htpasswd.py: add optional MD5-APR1 and BCRYPT support via passlib.

- Update docstring for optional MD5-APR1/BCRYPT support via passlib.
- Support the "md5" and "bcrypt" htpasswd_encryption config values.
- Conditionally import the required passlib components if either
  "md5" or "bcrypt" is requested in the configuration file.
- Test bcrypt backend availability upon import.
- First define verification functions, then conditionally import
  external dependencies.
- Consolidate: use context manager for reading credential file.
- Consolidate: save one call to strip() while parsing.
- Consolidate: break long lines, clarify comments and docstrings.
- Consolidate: use verification function mapping for improving maintainability.

radicale/auth/htpasswd.py

@@ -19,18 +19,44 @@
 # along with Radicale.  If not, see <http://www.gnu.org/licenses/>.
 
 """
-Htpasswd authentication.
+Implement htpasswd authentication.
 
-Load the list of login/password couples according a the configuration file
-created by Apache ``htpasswd`` command. Plain-text, crypt and sha1 are
-supported, but md5 is not (see ``htpasswd`` man page to understand why).
+Apache's htpasswd command (httpd.apache.org/docs/programs/htpasswd.html) manages
+a file for storing user credentials. It can encrypt passwords using different
+methods, e.g. BCRYPT, MD5-APR1 (a version of MD5 modified for Apache), SHA1, or
+by using the system's CRYPT routine. The CRYPT and SHA1 encryption methods
+implemented by htpasswd are considered as insecure. MD5-APR1 provides medium
+security as of 2015. Only BCRYPT can be considered secure by current standards.
 
+MD5-APR1-encrypted credentials can be written by all versions of htpasswd (its
+the default, in fact), whereas BCRYPT requires htpasswd 2.4.x or newer.
+
+The `is_authenticated(user, password)` function provided by this module
+verifies the user-given credentials by parsing the htpasswd credential file
+pointed to by the ``htpasswd_filename`` configuration value while assuming
+the password encryption method specified via the ``htpasswd_encryption``
+configuration value.
+
+The following htpasswd password encrpytion methods are supported by Radicale
+out-of-the-box:
+
+    - plain-text (created by htpasswd -p...) -- INSECURE
+    - CRYPT      (created by htpasswd -d...) -- INSECURE
+    - SHA1       (created by htpasswd -s...) -- INSECURE
+
+When passlib (https://pypi.python.org/pypi/passlib) is importable, the
+following significantly more secure schemes are parsable by Radicale:
+
+    - MD5-APR1   (htpasswd -m...) -- htpasswd's default method
+    - BCRYPT     (htpasswd -B...) -- Requires htpasswd 2.4.x
 """
 
+
 import base64
 import hashlib
 import os
 
+
 from .. import config
 
 
@@ -39,28 +65,30 @@ ENCRYPTION = config.get("auth", "htpasswd_encryption")
 
 
 def _plain(hash_value, password):
-    """Check if ``hash_value`` and ``password`` match using plain method."""
+    """Check if ``hash_value`` and ``password`` match, using plain method."""
     return hash_value == password
 
 
 def _crypt(hash_value, password):
-    """Check if ``hash_value`` and ``password`` match using crypt method."""
-    # The ``crypt`` module is only present on Unix, import if needed
-    import crypt
+    """Check if ``hash_value`` and ``password`` match, using crypt method."""
     return crypt.crypt(password, hash_value) == hash_value
 
 
 def _sha1(hash_value, password):
-    """Check if ``hash_value`` and ``password`` match using sha1 method."""
+    """Check if ``hash_value`` and ``password`` match, using sha1 method."""
     hash_value = hash_value.replace("{SHA}", "").encode("ascii")
     password = password.encode(config.get("encoding", "stock"))
     sha1 = hashlib.sha1()  # pylint: disable=E1101
     sha1.update(password)
     return sha1.digest() == base64.b64decode(hash_value)
 
+
 def _ssha(hash_salt_value, password):
-    """Check if ``hash_salt_value`` and ``password`` match using salted sha1 method."""
-    hash_salt_value = hash_salt_value.replace("{SSHA}", "").encode("ascii").decode('base64')
+    """Check if ``hash_salt_value`` and ``password`` match, using salted sha1
+    method. This method is not directly supported by htpasswd, but it can be
+    written with e.g. openssl, and nginx can parse it."""
+    hash_salt_value = hash_salt_value.replace(
+        "{SSHA}", "").encode("ascii").decode('base64')
     password = password.encode(config.get("encoding", "stock"))
     hash_value = hash_salt_value[:20]
     salt_value = hash_salt_value[20:]
@@ -69,11 +97,69 @@ def _ssha(hash_salt_value, password):
     sha1.update(salt_value)
     return sha1.digest() == hash_value
 
+
+def _bcrypt(hash_value, password):
+    return _passlib_bcrypt.verify(password, hash_value)
+
+
+def _md5apr1(hash_value, password):
+    return _passlib_md5apr1.verify(password, hash_value)   
+
+
+# Prepare mapping between encryption names and verification functions.
+# Pre-fill with methods that do not have external dependencies.
+_verifuncs = {
+    "ssha": _ssha,
+    "sha1": _sha1,
+    "plain": _plain
+}
+
+
+# Conditionally attempt to import external dependencies.
+if ENCRYPTION == "md5":
+    try:
+        from passlib.hash import apr_md5_crypt as _passlib_md5apr1
+    except ImportError:
+        raise RuntimeError(("The htpasswd_encryption method 'md5' requires "
+            "availability of the passlib module."))
+    _verifuncs["md5"] = _md5apr1
+elif ENCRYPTION == "bcrypt":
+    try:
+        from passlib.hash import bcrypt as _passlib_bcrypt
+    except ImportError:
+        raise RuntimeError(("The htpasswd_encryption method 'bcrypt' requires "
+            "availability of the passlib module with bcrypt support."))
+    # A call to `encrypt` raises passlib.exc.MissingBackendError with a good
+    # error message if bcrypt backend is not available. Trigger this here.
+    _passlib_bcrypt.encrypt("test-bcrypt-backend")
+    _verifuncs["bcrypt"] = _bcrypt
+elif ENCRYPTION == "crypt":
+    try:
+        import crypt
+    except ImportError:
+        raise RuntimeError(("The htpasswd_encryption method 'crypt' requires "
+            "crypt() system support."))
+    _verifuncs["crypt"] = _crypt
+
+
+# Validate initial configuration.
+if ENCRYPTION not in _verifuncs:
+    raise RuntimeError(("The htpasswd encryption method '%s' is not "
+        "supported." % ENCRYPTION))
+ 
+
 def is_authenticated(user, password):
-    """Check if ``user``/``password`` couple is valid."""
-    for line in open(FILENAME).readlines():
-        if line.strip():
-            login, hash_value = line.strip().split(":")
+    """Validate credentials: iterate through htpasswd credential file until
+    user matches, extract hash (encrypted password) and check hash against
+    user-given password, using the method specified in the Radicale config.
+    """
+    with open(FILENAME) as f:
+        for line in f:
+            strippedline = line.strip()
+            if strippedline:
+                login, hash_value = strippedline.split(":")
                 if login == user:
-                return globals()["_%s" % ENCRYPTION](hash_value, password)
+                    # Allow encryption method to be overridden at runtime.
+                    return _verifuncs[ENCRYPTION](hash_value, password)
     return False
+