konidev20 / pyshamir

Shamir key Split and Combine methods.

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pyshamir's Issues

Remove generic type hints to support version < Python-3.9

Hi there

combine and split method in pyshamir have generic type hints list[bytearray], I am running into an error when using this on Python3.8 interpreter:

def combine(parts: list[bytearray]) -> bytearray:
TypeError: 'type' object is not subscriptable

I think this is because the generic type hints were introduced in Python 3.9

I can make the source code change myself but dont want to deviate from upstream. Would you consider removing the type hints to support Python version < 3.9?

Let me know if you are fine with removing them and I can send out a PR for the same. Please let me know if there is a way to go around the issue without making a code change.

Split method generating duplicate shares sometimes

Description of the issue
I need to use this module for sharing a secret key. To ensure the good working of it, I created a test function that generate a random key, split it in shares (3 parts and 2 thresholds), and then combining it back.

For some reason, it appears that sometimes I got the error Duplicate sample when combining the keys. In fact, 2 of my keys are exactly the same.

After a bit of investigating, I found out that it is caused by x_coordinates when generating the shares. If a number is duplicated in the parts firsts values (where parts = 3 in my case), then the shares at index of those duplicates will be the same.

Expected behaviour
Shares are all different

Random bad behaviour
Some shares are exactly the same

To Reproduce
Steps to reproduce the behaviour:

1. Go to shamir.py line 77
2. Replace x_coordinates = [secrets.randbelow(255) for _ in range(1, 256)] with some random numbers, except that there's a duplicate in the 3 firsts one. For example (notice the two 1 at the beginning of the list) :

x_coordinates = [1,1,57,119,170,112,72,53,195,195,131,192,11,101,31,200,190,241,209,89,228,99,82,125,78,208,199,62,225,97,136,19,152,115,66,53,245,214,98,78,165,113,182,159,8,64,223,201,118,117,246,207,22,136,48,50,218,73,107,43,219,83,188,234,104,1,108,206,221,38,183,77,96,138,35,1,213,184,246,241,242,64,239,246,37,206,22,224,159,133,221,64,207,211,24,128,114,227,236,96,42,254,165,64,103,233,143,45,225,97,187,140,98,174,183,209,132,225,233,94,64,180,200,109,190,40,143,153,210,207,220,130,139,75,114,87,45,45,93,111,102,42,247,50,82,208,233,52,67,242,193,218,158,239,22,220,47,136,114,99,244,150,39,198,21,108,64,228,180,214,215,24,248,81,193,215,6,106,84,128,26,196,232,9,247,119,203,64,247,158,16,45,233,108,85,67,94,39,157,64,240,184,129,151,158,117,85,105,254,116,203,252,160,90,56,226,118,105,157,106,239,162,39,50,117,187,187,102,177,101,114,96,250,70,103,26,113,168,2,140,5,151,105,136,129,235,167,75,154,199,204,243,149,156,251]

3. Run this simple piece of code (you can find it here) :

from base64 import b64decode
import pyshamir

secret = b64decode("a+m4G0kEkKDQK4MFGz7L0vLz5oViQkDSLThiC4zDRZU=")
shares = pyshamir.split(secret, 3, 2)
for share in shares:
    print(share.hex())

4. As a result, shares 1 and 2 will be exactly the same (as a result of the duplicates 1 in the list), and share 3 will be fine

How to resolve
I think that to solve this problem, the solution is as simple as ensure that the parts first numbers of this x_coordinates are unique. But as I don't know the deep working of this, I don't wanted to make a pull request.

Desktop

• OS: Ubuntu 22.04
• Python: 3.10.6
• Pyshamir: 1.0.0

Duplicates in EXP_TABLE in _constants.py

I am looking at https://github.com/konidev20/pyshamir/blob/main/pyshamir/_constants.py#L39C7-L39C7 and found some duplicates. The value for EXP_TABLE[0x00] and EXP_TABLE[0xFF] both point to 0x01. Is this by design?

If you dont mind, can I get the reduction polynomial used for generating these tables.