helper: client should verify ciphertext hashes and UEB

[warner]

Prompted by a question from David-Sarah Hopwood, I spent some time today reviewing the helper code, and realized that the client should be doing more verification of the data that the helper returns to it. Specifically, the client should:

• locally compute the ciphertext hashes (flat and Merkle tree), in upload.EncryptAnUploadable
• compare these against the versions returned by the helper, in AssistedUploader._build_verifycap upload_results.uri_extension_data
• compute and compare the resulting UEB hash

This would prevent the helper from causing an integrity violation. With the present behavior, the helper can flip bits in the ciphertext (or upload a completely unrelated ciphertext of the same length) and return the resulting ciphertext hash to the trusting client. Because the client doesn't perform any validation of the response, it will simply build and return the resulting filecap. Later, when someone attempts a download with this filecap, they will receive the altered ciphertext (but it will match the hash provided by the helper) and try to decrypt it. Since we removed the plaintext hashes in 7996131a0aa0b55c,7b21054c33d4651d,1e097766c9b4c873,db566db31a66e076, the downloader hash no way to check the plaintext either, and will return corrupted plaintext to the end user.

With the current codebase, this won't be too much work. But when ​pycryptopp#18 (allow random-access AES-CTR encryption) is fixed, I'd like to improve the assisted-uploader code to be more efficient in the resumed-upload case (by not encrypting-then-discarding all the previously-uploaded data), at which point this locally-generate-hashes fix would become more difficult. Or rather, I might have to forego the resumed-upload improvement to retain the don't-rely-on-helper-for-integrity property that this ticket would provide.