SHA-256 collision for a 48-bit prefix

The two distinct eight-byte inputs in collision.json have identical first 48 bits of their SHA-256 digests. Here 0x denotes hexadecimal byte encoding; it is not part of the hashed message.

Item	Input bytes (hex)	Full SHA-256 digest
inputA	000000000017211c	4e84dca19fa699674e41dc0ff0f92046577a96fd3bbc22ae6a036986dc268301
inputB	00000000014060cc	4e84dca19fa67ad46fd0f73243c0081834a4767ce3fc91b0b4dade47ab574d2e

The common six-byte (48-bit MSB) prefix is 4e84dca19fa6. The requested parameter is lambda = 24. The full digests differ.

Method and attributable evidence

The local search program implements SHA-256 for eight-byte big-endian counters, enumerates counters starting at zero, and stores digest prefixes in a hash table. It reported a collision after 20,996,301 evaluations. No external data or dependencies were downloaded.

The search was compiled with gcc -O3 -Wall -Wextra -Werror and exited successfully. Its candidate was separately recomputed with Python 3.12.3 hashlib.sha256 using the delivered verification script:

inputA SHA-256: 4e84dca19fa699674e41dc0ff0f92046577a96fd3bbc22ae6a036986dc268301
inputB SHA-256: 4e84dca19fa67ad46fd0f73243c0081834a4767ce3fc91b0b4dade47ab574d2e
Shared first 48 bits: 4e84dca19fa6
PASS: distinct inputs with equal 48-bit MSB prefixes

OpenSSL 3.0.13 openssl dgst -sha256, with each decoded eight-byte input passed directly on standard input, also returned exactly the full digests above. These are local observed results, attributable to the included inputs and code. Python and the OpenSSL command may share a cryptographic backend; they are separate recomputations from the custom search implementation, not two guaranteed independent cryptographic implementations.

Conclusion and limits

The byte inequality and matching first six digest bytes establish the requested truncated collision under the specified hexadecimal decoding. This is not a full SHA-256 collision. No SIMD verifier or external reviewer was available or run; its eventual acceptance is unobserved. These local checks provide reproducible evidence, not independent certification. No unresolved uncertainty remains in the locally checked prefix equality.