From 498084228b89d30462df0a5adfcc737fdc21d314 Mon Sep 17 00:00:00 2001
From: Dan Callahan <danc@element.io>
Date: Fri, 14 May 2021 10:58:46 +0100
Subject: Use Python's secrets module instead of random (#9984)

Functionally identical, but more obviously cryptographically secure.
...Explicit is better than implicit?

Avoids needing to know that SystemRandom() implies a CSPRNG, and
complies with the big scary red box on the documentation for random:

> Warning:
>   The pseudo-random generators of this module should not be used for
>   security purposes. For security or cryptographic uses, see the
>   secrets module.

https://docs.python.org/3/library/random.html

Signed-off-by: Dan Callahan <danc@element.io>
---
 synapse/util/stringutils.py | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

(limited to 'synapse/util/stringutils.py')

diff --git a/synapse/util/stringutils.py b/synapse/util/stringutils.py
index 4f25cd1d26..40cd51a8ca 100644
--- a/synapse/util/stringutils.py
+++ b/synapse/util/stringutils.py
@@ -13,8 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import itertools
-import random
 import re
+import secrets
 import string
 from collections.abc import Iterable
 from typing import Optional, Tuple
@@ -35,18 +35,21 @@ CLIENT_SECRET_REGEX = re.compile(r"^[0-9a-zA-Z\.=_\-]+$")
 #
 MXC_REGEX = re.compile("^mxc://([^/]+)/([^/#?]+)$")
 
-# random_string and random_string_with_symbols are used for a range of things,
-# some cryptographically important, some less so. We use SystemRandom to make sure
-# we get cryptographically-secure randoms.
-rand = random.SystemRandom()
-
 
 def random_string(length: int) -> str:
-    return "".join(rand.choice(string.ascii_letters) for _ in range(length))
+    """Generate a cryptographically secure string of random letters.
+
+    Drawn from the characters: `a-z` and `A-Z`
+    """
+    return "".join(secrets.choice(string.ascii_letters) for _ in range(length))
 
 
 def random_string_with_symbols(length: int) -> str:
-    return "".join(rand.choice(_string_with_symbols) for _ in range(length))
+    """Generate a cryptographically secure string of random letters/numbers/symbols.
+
+    Drawn from the characters: `a-z`, `A-Z`, `0-9`, and `.,;:^&*-_+=#~@`
+    """
+    return "".join(secrets.choice(_string_with_symbols) for _ in range(length))
 
 
 def is_ascii(s: bytes) -> bool:
-- 
cgit 1.5.1


From bd918d874f4eb459b9c2f9af6e8e994b6b19d264 Mon Sep 17 00:00:00 2001
From: Dan Callahan <danc@element.io>
Date: Fri, 14 May 2021 10:58:52 +0100
Subject: Simplify exception handling in is_ascii. (#9985)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

We can get away with just catching UnicodeError here.

    ⋮
    +-- ValueError
    |    +-- UnicodeError
    |         +-- UnicodeDecodeError
    |         +-- UnicodeEncodeError
    |         +-- UnicodeTranslateError
    ⋮

https://docs.python.org/3/library/exceptions.html#exception-hierarchy

Signed-off-by: Dan Callahan <danc@element.io>
---
 changelog.d/9985.misc       | 1 +
 synapse/util/stringutils.py | 4 +---
 2 files changed, 2 insertions(+), 3 deletions(-)
 create mode 100644 changelog.d/9985.misc

(limited to 'synapse/util/stringutils.py')

diff --git a/changelog.d/9985.misc b/changelog.d/9985.misc
new file mode 100644
index 0000000000..97bd747f26
--- /dev/null
+++ b/changelog.d/9985.misc
@@ -0,0 +1 @@
+Simplify a few helper functions.
diff --git a/synapse/util/stringutils.py b/synapse/util/stringutils.py
index 40cd51a8ca..f029432191 100644
--- a/synapse/util/stringutils.py
+++ b/synapse/util/stringutils.py
@@ -55,9 +55,7 @@ def random_string_with_symbols(length: int) -> str:
 def is_ascii(s: bytes) -> bool:
     try:
         s.decode("ascii").encode("ascii")
-    except UnicodeDecodeError:
-        return False
-    except UnicodeEncodeError:
+    except UnicodeError:
         return False
     return True
 
-- 
cgit 1.5.1