add entropy enforcement for optional encryption requirement

Shannon entropy check rejects low-entropy content when MIN_ENTROPY > 0.
Encrypted data ~7.5-8.0 bits/byte, plaintext ~4.0-5.0 bits/byte.
Configurable via FLASKPASTE_MIN_ENTROPY environment variable.

app/api/routes.py

@@ -3,6 +3,7 @@
 import hashlib
 import hmac
 import json
+import math
 import os
 import re
 import secrets
@@ -58,6 +59,33 @@ MAGIC_SIGNATURES = {
 }
 
 
+def _calculate_entropy(data: bytes) -> float:
+    """Calculate Shannon entropy in bits per byte.
+
+    Returns value between 0 (uniform) and 8 (perfectly random).
+    Encrypted/compressed data: ~7.5-8.0
+    English text: ~4.0-5.0
+    Binary executables: ~5.0-6.5
+    """
+    if not data:
+        return 0.0
+
+    # Count byte frequencies
+    freq = [0] * 256
+    for byte in data:
+        freq[byte] += 1
+
+    # Calculate entropy
+    length = len(data)
+    entropy = 0.0
+    for count in freq:
+        if count > 0:
+            p = count / length
+            entropy -= p * math.log2(p)
+
+    return entropy
+
+
 def _get_pow_secret() -> bytes:
     """Get or generate the PoW signing secret."""
     global _pow_secret_cache
@@ -425,6 +453,22 @@ def create_paste():
             "authenticated": owner is not None,
         }, 413)
 
+    # Check minimum entropy requirement (encryption enforcement)
+    min_entropy = current_app.config.get("MIN_ENTROPY", 0)
+    if min_entropy > 0:
+        entropy = _calculate_entropy(content)
+        if entropy < min_entropy:
+            current_app.logger.warning(
+                "Low entropy rejected: %.2f < %.2f from=%s",
+                entropy, min_entropy, request.remote_addr
+            )
+            return _json_response({
+                "error": "Content entropy too low",
+                "entropy": round(entropy, 2),
+                "min_entropy": min_entropy,
+                "hint": "Encrypt content before uploading (-e flag in fpaste)",
+            }, 400)
+
     # Check content deduplication threshold
     content_hash = hashlib.sha256(content).hexdigest()
     is_allowed, dedup_count = check_content_hash(content_hash)