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Easy secure protocol for passing encrypted structured data over unencrypted channels (such as URLs) while maintaining tamper-proof integrity.

Project description

Ax_Handoff: Easy secure protocol for passing encrypted structured data over unencrypted channels (such as URLs) while maintaining tamper-proof integrity.


About This Package

This package (axonchisel.handoff a.k.a. Ax_Handoff) provides a low level protocol and a high level wrapper encapsulating a number of complex features provided by other libraries and exposes a simple interface that allows a developer to exchange or “hand off” secure chunks of flexibly structured data (anything JSON-able) over untrusted communication channels between distributed components of a system.

The intricate details of thoroughly applied cryptography (AES encryption and SHA-family hashes) (more on crypto below) and compression (gzip) are hidden from the developer integrating with this code such that the requirement of handing off chunks of data in this manner becomes trivial and secure with little effort required and less than 5 lines of code (see code samples below!).

A primary motivation for this package is the case where a user is redirected from one web app to another on a different domain (i.e. where cookies cannot be shared) but important information must be transmitted with guaranteed integrity and total opaqueness, and other mechanisms (such as shared state or out-of-band data exchange) are not possible or not desired.


This library passes rigorous unit tests, is considered production-ready, and has been used in live production systems for over a year. As of this release, no security vulnerabilities or bugs have been identified.

Examples of Use

  • A user linking to a supplemental 3rd party support or download hosted web service site that offers content based on the user’s subscription level, location, and other metadata.

  • Exchanging session data (including logged in status) between two web apps managed by the same company but served by different platforms with no shared resources (often called “Single Sign On”).

  • Saving state in cookies that is not intended for users to see or modify in any way.

  • Embedding complex data in links included in e-mails.

How it Works

One or more distributed components maintain a shared secret pass phrase.

Ax_Handoff converts a (JSON-congruent) Python object to JSON, compresses it, encrypts it, signs it, and packs it up into a custom envelope resulting in a guaranteed URL-safe string suitable for passing as query params.

Upon unpacking, integrity is verified, and the original JSON restored as a Python object (or other object in non-Python environments).


The recommended way of installing this package is with “pip”:

$ pip install pycrypto
$ pip install Ax_Handoff

That’s it.

If you don’t have/want/like pip or that seems too easy for you, then you should first download and install PyCrypto from Then download this (Ax_Handoff) package source and either copy/symlink the axonchisel directory from this package into your Python path or run:

$ python install

Show me the code!

Encoding / decoding complex objects

This brief example shows how easy it is to encode and decode complex objects:

from axonchisel.handoff.object import Ax_Handoff

secret = "mY s3cR3t p@$s phr@s3! Unb-b-b-re@k@ble!!"
obj1 = {'foo': "Big Foo", 'bar': [10, 20.5, 30]}

# Encode:
enc = Ax_Handoff.encode(obj1, secret)
# enc = 'XHADPtqHlzJuuFBpFnTmBz8Uk3tYTczT1oChKQyho9flBqlRbSTSgXBybJ59CI1N4_wnGl3nsuMwJ7ItMxixm8H9bCIsjv5M00At1rElGvuuJ7u4v4WAHX'

# And decode back again:
obj2 = Ax_Handoff.decode(enc, secret)
# obj2 = {u'foo': u'Big Foo', u'bar': [10, 20.5, 30]}

Used with URLs for handing off between web apps

Use it in a URL:

url = "" + Ax_Handoff.encode(user_data, shared_secret)

And on the receiving end:

user_data = Ax_Handoff.decode(request.get('data'), shared_secret)

Details, Details


  • Python 2.6 is required.

  • PyCrypto (>=2.3) is required. To install PyCrypto, try one of:

Protocol Variants

Ax_Handoff currently (as of 1.1.2) supports two protocol variants. When using the recommended object level API, the variant may optionally be specified at encoding time, and is automatically detected when decoding.

  • Variant ‘A’ (default full):

    • Full original standard (default) Ax_Handoff protocol.

    • Includes encryption, compression, signing.

    • Compatible with previous versions of Ax_Handoff.

  • Variant ‘B’ (minimial):

    • Simplified concise version of Ax_Handoff protocol.

    • Includes compression and signing, but not encryption.

    • Faster to encode/decode due to lack of AES.

    • Shorter encoded strings (by ~20-40 chars) due to lack of AES iv + padding.

    • Easier integration with platforms without good AES support.


This open-source software is offered for free under standard MIT license as contained in the LICENSE.txt file and described here: See:


1.1.3 (2012-12-24)

  • Refactor to support multiple protocol variants.

  • Inclusion of new ‘B’ minimal (non-encrypting) variant.

  • Miscellaneous cleanup.

  • Status update to “production ready”.

1.0.1 (2011-06-11)

  • Fix over-aggressive type checking of encrypted text.

  • Fix README dates.

1.0.0 (2011-06-11)

  • Official v1 release.

0.9.4 (2011-06-10)

  • Support for unicode secret phrases.

  • Friendlier errors for invalid types.

  • Crypto doc clarifications following positive security review.

  • Major documentation update and formatting.

  • Code cleanup.

0.8.4 (2011-06-08)

  • First public preview release.

Bugs, Requests, Feedback, and Contributions

If you find any bugs or have feedback, please use our issue tracker:

You may also e-mail the author directly:

Dan Kamins <dos at axonchisel dot net>

While you’re free to fork this project, if you’d like to contribute, please send an e-mail first to one of the authors. If you have patches, let us know and we’ll roll them into the next release. Our source repository is at:

Lastly, if you use this code for something interesting, drop us a line too!

Additional Documentation

Extensive clear documentation, cryptographic analysis, protocol specification, module overview, and more are available in the docs directory of this distribution.

Cryptography Survey (or “Why should I trust this library?”)

How crypto is used

  • Data encryption uses AES-128 (CBC mode) with random initialization vector. AES-128 is chosen over AES-256 due to recently discovered attacks (Biryukov and Khovratovich, 2009), making AES-128 preferable for now (Schneier, 2009).

  • HMAC(SHA-1) is used for data integrity to sign the encrypted payload and prevent tampering, truncation, or errors in transit.

  • Because HMAC is verified prior to decryption, the known CBC attack “Padding Oracle” (Vaudenay, Eurocrypt 2002) is not applicable.

  • The AES initialization vector is random bytes (from os.urandom) which are then further hashed to avoid potential RNG pattern analysis attacks on potentially deficient random sources.

  • Keys for AES-128 and HMAC(SHA-1) are generated by extracting bits from the SHA-256 and SHA-512 hashes of the secret phrase, respectively.

Other notes

  • PBKDF2 is not used mainly to minimize external dependencies and keep code size and potential bugs down. Due to HMAC signature of the encrypted stream and sufficient entropy of arbitrary pass phrases, this is not considered to be a vulnerability.

  • As a further measure of precaution, clients are advised to avoid sharing details of possible decoding errors with end users who may in the future find ways of using this information for new attacks.

  • Details of the protocol specification sufficient to re-implement, interoperate with, or audit are provided in the docs/protocol.rst file.

Copyright (c) 2012 Dan Kamins,

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