Define Magic newtype as `[u8; 4]`, not `u32`.

The magic value isn't a u32, it's a byte string, so specifying it as
such avoids ambiguity about endianness.

zebra-network/src/constants.rs

@@ -9,7 +9,7 @@ pub const CURRENT_VERSION: Version = Version(170_007);
 pub mod magics {
     use super::*;
     /// The production mainnet.
-    pub const MAINNET: Magic = Magic(0x6427e924);
+    pub const MAINNET: Magic = Magic([0x24, 0xe9, 0x27, 0x64]);
     /// The testnet.
-    pub const TESTNET: Magic = Magic(0xbff91afa);
+    pub const TESTNET: Magic = Magic([0xfa, 0x1a, 0xf9, 0xbf]);
 }

zebra-network/src/message.rs

@@ -329,7 +329,7 @@ impl ZcashSerialization for Message {
         };
 
         // Write the header and then the body.
-        writer.write_u32::<LittleEndian>(magic.0)?;
+        writer.write_all(&magic.0)?;
         writer.write_all(command)?;
         writer.write_u32::<LittleEndian>(body.len() as u32)?;
         writer.write_all(&Sha256dChecksum::from(&body[..]).0)?;

zebra-network/src/types.rs

@@ -2,7 +2,7 @@
 
 /// A magic number identifying the network.
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
-pub struct Magic(pub u32);
+pub struct Magic(pub [u8; 4]);
 
 /// A protocol version number.
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]