From b86442d9ae23b91e81bcfa2d82ed90638f849578 Mon Sep 17 00:00:00 2001
From: drebs <drebs@leap.se>
Date: Tue, 21 May 2013 17:02:54 -0300
Subject: Increase storage secret size and use half of it to get the local
 storage passphrase.

---
 src/leap/soledad/__init__.py | 30 ++++++++++++++++++++++++------
 src/leap/soledad/crypto.py   | 11 +++++++----
 2 files changed, 31 insertions(+), 10 deletions(-)

diff --git a/src/leap/soledad/__init__.py b/src/leap/soledad/__init__.py
index 9824c5f6..ab8c03b4 100644
--- a/src/leap/soledad/__init__.py
+++ b/src/leap/soledad/__init__.py
@@ -136,9 +136,23 @@ class Soledad(object):
     The name of the file where the storage secrets will be stored.
     """
 
-    STORAGE_SECRET_LENGTH = 512
+    GENERATED_SECRET_LENGTH = 1024
     """
-    The length of the secret used for symmetric encryption.
+    The length of the generated secret used to derive keys for symmetric
+    encryption for local and remote storage.
+    """
+
+    LOCAL_STORAGE_SECRET_LENGTH = 512
+    """
+    The length of the secret used to derive a passphrase for the SQLCipher
+    database.
+    """
+
+    REMOTE_STORAGE_SECRET_LENGTH = \
+        GENERATED_SECRET_LENGTH - LOCAL_STORAGE_SECRET_LENGTH
+    """
+    The length of the secret used to derive an encryption key and a MAC auth
+    key for remote storage.
     """
 
     SALT_LENGTH = 64
@@ -302,11 +316,15 @@ class Soledad(object):
     def _init_db(self):
         """
         Initialize the U1DB SQLCipher database for local storage.
+
+        The local storage passphrase is hexlified version of the last
+        C{LOCAL_STORAGE_SECRET_LENGTH} bytes of the storage secret.
         """
         self._db = sqlcipher.open(
             self._local_db_path,
             # storage secret is binary but sqlcipher passphrase must be string
-            binascii.b2a_hex(self._get_storage_secret()),
+            binascii.b2a_hex(
+                self._get_storage_secret()[self.LOCAL_STORAGE_SECRET_LENGTH:]),
             create=True,
             document_factory=LeapDocument,
             crypto=self._crypto)
@@ -350,7 +368,7 @@ class Soledad(object):
         )
         # recover the initial value and ciphertext
         iv, ciphertext = self._secrets[self._secret_id][self.SECRET_KEY].split(
-                self.IV_SEPARATOR, 1)
+            self.IV_SEPARATOR, 1)
         iv = int(iv)
         ciphertext = binascii.a2b_base64(ciphertext)
         return self._crypto.decrypt_sym(ciphertext, key, iv=iv)
@@ -445,7 +463,7 @@ class Soledad(object):
         """
         events.signal(events.events_pb2.SOLEDAD_CREATING_KEYS, self._uuid)
         # generate random secret
-        secret = os.urandom(self.STORAGE_SECRET_LENGTH)
+        secret = os.urandom(self.GENERATED_SECRET_LENGTH)
         secret_id = sha256(secret).hexdigest()
         # generate random salt
         salt = os.urandom(self.SALT_LENGTH)
@@ -481,7 +499,7 @@ class Soledad(object):
                         'kdf_salt': '<salt>'
                         'kdf_length': <len>
                         'cipher': 'aes256',
-                        'length': 512,
+                        'length': 1024,
                         'secret': '<encrypted storage_secret 1>',
                     }
                 }
diff --git a/src/leap/soledad/crypto.py b/src/leap/soledad/crypto.py
index 0a459293..e020eee6 100644
--- a/src/leap/soledad/crypto.py
+++ b/src/leap/soledad/crypto.py
@@ -95,9 +95,10 @@ class SoledadCrypto(object):
         Generate a passphrase for symmetric encryption of document's contents.
 
         The password is derived using HMAC having sha256 as underlying hash
-        function. The key used for HMAC is Soledad's storage secret stripped
-        from the first MAC_KEY_LENGTH characters. The HMAC message is
-        C{doc_id}.
+        function. The key used for HMAC are the first
+        C{soledad.REMOTE_STORAGE_SECRET_KENGTH} bytes of Soledad's storage
+        secret stripped from the first MAC_KEY_LENGTH characters. The HMAC
+        message is C{doc_id}.
 
         @param doc_id: The id of the document that will be encrypted using
             this passphrase.
@@ -111,7 +112,9 @@ class SoledadCrypto(object):
         if self.secret is None:
             raise NoSymmetricSecret()
         return hmac.new(
-            self.secret[self.MAC_KEY_LENGTH:],
+            self.secret[
+                self.MAC_KEY_LENGTH:
+                self._soledad.REMOTE_STORAGE_SECRET_LENGTH],
             doc_id,
             hashlib.sha256).digest()
 
-- 
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