deflate compressed data format as described in the document         "DEFLATE Compressed Data Format Specification" by L. Peter         Deutsch. (See reference [3] in Chapter 3, below)         Other compressed data formats are not specified in this version         of the zlib specification.      ADLER32 (Adler-32 checksum)         This contains a checksum value of the uncompressed data         (excluding any dictionary data) computed according to Adler-32         algorithm. This algorithm is a 32-bit extension and improvement         of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073         standard. See references [4] and [5] in Chapter 3, below)         Adler-32 is composed of two sums accumulated per byte: s1 is         the sum of all bytes, s2 is the sum of all s1 values. Both sums         are done modulo 65521. s1 is initialized to 1, s2 to zero.  The         Adler-32 checksum is stored as s2*65536 + s1 in most-         significant-byte first (network) order.Deutsch & Gailly             Informational                      [Page 6]RFC 1950       ZLIB Compressed Data Format Specification        May 1996   2.3. Compliance      A compliant compressor must produce streams with correct CMF, FLG      and ADLER32, but need not support preset dictionaries.  When the      zlib data format is used as part of another standard data format,      the compressor may use only preset dictionaries that are specified      by this other data format.  If this other format does not use the      preset dictionary feature, the compressor must not set the FDICT      flag.      A compliant decompressor must check CMF, FLG, and ADLER32, and      provide an error indication if any of these have incorrect values.      A compliant decompressor must give an error indication if CM is      not one of the values defined in this specification (only the      value 8 is permitted in this version), since another value could      indicate the presence of new features that would cause subsequent      data to be interpreted incorrectly.  A compliant decompressor must      give an error indication if FDICT is set and DICTID is not the      identifier of a known preset dictionary.  A decompressor may      ignore FLEVEL and still be compliant.  When the zlib data format      is being used as a part of another standard format, a compliant      decompressor must support all the preset dictionaries specified by      the other format. When the other format does not use the preset      dictionary feature, a compliant decompressor must reject any      stream in which the FDICT flag is set.3. References   [1] Deutsch, L.P.,"GZIP Compressed Data Format Specification",       available in ftp://ftp.uu.net/pub/archiving/zip/doc/   [2] Thomas Boutell, "PNG (Portable Network Graphics) specification",       available in ftp://ftp.uu.net/graphics/png/documents/   [3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",       available in ftp://ftp.uu.net/pub/archiving/zip/doc/   [4] Fletcher, J. G., "An Arithmetic Checksum for Serial       Transmissions," IEEE Transactions on Communications, Vol. COM-30,       No. 1, January 1982, pp. 247-252.   [5] ITU-T Recommendation X.224, Annex D, "Checksum Algorithms,"       November, 1993, pp. 144, 145. (Available from       gopher://info.itu.ch). ITU-T X.244 is also the same as ISO 8073.Deutsch & Gailly             Informational                      [Page 7]RFC 1950       ZLIB Compressed Data Format Specification        May 19964. Source code   Source code for a C language implementation of a "zlib" compliant   library is available at ftp://ftp.uu.net/pub/archiving/zip/zlib/.5. Security Considerations   A decoder that fails to check the ADLER32 checksum value may be   subject to undetected data corruption.6. Acknowledgements   Trademarks cited in this document are the property of their   respective owners.   Jean-Loup Gailly and Mark Adler designed the zlib format and wrote   the related software described in this specification.  Glenn   Randers-Pehrson converted this document to RFC and HTML format.7. Authors' Addresses   L. Peter Deutsch   Aladdin Enterprises   203 Santa Margarita Ave.   Menlo Park, CA 94025   Phone: (415) 322-0103 (AM only)   FAX:   (415) 322-1734   EMail: <ghost@aladdin.com>   Jean-Loup Gailly   EMail: <gzip@prep.ai.mit.edu>   Questions about the technical content of this specification can be   sent by email to   Jean-Loup Gailly <gzip@prep.ai.mit.edu> and   Mark Adler <madler@alumni.caltech.edu>   Editorial comments on this specification can be sent by email to   L. Peter Deutsch <ghost@aladdin.com> and   Glenn Randers-Pehrson <randeg@alumni.rpi.edu>Deutsch & Gailly             Informational                      [Page 8]RFC 1950       ZLIB Compressed Data Format Specification        May 19968. Appendix: Rationale   8.1. Preset dictionaries      A preset dictionary is specially useful to compress short input      sequences. The compressor can take advantage of the dictionary      context to encode the input in a more compact manner. The      decompressor can be initialized with the appropriate context by      virtually decompressing a compressed version of the dictionary      without producing any output. However for certain compression      algorithms such as the deflate algorithm this operation can be      achieved without actually performing any decompression.      The compressor and the decompressor must use exactly the same      dictionary. The dictionary may be fixed or may be chosen among a      certain number of predefined dictionaries, according to the kind      of input data. The decompressor can determine which dictionary has      been chosen by the compressor by checking the dictionary      identifier. This document does not specify the contents of      predefined dictionaries, since the optimal dictionaries are      application specific. Standard data formats using this feature of      the zlib specification must precisely define the allowed      dictionaries.   8.2. The Adler-32 algorithm      The Adler-32 algorithm is much faster than the CRC32 algorithm yet      still provides an extremely low probability of undetected errors.      The modulo on unsigned long accumulators can be delayed for 5552      bytes, so the modulo operation time is negligible.  If the bytes      are a, b, c, the second sum is 3a + 2b + c + 3, and so is position      and order sensitive, unlike the first sum, which is just a      checksum.  That 65521 is prime is important to avoid a possible      large class of two-byte errors that leave the check unchanged.      (The Fletcher checksum uses 255, which is not prime and which also      makes the Fletcher check insensitive to single byte changes 0 <->      255.)      The sum s1 is initialized to 1 instead of zero to make the length      of the sequence part of s2, so that the length does not have to be      checked separately. (Any sequence of zeroes has a Fletcher      checksum of zero.)Deutsch & Gailly             Informational                      [Page 9]RFC 1950       ZLIB Compressed Data Format Specification        May 19969. Appendix: Sample code   The following C code computes the Adler-32 checksum of a data buffer.   It is written for clarity, not for speed.  The sample code is in the   ANSI C programming language. Non C users may find it easier to read   with these hints:      &      Bitwise AND operator.      >>     Bitwise right shift operator. When applied to an             unsigned quantity, as here, right shift inserts zero bit(s)             at the left.      <<     Bitwise left shift operator. Left shift inserts zero             bit(s) at the right.      ++     "n++" increments the variable n.      %      modulo operator: a % b is the remainder of a divided by b.      #define BASE 65521 /* largest prime smaller than 65536 */      /*         Update a running Adler-32 checksum with the bytes buf[0..len-1]       and return the updated checksum. The Adler-32 checksum should be       initialized to 1.       Usage example:         unsigned long adler = 1L;         while (read_buffer(buffer, length) != EOF) {           adler = update_adler32(adler, buffer, length);         }         if (adler != original_adler) error();      */      unsigned long update_adler32(unsigned long adler,         unsigned char *buf, int len)      {        unsigned long s1 = adler & 0xffff;        unsigned long s2 = (adler >> 16) & 0xffff;        int n;        for (n = 0; n < len; n++) {          s1 = (s1 + buf[n]) % BASE;          s2 = (s2 + s1)     % BASE;        }        return (s2 << 16) + s1;      }      /* Return the adler32 of the bytes buf[0..len-1] */Deutsch & Gailly             Informational                     [Page 10]RFC 1950       ZLIB Compressed Data Format Specification        May 1996      unsigned long adler32(unsigned char *buf, int len)      {        return update_adler32(1L, buf, len);      }Deutsch & Gailly             Informational                     [Page 11]