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Specify that the computation of checksums includes counts. #35

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19 changes: 14 additions & 5 deletions 01_data-model-and-serialized-rep.adoc
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Expand Up @@ -1559,11 +1559,6 @@ The checksum is made visible to the client by adding an attribute to
each top-level variable in the DMR. This attribute is named
"`_DAP4_Checksum_CRC32`".

In all cases, the checksum is computed over the serialized
representation of each top-level variable. The checksum is computed
before any chunking Section link:#_dap4_chunked_data_representation[[1.7]])
is applied.

If the request to the server is a dmr-only request, then the server will
compute the checksum for each variable mentioned in the DMR and will
insert the "`_DAP4_Checksum_CRC32`" attribute in the DMR. Note that this
Expand All @@ -1583,6 +1578,20 @@ integer will use the same endian representation as for the all other
data. Note that CRC32 is not a cryptographically strong checksum, so it
is not suitable for detecting man-in-the-middle attacks.

==== Computing Checksums ====

On the server side, the checksum is computed before any chunking
Section link:#_dap4_chunked_data_representation[[1.7]]) is
applied. On the client side, the checksum is computed after any
chunking is removed from the response.

The checksum is computed over the whole serialized
representation of each top-level variable. This means
specifically that the counts for variable-length variables
are included in the checksum. A variable is variable length
if it's type is String type or is Sequence type, or is a Structure type
that has variable-length fields.

_[#Historical_Note]#Historical Note#_

The encoding described in Section link:#_response_format[1.6.1] is similar
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