fix: use 256-bit entropy for pairing tokens (#351)

Merges #413

Cargo.lock

@@ -4843,6 +4843,7 @@ dependencies = [
  "pdf-extract",
  "probe-rs",
  "prometheus",
+ "rand 0.8.5",
  "reqwest",
  "rppal",
  "rusqlite",

Cargo.toml

@@ -57,6 +57,9 @@ hmac = "0.12"
 sha2 = "0.10"
 hex = "0.4"
 
+# CSPRNG for secure token generation
+rand = "0.8"
+
 # Landlock (Linux sandbox) - optional dependency
 landlock = { version = "0.4", optional = true }
 

src/security/pairing.rs

@@ -201,9 +201,17 @@ fn generate_code() -> String {
     }
 }
 
-/// Generate a cryptographically-adequate bearer token (hex-encoded).
+/// Generate a cryptographically-adequate bearer token with 256-bit entropy.
+///
+/// Uses `rand::thread_rng()` which is backed by the OS CSPRNG
+/// (/dev/urandom on Linux, BCryptGenRandom on Windows, SecRandomCopyBytes
+/// on macOS). The 32 random bytes (256 bits) are hex-encoded for a
+/// 64-character token, providing 256 bits of entropy.
 fn generate_token() -> String {
-    format!("zc_{}", uuid::Uuid::new_v4().as_simple())
+    use rand::RngCore;
+    let mut bytes = [0u8; 32];
+    rand::thread_rng().fill_bytes(&mut bytes);
+    format!("zc_{}", hex::encode(&bytes))
 }
 
 /// SHA-256 hash a bearer token for storage. Returns lowercase hex.