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checksum.md

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Owncloud protocol extension: checksumming

Scope and purpose

Add checksum capability to verify end-to-end integrity of file uploads and downloads operations.

NOT in scope: using checksums as ETAG

Enabling transfer checksums in the client

As of version 1.7.2-cernbox and port to 1.8(.2) the type of the checksum is defined in the main config file as:

[General]
tranmissionChecksum=Adler32

Since client 2.2.0 checksumming is configured as server capability: owncloud/client#4638 (comment)
Client discovers from the server if server supports this capability.

Checksum functionality in the client is enabled by the response to capabilities call: https://github.com/cernbox/smashbox/blob/master/protocol/protocol.md#capabilities-call

Supported checksum types are defined here: https://github.com/owncloud/client/blob/d7bd1300a8397c2782e8d75cf7c595b1ada70d88/src/libsync/propagatorjobs.h#L24

For example this response will enable Adler32 checksum on file upload and download:

  {
      "ocs": {
          "data": {
              "capabilities": {   
                  "checksums" : {"supportedTypes" : ["Adler32"], "preferredUploadType":"Adler32"},
                  ...
                              }
   }}}

Assumptions on the checksum value

The value of Adler32 checksum MUST NOT be zero padded (2.2.4 client will complain).

Simple PUT (not-chunked)

Client computes the checksum and sends it in the request header OC-Checksum. The OC-Checksum is defined as: checkum_type:checksum_value

Examples:

PUT /file HTTP/1.1
OC-Checksum: Adler32:xxxxxxxxxxxxxxxxxx

PUT /file HTTP/1.1
OC-Checksum: MD5:xxxxxxxxxxxxxxxxxx

If the checksum does not match the content on the server then the server returns 412 (Precondition Failed).

Response: 412

BITS NOT YET IMPLEMENTED/UNDER DISCUSSION: [see comments in the source of this file]

GET

Server may provide the OC-Checksum response header with the GET request. If OC-Checksum is provides then client may use it to verify the checksum on the final destination.

In case of byte-range request the OC-Checksum response header is the checksum of the entire file (like for the GET of the entire file).

Chunked PUT

OC-Checksum of the entire file content is sent with the last chunk PUT request (and of course should not change during the upload).

Remarks

The checksumming feature is optional. Client may decide NOT to provide OC-Checksum header for PUT request and ignore OC-Checksum header in the GET reponse. If the type of the checksum is not understood or supported by the client or by the server then the checksum should be ignored.

Transfers should fail if the checksum type is understood and supported but the checksum value does not match.

BITS NOT YET IMPLEMENTED/UNDER DISCUSSION: [see comments in the source of this file]