Sealing a file so nobody can argue you touched it

An argument about a digital file is almost never lost over what the file says. It is lost one question earlier: How do we know that is the file you received, and not the one you edited last night? If the answer is "trust me", you have already lost. However right you are on the substance. This problem is not exclusive to a courtroom. The auditor receiving a log dump has it. So does the team documenting an incident, or anyone keeping a copy of a contract signed over email. In every case the need is the same: being able to prove that a set of bytes has not changed since a given moment — and having that proved by someone who is not you . That is why I wrote Tunjo : a Rust tool that walks material read-only, computes its fingerprint, and signs a record anyone can verify. Why a tree and not a hash The obvious approach would be to concatenate everything and take one SHA-256. It works, and it is useless in practice. When someone disputes one file — a specific email out of four thousand — a single hash leaves you two options: hand over the complete set so it can be recomputed, or ask to be believed. The first exposes material that has no business being exposed; the second is not evidence. A Merkle tree solves exactly that. Each file is a leaf, each pair of nodes combines upward, and a root remains. To prove a leaf belongs to that root, you only need to show that leaf and the path of hashes to the top: a few kilobytes. The rest of the set is never touched. Two details of the tree that are not optional: // Domain separation: a leaf can never pass itself off as an internal node. h .update ([ 0x00 ]); // leaf h .update ([ 0x01 ]); // internal node // And the root binds the number of leaves. h .update ([ 0x02 ]); h .update ( n .to_be_bytes ()); Without the first, a leaf hash could be presented as if it were a node of the tree. Without the second you get the classic ambiguity of trees with an odd number of leaves: two different sets can produce the same root. It is an old, well-kn