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[Imported from Trac: page TaggedHash, version 1]

warner 2007-04-26 22:43:35 +00:00
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Each place a hash function is used within Tahoe, the data to be hashed is first prepended with a distinct "tag", one per purpose.
These tags are encoded as a [NetString](NetString), to insure that they cannot be confused with the data being hashed. For example, the [VerifierId](VerifierId) is computed as the SHA-256 hash of the following string:
```
"23:allmydata_verifierid_v1," + DATA
```
This technique is especially important to distinguish the leaf nodes from the internal nodes in a [MerkleHashTree](MerkleHashTree), because otherwise an attacker can trivially construct leaf data that results in the same root hash as a previously constructed tree.