RIPEMD-160 50-bit collision

Answer

The deliverable is collision.json. It contains two distinct 8-byte inputs, encoded as hex. For both inputs, the first 50 most significant bits of the RIPEMD-160 digest are 0x0310d70365593.

Facts and evidence

• inputA is the byte string 00 00 00 00 04 52 50 2f; its full RIPEMD-160 digest is 0c435c0d9564cbf2e44100c312b2ce4afe803d9f.
• inputB is the byte string 00 00 00 00 04 54 c2 05; its full RIPEMD-160 digest is 0c435c0d9564c99246e3ab6f6645df758ad3c7a7.
• These full digests were independently reproduced with Python hashlib.new("ripemd160", bytes.fromhex(...)) and with OpenSSL 3.5.7 openssl dgst -ripemd160 reading the raw bytes from stdin. OpenSSL documents dgst as a command for computing message digests; its RIPEMD-160 API documentation specifies a 160-bit digest. (OpenSSL dgst documentation, OpenSSL RIPEMD-160 documentation)
• Taking the first 50 bits means reading the first 12 hex digits and the top two bits of the 13th hex digit. Both full digests begin 0c435c0d9564c; they therefore share at least those 50 bits. The full 160-bit digests differ.
• The search enumerated 8-byte big-endian encodings of consecutive integers beginning at zero and keyed an in-memory table by the first 50 digest bits. It found this pair at indices 72,503,343 and 72,663,557, after 72,663,558 inputs had been evaluated.

Inference

The result is a collision for the task's truncated hash: the two inputs are different, and their digest prefixes agree through bit 50. This is not a collision in the full 160-bit RIPEMD-160 output, as the digest values above show.

Uncertainty and limits

The report records a local recomputation with two installed implementations; it does not claim an independent third-party audit of the search program or of SIMD's verifier. The cited OpenSSL documentation supports digest behavior and the verification command; the measured collision evidence is the local digest output listed above.

Unanswered questions

None for constructing the requested file. SIMD's own recomputation remains outside this local check.

RIPEMD-160 was described by its authors in the original design paper, “RIPEMD-160: A Strengthened Version of RIPEMD”.