The browser app must read and write the same encrypted files as the
desktop Pithos. web/lib/crypt.js reimplements core/crypt.js on Web
Crypto (AES-SIV and CMAC built from AES-CBC with a zero IV and AES-CTR,
since Web Crypto has neither), and web/lib/sigv4.js the SigV4 signing.
Cross-tests: the RFC 5297 vector, identical drive keys and names, bodies
encrypted on one side and opened on the other (empty, chunk-edge and
multi-chunk files, ranges), tamper and wrong-path rejection, and equal
SigV4 headers and presigned URLs.
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