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[WIP] [RFC] add cryptographic hash to seekable format #2737

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[WIP] [RFC] add cryptographic hash to seekable format #2737

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30 changes: 20 additions & 10 deletions contrib/seekable_format/zstd_seekable_compression_format.md
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Expand Up @@ -14,7 +14,7 @@ are clearly marked.
Distribution of this document is unlimited.

### Version
0.1.0 (11/04/17)
0.2.0 (31/07/21)

## Introduction
This document defines a format for compressed data to be stored so that subranges of the data can be efficiently decompressed without requiring the entire document to be decompressed.
Expand Down Expand Up @@ -78,26 +78,31 @@ A bitfield describing the format of the seek table.

| Bit number | Field name |
| ---------- | ---------- |
| 7 | `Checksum_Flag` |
| 6-2 | `Reserved_Bits` |
| 7 | `XXH64_Checksum_Flag` |
| 6 | `SHA512-256_Checksum_Flag`|
| 5-2 | `Reserved_Bits` |
| 1-0 | `Unused_Bits` |

While only `Checksum_Flag` currently exists, there are 7 other bits in this field that can be used for future changes to the format,
While only `Checksum_Flag` currently exists, there are 6 other bits in this field that can be used for future changes to the format,
for example the addition of inline dictionaries.

__`Checksum_Flag`__
__`XXH64_Checksum_Flag`__

If the checksum flag is set, each of the seek table entries contains a 4 byte checksum of the uncompressed data contained in its frame.

__`SHA512-256_Checksum_Flag`__

If the checksum flag is set, each of the seek table entries contains a 32 byte SHA-512/256 checksum of the uncompressed data contained in its frame.

`Reserved_Bits` are not currently used but may be used in the future for breaking changes, so a compliant decoder should ensure they are set to 0. `Unused_Bits` may be used in the future for non-breaking changes, so a compliant decoder should not interpret these bits.

#### __`Seek_Table_Entries`__

`Seek_Table_Entries` consists of `Number_Of_Frames` (one for each frame in the data, not including the seek table frame) entries of the following form, in sequence:

|`Compressed_Size`|`Decompressed_Size`|`[Checksum]`|
|-----------------|-------------------|------------|
| 4 bytes | 4 bytes | 4 bytes |
|`Compressed_Size`|`Decompressed_Size`|`[XXH64_Checksum]`|`[SHA512-256_Checksum]`|
|-----------------|-------------------|------------------|-----------------------|
| 4 bytes | 4 bytes | 4 bytes | 32 bytes |

__`Compressed_Size`__

Expand All @@ -108,9 +113,14 @@ __`Decompressed_Size`__

The size of the decompressed data contained in the frame. For skippable or otherwise empty frames, this value is 0.

__`Checksum`__
__`XXH64_Checksum`__

Only present if `XXH64_Checksum_Flag` is set in the `Seek_Table_Descriptor`. Value : the least significant 32 bits of the XXH64 digest of the uncompressed data, stored in little-endian format.

__`SHA512-256_Checksum`__

Only present if `Checksum_Flag` is set in the `Seek_Table_Descriptor`. Value : the least significant 32 bits of the XXH64 digest of the uncompressed data, stored in little-endian format.
Only present if `SHA512-256_Checksum_Flag` is set in the `Seek_Table_Descriptor`. Value : the 256 bits of the SHA-512/256 digest of the uncompressed data, stored in little-endian format.

## Version Changes
- 0.1.0: initial version
- 0.2.0: add cryptographic content hash