::GetSalt => {
+                data[0] = 0;
+                true
+            }
+            Self::Salt(salt) => {
+                data[0] = 1;
+                data[1..9].copy_from_slice(&salt.to_le_bytes());
+                true
+            }
+            Self::Report(report) => {
+                // TODO
+                false
+            }
+            Self::GetValues(get_values) => {
+                // TODO
+                false
+            }
+            Self::Values(values) => {
+                // TODO
+                false
+            }
+        }
+    }
 }
 
 #[derive(Debug, Clone)]
@@ -144,6 +181,16 @@ pub enum ValueType {
     Humidity,
 }
 
+fn encrypt_cbc(key: &[u8], data: &mut [u8]) {
+    for i in 0..data.len() / 8 - 1 {
+        let (prev, block) = data[i * 8..i * 8 + 16].split_at_mut(8);
+        for j in 0..8 {
+            block[j] ^= prev[j];
+        }
+        encrypt(&key, block);
+    }
+}
+
 fn decrypt_cbc(key: &[u8], data: &mut [u8]) {
     let mut blocks = data.rchunks_mut(8).peekable();
     loop {