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diff --git a/doc/file-format.txt b/doc/file-format.txt new file mode 100644 index 00000000..4a90a67d --- /dev/null +++ b/doc/file-format.txt @@ -0,0 +1,1861 @@ + +The .lzma File Format +--------------------- + + 0. Preface + 0.1. Copyright Notices + 0.2. Changes + 1. Conventions + 1.1. Byte and Its Representation + 1.2. Multibyte Integers + 2. Stream + 2.1. Stream Types + 2.1.1. Single-Block Stream + 2.1.2. Multi-Block Stream + 2.2. Stream Header + 2.2.1. Header Magic Bytes + 2.2.2. Stream Flags + 2.2.3. CRC32 + 3. Block + 3.1. Block Header + 3.1.1. Block Flags + 3.1.2. Compressed Size + 3.1.3. Uncompressed Size + 3.1.4. List of Filter Flags + 3.1.4.1. Misc + 3.1.4.2. External ID + 3.1.4.3. External Size of Properties + 3.1.4.4. Filter Properties + 3.1.5. CRC32 + 3.1.6. Header Padding + 3.2. Compressed Data + 3.3. Block Footer + 3.3.1. Check + 3.3.2. Stream Footer + 3.3.2.1. Uncompressed Size + 3.3.2.2. Backward Size + 3.3.2.3. Stream Flags + 3.3.2.4. Footer Magic Bytes + 3.3.3. Footer Padding + 4. Filters + 4.1. Detecting when All Data Has Been Decoded + 4.1.1. With Uncompressed Size + 4.1.2. With End of Input + 4.1.3. With End of Payload Marker + 4.2. Alignment + 4.3. Filters + 4.3.1. Copy + 4.3.2. Subblock + 4.3.2.1. Format of the Encoded Output + 4.3.3. Delta + 4.3.3.1. Format of the Encoded Output + 4.3.4. LZMA + 4.3.4.1. LZMA Properties + 4.3.4.2. Dictionary Flags + 4.3.5. Branch/Call/Jump Filters for Executables + 5. Metadata + 5.1. Metadata Flags + 5.2. Size of Header Metadata Block + 5.3. Total Size + 5.4. Uncompressed Size + 5.5. Index + 5.5.1. Number of Data Blocks + 5.5.2. Total Sizes + 5.5.3. Uncompressed Sizes + 5.6. Extra + 5.6.1. 0x00: Dummy/Padding + 5.6.2. 0x01: OpenPGP Signature + 5.6.3. 0x02: Filter Information + 5.6.4. 0x03: Comment + 5.6.5. 0x04: List of Checks + 5.6.6. 0x05: Original Filename + 5.6.7. 0x07: Modification Time + 5.6.8. 0x09: High-Resolution Modification Time + 5.6.9. 0x0B: MIME Type + 5.6.10. 0x0D: Homepage URL + 6. Custom Filter and Extra Record IDs + 6.1. Reserved Custom Filter ID Ranges + 7. Cyclic Redundancy Checks + 8. References + 8.1. Normative References + 8.2. Informative References + + +0. Preface + + This document describes the .lzma file format (filename suffix + `.lzma', MIME type `application/x-lzma'). It is intended that + this format replace the format used by the LZMA_Alone tool + included in LZMA SDK up to and including version 4.43. + + IMPORTANT: The version described in this document is a + draft, NOT a final, official version. Changes + are possible. + + +0.1. Copyright Notices + + Copyright (C) 2006, 2007 Lasse Collin <lasse.collin@tukaani.org> + Copyright (C) 2006 Ville Koskinen <w-ber@iki.fi> + + Copying and distribution of this file, with or without + modification, are permitted in any medium without royalty + provided the copyright notice and this notice are preserved. + Modified versions must be marked as such. + + All source code examples given in this document are put into + the public domain by the authors of this document. + + Thanks for helping with this document goes to Igor Pavlov, + Mark Adler and Mikko Pouru. + + +0.2. Changes + + Last modified: 2007-12-02 22:40+0200 + + (A changelog will be kept once the first official version + is made.) + + +1. Conventions + + The keywords `must', `must not', `required', `should', + `should not', `recommended', `may', and `optional' in this + document are to be interpreted as described in [RFC-2119]. + These words are not capitalized in this document. + + Indicating a warning means displaying a message, returning + appropriate exit status, or 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 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 `nul byte' has all bits unset. That is, the value of a nul + 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 called `the Foo field' or plain `Foo'. + + +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 (e.g. + file sizes), multibyte integers are encoded in a simple + 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. The lowest + seven bits of every byte are used for data; the highest + (eighth) bit indicates either that + 0) the byte is in the middle of the byte sequence, or + 1) the byte is the first or the last byte. + + 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 future if needed. + + Note that the encoding is not as optimal as it could be. For + example, it is possible to encode the number 42 using any + number of bytes between one and nine. This is convenient + for non-streamed encoders, that write Compressed Size or + Uncompressed Size fields to the Block Header (see Section 3.1) + after the Compressed Data field is written to the disk. + + In several situations, the decoder needs to compare that two + fields contain identical information. When comparing fields + using the encoding described in this Section, the decoder must + consider two fields identical if their decoded values are + identical; it does not matter if the encoded variable-length + representations differ. + + The following C code illustrates encoding and decoding 63-bit + variables; the highest bit of uint64_t must be unset. The + functions return the number of bytes occupied by the integer + (1-9), or zero on error. + + #include <sys/types.h> + #include <inttypes.h> + + size_t + encode(uint8_t buf[static 9], uint64_t num) + { + if (num >= (UINT64_C(1) << (9 * 7))) + return 0; + if (num <= 0x7F) { + buf[0] = num; + return 1; + } + buf[0] = (num & 0x7F) | 0x80; + num >>= 7; + size_t i = 1; + while (num >= 0x80) { + buf[i++] = num & 0x7F; + num >>= 7; + } + buf[i++] = num | 0x80; + 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; + if (!(buf[0] & 0x80)) + return 1; + size_t i = 1; + do { + if (i == size_max) + return 0; + *num |= (uint64_t)(buf[i] & 0x7F) << (7 * i); + } while (!(buf[i++] & 0x80)); + return i; + } + + size_t + decode_reverse(const uint8_t buf[], size_t size_max, + uint64_t *num) + { + if (size_max == 0) + return 0; + const size_t end = size_max > 9 ? size_max - 9 : 0; + size_t i = size_max - 1; + *num = buf[i] & 0x7F; + if (!(buf[i] & 0x80)) + return 1; + do { + if (i-- == end) + return 0; + *num <<= 7; + *num |= buf[i] & 0x7F; + } while (!(buf[i] & 0x80)); + return size_max - i; + } + + +2. Stream + + +========+========+========+ + | Stream | Stream | Stream | ... + +========+========+========+ + + A file contains usually only one Stream. However, it is + possible to concatenate multiple Streams together with no + additional processing. It is up to the implementation to + decide if the decoder will continue decoding from the next + Stream once the end of the first Stream has been reached. + + +2.1. Stream Types + + There are two types of Streams: Single-Block Streams and + Multi-Block Streams. Decoders conforming to this specification + must support at least Single-Block Streams. Supporting + Multi-Block Streams is optional. If the decoder supports only + Single-Block Streams, the documentation of the decoder should + mention this fact clearly. + + +2.1.1. Single-Block Stream + + +===============+============+ + | Stream Header | Data Block | + +===============+============+ + + As the name says, a Single-Block Stream has exactly one Block. + The Block must be a Data Block; Metadata Blocks are not allowed + in Single-Block Streams. + + +2.1.2. Multi-Block Stream + + +===============+=======================+ + | Stream Header | Header Metadata Block | + +===============+=======================+ + + +============+ +============+=======================+ + ---> | Data Block | ... | Data Block | Footer Metadata Block | + +============+ +============+=======================+ + + Notes: + - Stream Header is mandatory. + - Header Metadata Block is optional. + - Each Multi-Block Stream has at least one Data Block. The + maximum number of Data Blocks is not limited. + - Footer Metadata Block is mandatory. + + +2.2. Stream Header + + +---+---+---+---+---+---+--------------+--+--+--+--+ + | Header Magic Bytes | Stream Flags | CRC32 | + +---+---+---+---+---+---+--------------+--+--+--+--+ + + +2.2.1. 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] + = { 0xFF, 'L', 'Z', 'M', 'A', 0x00 }; + + In plain hexadecimal: + FF 4C 5A 4D 41 00 + + Notes: + - The first byte (0xFF) was chosen so that the files cannot + be erroneously detected as being in LZMA_Alone format, in + which the first byte is in the the range [0x00, 0xE0]. + - The sixth byte (0x00) was chosen to prevent applications + from misdetecting the file as a text file. + + +2.2.2. Stream Flags + + Bit(s) Mask Description + 0-2 0x07 Type of Check (see Section 3.3.1): + ID Size Check name + 0x00 0 bytes None + 0x01 4 bytes CRC32 + 0x02 4 bytes (Reserved) + 0x03 8 bytes CRC64 + 0x04 16 bytes (Reserved) + 0x05 32 bytes SHA-256 + 0x06 32 bytes (Reserved) + 0x07 64 bytes (Reserved) + 3 0x08 The CRC32 field is present in Block Headers. + 4 0x10 If unset, this is a Single-Block Stream; if set, + this is a Multi-Block Stream. + 5-7 0xE0 Reserved for future use; must be zero for now. + + Implementations must 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 must 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. + + +2.2.3. 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. + + Note that this field is always present; the bit in Stream Flags + controls only presence of CRC32 in Block Headers. + + +3. Block + + +==============+=================+==============+ + | Block Header | Compressed Data | Block Footer | + +==============+=================+==============+ + + There are two types of Blocks: + - Data Blocks hold the actual compressed data. + - Metadata Blocks hold the Index, Extra, and a few other + non-data fields (see Section 5). + + The type of the Block is indicated by the corresponding bit + in the Block Flags field (see Section 3.1.1). + + +3.1. Block Header + + +------+------+=================+===================+ + | Block Flags | Compressed Size | Uncompressed Size | + +------+------+=================+===================+ + + +======================+--+--+--+--+================+ + ---> | List of Filter Flags | CRC32 | Header Padding | + +======================+--+--+--+--+================+ + + +3.1.1. Block Flags + + The first byte of the Block Flags field is a bit field: + + Bit(s) Mask Description + 0-2 0x07 Number of filters (0-7) + 3 0x08 Use End of Payload Marker (even if + Uncompressed Size is stored to Block Header). + 4 0x10 The Compressed Size field is present. + 5 0x20 The Uncompressed Size field is present. + 6 0x40 Reserved for future use; must be zero for now. + 7 0x80 This is a Metadata Block. + + The second byte of the Block Flags field is also a bit field: + + Bit(s) Mask Description + 0-4 0x1F Size of the Header Padding field (0-31 bytes) + 5-7 0xE0 Reserved for future use; must be zero for now. + + The decoder must indicate an error if End of Payload Marker + is not used and Uncompressed Size is not stored to the Block + Header. Because of this, the first byte of Block Flags can + never be a nul byte. This is useful when detecting beginning + of the Block after Footer Padding (see Section 3.3.3). + + 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. + + +3.1.2. Compressed Size + + This field is present only if the appropriate bit is set in + the Block Flags field (see Section 3.1.1). + + This field contains the size of the Compressed Data field. + The size is stored using the encoding described in Section 1.2. + If the Compressed Size does not match the real size of the + Compressed Data field, the decoder must indicate an error. + + Having the Compressed Size field in the Block Header can be + useful for multithreaded decoding when seeking is not possible. + If the Blocks are small enough, the decoder can read multiple + Blocks into its internal buffer, and decode the Blocks in + parallel. + + Compressed Size can also be useful when seeking forwards to + a specific location in streamed mode: the decoder can quickly + skip over irrelevant Blocks, without decoding them. + + +3.1.3. Uncompressed Size + + This field is present only if the appropriate bit is set in + the Block Flags field (see Section 3.1.1). + + The Uncompressed Size field contains the size of the Block + after uncompressing. + + Storing Uncompressed Size serves several purposes: + - The decoder will know when all of the data has been + decoded without an explicit End of Payload Marker. + - 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. + - Sometimes it is useful to know the file size without + uncompressing the file. + + It should be noted that the only reliable way to find out what + the real uncompressed size is is to uncompress the Block, + because the Block Header and Metadata Block fields may contain + (intentionally or unintentionally) invalid information. + + 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. + + +3.1.4. 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 3.1.1). As a special case, if the number of + Filter Flags fields is zero, it is equivalent to having the + Copy filter as the only filter. + + The format of each Filter Flags field is as follows: + + +------+=============+=============================+ + | Misc | External ID | External Size of Properties | + +------+=============+=============================+ + + +===================+ + ---> | Filter Properties | + +===================+ + + The list of officially defined Filter IDs and the formats of + their Filter Properties are described in Section 4.3. + + +3.1.4.1. Misc + + To save space, the most commonly used Filter IDs and the + Size of Filter Properties are encoded in a single byte. + Depending on the contents of the Misc field, Filter ID is + the value of the Misc or External ID field. + + Value Filter ID Size of Filter Properties + 0x00 - 0x1F Misc 0 bytes + 0x20 - 0x3F Misc 1 byte + 0x40 - 0x5F Misc 2 bytes + 0x60 - 0x7F Misc 3 bytes + 0x80 - 0x9F Misc 4 bytes + 0xA0 - 0xBF Misc 5 bytes + 0xC0 - 0xDF Misc 6 bytes + 0xE0 - 0xFE External ID 0-30 bytes + 0xFF External ID External Size of Properties + + The following code demonstrates parsing the Misc field and, + when needed, the External ID and External Size of Properties + fields. + + uint64_t id; + uint64_t properties_size; + uint8_t misc = read_byte(); + + if (misc >= 0xE0) { + id = read_variable_length_integer(); + + if (misc == 0xFF) + properties_size = read_variable_length_integer(); + else + properties_size = misc - 0xE0; + + } else { + id = misc; + properties_size = misc / 0x20; + } + + +3.1.4.2. External ID + + This field is present only if the Misc field contains a value + that indicates usage of External ID. The External ID is stored + using the encoding described in Section 1.2. + + +3.1.4.3. External Size of Properties + + This field is present only if the Misc field contains a value + that indicates usage of External Size of Properties. The size + of Filter Properties is stored using the encoding described in + Section 1.2. + + +3.1.4.4. Filter Properties + + Size of this field depends on the Misc field (Section 3.1.4.1) + and, if present, External Size of Properties field (Section + 3.1.4.3). The format of this field is depends on the selected + filter; see Section 4.3 for details. + + +3.1.5. CRC32 + + This field is present only if the appropriate bit is set in + the Stream Flags field (see Section 2.2.2). + + The CRC32 is calculated over everything in the Block Header + field except the Header Padding field and 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. + + +3.1.6. Header Padding + + This field contains as many nul bytes as indicated by the value + stored in the Header Flags field. If the Header Padding field + contains any non-nul bytes, the decoder must indicate an error. + + The intent of the Header Padding field is to allow alignment + of Compressed Data. The usefulness of alignment is described + in Section 4.3. + + +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 4, the format of the filter-specific encoded + data is out of scope of this document. + + Note a special case: if End of Payload Marker (see Section + 3.1.1) is not used and Uncompressed Size is zero, the size + of the Compressed Data field is always zero. + + +3.3. Block Footer + + +=======+===============+================+ + | Check | Stream Footer | Footer Padding | + +=======+===============+================+ + + +3.3.1. Check + + The type and size of the Check field depends on which bits + are set in the Stream Flags field (see Section 2.2.2). + + 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 must indicate + a warning or error. + + +3.3.2. Stream Footer + + +===================+===============+--------------+ + | Uncompressed Size | Backward Size | Stream Flags | + +===================+===============+--------------+ + + +----------+---------+ + ---> | Footer Magic Bytes | + +----------+---------+ + + Stream Footer is present only in + - Data Block of a Single-Block Stream; and + - Footer Metadata Block of a Multi-Block Stream. + + The Stream Footer field is placed inside Block Footer, because + no padding is allowed between Check and Stream Footer. + + +3.3.2.1. Uncompressed Size + + This field is present only in the Data Block of a Single-Block + Stream if Uncompressed Size is not stored to the Block Header + (see Section 3.1.1). Without the Uncompressed Size field in + Stream Footer it would not be possible to quickly find out + the Uncompressed Size of the Stream in all cases. + + Uncompressed Size is stored using the encoding described in + Section 1.2. If the stored value does not match the real + uncompressed size of the Single-Block Stream, the decoder must + indicate an error. + + +3.3.2.2. Backward Size + + This field contains the total size of the Block Header, + Compressed Data, Check, and Uncompressed Size fields. The + value is stored using the encoding described in Section 1.2. + If the Backward Size does not match the real total size of + the appropriate fields, the decoder must indicate an error. + + Implementations reading the Stream backwards should notice + that the value in this field can never be zero. + + +3.3.2.3. 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. + + +3.3.2.4. Footer Magic Bytes + + As the last step of the decoding process, the decoder must + verify the existence of Footer Magic Bytes. If they are not + found, 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 Footer Padding described in Section 3.3.3), it + cannot be undamaged, unless someone has intentionally appended + garbage after the end of the Stream. (Appending garbage at the + end of the file does not prevent uncompressing the file, but + may give a warning or error depending on the decoder + implementation.) + + +3.3.3. Footer Padding + + In certain situations it is convenient to be able to pad + Blocks or Streams to be multiples of, for example, 512 bytes. + Footer Padding makes this possible. Note that this is in no + way required to enforce alignment in the way described in + Section 4.3; the Header Padding field is enough for that. + + When Footer Padding is used, it must contain only nul bytes. + Any non-nul byte should be considered as the beginning of + a new Block or Stream. + + The possibility of Padding should be taken into account when + designing an application that wants to find out information + about a Stream by parsing Footer Metadata Block. + + Support for Padding was inspired by a related note in + [GNU-tar]. + + +4. Filters + + 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 ^ + + The filters are independent from each other, except that they + must cooperate a little to make it possible, in all cases, to + detect when all of the data has been decoded. In addition, the + filters should cooperate in the encoder to keep the alignment + optimal. + + +4.1. Detecting when All Data Has Been Decoded + + There must be a way for the decoder to detect when all of the + Compressed Data has been decoded. This is simple when only + one filter is used, but a bit more complex when multiple + filters are chained. + + This file format supports three methods to detect when all of + the data has been decoded: + - Uncompressed size + - End of Input + - End of Payload Marker + + In both encoder and decoder, filters are initialized starting + from the first filter in the chain. For each filter, one of + these three methods is used. + + +4.1.1. With Uncompressed Size + + This method is the only method supported by all filters. + It must be used when uncompressed size is known by the + filter-specific encoder or decoder. In practice this means + that Uncompressed Size has been stored to the Block Header. + + In case of the first filter in the chain, the uncompressed size + given to the filter-specific encoder or decoder equals the + Uncompressed Size stored in the Block Header. For the rest of + the filters in the chain, uncompressed size is the size of the + output data of the previous filter in the chain. + + Note that when Use End of Payload Marker bit is set in Block + Flags, Uncompressed Size is considered to be unknown even if + it was present in the Block Header. Thus, if End of Payload + Marker is used, uncompressed size of all of the filters in + the chain is unknown, and can never be used to detect when + all of the data has been decoded. + + Once the correct number of bytes has been written out, the + filter-specific decoder indicates to its caller that all of + the data has been decoded. If the filter-specific decoder + detects End of Input or End of Payload Marker before the + correct number of bytes is decoded, the decoder must indicate + an error. + + +4.1.2. With End of Input + + Most filters will know that all of the data has been decoded + when the End of Input data has been reached. Once the filter + knows that it has received the input data in its entirety, + it finishes its job, and indicates to its caller that all of + the data has been decoded. The filter-specific decoder must + indicate an error if it detects End of Payload Marker. + + Note that this method can work only when the filter is not + the last filter in the chain, because only another filter + can indicate the End of Input data. In practice this means, + that a filter later in the chain must support embedding + End of Payload Marker. + + When a filter that cannot embed End of Payload Marker is the + last filter in the chain, Subblock filter is appended to the + chain as an implicit filter. In the simplest case, this occurs + when no filters are specified, and Uncompressed Size is unknown + or the End of Payload Marker bit is set in Block Flags. + + +4.1.3. With End of Payload Marker + + End of Payload Marker is a filter-specific bit sequence that + indicates the end of data. It is supported by only a few + filters. It is used when uncompressed size is unknown, and + the filter + - doesn't support End of Input; or + - is the last filter in the chain. + + End of Payload Marker is embedded at the end of the encoded + data by the filter-specific encoder. When the filter-specific + decoder detects the embedded End of Payload Marker, the decoder + knows that all of the data has been decoded. Then it finishes + its job, and indicates to its caller that all of the data has + been decoded. If the filter-specific decoder detects End of + Input before End of Payload Marker, the decoder must indicate + an error. + + If the filter supports both End of Input and End of Payload + Marker, the former is used, unless the filter is the last + filter in the chain. + + +4.2. Alignment + + Some filters give better compression ratio or are faster + when the input or output data is aligned. For optimal results, + the encoder should try to enforce proper alignment when + possible. Not enforcing alignment in the encoder is not + an error. Thus, the decoder must be able to handle files with + suboptimal 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 LZMA, can be configured to take + advantage of specified alignment of input data. Note that + taking advantage of aligned input can be benefical 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 LZMA filter. Because not + only the input but also the output alignment of the PowerPC + filter is four bytes, it is now benefical to set LZMA settings + so that the LZMA 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. Aligning Compressed Data appropriately + 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. + + Compressed Data in a Stream can be aligned by using the Header + Padding field in the Block Header. + + +4.3. Filters + +4.3.1. Copy + + This is a dummy filter that simply copies all data from input + to output unmodified. + + Filter ID: 0x00 + Size of Filter Properties: 0 bytes + Changes size of data: No + + Detecting when all of the data has been decoded: + Uncompressed size: Yes + End of Payload Marker: No + End of Input: Yes + + Preferred alignment: + Input data: 1 byte + Output data: 1 byte + + +4.3.2. Subblock + + The Subblock filter can be used to + - embed End of Payload Marker when the otherwise last + filter in the chain does not support embedding it; and + - apply additional filters in the middle of a Block. + + Filter ID: 0x01 + Size of Filter Properties: 0 bytes + Changes size of data: Yes, unpredictably + + Detecting when all of the data has been decoded: + Uncompressed size: Yes + End of Payload Marker: Yes + End of Input: Yes + + Preferred alignment: + Input data: 1 byte + Output data: Freely adjustable + + +4.3.2.1. Format of the Encoded Output + + The encoded data from the Subblock filter consist of zero or + more Subblocks: + + +==========+==========+ + | Subblock | Subblock | ... + +==========+==========+ + + Each Subblock contains two fields: + + +----------------+===============+ + | Subblock Flags | Subblock Data | + +----------------+===============+ + + Subblock Flags is a bitfield: + + Bits Mask Description + 0-3 0x0F The interpretation of these bits depend on + the Subblock Type: + - 0x20 Bits 0-3 for Size + - 0x30 Bits 0-3 for Repeat Count + - Other These bits must be zero. + 4-7 0xF0 Subblock Type: + - 0x00: Padding + - 0x10: End of Payload Marker + - 0x20: Data + - 0x30: Repeating Data + - 0x40: Set Subfilter + - 0x50: Unset Subfilter + If some other value is detected, the decoder + must indicate an error. + + The format of the Subblock Data field depends on Subblock Type. + + Subblocks with the Subblock Type 0x00 (Padding) don't have a + Subblock Data field. These Subblocks can be useful for fixing + alignment. There can be at maximum of 31 consecutive Subblocks + with this Subblock Type; if there are more, the decoder must + indicate an error. + + Subblock with the Subblock Type 0x10 (End of Payload Marker) + doesn't have a Subblock Data field. The decoder must indicate + an error if this Subblock Type is detected when Subfilter is + enabled, or when the Subblock filter is not supposed to embed + the End of Payload Marker. + + Subblocks with the Subblock Type 0x20 (Data) contain the rest + of the Size, which is followed by Size + 1 bytes in the Data + field (that is, Data can never be empty): + + +------+------+------+======+ + | Bits 4-27 for Size | Data | + +------+------+------+======+ + + Subblocks with the Subblock Type 0x30 (Repeating Data) contain + the rest of the Repeat Count, the Size of the Data, and finally + the actual Data to be repeated: + + +---------+---------+--------+------+======+ + | Bits 4-27 for Repeat Count | Size | Data | + +---------+---------+--------+------+======+ + + The size of the Data field is Size + 1. It is repeated Repeat + Count + 1 times. That is, the minimum size of Data is one byte; + the maximum size of Data is 256 bytes. The minimum number of + repeats is one; the maximum number of repeats is 2^28. + + If Subfilter is not used, the Data field of Subblock Types 0x20 + and 0x30 is the output of the decoded Subblock filter. If + Subfilter is used, Data is the input of the Subfilter, and the + decoded output of the Subfilter is the decoded output of the + Subblock filter. + + Subblocks with the Subblock Type 0x40 (Set Subfilter) contain + a Filter Flags field in Subblock Data: + + +==============+ + | Filter Flags | + +==============+ + + It is an error to set the Subfilter to Filter ID 0x00 (Copy) + or 0x01 (Subblock). All the other Filter IDs are allowed. + The decoder must indicate an error if this Subblock Type is + detected when a Subfilter is already enabled. + + Subblocks with the Subblock Type 0x50 (Unset Subfilter) don't + have a Subblock Data field. There must be at least one Subblock + with Subblock Type 0x20 or 0x30 between Subblocks with Subblock + Type 0x40 and 0x50; if there isn't, the decoder must indicate + an error. + + Subblock Types 0x40 and 0x50 are always used as a pair: If the + Subblock filter has been enabled with Subblock Type 0x40, it + must always be disabled later with Subblock Type 0x50. + Disabling must be done even if the Subfilter used End of + Payload Marker; after the Subfilter has detected End of Payload + Marker, the next Subblock that is not Padding must unset the + Subfilter. + + When the Subblock filter is used as an implicit filter to embed + End of Payload marker, the Subblock Types 0x40 and 0x50 (Set or + Unset Subfilter) must not be used. The decoder must indicate an + error if it detects any of these Subblock Types in an implicit + Subblock filter. + + The following code illustrates the basic structure of a + Subblock decoder. + + uint32_t consecutive_padding = 0; + bool got_output_with_subfilter = false; + + while (true) { + uint32_t size; + uint32_t repeat; + uint8_t flags = read_byte(); + + if (flags != 0) + consecutive_padding = 0; + + switch (flags >> 4) { + case 0: + // Padding + if (flags & 0x0F) + return DATA_ERROR; + if (++consecutive_padding == 32) + return DATA_ERROR; + break; + + case 1: + // End of Payload Marker + if (flags & 0x0F) + return DATA_ERROR; + if (subfilter_enabled || !allow_eopm) + return DATA_ERROR; + break; + + case 2: + // Data + size = flags & 0x0F; + for (size_t i = 4; i < 28; i += 8) + size |= (uint32_t)(read_byte()) << i; + + // If any output is produced, this will + // set got_output_with_subfilter to true. + copy_data(size); + break; + + case 3: + // Repeating Data + repeat = flags & 0x0F; + for (size_t i = 4; i < 28; i += 8) + repeat |= (uint32_t)(read_byte()) << i; + size = read_byte(); + + // If any output is produced, this will + // set got_output_with_subfilter to true. + copy_repeating_data(size, repeat); + break; + + case 4: + // Set Subfilter + if (flags & 0x0F) + return DATA_ERROR; + if (subfilter_enabled) + return DATA_ERROR; + got_output_with_subfilter = false; + set_subfilter(); + break; + + case 5: + // Unset Subfilter + if (flags & 0x0F) + return DATA_ERROR; + if (!subfilter_enabled) + return DATA_ERROR; + if (!got_output_with_subfilter) + return DATA_ERROR; + unset_subfilter(); + break; + + default: + return DATA_ERROR; + } + } + + +4.3.3. 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: 0x20 + Size of Filter Properties: 1 byte + Changes size of data: No + + Detecting when all of the data has been decoded: + Uncompressed size: Yes + End of Payload Marker: No + End of Input: Yes + + 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. + + +4.3.3.1. 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; + } + + +4.3.4. LZMA + + LZMA (Lempel-Ziv-Markov chain-Algorithm) is a general-purporse + compression algorithm with high compression ratio and fast + decompression. LZMA based on LZ77 and range coding algorithms. + + Filter ID: 0x40 + Size of Filter Properties: 2 bytes + Changes size of data: Yes, unpredictably + + Detecting when all of the data has been decoded: + Uncompressed size: Yes + End of Payload Marker: Yes + End of Input: No + + Preferred alignment: + Input data: Adjustable to 1/2/4/8/16 byte(s) + Output data: 1 byte + + At the time of writing, there is no other documentation about + how LZMA works than the source code in LZMA SDK. Once such + documentation gets written, it will probably be published as + a separate document, because including the documentation here + would lengthen this document considerably. + + The format of the Filter Properties field is as follows: + + +-----------------+------------------+ + | LZMA Properties | Dictionary Flags | + +-----------------+------------------+ + + +4.3.4.1. LZMA Properties + + The LZMA Properties bits contain three properties. An + abbreviation is given in parentheses, followed by the value + range of the property. The field consists of + + 1) the number of literal context bits (lc, [0, 8]); + 2) the number of literal position bits (lp, [0, 4]); and + 3) the number of position bits (pb, [0, 4]). + + They are encoded using the following formula: + + LZMA Properties = (pb * 5 + lp) * 9 + lc + + The following C code illustrates a straightforward way to + decode the properties: + + uint8_t lc, lp, pb; + uint8_t prop = get_lzma_properties() & 0xFF; + if (prop > (4 * 5 + 4) * 9 + 8) + return LZMA_PROPERTIES_ERROR; + + pb = prop / (9 * 5); + prop -= pb * 9 * 5; + lp = prop / 9; + lc = prop - lp * 9; + + +4.3.4.2. Dictionary Flags + + Currently the lowest six bits of the Dictionary Flags field + are in use: + + 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. To avoid wasting space, one-byte dictionary has its + own special value. + + Raw value Mantissa Exponent Dictionary size + 0 1 0 1 byte + 1 2 0 2 bytes + 2 3 0 3 bytes + 3 2 1 4 bytes + 4 3 1 6 bytes + 5 2 2 8 bytes + 6 3 2 12 bytes + 7 2 3 16 bytes + 8 3 3 24 bytes + 9 2 4 32 bytes + ... ... ... ... + 61 2 30 2 GiB + 62 3 30 3 GiB + 63 2 31 4 GiB (*) + + (*) The real maximum size of the dictionary is one byte + less than 4 GiB, because the distance of 4 GiB is + reserved for End of Payload Marker. + + Instead of having a table in the decoder, the dictionary size + can be decoded using the following C code: + + uint64_t dictionary_size; + const uint8_t bits = get_dictionary_flags() & 0x3F; + if (bits == 0) { + dictionary_size = 1; + } else { + dictionary_size = 2 | ((bits + 1) & 1); + dictionary_size = dictionary_size << ((bits - 1) / 2); + } + + +4.3.5. 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 + + Detecting when all of the data has been decoded: + Uncompressed size: Yes + End of Payload Marker: No + End of Input: Yes + + 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. 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. + + +5. Metadata + + Metadata is stored in Metadata Blocks, which can be in the + beginning or at the end of a Multi-Block Stream. Because of + Blocks, it is possible to compress Metadata in the same way + as the actual data is compressed. This Section describes the + format of the data stored in Metadata Blocks. + + +----------------+===============================+ + | Metadata Flags | Size of Header Metadata Block | + +----------------+===============================+ + + +============+===================+=======+=======+ + ---> | Total Size | Uncompressed Size | Index | Extra | + +============+===================+=======+=======+ + + Stream must be parseable backwards. That is, there must be + a way to locate the beginning of the Stream by starting from + the end of the Stream. Thus, the Footer Metadata Block must + contain the Total Size field or the Index field. If the Stream + has Header Metadata Block, also the Size of Header Metadata + Block field must be present in Footer Metadata Block. + + It must be possible to quickly locate the Blocks in + non-streamed mode. Thus, the Index field must be present + at least in one Metadata Block. + + If the above conditions are not met, the decoder must indicate + an error. + + There should be no additional data after the last field. If + there is, the the decoder should indicate an error. + + +5.1. Metadata Flags + + This field describes which fields are present in a Metadata + Block: + + Bit(s) Mask Desription + 0 0x01 Size of Header Metadata Block is present. + 1 0x02 Total Size is present. + 2 0x04 Uncompressed Size is present. + 3 0x08 Index is present. + 4-6 0x70 Reserve for future use; must be zero for now. + 7 0x80 Extra 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 Metadata + incorrectly. + + +5.2. Size of Header Metadata Block + + This field is present only if the appropriate bit is set in + the Metadata Flags field (see Section 5.1). + + Size of Header Metadata Block is needed to make it possible to + parse the Stream backwards. The size is stored using the + encoding described in Section 1.2. The decoder must verify that + that the value stored in this field is non-zero. In Footer + Metadata Block, the decoder must also verify that the stored + size matches the real size of Header Metadata Block. In the + Header Meatadata Block, the value of this field is ignored as + long as it is not zero. + + +5.3. Total Size + + This field is present only if the appropriate bit is set in the + Metadata Flags field (see Section 5.1). + + This field contains the total size of the Data Blocks in the + Stream. Total Size is stored using the encoding described in + Section 1.2. If the stored value does not match the real total + size of the Data Blocks, the decoder must indicate an error. + The value of this field must be non-zero. + + Total Size can be used to quickly locate the beginning or end + of the Stream. This can be useful for example when doing + random-access reading, and the Index field is not in the + Metadata Block currently being read. + + It is useless to have both Total Size and Index in the same + Metadata Block, because Total Size can be calculated from the + Index field. + + +5.4. Uncompressed Size + + This field is present only if the appropriate bit is set in the + Metadata Flags field (see Section 5.1). + + This field contains the total uncompressed size of the Data + Blocks in the Stream. Uncompresssed Size is stored using the + encoding described in Section 1.2. If the stored value does not + match the real uncompressed size of the Data Blocks, the + decoder must indicate an error. + + It is useless to have both Uncompressed Size and Index in + the same Metadata Block, because Uncompressed Size can be + calculated from the Index field. + + +5.5. Index + + +=======================+=============+====================+ + | Number of Data Blocks | Total Sizes | Uncompressed Sizes | + +=======================+=============+====================+ + + Index serves several purporses. 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). + + +5.5.1. Number of Data Blocks + + This field contains the number of Data Blocks in the Stream. + The value is stored using the encoding described in Section + 1.2. If the decoder has decoded all the Data Blocks of the + Stream, and then notices that the Number of Records doesn't + match the real number of Data Blocks, the decoder must + indicate an error. The value of this field must be non-zero. + + +5.5.2. Total Sizes + + +============+============+ + | Total Size | Total Size | ... + +============+============+ + + This field lists the Total Sizes of every Data Block in the + Stream. There are as many Total Size fields as indicated by + the Number of Data Blocks field. + + Total Size is the size of Block Header, Compressed Data, and + Block Footer. It is stored using the encoding described in + Section 1.2. If the Total Sizes do not match the real sizes + of respective Blocks, the decoder should indicate an error. + All the Total Size fields must have a non-zero value. + + +5.5.3. Uncompressed Sizes + + +===================+===================+ + | Uncompressed Size | Uncompressed Size | ... + +===================+===================+ + + This field lists the Uncompressed Sizes of every Data Block + in the Stream. There are as many Uncompressed Size fields as + indicated by the Number of Records field. + + Uncompressed Sizes are stored using the encoding described + in Section 1.2. If the Uncompressed Sizes do not match the + real sizes of respective Blocks, the decoder shoud indicate + an error. + + +5.6. Extra + + This field is present only if the appropriate bit is set in the + Metadata Flags field (see Section 5.1). Note that the bit does + not indicate that there is any data in the Extra field; it only + indicates that Extra may be non-empty. + + The Extra field contains only information that is not required + to properly uncompress the Stream or to do random-access + reading. Supporting the Extra field is optional. In case the + decoder doesn't support the Extra field, it should silently + ignore it. + + Extra consists of zero or more Records: + + +========+========+ + | Record | Record | ... + +========+========+ + + Excluding Records with Record ID 0x00, each Record contains + three fields: + + +==========+==============+======+ + | Reord ID | Size of Data | Data | + +==========+==============+======+ + + The Record ID and Size of Data are stored using the encoding + described in Section 1.2. Data can be binary or UTF-8 + [RFC-3629] strings. Non-UTF-8 strings should be avoided. + Because the Size of Data is known, there is no need to + terminate strings with a nul byte, although doing so should + not be considered an error. + + The Record IDs are divided in two categories: + - Safe-to-Copy Records may be preserved as is when the + Stream is modified in ways that don't change the actual + uncompressed data. Examples of such operatings include + recompressing and adding, modifying, or deleting unrelated + Extra Records. + - Unsafe-to-Copy Records should be removed (and possibly + recreated) when any kind of changes are made to the Stream. + + When the actual uncompressed data is modified, all Records + should be removed (and possibly recreated), unless the + application knows that the Data stored to the Record(s) is + still valid. + + The following subsections describe the standard Record IDs and + the format of their Data fields. Safe-to-Copy Records have an + odd ID, while Unsafe-to-Copy Records have an even ID. + + +5.6.1. 0x00: Dummy/Padding + + This Record is special, because it doesn't have the Size of + Data or Data fields. + + Dummy Records can be used, for example, to fill Metadata Block + when a few bytes of extra space has been reserved for it. There + can be any number of Dummy Records. + + +5.6.2. 0x01: OpenPGP Signature + + OpenPGP signature is computed from uncompressed data. The + signature can be used to verify that the contents of a Stream + has been created by a trustworthy source. + + If the decoder supports decoding concatenated Streams, it + must indicate an error when verifying OpenPGP signatures if + there is more than one Stream. + + OpenPGP format is documented in [RFC-2440]. + + +5.6.3. 0x02: Filter Information + + The Filter Information Record contains information about the + filters used in the Stream. This field can be used to quickly + - display which filters are used in each Block; + - check if all the required filters are supported by the + current decoder version; and + - check how much memory is required to decode each Block. + + The format of the Filter Information field is as follows: + + +=================+=================+ + | Block 0 Filters | Block 1 Filters | ... + +=================+=================+ + + There can be at maximum of as many Block Filters fields as + there are Data Blocks in the Stream. The format of the Block + Filters field is as follows: + + +------------------+======================+============+ + | Block Properties | List of Filter Flags | Subfilters | + +------------------+======================+============+ + + Block Properties is a bitfield: + + Bit(s) Mask Description + 0-2 0x07 Number of filters (0-7) + 3 0x08 End of Payload Marker is used. + 4 0x10 The Subfilters field is present. + 5-7 0xE0 Reserved for future use; must be zero for now. + + The contents of the List of Filter Flags field must match the + List of Filter Flags field in the respective Block Header. + + The Subfilters field may be present only if the List of Filter + Flags contains a Filter Flags field for a Subblock filter. The + format of the Subfilters field is as follows: + + +======================+=========================+ + | Number of Subfilters | List of Subfilter Flags | + +======================+=========================+ + + The value stored in the Number of Subfilters field is stored + using the encoding described in Section 1.2. The List of + Subfilter Flags field contains as many Filter Flags fields + as indicated by the Number of Subfilters field. These Filter + Flags fields list some or all the Subfilters used via the + Subblock filter. The order of the listed Subfilters is not + significant. + + Decoders supporting this Record should indicate a warning or + error if this Record contains Filter Flags that are not + actually used by the respective Blocks. + + +5.6.4. 0x03: Comment + + Free-form comment is stored in UTF-8 [RFC-3629] encoding. + + The beginning of a new line should be indicated using the + ASCII Line Feed character (0x0A). When the Line Feed character + is not the native way to indicate new line in the underlying + operating system, the encoder and decoder should convert the + newline characters to and from Line Feeds. + + +5.6.5. 0x04: List of Checks + + +=======+=======+ + | Check | Check | ... + +=======+=======+ + + There are as many Check fields as there are Blocks in the + Stream. The size of Check fields depend on Stream Flags + (see Section 2.2.2). + + Decoders supporting this Record should indicate a warning or + error if the Checks don't match the respective Blocks. + + +5.6.6. 0x05: Original Filename + + Original filename is stored in UTF-8 [RFC-3629] encoding. + + The filename must not include any path, only the filename + itself. Special care must be taken to prevent directory + traversal vulnerabilities. + + When files are moved between different operating systems, it + is possible that filename valid in the source system is not + valid in the target system. It is implementation defined how + the decoder handles this kind of situations. + + +5.6.7. 0x07: Modification Time + + Modification time is stored as POSIX time, as an unsigned + little endian integer. The number of bits depends on the + Size of Data field. Note that the usage of unsigned integer + limits the earliest representable time to 1970-01-01T00:00:00. + + +5.6.8. 0x09: High-Resolution Modification Time + + This Record extends the `0x04: Modification time' Record with + a subsecond time information. There are two supported formats + of this field, which can be distinguished by looking at the + Size of Data field. + + Size Data + 3 [0; 9,999,999] times 100 nanoseconds + 4 [0; 999,999,999] nanoseconds + + The value is stored as an unsigned 24-bit or 32-bit little + endian integer. + + +5.6.9. 0x0B: MIME Type + + MIME type of the uncompressed Stream. This can be used to + detect the content type. [IANA-MIME] + + +5.6.10. 0x0D: Homepage URL + + This field can be used, for example, when distributing software + packages (sources or binaries). The field would indicate the + homepage of the program. + + For details on how to encode URLs, see [RFC-1738]. + + +6. Custom Filter and Extra Record IDs + + If a developer wants to use custom Filter or Extra Record 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 minimalize 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 and Extra Record IDs. + + Bits Mask Description + 0-15 0x0000_0000_0000_FFFF Filter or Extra Record ID + 16-55 0x00FF_FFFF_FFFF_0000 Developer ID + 56-62 0x7F00_0000_0000_0000 Static prefix: 0x7F + + 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. + + Note that Filter and Metadata Record IDs are in their own + namespaces. That is, you can use the same ID value as Filter ID + and Metadata Record ID, and the meanings of the IDs do not need + to be related to each other. + + +6.1. Reserved Custom Filter ID Ranges + + Range Description + 0x0000_0000 - 0x0000_00DF IDs fitting into the Misc field + 0x0002_0000 - 0x0007_FFFF Reserved to ease .7z compatibility + 0x0200_0000 - 0x07FF_FFFF Reserved to ease .7z compatibility + + +7. 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 <sys/types.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, 8192, 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; + } + + +8. References + +8.1. Normative References + + [RFC-1738] + Uniform Resource Locators (URL) + http://www.ietf.org/rfc/rfc1738.txt + + [RFC-2119] + Key words for use in RFCs to Indicate Requirement Levels + http://www.ietf.org/rfc/rfc2119.txt + + [RFC-2440] + OpenPGP Message Format + http://www.ietf.org/rfc/rfc2440.txt + + [RFC-3629] + UTF-8, a transformation format of ISO 10646 + http://www.ietf.org/rfc/rfc3629.txt + + [IANA-MIME] + MIME Media Types + http://www.iana.org/assignments/media-types/ + + +8.2. Informative 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/ + + [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' + + [GNU-tar] + GNU tar 1.16.1 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 + 1.16.1. For the exact version of the manual, download GNU + tar 1.16.1: ftp://ftp.gnu.org/pub/gnu/tar/tar-1.16.1.tar.gz + |