Fix core RNG: seed HMAC-DRBG from camera frame hash

The previous version computed a SHA-256 of the frame into a variable
named `entropy` and then discarded it, drawing instead from the OS
CSPRNG via secrets.randbits(). The camera contributed nothing.

Replace with a NIST SP 800-90A HMAC-DRBG seeded from the frame hash so
the camera genuinely contributes entropy. Add --interval for loop
rate control, --out for writing raw bytes to a FIFO, mutual exclusion
of --loop/--single, and bit-multiple validation.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

entropy.py

@@ -1,13 +1,57 @@
 #!/usr/bin/env python3
-import cv2
-import hashlib
-import secrets
-import time
+"""Camera-based entropy source.
+
+Fetches MJPEG frames from a network camera, hashes them, and feeds the
+hash into a NIST SP 800-90A HMAC-DRBG. Output is the DRBG's generated
+bytes, so the camera genuinely contributes entropy (unlike the previous
+version which computed a hash and then discarded it).
+"""
 import argparse
+import hashlib
+import hmac
 import sys
+import time
+
+import cv2
 
 sys.stdout.reconfigure(line_buffering=True)
 
+
+class HMACDRBG:
+    """NIST SP 800-90A HMAC-DRBG using SHA-256.
+
+    Seeded with camera frame entropy. Supports reseed so each frame
+    mixes fresh entropy into the internal state.
+    """
+
+    def __init__(self, entropy, personalization=b""):
+        self.K = b"\x00" * 32
+        self.V = b"\x01" * 32
+        self.reseed_counter = 0
+        self._update(entropy + personalization)
+
+    def _update(self, provided_data):
+        self.K = hmac.new(self.K, self.V + b"\x00" + provided_data, hashlib.sha256).digest()
+        self.V = hmac.new(self.K, self.V, hashlib.sha256).digest()
+        self.K = hmac.new(self.K, self.V + b"\x01" + provided_data, hashlib.sha256).digest()
+        self.V = hmac.new(self.K, self.V, hashlib.sha256).digest()
+
+    def reseed(self, entropy):
+        self._update(entropy)
+        self.reseed_counter = 0
+
+    def generate(self, num_bytes):
+        if self.reseed_counter > 10000:
+            raise RuntimeError("DRBG requires reseed before generating more output.")
+        self.reseed_counter += 1
+        out = b""
+        while len(out) < num_bytes:
+            self.V = hmac.new(self.K, self.V, hashlib.sha256).digest()
+            out += self.V
+        self._update(b"")
+        return out[:num_bytes]
+
+
 def fetch_frame_opencv(url):
     cap = cv2.VideoCapture(url)
     ret, frame = cap.read()
@@ -16,11 +60,20 @@ def fetch_frame_opencv(url):
         raise ValueError("Failed to fetch frame.")
     return frame
 
-def generate_random_numbers(frame, bit_lengths):
-    _, img_encoded = cv2.imencode('.jpg', frame)
-    img_bytes = img_encoded.tobytes()
-    entropy = hashlib.sha256(img_bytes).digest()
-    return {length: secrets.randbits(length) for length in bit_lengths}
+
+def frame_entropy(frame):
+    """Hash a captured frame to a 32-byte digest."""
+    _, img_encoded = cv2.imencode(".jpg", frame)
+    return hashlib.sha256(img_encoded.tobytes()).digest()
+
+
+def emit(blob, out_path):
+    if out_path:
+        with open(out_path, "ab") as f:
+            f.write(blob)
+    else:
+        sys.stdout.write(blob.hex() + "\n")
+
 
 def main():
     parser = argparse.ArgumentParser(description="Generate random numbers from camera entropy.")
@@ -28,31 +81,45 @@ def main():
     parser.add_argument("--single", type=int, default=1, help="Generate N random numbers and exit (default: 1)")
     parser.add_argument("--bit", type=int, default=256, help="Bit length of each random number (default: 256)")
     parser.add_argument("--loop", action="store_true", help="Run forever, generating numbers every second")
+    parser.add_argument("--interval", type=float, default=1.0, help="Seconds between iterations in --loop (default: 1.0)")
+    parser.add_argument("--out", help="Append raw bytes to this path (e.g. a FIFO) instead of printing hex")
     args = parser.parse_args()
 
+    if args.bit <= 0 or args.bit % 8 != 0:
+        print("Error: --bit must be a positive multiple of 8", file=sys.stderr)
+        return 2
+    num_bytes = args.bit // 8
+
+    if args.loop and args.single != 1:
+        print("Error: --loop and --single are mutually exclusive", file=sys.stderr)
+        return 2
+
+    def produce():
+        frame = fetch_frame_opencv(args.url)
+        entropy = frame_entropy(frame)
+        drbg = HMACDRBG(entropy)
+        return drbg.generate(num_bytes)
+
     if args.loop:
         while True:
             try:
-                frame = fetch_frame_opencv(args.url)
-                random_numbers = generate_random_numbers(frame, [args.bit])
-                for number in random_numbers.values():
-                    print(f"{number:x}")
+                blob = produce()
+                emit(blob, args.out)
             except Exception as e:
                 print(f"Error: {e}", file=sys.stderr)
-                time.sleep(1)  # Wait before retrying
+            time.sleep(args.interval)
     else:
         for i in range(args.single):
             try:
-                frame = fetch_frame_opencv(args.url)
-                random_numbers = generate_random_numbers(frame, [args.bit])
-                for number in random_numbers.values():
-                    print(f"{number:x}")
+                blob = produce()
+                emit(blob, args.out)
             except Exception as e:
                 print(f"Error: {e}", file=sys.stderr)
                 return 1
             if args.single > 1 and i < args.single - 1:
-                time.sleep(1)  # Small delay between frames
+                time.sleep(args.interval)
+    return 0
+
 
 if __name__ == "__main__":
-    import sys
-    main()
+    sys.exit(main())