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+Converting werkzeug’s hashes to Passlib format
+##############################################
+
+:date: 2020-07-19T05:02Z
+:category: blog
+:tags: python,security
+:url: blog/converting-werkzeug-hashes-to-passlib/
+:save_as: blog/converting-werkzeug-hashes-to-passlib/index.html
+:status: published
+:author: Gergely Polonkai
+
+
+In one of my projects i use `Werkzeug <https://palletsprojects.com/p/werkzeug/>`_’s
+``generate_password_hash`` to, well, generate password hashes.  Werkzeug uses Python’s
+`hashlib <https://docs.python.org/3.8/library/hashlib.html>`_ module under the hood which,
+unfortunately to me, doesn’t support Argon2 (which i want to transition to for greater security).
+
+Enter `Passlib <https://passlib.readthedocs.io/>`_.
+
+Passlib conveniently supports both `PBKDF2
+<https://passlib.readthedocs.io/en/stable/lib/passlib.hash.pbkdf2_digest.html>`_ (what i currently
+use in the project, using SHA256 digests), and also supports `Argon2
+<https://passlib.readthedocs.io/en/stable/lib/passlib.hash.argon2.html>`_.  Here are the
+differences:
+
+.. table::
+   :widths: auto
+
+   ==================  ====================  ============================
+   Feature             Werkzeug              Passlib
+   ==================  ====================  ============================
+   Iteration count     150000, hardcoded     29000, changeable
+   Default method      PBKDF2                None, must be set explicitly
+   Default digest      SHA-256               None, must be set explicitly
+   Default salt size   8, changeable         16, changeable
+   Salt character set  ASCII only            Binary
+   Salt storage        Plain text (shorter)  Adapted Base64
+   Hash storage        Hex string            Adapted Base64 (shorter)
+   ==================  ====================  ============================
+
+But even if i force the same settings on Passlib, their format is different and, obviously,
+Passlib doesn’t understand Werkzeug’s format so i had to convert all my hashes to the new format.
+
+.. code-block:: python
+
+   >>> generate_password_hash('password', method='pbkdf2:sha256', salt_length=8)
+   'pbkdf2:sha256:150000$2dFFA24B$1602ed71733451acaf29abd26b1d1a78aced4442467f9efbab9b1fa21ae39d68'
+   >>> pbkdf2_sha256.using(rounds=150000, salt_size=8).hash('password')
+   '$pbkdf2-sha256$29000$tdZ6731vzTnn/H9vTSnl3A$maWqohBS0ghQEjIoJWnYC1zGF2T/qOwRmHzVzHt3NqU'
+
+First, let’s split the old hash by the ``$`` characters, and also split the first part by colons:
+
+.. code-block:: python
+
+   full_method, salt, hashed_value = old_hash.split('$')
+   method, digest, rounds = full_method.split(':')
+
+As it soon turned out, the two libraries even store the actual data in different formats.
+Werkzeug stores the salt in plain text (it’s always ASCII characters), and the resulting hash in a
+hex string.  Passlib, however, aims for greater security with a binary salt, so it’s Base64
+encoded.  This encoding is, however, a bit different from what you’d expect, as it’s “using
+shortened base64 format which omits padding & whitespace” and also “uses custom ``./`` altchars”.
+It even has its own `ab64_encode
+<https://passlib.readthedocs.io/en/stable/lib/passlib.utils.binary.html?highlight=ab64_encode#passlib.utils.binary.ab64_encode>`_
+and `ab64_decode
+<https://passlib.readthedocs.io/en/stable/lib/passlib.utils.binary.html?highlight=ab64_encode#passlib.utils.binary.ab64_decode>`_
+functions to handle this encoding.  So i first need to re-encode both the salt string and the hash
+value hex string, so i have raw bytes.
+
+.. code-block:: python
+
+   salt_bytes = salt.encode('ascii')
+   hash_bytes = bytes.fromhex(hash)
+
+Then encode them to the adapted Base64 format (i also convert it back to ``str`` for easier
+concatenation later):
+
+.. code-block:: python
+
+   passlib_salt = ab64_encode(salt_bytes).decode('ascii')
+   passlib_hash = ab64_encode(hash_bytes).decode('ascii')
+
+Now we just need to concatenate all the things with the right separators.
+
+.. code-block:: python
+
+   passlib_final_hash = f'${method}-{digest}${rounds}${passlib_salt}${passlib_hash}'
+
+Finally, let’s verify everything went right:
+
+.. code-block:: python
+
+   >>> pbkdf2_sha256.verify('password', passlib_final_hash)
+   True
+
+Here’s the whole series of command, converted to a Python function (with slightly altered variable
+names) for your copy-pasting convenience (plus, it’s not using f-strings, so you can use it with
+Python <3.6):
+
+.. code-block:: python
+
+   def werkzeug_to_passlib_hash(old_hash):
+       """Convert Werkzeug’s password hashes to Passlib format.
+
+       Copied from https://gergely.polonkai.eu/blog/converting-werkzeug-hashes-to-passlib/
+       """
+
+       from passlib.utils import ab64_encode
+
+       # Werkzeug hashes look like full_method$salt$hash.  We handle full_method later;  salt is
+       # an ASCII string; hashed value is the hex string representation of the hashed value
+       full_method, salt, hashed_value = old_hash.split('$')
+
+       # Werkzeug’s full_method is a colon delimited list of the method, digest, and rounds
+       method, digest, rounds = full_method.split(':')
+
+       new_parts = [
+           # Passlib expects the hashed value to starts with a $ sign (hence the empty string at the
+           # beginning of this list).
+           '',
+           # Passlib’s method and digest is concatenated together with a hyphen.
+           f'{method}-{digest}',
+           rounds,
+           # Passlib expects the salt and the actual hash to be in the adapted base64 encoding format.
+           ab64_encode(salt.encode('ascii')).decode('ascii'),
+           ab64_encode(bytes.fromhex(hashed_value)).decode('utf-8'),
+       ]
+
+       return '$'.join(new_parts)