HashLog

Principles

• Records are immutable
• Records are ordered
• It must be possible to check hash(Xi) for any i
• hash(Xi) can be sent to anyone as a proof
• hash(Xi) == hash(X, hash(Xi-1))
• You do not need X to perform the check

Installation

$ pip install hlog

Usage

Getting started

# create a chain
c = Chain()

# send some messages
c.append(message="Alice gives 10.36 euros to Bob", amount=10.36, currency="EUR")
c.append(message="Bob gives 2 dollars to Alice", amount=2, currency="USD")

# you can loop through the chain records
used_currencies = set(r.fields["currency"] for r in c.records)

# call verify() to ensure records have not been modified
c.verify()

Raising an exception during validation

c.verify(raise_on_error=True)

Verify will raise a ModifiedRecordException. It has 3 interesting attributes:

• index: the index in the chain where validation starts to fail
• message: a user-friendly message to indicate the error
• record: the Record object itself

Dumping

c = Chain()
c.append(message="hello")
c.append(message="wonderful")
c.append(message="world")

rows = c.dump()

rows is a tuple of bytes, each item being one record of the chain.

Restoring from a dump

If you want to reconstruct your chain based on a file or database records, you can use Chain.from_dump()

c = Chain()
c.append(message="hello")
c.append(message="wonderful")
c.append(message="world")

d = c.dump()

c2 = Chain.from_dump(d)

c2.verify()