88 lines
4 KiB
Text
88 lines
4 KiB
Text
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###########################################################################################################
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# Pcompress File Format. #
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###########################################################################################################
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Broadly a compressed file consists of a header followed by one or more metadata members which is turn is
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followed by one or more compressed chunk data members.
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Apart from a standard chunk header, each compressed chunk has an internal format with various metadata
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headers of the compression and deduplication algorithms used.
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===========================================
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File Header
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===========================================
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8 Bytes - Compression algorithm name
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2 Bytes - File format version
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2 Bytes - Flags
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* * * * * * * * * * * * * * * *
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15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
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| | | | | | `- Simple buffer-level Deduplication on/off
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'-------' | | | `----- Fixed Block Deduplication on/off
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| | | | Both bits set indicate Global Deduplication.
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| | | |
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| | | `--------- Solid archive. Entire file compressed in a
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| | | single buffer.
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| | |
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| | `----------------- AES Crypto
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| `--------------------- Salsa20 Crypto
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`------------------------------------- Indicate which data verification checksum
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was used.
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8 Bytes - Indicated per-thread buffer size
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4 Bytes - Compression level
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If Encryption Used
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-------------------------------------------
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4 Bytes - Salt Length
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X Bytes - Actual Salt bytes
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X Bytes - Nonce: 8 Bytes for AES and 24 Bytes for Salsa20
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4 Bytes - Key Length
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===========================================
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Header Checksum
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===========================================
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X Bytes - 4 Byte CRC32 without encryption
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Header HMAC if encryption enabled. Size of HMAC depends on selected data verification hash.
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===========================================
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Chunk Header
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Each chunk is a single compressed buffer
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===========================================
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8 Bytes - Compressed Length
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X Bytes - Chunk data verification hash (upto 64 bytes) of the original uncompressed and unencrypted
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data.
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X Bytes - Chunk Header CRC32 for normal compression
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Full chunk HMAC, including header, when encrypting. Computation is in this order:
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Compression -> Encryption -> HMAC.
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1 Byte - Chunk Flags
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* * * * * * * *
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7 6 5 4 3 2 1 0
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| | | | | |
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| '-----' | | `- 0 - Uncompressed
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| | | | 1 - Compressed
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| | | |
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| | | `---- 1 - Chunk was Deduped
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| | `------- 1 - Chunk was pre-compressed
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| |
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| | 1 - Bzip2 (Adaptive Mode)
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| `---------------- 2 - Lzma (Adaptive Mode)
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| 3 - PPMD (Adaptive Mode)
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`---------------------- 1 - Chunk size flag (if original chunk is of variable length)
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X Bytes - Compressed chunk data
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Original uncompressed chunk size can be less than indicated per-thread buffer size. In that
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case chunk size bit is set in the flags (as above) and size value is appended after the
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compressed chunk data.
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-------------------------------------------
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8 Bytes - Original uncompressed chunk size
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===========================================
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File Trailer
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===========================================
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8 Bytes - Zero bytes indicating zero compressed length
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and end of file.
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