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I found a real collision: the strings imd-12192837 and imd-19093962 are distinct, but their SHA-256 hashes start with the same 48 bits, c94563ba4883.

collision.json in the repository root holds:

{"algo":"sha256","lambda":24,"inputA":"imd-12192837","inputB":"imd-19093962"}

Check: I hashed both inputs with two separate tools, Node.js's built-in crypto module and the openssl command line. Both give:

• imd-12192837 → c94563ba4883 4c66186f…74d9
• imd-19093962 → c94563ba4883 0d17ce78…2211

The first 6 bytes match and the rest of each hash differs.

How it was found: tools/find_collision.js hashes imd-0, imd-1, … and stores the first 48 bits of each result until one repeats. The first repeat came after 19,093,963 hashes (about 2^24.2), in about 43 seconds. That is about what the birthday bound predicts (around 2^24.3). Python isn't installed here, so the search uses Node.js with no extra packages.

Assumptions: I'm relying on two readings of the task text that I couldn't check against the verifier itself:

• Inputs without a 0x prefix are read as UTF-8 text.
• "First 48 bits" means the first 6 bytes of the hash as it prints in hex.

Files:

• collision.json and tools/find_collision.js, both untracked. Nothing is committed.
• artifacts/report.md: the report, with facts, assumptions and open questions in separate sections.
• artifacts/README.md: the question and its limits.
• artifacts/collision.json: a copy of the answer.

The repository's own git settings already ignore artifacts/, so it stays out of git as the instructions for the uploaded outputs require. No scratch files were left behind.