RIPEMD-160 first-48-bit collision

The requested collision was found. The exact deliverable is collision.json, with algo equal to ripemd160 and lambda equal to 24. Inputs below are hexadecimal encodings of eight raw bytes.

Value	Input A	Input B
Input	0xd5a30b0100000000	0xf30dac0100000000
Full RIPEMD-160 digest	701296ae0b3717c357ba2d5340e1ef85b9ef25a0	701296ae0b377c964cd6fc566b79717d1236c391
First 48 bits (six leading bytes)	701296ae0b37	701296ae0b37

Method and attributable local evidence

A local C search used OpenSSL 3.0.13's RIPEMD160 function to hash 33,554,432 inputs: integers 0 through 33,554,431 encoded as unsigned eight-byte little-endian values. Sorting by the first six digest bytes found a match at integers 17,540,053 and 28,052,979. No dependencies were installed.

The delivered verification script, run with python3 artifacts/verify.py, checked the JSON fields, algorithm and lambda, decoded both inputs, asserted distinctness, recomputed their full digests using Python hashlib, and asserted equality of the first six bytes. It exited successfully with:

inputA bytes: d5a30b0100000000
inputB bytes: f30dac0100000000
RIPEMD160(A): 701296ae0b3717c357ba2d5340e1ef85b9ef25a0
RIPEMD160(B): 701296ae0b377c964cd6fc566b79717d1236c391
PASS: distinct inputs; shared first 48 bits: 701296ae0b37

A separate local recomputation called libgcrypt 1.10.3's gcry_md_hash_buffer through Python ctypes, selecting RIPEMD160 using gcry_md_map_name. Both full digests matched the values above byte for byte. This provides a second library implementation check; hashlib and the search both use OpenSSL and are not independent implementations of one another.

Findings and limits

Observed facts: the decoded inputs differ, their computed six-byte prefixes are equal, and their full digests differ. These observations establish the requested truncated collision under the tested implementations. This is not a full RIPEMD-160 collision.

The evidence is local execution, not an external citation or independent reviewer's certification. The assignment's SIMD verifier was not available or run locally; its eventual acceptance remains unobserved. No statistical claim or inference is needed to establish this particular collision.