Add sha512 hmac check for file upload/download, and encrypt metadata

[braydonf]

For compatibility with decryption checks with libstorj:

Scheme for generating hmac:

The hmac is a hash of all shard hashes sequentially, as all that is necessary is all shard hashes, and to update the create bucket entry JSON body with the hmac.

{
  "hmac": {
    "type": "sha512",
    "value": "<hex-string-of-hmac-value>"
  }
}

• Use hmac to verify decryption was successful on download

References:

• https://github.com/Storj/libstorj-c/blob/master/src/uploader.c
• https://github.com/Storj/libstorj-c/blob/master/src/downloader.c

Scheme for encrypting metadata:

1. Generate the deterministic bucket key
2. Take the left half of a sha512 hmac with bucket key of a known constant
3. Take the left half of a sha512 hmac with bucket key of the bucket id and filename for the iv
4. Encrypt with AES-256-GCM
5. Digest (GCM_DIGEST_SIZE) is concatenated with iv (SHA512_DIGEST_SIZE/2) and ciphertext (variable length)

References:

• https://github.com/Storj/libstorj/blob/master/src/uploader.c#L1950-L1984
• https://github.com/Storj/libstorj/blob/master/src/storj.c#L31-L58

[braydonf]

As BridgeClient is deprecated, closing.