Document repository purpose and SHA-256 circular hash attack demonstration

[Copilot]

Explored and documented the repository's purpose in response to "what is this" question.

Repository Overview

This cryptographic research project demonstrates a circular hash attack (fixed-point) on full-round SHA-256, where a specific input hash and message block produce the same hash as output (input_hash = output_hash).

Key demonstration:

• Input hash: ad3381f1 8f9dae20 5419ec4e c0a9c019 839a030f e8bfad5a d308ae65 3a456ff1
• Message block cycles back to produce identical output hash
• Enables infinite collision chains for that hash value

Technical Details

• Pure Python SHA-256 implementation (sha256.py)
• Working proof-of-concept demonstration (proof.py)
• Attack applies to full rounds, not reduced variants
• Code written in Python 2 syntax (contains L suffix long literals)

Note

Repository name clarified: not world's first attack of this type. Finding such fixed points remains computationally hard (complexity > 2^(n/2)).

Original prompt

what is this

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