hippiepug¶

Sublinear-lookup blockchains and efficient key-value Merkle trees

This library provides implementations of two cryptographic data structures:

Blockchains with log(n) sublinear traversal, implemented as high-integrity deterministic skip-lists (skipchains). In this kind of blockchain verifying that block b extends block a does not require to download and process all blocks between a and b, but only a logarithmic amount of them.
Verifiable dictionary, implemented as a key-value Merkle tree that guarantees unique resolution. A proof of inclusion of a key-value pair in such a tree also proves that there does not exist another value for a given key somewhere else in the tree.

Both are meant to be used with a content-addressable storage. Each data structure supports logarithmic queries, and logarithmic proofs of inclusion:

	Retrievals per lookup	Inclusion proof size	Append
Skipchain	O(log(n))	O(log(n))	O(1)
Key-value Merkle tree	O(log(n))	O(log(n))	Immutable

with n being the size of the dictionary, or the number of blocks in the case of a chain.

The theoretical details are in the paper.

Getting started¶

You can install the library from PyPI:

pip install hippiepug

Then, the easiest way to run the tests is:

python setup.py test

Be sure to check out the usage guide.

Usage guide¶

Overview¶

Skipchain. hippiepug.chain implements a blockchain that only requires logarithmic-size proofs of inclusion. Each block of such blockchain has not one but many hash-pointers to previous blocks.

Key-value Merkle tree. hippiepug.tree implements a verifiable dictionary as a key-value Merkle tree that guarantees unique resolution. For each lookup key, one can produce a proof of inclusion of the key in the tree. Moreover, unique resolution ensures that a proof of inclusion also proves that no other value with the same lookup key exists somewhere else in the tree.

Object store. hippiepug.store implements a content-addressable key-value store in which keys are cryptographic hashes of values. Using such storage with hippiepug data structures is convenient, because a creator of a chain or a tree only needs to provide a querier with a hash of a chain head or a tree root. That is, there is no need to explicitly produce and transmit inclusion proofs. Queriers will be able to verify inclusion on the fly, provided the storage is available. See section “Producing and verifying proofs” for more.

Using object stores¶

hippiepug includes an instantation of an in-memory content-addressable storage that uses SHA256 for hashes: hippiepug.store.Sha256DictStore. By default, the hashes are truncated to 8 bytes.

from hippiepug.store import Sha256DictStore

store = Sha256DictStore()
obj = b'dummy'
obj_hash = store.hash_object(obj)  # 'b5a2c96250612366'

store.add(obj) == obj_hash  # True

The store verifies hashes internally on each lookup.

obj_hash in store  # True
store.get(obj_hash) == obj   # True

If you want to use external storage, you can provide a dict-like facade to it and pass as a backend parameter:

class CustomBackend(object):

    def get(self, k):
        return 'stub'

    def __setitem__(self, k, v):
        pass

store = Sha256DictStore(backend=CustomBackend())

To change the hash function, subclass hippiepug.store.BaseDictStore, and implement the hash_object method.

You can also define a completely different store by implementing abstract base hippiepug.store.BaseStore.

Building the data structures¶

Chain¶

To append a new block to a chain, first obtain an existing chain, or initialize a new empty hippiepug.chain.Chain object:

from hippiepug.chain import Chain

chain = Chain(store)
chain.head  # None

Then, add chain blocks ony by one.

from hippiepug.chain import BlockBuilder

block_builder = BlockBuilder(chain)
block_builder.payload = b'This is the first block!'
block_builder.commit()

chain.head  # '154bdee593d8c9b2'

You can continue adding blocks using the same builder instance.

block_builder.payload  # None
block_builder.payload = b'This is the second block!'
block_builder.commit()

chain.head  # '48e399de59796ab1'

The builder automatically fills all the skipchain special block attributes, like hashes of previous blocks.

Tree¶

Unlike chains, hippiepug trees can not be extended. To build a new tree, initialize the tree builder on a store, and set the key-value pairs to be committed.

from hippiepug.tree import TreeBuilder

tree_builder = TreeBuilder(store)
tree_builder['foo'] = b'bar'
tree_builder['baz'] = b'wow'

Once all key-value pairs are added, commit them to store and obtain a view of the committed tree:

tree = tree_builder.commit()
tree.root  # '150cc8da6d6cfa17'

Querying the data structures¶

Chain¶

To get a queryable view of a chain, you need to specify the storage where its blocks reside, and the head of the chain (hash of the latest block). You can then retrieve blocks by their indices, or iterate.

chain = Chain(store, head='48e399de59796ab1')
first_block = chain[0]
first_block.payload  # b'This is the first block!'

for block in chain:
    print(block.index)  # will print 1, and then 0

You can also get the latest view of a current chain while building a block in block_builder.chain.

Tree¶

Similarly, to get a view of a tree, you need to specify the storage, and the root of the tree (hash of the root node). You can then retrieve stored values by corresponding lookup keys.

from hippepug.tree import Tree

tree = Tree(store, root='150cc8da6d6cfa17')
tree['foo']  # b'bar'
'baz' in tree  # True

Producing and verifying proofs¶

When the creator of a data structure and the querier use the same storage (e.g., external database), no additional work regarding inclusion proofs needs to be done, since queries produce inclusion proofs on the fly. This scenario, however, is not always possible. In such case, hippiepug allows to produce and verify proofs explicitly.

Chain¶

You can get the proof of block inclusion from a chain view:

block, proof = chain.get_block_by_index(0, return_proof=True)

A proof is a subset of blocks between head block and the requested block.

To verify the proof, the querier needs to locally reproduce a store, populating it with the blocks in the proof, and then query the chain in the reproduced store normally. A convenience utility hippiepug.chain.verify_chain_inclusion_proof() does all of this internally, and only returns the verification result:

from hippiepug.chain import verify_chain_inclusion_proof

verification_store = Sha256DictStore()
verify_chain_inclusion_proof(verification_store,
                             chain.head, block, proof)  # True.

Tree¶

You can get the proof of value and lookup key inclusion from a tree view:

value, proof = tree.get_value_by_lookup_key('foo', return_proof=True)

For trees, a proof is the list of nodes on the path from root to the leaf containing the lookup key.

The mechanism of verifying an explicit proof is the same as with chains: locally reproduce a store populating it with all the nodes in the proof, and then query normally the tree in the reproduced store. Similarly, a utility hippiepug.tree.verify_tree_inclusion_proof() does this internally and returns the verification result:

from hippiepug.tree import verify_tree_inclusion_proof

verification_store = Sha256DictStore()
verify_tree_inclusion_proof(verification_store, tree.head,
                            lookup_key='foo', value=b'bar',
                            proof=proof)  # True.

Serialization¶

hippiepug includes default binary serialization using msgpack library.

from hippiepug.pack import decode, encode

block = chain[0]
decode(encode(block)) == block  # True

If you want to define custom serializers, be sure to check the documentation of hippiepug.pack. You need to be careful with custom encoders to not jeopardize security of the data structure.

Once you have defined a custom encoder and decoder, you can set them to global defaults like this:

from hippiepug.pack import EncodingParams

my_params = EncodingParams()
my_params.encoder = lambda obj: b'encoded!'
my_params.decoder = lambda encoded: b'decoded!'

EncodingParams.set_global_default(my_params)

Alternatively, you can also limit their usage to a specific context:

with my_params.as_default():
    encode(b'stub')  # b'encoded!'

API¶

Chain¶

Tools for building and interpreting skipchains.

class hippiepug.chain.BlockBuilder(chain)¶

Customizable builder of skipchain blocks.

You can override the pre-commit hook (BlockBuilder.pre_commit()) to modify the payload before the block is committed to a chain. This is needed, say, if you want to sign the payload before commiting.

Parameters:	chain – Chain to which the block should belong.

Set the payload before committing:

>>> from .store import Sha256DictStore
>>> store = Sha256DictStore()
>>> chain = Chain(store)
>>> builder = BlockBuilder(chain)
>>> builder.payload = b'Hello, world!'
>>> block = builder.commit()
>>> block == chain.head_block
True

chain¶: The associated chain.

commit()¶

Commit the block to the associated chain.

Returns:	The block that was committed.

fingers¶: Anticipated skip-list fingers (back-pointers to previous blocks).

index¶: Anticipated index of the block being built.

payload¶: Anticipated block payload.

pre_commit()¶

Pre-commit hook.

This can be overriden. For example, you can add a signature that includes index and fingers into your payload before the block is committed.

static skipchain_indices(index)¶

Finger indices for a given index.

Parameters:	index (int>=0) – Block index

class hippiepug.chain.Chain(object_store, head=None, cache=None)¶

Skipchain (hash chain with skip-list pointers).

To add a new block to a chain, use BlockBuilder.

Warning

All read accesses are cached. The cache is assumed to be trusted, so blocks retrieved from cache are not checked for integrity, unlike when they are retrieved from the object store.

Tree¶

Tools for building and interpreting key-value Merkle trees.

class hippiepug.tree.Tree(object_store, root, cache=None)¶

View of a Merkle tree.

Use TreeBuilder to build a Merkle tree first.

Parameters:	object_store – Object store root – The hash of the root node cache (dict) – Cache

Warning

All read accesses are cached. The cache is assumed to be trusted, so blocks retrieved from cache are not checked for integrity, unlike when they are retrieved from the object store.

Store¶

class hippiepug.store.BaseDictStore(backend=None)¶

Store with dict-like backend.

Parameters:	backend (dict-like) – Backend

__contains__(obj_hash)¶: Check if obj with a given hash is in the store.

add(serialized_obj)¶

Add an object to the store.

If an object with this hash already exists, silently does nothing.

get(obj_hash, check_integrity=True)¶

Get an object with a given hash from the store.

If the object does not exist, returns None.

Parameters:	obj_hash – ASCII hash of the object check_integrity – Whether to check the hash of the retrieved object against the given hash.

class hippiepug.store.BaseStore¶

Abstract base class for a content-addresable store.

__contains__(obj_hash)¶

Check whether the store contains an object with a give hash.

Parameters:	obj_hash – ASCII hash

add(serialized_obj)¶

Put the object in the store.

Parameters:	serialized_obj – Object, serialized to bytes
Returns:	Hash of the object.

get(obj_hash, check_integrity=True)¶

Return the object by its ASCII hash value.

Parameters:	obj_hash – ASCII hash check_integrity – Whether to check the hash upon retrieval

classmethod hash_object(serialized_obj)¶

Return the ASCII hash of the object.

Parameters:	obj – Object, serialized to bytes

exception hippiepug.store.IntegrityValidationError¶

class hippiepug.store.Sha256DictStore(backend=None)¶

Dict-based store using truncated SHA256 hex-encoded hashes.

>>> store = Sha256DictStore()
>>> obj = b'dummy'
>>> obj_hash = store.hash_object(obj)
>>> store.add(obj) == obj_hash
True
>>> obj_hash in store
True
>>> b'nonexistent' not in store
True
>>> store.get(obj_hash) == obj
True

hash_object(serialized_obj)¶: Return a SHA256 hex-encoded hash of a serialized object.

Basic containers¶

Basic building blocks.

class hippiepug.struct.ChainBlock(payload, index=0, fingers=NOTHING)¶

Skipchain block.

Parameters:	payload – Block payload index – Block index fingers – Back-pointers to previous blocks

class hippiepug.struct.TreeLeaf(lookup_key=None, payload_hash=None)¶

Merkle tree leaf.

Parameters:	lookup_key – Lookup key payload_hash – Hash of the payload

class hippiepug.struct.TreeNode(pivot_prefix, left_hash=None, right_hash=None)¶

Merkle tree intermediate node.

Parameters:	pivot_prefix – Pivot key for the subtree left_hash – Hash of the left child right_hash – Hash of the right child

Serialization¶

Serializers for chain blocks and tree nodes.

Warning

You need to take extra care when defining custom serializations. Be sure that your serialization includes all the fields in the original structure. E.g., for chain blocks:

self.index
self.fingers
Your payload

Unless this is done, the integrity of the data structures is screwed, since it’s the serialized versions of nodes and blocks that are hashed.

class hippiepug.pack.EncodingParams(encoder=NOTHING, decoder=NOTHING)¶

Thread-local container for default encoder and decoder funcs.

Parameters:	encoder – Default encoder decoder – Default decoder

This is how you can override the defaults using this class:

>>> my_params = EncodingParams()
>>> my_params.encoder = lambda obj: b'encoded!'
>>> my_params.decoder = lambda encoded: b'decoded!'
>>> EncodingParams.set_global_default(my_params)
>>> encode(b'dummy') == b'encoded!'
True
>>> decode(b'encoded!') == b'decoded!'
True
>>> EncodingParams.reset_defaults()

hippiepug.pack.decode(serialized, decoder=None)¶

Deserialize object.

Parameters:	serialized – Encoded structure encoder – Custom de-serializer

hippiepug.pack.encode(obj, encoder=None)¶

Serialize object.

Parameters:	obj – Chain block, tree node, or bytes encoder – Custom serializer

hippiepug.pack.msgpack_decoder(serialized_obj)¶

Deserialize structure from msgpack-encoded tuple.

Default decoder.

hippiepug.pack.msgpack_encoder(obj)¶

Represent structure as tuple and serialize using msgpack.

Default encoder.

Contributing¶

Dev setup¶

To install the development dependencies, clone the package from Github, and run within the folder:

pip install -e ".[dev]"

You can then run the tests from the root folder:

pytest

You can also run the tests against multiple pythons:

tox

Note that this invocation is expected to fail in the coverage upload stage (it needs access token to upload coverage report)

To build the documentation, run make html from the docs folder:

cd docs
make html

Then you can run a static HTML server from docs/build/html.

cd build/html
python -m http.server

License¶

The MIT License (MIT) Copyright (c) 2018 Bogdan Kulynych (EPFL SPRING Lab)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Notice¶

Some of the code was adapted from G.Danezis’s hippiehug library: https://github.com/gdanezis/rousseau-chain

The license of hippiehug is reproduced below:

Copyright (c) 2015, George Danezis for Escape Goat Productions All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Acknowledgements¶

The library is a reimplementation of G. Danezis’s hippiehug (hence the name).
This work is funded by the NEXTLEAP project within the European Union’s Horizon 2020 Framework Programme for Research and Innovation (H2020-ICT-2015, ICT-10-2015) under grant agreement 688722.
The hippie pug logo kindly donated by M. Naiem.

Parameters:	index (int>=0) – Block index return_proof (bool) – Whether to return inclusion proof
Returns:	Found block or None, or (block, proof) tuple if return_proof is True.
Raises:	If the index is out of bounds, raises `IndexError`.

Returns:	Corresponding value
Raises:	`KeyError` when the lookup key was not found.

hippiepug¶

Getting started¶

Usage guide¶

Overview¶

Using object stores¶

Building the data structures¶

Chain¶

Tree¶

Querying the data structures¶

Chain¶

Tree¶

Producing and verifying proofs¶

Chain¶

Tree¶

Serialization¶

API¶

Chain¶

Tree¶

Store¶

Basic containers¶

Serialization¶

Contributing¶

Dev setup¶

License¶

Notice¶

Acknowledgements¶

Indices and tables¶