Share a secret securely implementing Shamir's secret share

# SecretShare

A simple library implementing Adi Shamir's "How to share a secret" algorithm. It is currently very limited since it uses integer arithmetic with primes, thus limiting the size of the shared secret to 8192 bits (1024 bytes). A future version might implement some form of unlimited stream share, but it is not its current goal, which is being used on cryptographic applications.

This library is part of the Dungeon Password Manager project.

## Installation

Clone the repo an run make package-install or, for development purposes, make devenvironment. Install dependencies with pipenv install or pip install -r requirements.txt.

## Usage

from secretshare import Secret, SecretShare, Share

def email(recipient, body):
"""Simulate sending an email."""
print(f'Recipient: {recipient} - Body: {body}')

# Generate a new random secret
secret = Secret()
# Or use an existing one
secret.value = 12345267890
# Or directly: secret = Secret(1234567890)
# If your secret is not an integer, you can use these methods
# from_bytes()
# from_base64()
# from_hex()

# Share the secret
share_count = 5  # How many pieces will the secret be split into?
threshold = 3    # How many pieces are required to recover the secret?
shamir = SecretShare(threshold, share_count, secret=secret)
shares = shamir.split()
print(shamir.shares)  # [Share(point=1, value=110014556089737955654312725615756332615), Share(point=2, value=270561733948920165984210681299343881767), Share(point=3, value=141359166656608167526319259631339703839), Share(point=4, value=62689221133740423744013068043512010338), Share(point=5, value=34551897380316934637292106535860801264)]
# shamir.shares are the same as shares, returned by the split() method

# Now deliver the shares to each recipient
recipients = ('r1@email.com', 'r2@email.com', 'r3@email.com', 'r4@email.com',
'r5@email.com')
for i, share in enumerate(shares):
email(recipients[i], str(share))  # Send in base64
# A Share can be converted to several convenient formats:
# bytes(share), share.to_bytes()
# str(share)
# share.to_hex(), hexlify(bytes(share))
# share.to_base64()
# Note: a Secret can be converted the very same way

# To recover the secret, get the share from each holder
# Each Share has a point and a value
s1 = Share(1, 335597737083070970356431407479895583486)
s2 = Share(2, 138424231339574140617068979728677909704)
s4 = Share()
# They are both encoded together for convenience
s4.from_base64('BQBGQMaGiRzWb0gO3gGEotVL')
# The share count and threshold information is NOT saved anywhere
# so the developer must save it somewhere as it is public
# information and there's no risk in storing it.
# As a matter of fact, those parameters are not really needed to recover a
# secret. If you provide less shares than the threshold then you will get a
# secret value that will be incorrect. There's no way to tell from the result
# whether the amount of shares are actually correct or not.
shamir = SecretShare(threshold, share_count, shares=[s1, s2, s4])
secret = shamir.combine()
print(secret)  # At/V1rI=
print(int(secret))  # 1234567890
# If the wrong number of shares is provided, an incorrect result
# is obtained. This is because this algorithm can't validate the result.
# The developer should verify the secret obtained by comparing into
# a hash, preferably using a secure KDF such as Argon2.
# Alternatively, it might be more convenient to verify each share
# against a hash: if the provided shares are valid the result
# will be correct.


## Developing

Install the development requirements, run tests with make test and lint with make lint. Check for tests coverage with make coverage (must be 100%).

SecretShare is made by HacKan under GNU GPL v3.0+. You are free to use, share, modify and share modifications under the terms of that license.

Copyright (C) 2018 HacKan (https://hackan.net)

This program is free software: you can redistribute it and/or modify
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.


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