Sign form to ensure it is not tampered with

This adds a server secret and uses it to sign the generated url with a
timestamp to prevent client-side tampering.

fixes atoponce#57

dnote/__init__.py

@@ -12,7 +12,7 @@ def index():
     """Return the index.html for the main application."""
     error = request.args.get('error', None)
     note = Note()
-    return render_template('index.html', random=note.url, error=error)
+    return render_template('index.html', note=note, error=error)
 
 @DNOTE.route('/security/', methods=['GET'])
 def security():
@@ -33,6 +33,9 @@ def about():
 def show_post():
     """Return the random URL after posting the plaintext."""
     new_url = request.form["new_url"]
+    timestamp = request.form["timestamp"]
+    signature = request.form["signature"]
+
     note = Note(new_url)
     note.plaintext = request.form['paste']
 
@@ -42,6 +45,12 @@ def show_post():
     if not utils.verify_hashcash(token):
         return redirect(url_for('index', error='hashcash'))
 
+    try:
+        note.validate_signature(url=new_url, timestamp=timestamp,
+                                provided_signature=signature)
+    except Exception as e:
+        return render_template('signerror.html', error=e)
+
     if passphrase:
         note.set_passphrase(passphrase)
         note.encrypt()

dnote/note.py

@@ -2,9 +2,10 @@
 import base64
 import os
 import sys
+import time
 import zlib
 from Crypto.Cipher import AES
-from Crypto.Hash import HMAC, SHA512
+from Crypto.Hash import HMAC, SHA512, SHA384
 from Crypto.Protocol import KDF
 from Crypto.Util import Counter
 
@@ -60,13 +61,48 @@ class Note(object):
     dkey = None         # Duress passphrase
     plaintext = None    # Plain text note
     ciphertext = None   # Encrypted text
+    timestamp = None    # current time
+    signature = None    # signature
 
     def __init__(self, url=None):
         if url is None:
             self.create_url()
+            self.sign_note()
         else:
             self.decode_url(url)
 
+    @staticmethod
+    def validate_signature(url, timestamp, provided_signature):
+        # Raise an exception for invalid signature
+
+        if url is None or timestamp is None:
+            raise ValueError("timestamp and/or url are None")
+
+        delta = int(time.time()) - int(float(timestamp))
+        if delta < 0 or delta > dconfig.signature_validity:
+            raise ValueError("Signature has expired")
+
+        computed_signature = Note.get_signature(url=url, timestamp=timestamp)
+
+        # FIXME: requires python3.3
+        #if not HMAC.compare_digest(computed_signature, provided_signature):
+        if computed_signature != provided_signature:
+            raise ValueError("Invalid signature provided!")
+
+    def sign_note(self):
+        if self.url is None:
+            raise ValueError("url has not been set")
+
+        self.timestamp = str(time.time())
+
+        self.signature = Note.get_signature(url=self.url, timestamp=self.timestamp)
+
+    @staticmethod
+    def get_signature(url, timestamp):
+        data = "{timestamp}?{url}".format(timestamp=timestamp, url=url)
+        sig = HMAC.new(dconfig.server_secret, data, SHA384)
+        return sig.hexdigest()
+
     def exists(self):
         """Checks if note already exists"""
         return os.path.exists(self.path())

dnote/templates/index.html

@@ -19,7 +19,9 @@ <h1>Self destructing encrypted notes</h1>
             {% endif %}
             </p>
         {% endif %}
-        <input name="new_url" type="hidden" value="{{ random }}">
+        <input name="new_url" type="hidden" value="{{ note.url }}"/>
+        <input name="timestamp" type="hidden" value="{{ note.timestamp }}"/>
+        <input name="signature" type="hidden" value="{{ note.signature }}"/>
         <p>Type or paste your text below:</p>
         <textarea style="width:100%; height:250px" id="paste" name="paste"></textarea>
         <br/>

dnote/templates/signerror.html

@@ -0,0 +1,7 @@
+{% extends "base.html" %}
+{% block title %}Signature Error{% endblock %}
+{% block content %}
+    <h1>Error</h1>
+    <p>Your request appears to have been tampered with. Please try again.</p>
+    <p>error: {{ error }}</p>
+{% endblock %}

scripts/generate_dnote_hashes

@@ -45,4 +45,6 @@ if not os.path.exists(dconfig):
         f.write('mac_salt = "%s"\n' % os.urandom(16).encode('hex'))
         f.write('nonce_salt = "%s"\n' % os.urandom(16).encode('hex'))
         f.write('duress_salt = "%s"\n' % os.urandom(16).encode('hex'))
+        f.write('server_secret = "%s"\n' % os.urandom(64).encode('hex'))
+        f.write('signature_validity = 300  # seconds\n')
     os.chmod(dconfig, 0440)