553-EN, 5. Statistical Uniformity and Cryptographic Collision Resistance - The Hidden Decorrelation
The Emergence of a Decodable Imprint in SHA-256 - A Possible Message Left by Satoshi
Author: The Two Goddesses
5. Statistical Uniformity and Cryptographic Collision Resistance - The Hidden Decorrelation
For a cryptographic hash function to be considered secure, the avalanche effect is indispensable.
That is, the output must be completely unpredictable with respect to the input, exhibiting no semantic correspondence whatsoever.
Hence, the very observation that a readable imprint appears in SHA-256 represents a clear deviation from this fundamental principle.
However, the focus of this section lies in a deeper structural issue - an intrinsic and highly complex decorrelation between statistical uniformity and cryptographic collision resistance.
We investigate this decorrelation both theoretically and through direct empirical observation.
A critical misconception arises in the evaluation of avalanche properties.
Even if the bit-flip rate approaches the ideal value of 0.5, meaning that each bit flips with a probability of one-half,
this statistical optimum does not guarantee cryptographic collision resistance.
This is not merely a theoretical misunderstanding but a source of practical misuse in implementation.
The underlying cause lies in deterministic structure:
to preserve determinism, invisible correlations inevitably arise among bits.
As a result, although the distribution may appear statistically random and uniform, the bits are in fact not completely independent.
They are instead constrained by "invisible chains", interacting within a hidden dependency network that restricts their apparent randomness.
Consequently, there exists no direct correlation between statistical uniformity and cryptographic collision resistance.
This limitation is not unique to hash functions:
it also manifests in cryptographically secure pseudo-random number generators (CSPRNG).
True independence can only emerge when determinism is fully broken - for instance,
in quantum randomness, where probabilistic convergence occurs precisely at the moment of measurement.
Therefore, the most fundamental principle in designing and evaluating a cryptographic hash function is that bit-level independence must not be assumed.
True cryptographic behavior emerges only when all bits are treated as an interdependent whole - a single correlated structure rather than an aggregate of isolated components.
This implies a direct operational consequence:
partial bit-wise comparison is invalid for evaluating or verifying cryptographic safety.
A secure hash function preserves collision resistance through carefully balanced inter-bit correlations:
any partial evaluation inevitably destroys this balance.
The principle can be verified through concrete numerical experiments.
In our test, 24 bits were partially extracted from the SHA-256 output to observe collision behavior.
At this bit length, collisions can be induced easily-both theoretically and experimentally.
However, when the resulting collision data are integrated into the overall distribution,
their effects vanish as statistical noise, leaving no visible deformation in the distribution shape.
Thus, even when "good uniformity" is observed statistically,
it does not necessarily imply cryptographic collision resistance.
From these results, we infer that the security of a hash function depends critically on the entire bit-level structure of its output.
Partial evaluations risk overlooking essential collision behavior.
Accordingly, the analysis of collision resistance must encompass the complete representational space of all output bits.
Worse still, once such structural "gaps" are introduced, periodic patterns may begin to emerge.
Predictability, even in the slightest degree, is fatal to any cryptographic system.
Periodicity-even if weak-constitutes a critical warning signal,
and its appearance serves as empirical evidence of the fundamental decorrelation between avalanche effect and collision resistance.
