# RIPEMD-160 collision at 48-bit truncation

## Finding

The distinct four-byte inputs `0x4b3fd300` and `0x942def00` have RIPEMD-160 digests with the same first 48 bits: `a753ebe0c45a`. The required machine-readable result is in `collision.json` at the repository root.

## Evidence

The following values were calculated locally from the literal bytes represented by each `0x`-prefixed input. The first six bytes of each 20-byte digest are the most significant 48 bits in the usual hexadecimal digest representation.

| Input | Full RIPEMD-160 digest | First 48 bits |
| --- | --- | --- |
| `0x4b3fd300` | `a753ebe0c45aae16f469e0777f027348ee2bd8d0` | `a753ebe0c45a` |
| `0x942def00` | `a753ebe0c45ac7e92464fbe55b9f6a6e260405f5` | `a753ebe0c45a` |

A deterministic birthday search hashed sequential four-byte little-endian counters with Node's `crypto.createHash('ripemd160')` and stopped at counter 15,674,772, after 15,674,773 evaluations. The earlier matching counter was 13,844,299. Python 3.12.3 `hashlib` and the OpenSSL 3.0.13 command-line tool each recomputed the two full digests shown in the table. Their matching first 12 hexadecimal characters establish the claimed truncated collision.

To reproduce the check with Python's `hashlib`:

```python
import hashlib

a = bytes.fromhex('4b3fd300')
b = bytes.fromhex('942def00')
assert a != b
da = hashlib.new('ripemd160', a).digest()
db = hashlib.new('ripemd160', b).digest()
assert da[:6] == db[:6]
print(da.hex())
print(db.hex())
```

## Scope and limits

**Fact:** The listed inputs differ, and local RIPEMD-160 calculations return the full digests shown above. **Inference:** Since the first six digest bytes agree, these inputs satisfy the requested 48-bit truncation collision. **Uncertainty:** This is a local calculation, not an independently certified result; the included reproduction check allows the values to be recalculated. The checks may share an OpenSSL implementation. **Unanswered questions:** None affect the stated collision property. This does not claim a collision in all 160 bits.