The .xz File Format =================== Version 1.0.4 (2009-08-27) 0. Preface 0.1. Notices and Acknowledgements 0.2. Getting the Latest Version 0.3. Version History 1. Conventions 1.1. Byte and Its Representation 1.2. Multibyte Integers 2. Overall Structure of .xz File 2.1. Stream 2.1.1. Stream Header 2.1.1.1. Header Magic Bytes 2.1.1.2. Stream Flags 2.1.1.3. CRC32 2.1.2. Stream Footer 2.1.2.1. CRC32 2.1.2.2. Backward Size 2.1.2.3. Stream Flags 2.1.2.4. Footer Magic Bytes 2.2. Stream Padding 3. Block 3.1. Block Header 3.1.1. Block Header Size 3.1.2. Block Flags 3.1.3. Compressed Size 3.1.4. Uncompressed Size 3.1.5. List of Filter Flags 3.1.6. Header Padding 3.1.7. CRC32 3.2. Compressed Data 3.3. Block Padding 3.4. Check 4. Index 4.1. Index Indicator 4.2. Number of Records 4.3. List of Records 4.3.1. Unpadded Size 4.3.2. Uncompressed Size 4.4. Index Padding 4.5. CRC32 5. Filter Chains 5.1. Alignment 5.2. Security 5.3. Filters 5.3.1. LZMA2 5.3.2. Branch/Call/Jump Filters for Executables 5.3.3. Delta 5.3.3.1. Format of the Encoded Output 5.4. Custom Filter IDs 5.4.1. Reserved Custom Filter ID Ranges 6. Cyclic Redundancy Checks 7. References 0. Preface This document describes the .xz file format (filename suffix ".xz", MIME type "application/x-xz"). It is intended that this this format replace the old .lzma format used by LZMA SDK and LZMA Utils. 0.1. Notices and Acknowledgements This file format was designed by Lasse Collin <lasse.collin@tukaani.org> and Igor Pavlov. Special thanks for helping with this document goes to Ville Koskinen. Thanks for helping with this document goes to Mark Adler, H. Peter Anvin, Mikko Pouru, and Lars Wirzenius. This document has been put into the public domain. 0.2. Getting the Latest Version The latest official version of this document can be downloaded from <http://tukaani.org/xz/xz-file-format.txt>. Specific versions of this document have a filename xz-file-format-X.Y.Z.txt where X.Y.Z is the version number. For example, the version 1.0.0 of this document is available at <http://tukaani.org/xz/xz-file-format-1.0.0.txt>. 0.3. Version History Version Date Description 1.0.4 2009-08-27 Language improvements in Sections 1.2, 2.1.1.2, 3.1.1, 3.1.2, and 5.3.1 1.0.3 2009-06-05 Spelling fixes in Sections 5.1 and 5.4 1.0.2 2009-06-04 Typo fixes in Sections 4 and 5.3.1 1.0.1 2009-06-01 Typo fix in Section 0.3 and minor clarifications to Sections 2, 2.2, 3.3, 4.4, and 5.3.2 1.0.0 2009-01-14 The first official version 1. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC-2119]. Indicating a warning means displaying a message, returning appropriate exit status, or doing something else to let the user know that something worth warning occurred. The operation SHOULD still finish if a warning is indicated. Indicating an error means displaying a message, returning appropriate exit status, or doing something else to let the user know that something prevented successfully finishing the operation. The operation MUST be aborted once an error has been indicated. 1.1. Byte and Its Representation In this document, byte is always 8 bits. A "null byte" has all bits unset. That is, the value of a null byte is 0x00. To represent byte blocks, this document uses notation that is similar to the notation used in [RFC-1952]: +-------+ | Foo | One byte. +-------+ +---+---+ | Foo | Two bytes; that is, some of the vertical bars +---+---+ can be missing. +=======+ | Foo | Zero or more bytes. +=======+ In this document, a boxed byte or a byte sequence declared using this notation is called "a field". The example field above would be called "the Foo field" or plain "Foo". If there are many fields, they may be split to multiple lines. This is indicated with an arrow ("--->"): +=====+ | Foo | +=====+ +=====+ ---> | Bar | +=====+ The above is equivalent to this: +=====+=====+ | Foo | Bar | +=====+=====+ 1.2. Multibyte Integers Multibyte integers of static length, such as CRC values, are stored in little endian byte order (least significant byte first). When smaller values are more likely than bigger values (for example file sizes), multibyte integers are encoded in a variable-length representation: - Numbers in the range [0, 127] are copied as is, and take one byte of space. - Bigger numbers will occupy two or more bytes. All but the last byte of the multibyte representation have the highest (eighth) bit set. For now, the value of the variable-length integers is limited to 63 bits, which limits the encoded size of the integer to nine bytes. These limits may be increased in the future if needed. The following C code illustrates encoding and decoding of variable-length integers. The functions return the number of bytes occupied by the integer (1-9), or zero on error. #include <stddef.h> #include <inttypes.h> size_t encode(uint8_t buf[static 9], uint64_t num) { if (num > UINT64_MAX / 2) return 0; size_t i = 0; while (num >= 0x80) { buf[i++] = (uint8_t)(num) | 0x80; num >>= 7; } buf[i++] = (uint8_t)(num); return i; } size_t decode(const uint8_t buf[], size_t size_max, uint64_t *num) { if (size_max == 0) return 0; if (size_max > 9) size_max = 9; *num = buf[0] & 0x7F; size_t i = 0; while (buf[i++] & 0x80) { if (i >= size_max || buf[i] == 0x00) return 0; *num |= (uint64_t)(buf[i] & 0x7F) << (i * 7); } return i; } 2. Overall Structure of .xz File A standalone .xz files consist of one or more Streams which may have Stream Padding between or after them: +========+================+========+================+ | Stream | Stream Padding | Stream | Stream Padding | ... +========+================+========+================+ The sizes of Stream and Stream Padding are always multiples of four bytes, thus the size of every valid .xz file MUST be a multiple of four bytes. While a typical file contains only one Stream and no Stream Padding, a decoder handling standalone .xz files SHOULD support files that have more than one Stream or Stream Padding. In contrast to standalone .xz files, when the .xz file format is used as an internal part of some other file format or communication protocol, it usually is expected that the decoder stops after the first Stream, and doesn't look for Stream Padding or possibly other Streams. 2.1. Stream +-+-+-+-+-+-+-+-+-+-+-+-+=======+=======+ +=======+ | Stream Header | Block | Block | ... | Block | +-+-+-+-+-+-+-+-+-+-+-+-+=======+=======+ +=======+ +=======+-+-+-+-+-+-+-+-+-+-+-+-+ ---> | Index | Stream Footer | +=======+-+-+-+-+-+-+-+-+-+-+-+-+ All the above fields have a size that is a multiple of four. If Stream is used as an internal part of another file format, it is RECOMMENDED to make the Stream start at an offset that is a multiple of four bytes. Stream Header, Index, and Stream Footer are always present in a Stream. The maximum size of the Index field is 16 GiB (2^34). There are zero or more Blocks. The maximum number of Blocks is limited only by the maximum size of the Index field. Total size of a Stream MUST be less than 8 EiB (2^63 bytes). The same limit applies to the total amount of uncompressed data stored in a Stream. If an implementation supports handling .xz files with multiple concatenated Streams, it MAY apply the above limits to the file as a whole instead of limiting per Stream basis. 2.1.1. Stream Header +---+---+---+---+---+---+-------+------+--+--+--+--+ | Header Magic Bytes | Stream Flags | CRC32 | +---+---+---+---+---+---+-------+------+--+--+--+--+ 2.1.1.1. Header Magic Bytes The first six (6) bytes of the Stream are so called Header Magic Bytes. They can be used to identify the file type. Using a C array and ASCII: const uint8_t HEADER_MAGIC[6] = { 0xFD, '7', 'z', 'X', 'Z', 0x00 }; In plain hexadecimal: FD 37 7A 58 5A 00 Notes: - The first byte (0xFD) was chosen so that the files cannot be erroneously detected as being in .lzma format, in which the first byte is in the range [0x00, 0xE0]. - The sixth byte (0x00) was chosen to prevent applications from misdetecting the file as a text file. If the Header Magic Bytes don't match, the decoder MUST indicate an error. 2.1.1.2. Stream Flags The first byte of Stream Flags is always a null byte. In the future, this byte may be used to indicate a new Stream version or other Stream properties. The second byte of Stream Flags is a bit field: Bit(s) Mask Description 0-3 0x0F Type of Check (see Section 3.4): ID Size Check name 0x00 0 bytes None 0x01 4 bytes CRC32 0x02 4 bytes (Reserved) 0x03 4 bytes (Reserved) 0x04 8 bytes CRC64 0x05 8 bytes (Reserved) 0x06 8 bytes (Reserved) 0x07 16 bytes (Reserved) 0x08 16 bytes (Reserved) 0x09 16 bytes (Reserved) 0x0A 32 bytes SHA-256 0x0B 32 bytes (Reserved) 0x0C 32 bytes (Reserved) 0x0D 64 bytes (Reserved) 0x0E 64 bytes (Reserved) 0x0F 64 bytes (Reserved) 4-7 0xF0 Reserved for future use; MUST be zero for now. Implementations SHOULD support at least the Check IDs 0x00 (None) and 0x01 (CRC32). Supporting other Check IDs is OPTIONAL. If an unsupported Check is used, the decoder SHOULD indicate a warning or error. If any reserved bit is set, the decoder MUST indicate an error. It is possible that there is a new field present which the decoder is not aware of, and can thus parse the Stream Header incorrectly. 2.1.1.3. CRC32 The CRC32 is calculated from the Stream Flags field. It is stored as an unsigned 32-bit little endian integer. If the calculated value does not match the stored one, the decoder MUST indicate an error. The idea is that Stream Flags would always be two bytes, even if new features are needed. This way old decoders will be able to verify the CRC32 calculated from Stream Flags, and thus distinguish between corrupt files (CRC32 doesn't match) and files that the decoder doesn't support (CRC32 matches but Stream Flags has reserved bits set). 2.1.2. Stream Footer +-+-+-+-+---+---+---+---+-------+------+----------+---------+ | CRC32 | Backward Size | Stream Flags | Footer Magic Bytes | +-+-+-+-+---+---+---+---+-------+------+----------+---------+ 2.1.2.1. CRC32 The CRC32 is calculated from the Backward Size and Stream Flags fields. It is stored as an unsigned 32-bit little endian integer. If the calculated value does not match the stored one, the decoder MUST indicate an error. The reason to have the CRC32 field before the Backward Size and Stream Flags fields is to keep the four-byte fields aligned to a multiple of four bytes. 2.1.2.2. Backward Size Backward Size is stored as a 32-bit little endian integer, which indicates the size of the Index field as multiple of four bytes, minimum value being four bytes: real_backward_size = (stored_backward_size + 1) * 4; If the stored value does not match the real size of the Index field, the decoder MUST indicate an error. Using a fixed-size integer to store Backward Size makes it slightly simpler to parse the Stream Footer when the application needs to parse the Stream backwards. 2.1.2.3. Stream Flags This is a copy of the Stream Flags field from the Stream Header. The information stored to Stream Flags is needed when parsing the Stream backwards. The decoder MUST compare the Stream Flags fields in both Stream Header and Stream Footer, and indicate an error if they are not identical. 2.1.2.4. Footer Magic Bytes As the last step of the decoding process, the decoder MUST verify the existence of Footer Magic Bytes. If they don't match, an error MUST be indicated. Using a C array and ASCII: const uint8_t FOOTER_MAGIC[2] = { 'Y', 'Z' }; In hexadecimal: 59 5A The primary reason to have Footer Magic Bytes is to make it easier to detect incomplete files quickly, without uncompressing. If the file does not end with Footer Magic Bytes (excluding Stream Padding described in Section 2.2), it cannot be undamaged, unless someone has intentionally appended garbage after the end of the Stream. 2.2. Stream Padding Only the decoders that support decoding of concatenated Streams MUST support Stream Padding. Stream Padding MUST contain only null bytes. To preserve the four-byte alignment of consecutive Streams, the size of Stream Padding MUST be a multiple of four bytes. Empty Stream Padding is allowed. If these requirements are not met, the decoder MUST indicate an error. Note that non-empty Stream Padding is allowed at the end of the file; there doesn't need to be a new Stream after non-empty Stream Padding. This can be convenient in certain situations [GNU-tar]. The possibility of Stream Padding MUST be taken into account when designing an application that parses Streams backwards, and the application supports concatenated Streams. 3. Block +==============+=================+===============+=======+ | Block Header | Compressed Data | Block Padding | Check | +==============+=================+===============+=======+ 3.1. Block Header +-------------------+-------------+=================+ | Block Header Size | Block Flags | Compressed Size | +-------------------+-------------+=================+ +===================+======================+ ---> | Uncompressed Size | List of Filter Flags | +===================+======================+ +================+--+--+--+--+ ---> | Header Padding | CRC32 | +================+--+--+--+--+ 3.1.1. Block Header Size This field overlaps with the Index Indicator field (see Section 4.1). This field contains the size of the Block Header field, including the Block Header Size field itself. Valid values are in the range [0x01, 0xFF], which indicate the size of the Block Header as multiples of four bytes, minimum size being eight bytes: real_header_size = (encoded_header_size + 1) * 4; If a Block Header bigger than 1024 bytes is needed in the future, a new field can be added between the Block Header and Compressed Data fields. The presence of this new field would be indicated in the Block Header field. 3.1.2. Block Flags The Block Flags field is a bit field: Bit(s) Mask Description 0-1 0x03 Number of filters (1-4) 2-5 0x3C Reserved for future use; MUST be zero for now. 6 0x40 The Compressed Size field is present. 7 0x80 The Uncompressed Size field is present. If any reserved bit is set, the decoder MUST indicate an error. It is possible that there is a new field present which the decoder is not aware of, and can thus parse the Block Header incorrectly. 3.1.3. Compressed Size This field is present only if the appropriate bit is set in the Block Flags field (see Section 3.1.2). The Compressed Size field contains the size of the Compressed Data field, which MUST be non-zero. Compressed Size is stored using the encoding described in Section 1.2. If the Compressed Size doesn't match the size of the Compressed Data field, the decoder MUST indicate an error. 3.1.4. Uncompressed Size This field is present only if the appropriate bit is set in the Block Flags field (see Section 3.1.2). The Uncompressed Size field contains the size of the Block after uncompressing. Uncompressed Size is stored using the encoding described in Section 1.2. If the Uncompressed Size does not match the real uncompressed size, the decoder MUST indicate an error. Storing the Compressed Size and Uncompressed Size fields serves several purposes: - The decoder knows how much memory it needs to allocate for a temporary buffer in multithreaded mode. - Simple error detection: wrong size indicates a broken file. - Seeking forwards to a specific location in streamed mode. It should be noted that the only reliable way to determine the real uncompressed size is to uncompress the Block, because the Block Header and Index fields may contain (intentionally or unintentionally) invalid information. 3.1.5. List of Filter Flags +================+================+ +================+ | Filter 0 Flags | Filter 1 Flags | ... | Filter n Flags | +================+================+ +================+ The number of Filter Flags fields is stored in the Block Flags field (see Section 3.1.2). The format of each Filter Flags field is as follows: +===========+====================+===================+ | Filter ID | Size of Properties | Filter Properties | +===========+====================+===================+ Both Filter ID and Size of Properties are stored using the encoding described in Section 1.2. Size of Properties indicates the size of the Filter Properties field as bytes. The list of officially defined Filter IDs and the formats of their Filter Properties are described in Section 5.3. Filter IDs greater than or equal to 0x4000_0000_0000_0000 (2^62) are reserved for implementation-specific internal use. These Filter IDs MUST never be used in List of Filter Flags. 3.1.6. Header Padding This field contains as many null byte as it is needed to make the Block Header have the size specified in Block Header Size. If any of the bytes are not null bytes, the decoder MUST indicate an error. It is possible that there is a new field present which the decoder is not aware of, and can thus parse the Block Header incorrectly. 3.1.7. CRC32 The CRC32 is calculated over everything in the Block Header field except the CRC32 field itself. It is stored as an unsigned 32-bit little endian integer. If the calculated value does not match the stored one, the decoder MUST indicate an error. By verifying the CRC32 of the Block Header before parsing the actual contents allows the decoder to distinguish between corrupt and unsupported files. 3.2. Compressed Data The format of Compressed Data depends on Block Flags and List of Filter Flags. Excluding the descriptions of the simplest filters in Section 5.3, the format of the filter-specific encoded data is out of scope of this document. 3.3. Block Padding Block Padding MUST contain 0-3 null bytes to make the size of the Block a multiple of four bytes. This can be needed when the size of Compressed Data is not a multiple of four. If any of the bytes in Block Padding are not null bytes, the decoder MUST indicate an error. 3.4. Check The type and size of the Check field depends on which bits are set in the Stream Flags field (see Section 2.1.1.2). The Check, when used, is calculated from the original uncompressed data. If the calculated Check does not match the stored one, the decoder MUST indicate an error. If the selected type of Check is not supported by the decoder, it SHOULD indicate a warning or error. 4. Index +-----------------+===================+ | Index Indicator | Number of Records | +-----------------+===================+ +=================+===============+-+-+-+-+ ---> | List of Records | Index Padding | CRC32 | +=================+===============+-+-+-+-+ Index serves several purposes. Using it, one can - verify that all Blocks in a Stream have been processed; - find out the uncompressed size of a Stream; and - quickly access the beginning of any Block (random access). 4.1. Index Indicator This field overlaps with the Block Header Size field (see Section 3.1.1). The value of Index Indicator is always 0x00. 4.2. Number of Records This field indicates how many Records there are in the List of Records field, and thus how many Blocks there are in the Stream. The value is stored using the encoding described in Section 1.2. If the decoder has decoded all the Blocks of the Stream, and then notices that the Number of Records doesn't match the real number of Blocks, the decoder MUST indicate an error. 4.3. List of Records List of Records consists of as many Records as indicated by the Number of Records field: +========+========+ | Record | Record | ... +========+========+ Each Record contains information about one Block: +===============+===================+ | Unpadded Size | Uncompressed Size | +===============+===================+ If the decoder has decoded all the Blocks of the Stream, it MUST verify that the contents of the Records match the real Unpadded Size and Uncompressed Size of the respective Blocks. Implementation hint: It is possible to verify the Index with constant memory usage by calculating for example SHA-256 of both the real size values and the List of Records, then comparing the hash values. Implementing this using non-cryptographic hash like CRC32 SHOULD be avoided unless small code size is important. If the decoder supports random-access reading, it MUST verify that Unpadded Size and Uncompressed Size of every completely decoded Block match the sizes stored in the Index. If only partial Block is decoded, the decoder MUST verify that the processed sizes don't exceed the sizes stored in the Index. 4.3.1. Unpadded Size This field indicates the size of the Block excluding the Block Padding field. That is, Unpadded Size is the size of the Block Header, Compressed Data, and Check fields. Unpadded Size is stored using the encoding described in Section 1.2. The value MUST never be zero; with the current structure of Blocks, the actual minimum value for Unpadded Size is five. Implementation note: Because the size of the Block Padding field is not included in Unpadded Size, calculating the total size of a Stream or doing random-access reading requires calculating the actual size of the Blocks by rounding Unpadded Sizes up to the next multiple of four. The reason to exclude Block Padding from Unpadded Size is to ease making a raw copy of Compressed Data without Block Padding. This can be useful, for example, if someone wants to convert Streams to some other file format quickly. 4.3.2. Uncompressed Size This field indicates the Uncompressed Size of the respective Block as bytes. The value is stored using the encoding described in Section 1.2. 4.4. Index Padding This field MUST contain 0-3 null bytes to pad the Index to a multiple of four bytes. If any of the bytes are not null bytes, the decoder MUST indicate an error. 4.5. CRC32 The CRC32 is calculated over everything in the Index field except the CRC32 field itself. The CRC32 is stored as an unsigned 32-bit little endian integer. If the calculated value does not match the stored one, the decoder MUST indicate an error. 5. Filter Chains The Block Flags field defines how many filters are used. When more than one filter is used, the filters are chained; that is, the output of one filter is the input of another filter. The following figure illustrates the direction of data flow. v Uncompressed Data ^ | Filter 0 | Encoder | Filter 1 | Decoder | Filter n | v Compressed Data ^ 5.1. Alignment Alignment of uncompressed input data is usually the job of the application producing the data. For example, to get the best results, an archiver tool should make sure that all PowerPC executable files in the archive stream start at offsets that are multiples of four bytes. Some filters, for example LZMA2, can be configured to take advantage of specified alignment of input data. Note that taking advantage of aligned input can be beneficial also when a filter is not the first filter in the chain. For example, if you compress PowerPC executables, you may want to use the PowerPC filter and chain that with the LZMA2 filter. Because not only the input but also the output alignment of the PowerPC filter is four bytes, it is now beneficial to set LZMA2 settings so that the LZMA2 encoder can take advantage of its four-byte-aligned input data. The output of the last filter in the chain is stored to the Compressed Data field, which is is guaranteed to be aligned to a multiple of four bytes relative to the beginning of the Stream. This can increase - speed, if the filtered data is handled multiple bytes at a time by the filter-specific encoder and decoder, because accessing aligned data in computer memory is usually faster; and - compression ratio, if the output data is later compressed with an external compression tool. 5.2. Security If filters would be allowed to be chained freely, it would be possible to create malicious files, that would be very slow to decode. Such files could be used to create denial of service attacks. Slow files could occur when multiple filters are chained: v Compressed input data | Filter 1 decoder (last filter) | Filter 0 decoder (non-last filter) v Uncompressed output data The decoder of the last filter in the chain produces a lot of output from little input. Another filter in the chain takes the output of the last filter, and produces very little output while consuming a lot of input. As a result, a lot of data is moved inside the filter chain, but the filter chain as a whole gets very little work done. To prevent this kind of slow files, there are restrictions on how the filters can be chained. These restrictions MUST be taken into account when designing new filters. The maximum number of filters in the chain has been limited to four, thus there can be at maximum of three non-last filters. Of these three non-last filters, only two are allowed to change the size of the data. The non-last filters, that change the size of the data, MUST have a limit how much the decoder can compress the data: the decoder SHOULD produce at least n bytes of output when the filter is given 2n bytes of input. This limit is not absolute, but significant deviations MUST be avoided. The above limitations guarantee that if the last filter in the chain produces 4n bytes of output, the chain as a whole will produce at least n bytes of output. 5.3. Filters 5.3.1. LZMA2 LZMA (Lempel-Ziv-Markov chain-Algorithm) is a general-purpose compression algorithm with high compression ratio and fast decompression. LZMA is based on LZ77 and range coding algorithms. LZMA2 is an extension on top of the original LZMA. LZMA2 uses LZMA internally, but adds support for flushing the encoder, uncompressed chunks, eases stateful decoder implementations, and improves support for multithreading. Thus, the plain LZMA will not be supported in this file format. Filter ID: 0x21 Size of Filter Properties: 1 byte Changes size of data: Yes Allow as a non-last filter: No Allow as the last filter: Yes Preferred alignment: Input data: Adjustable to 1/2/4/8/16 byte(s) Output data: 1 byte The format of the one-byte Filter Properties field is as follows: Bits Mask Description 0-5 0x3F Dictionary Size 6-7 0xC0 Reserved for future use; MUST be zero for now. Dictionary Size is encoded with one-bit mantissa and five-bit exponent. The smallest dictionary size is 4 KiB and the biggest is 4 GiB. Raw value Mantissa Exponent Dictionary size 0 2 11 4 KiB 1 3 11 6 KiB 2 2 12 8 KiB 3 3 12 12 KiB 4 2 13 16 KiB 5 3 13 24 KiB 6 2 14 32 KiB ... ... ... ... 35 3 27 768 MiB 36 2 28 1024 MiB 37 3 29 1536 MiB 38 2 30 2048 MiB 39 3 30 3072 MiB 40 2 31 4096 MiB - 1 B Instead of having a table in the decoder, the dictionary size can be decoded using the following C code: const uint8_t bits = get_dictionary_flags() & 0x3F; if (bits > 40) return DICTIONARY_TOO_BIG; // Bigger than 4 GiB uint32_t dictionary_size; if (bits == 40) { dictionary_size = UINT32_MAX; } else { dictionary_size = 2 | (bits & 1); dictionary_size <<= bits / 2 + 11; } 5.3.2. Branch/Call/Jump Filters for Executables These filters convert relative branch, call, and jump instructions to their absolute counterparts in executable files. This conversion increases redundancy and thus compression ratio. Size of Filter Properties: 0 or 4 bytes Changes size of data: No Allow as a non-last filter: Yes Allow as the last filter: No Below is the list of filters in this category. The alignment is the same for both input and output data. Filter ID Alignment Description 0x04 1 byte x86 filter (BCJ) 0x05 4 bytes PowerPC (big endian) filter 0x06 16 bytes IA64 filter 0x07 4 bytes ARM (little endian) filter 0x08 2 bytes ARM Thumb (little endian) filter 0x09 4 bytes SPARC filter If the size of Filter Properties is four bytes, the Filter Properties field contains the start offset used for address conversions. It is stored as an unsigned 32-bit little endian integer. The start offset MUST be a multiple of the alignment of the filter as listed in the table above; if it isn't, the decoder MUST indicate an error. If the size of Filter Properties is zero, the start offset is zero. Setting the start offset may be useful if an executable has multiple sections, and there are many cross-section calls. Taking advantage of this feature usually requires usage of the Subblock filter, whose design is not complete yet. 5.3.3. Delta The Delta filter may increase compression ratio when the value of the next byte correlates with the value of an earlier byte at specified distance. Filter ID: 0x03 Size of Filter Properties: 1 byte Changes size of data: No Allow as a non-last filter: Yes Allow as the last filter: No Preferred alignment: Input data: 1 byte Output data: Same as the original input data The Properties byte indicates the delta distance, which can be 1-256 bytes backwards from the current byte: 0x00 indicates distance of 1 byte and 0xFF distance of 256 bytes. 5.3.3.1. Format of the Encoded Output The code below illustrates both encoding and decoding with the Delta filter. // Distance is in the range [1, 256]. const unsigned int distance = get_properties_byte() + 1; uint8_t pos = 0; uint8_t delta[256]; memset(delta, 0, sizeof(delta)); while (1) { const int byte = read_byte(); if (byte == EOF) break; uint8_t tmp = delta[(uint8_t)(distance + pos)]; if (is_encoder) { tmp = (uint8_t)(byte) - tmp; delta[pos] = (uint8_t)(byte); } else { tmp = (uint8_t)(byte) + tmp; delta[pos] = tmp; } write_byte(tmp); --pos; } 5.4. Custom Filter IDs If a developer wants to use custom Filter IDs, he has two choices. The first choice is to contact Lasse Collin and ask him to allocate a range of IDs for the developer. The second choice is to generate a 40-bit random integer, which the developer can use as his personal Developer ID. To minimize the risk of collisions, Developer ID has to be a randomly generated integer, not manually selected "hex word". The following command, which works on many free operating systems, can be used to generate Developer ID: dd if=/dev/urandom bs=5 count=1 | hexdump The developer can then use his Developer ID to create unique (well, hopefully unique) Filter IDs. Bits Mask Description 0-15 0x0000_0000_0000_FFFF Filter ID 16-55 0x00FF_FFFF_FFFF_0000 Developer ID 56-62 0x3F00_0000_0000_0000 Static prefix: 0x3F The resulting 63-bit integer will use 9 bytes of space when stored using the encoding described in Section 1.2. To get a shorter ID, see the beginning of this Section how to request a custom ID range. 5.4.1. Reserved Custom Filter ID Ranges Range Description 0x0000_0300 - 0x0000_04FF Reserved to ease .7z compatibility 0x0002_0000 - 0x0007_FFFF Reserved to ease .7z compatibility 0x0200_0000 - 0x07FF_FFFF Reserved to ease .7z compatibility 6. Cyclic Redundancy Checks There are several incompatible variations to calculate CRC32 and CRC64. For simplicity and clarity, complete examples are provided to calculate the checks as they are used in this file format. Implementations MAY use different code as long as it gives identical results. The program below reads data from standard input, calculates the CRC32 and CRC64 values, and prints the calculated values as big endian hexadecimal strings to standard output. #include <stddef.h> #include <inttypes.h> #include <stdio.h> uint32_t crc32_table[256]; uint64_t crc64_table[256]; void init(void) { static const uint32_t poly32 = UINT32_C(0xEDB88320); static const uint64_t poly64 = UINT64_C(0xC96C5795D7870F42); for (size_t i = 0; i < 256; ++i) { uint32_t crc32 = i; uint64_t crc64 = i; for (size_t j = 0; j < 8; ++j) { if (crc32 & 1) crc32 = (crc32 >> 1) ^ poly32; else crc32 >>= 1; if (crc64 & 1) crc64 = (crc64 >> 1) ^ poly64; else crc64 >>= 1; } crc32_table[i] = crc32; crc64_table[i] = crc64; } } uint32_t crc32(const uint8_t *buf, size_t size, uint32_t crc) { crc = ~crc; for (size_t i = 0; i < size; ++i) crc = crc32_table[buf[i] ^ (crc & 0xFF)] ^ (crc >> 8); return ~crc; } uint64_t crc64(const uint8_t *buf, size_t size, uint64_t crc) { crc = ~crc; for (size_t i = 0; i < size; ++i) crc = crc64_table[buf[i] ^ (crc & 0xFF)] ^ (crc >> 8); return ~crc; } int main() { init(); uint32_t value32 = 0; uint64_t value64 = 0; uint64_t total_size = 0; uint8_t buf[8192]; while (1) { const size_t buf_size = fread(buf, 1, sizeof(buf), stdin); if (buf_size == 0) break; total_size += buf_size; value32 = crc32(buf, buf_size, value32); value64 = crc64(buf, buf_size, value64); } printf("Bytes: %" PRIu64 "\n", total_size); printf("CRC-32: 0x%08" PRIX32 "\n", value32); printf("CRC-64: 0x%016" PRIX64 "\n", value64); return 0; } 7. References LZMA SDK - The original LZMA implementation http://7-zip.org/sdk.html LZMA Utils - LZMA adapted to POSIX-like systems http://tukaani.org/lzma/ XZ Utils - The next generation of LZMA Utils http://tukaani.org/xz/ [RFC-1952] GZIP file format specification version 4.3 http://www.ietf.org/rfc/rfc1952.txt - Notation of byte boxes in section "2.1. Overall conventions" [RFC-2119] Key words for use in RFCs to Indicate Requirement Levels http://www.ietf.org/rfc/rfc2119.txt [GNU-tar] GNU tar 1.21 manual http://www.gnu.org/software/tar/manual/html_node/Blocking-Factor.html - Node 9.4.2 "Blocking Factor", paragraph that begins "gzip will complain about trailing garbage" - Note that this URL points to the latest version of the manual, and may some day not contain the note which is in 1.21. For the exact version of the manual, download GNU tar 1.21: ftp://ftp.gnu.org/pub/gnu/tar/tar-1.21.tar.gz