[tahoe-dev] Choice of tree-hash

[CodesInChaos]

On Sun, Sep 23, 2012 at 9:11 PM, Tony Arcieri <tony.arcieri at gmail.com> wrote:
> Why not use a hash (tree) of the ciphertext for this purpose? I suppose
> encrypting the hash of the plaintext accomplishes the same thing...

The hash of the ciphertext depends on both the key and on how you
encrypt. So it can't answer the question if two files are the same
without downloading and rehashing at least one of them. Even when
using convergent encryption, Tahoe uses a different convergence secret
for each user, leading to different ciphertext hashes.

My objective is that when you know a plaintext hash, you can download
the file efficiently from whereever you have access to it, independent
from the details of the storage systems.