Defined model

Two distinct inputs share one SHA-256 hash

Probability:
8.64 × 10⁻⁷⁶%
Odds:
1 to (1.16 × 10⁷⁷)
dBan
−770.6 dBan

Take two distinct inputs: in the ideal SHA-256 model, this particular pair sharing one digest would be an extraordinarily rare coincidence.

Calculation

One fixed pair: p = 1 / 2²⁵⁶ ≈ 1 / (1.158 × 10⁷⁷)

Assumptions

The two inputs are distinct, and their SHA-256 digests are modelled as independent uniform 256-bit values. This model covers one specified pair. A many-hash collision search counts n(n − 1)/2 pairs; its roughly 2¹²⁸-hash birthday scale describes the collection, while this specified pair remains at 2⁻²⁵⁶.

Sources

  1. NIST FIPS 180-4: Secure Hash Standard updated August 2015; checked 27 October 2026
  2. NIST SP 800-57 Part 1 Rev. 5: hash security strengths published May 2020; checked 27 October 2026