Ubuntu Manpage: File::RsyncP::Digest - Perl interface to rsync message digest algorithms

Provided by: libfile-rsyncp-perl_0.70-1ubuntu3_amd64

NAME

       File::RsyncP::Digest - Perl interface to rsync message digest algorithms

SYNOPSIS

           use File::RsyncP::Digest;

           $rsDigest = new File::RsyncP::Digest;

           # specify rsync protocol version (default is <= 26 -> buggy digests).
           $rsDigest->protocol(version);

           # file MD4 digests
           $rsDigest->reset();
           $rsDigest->add(LIST);
           $rsDigest->addfile(HANDLE);

           $digest = $rsDigest->digest();
           $string = $rsDigest->hexdigest();

           # Return 32 byte pair of digests (protocol <= 26 and >= 27).
           $digestPair = $rsDigest->digest2();

           $digest = File::RsyncP::Digest->hash(SCALAR);
           $string = File::RsyncP::Digest->hexhash(SCALAR);

           # block digests
           $digests = $rsDigest->blockDigest($data, $blockSize, $md4DigestLen,
                                             $checksumSeed);

           $digests = $rsDigest->blockDigestUpdate($state, $blockSize,
                                       $blockLastLen, $md4DigestLen, $checksumSeed);

           $digests2 = $rsDigest->blockDigestExtract($digests16, $md4DigestLen);

DESCRIPTION

       The File::RsyncP::Digest module allows you to compute rsync digests, including the RSA
       Data Security Inc. MD4 Message Digest algorithm, and Adler32 checksums from within Perl
       programs.

   Rsync Digests
       Rsync uses two main digests (or checksums), for checking with very high probability that
       the underlying data is identical, without the need to exchange the underlying data.

       The server (remote) side of rsync generates a checksumSeed (usually unix time()) that is
       exchanged during the protocol startup.  This seed is used in both the file and MD4
       checksum calculations.  This causes the block and file checksums to change every time
       Rsync is run.

       File Digest
           This is an MD4 digest of the checksum seed, followed by the entire file's contents.
           This digest is 128 bits long.  The file digest is sent at the end of a file's deltas
           to ensure that the reconstructed file is correct.  This digest is also optionally
           computed and sent as part of the file list if the --checksum option is specified to
           rsync.

       Block digest
           Each file is divided into blocks of default length 700 bytes.  The digest of each
           block is formed by computing the Adler32 checksum of the block, and also the MD4
           digest of the block followed by the checksum seed.  During phase 1, just the first two
           bytes of the MD4 digest are sent, meaning the total digest is 6 bytes or 48 bits (4
           bytes for Adler32 and the first 2 bytes of the MD4 digest).  During phase 2 (which is
           necessary for received files that have an incorrect file digest), the entire MD4
           checksum is used (128 bits) meaning the block digest is 20 bytes or 160 bits.  (Prior
           to rsync protocol XXX, the full 20 byte digest was sent every time and there was only
           a single phase.)

       This module contains routines for computing file and block digests in a manner that is
       identical to rsync.

       Incidentally, rsync contains two bugs in its implementation of MD4 (up to and including
       rsync protocol version 26):

       •   MD4Final() is not called when the data size (ie: file or block size plus 4 bytes for
           the checksum seed) is a multiple of 64.

       •   MD4 is not correct for total data sizes greater than 512MB (2^32 bits).  Rsync's MD4
           only maintains the data size using a 32 bit counter, so it overflows for file sizes
           bigger than 512MB.

       The effects of these bugs are benign: the MD4 digest should not be cryptographically
       weakened and both sides are consistent.

       This module implements both versions of the MD4 digest: the buggy version for protocol
       versions <= 26 and the correct version for protocol versions >= 27.  The default mode is
       the buggy version (protocol versions <= 26).

       You can specify the rsync protocol version to determine which MD4 version is used:

           # specify rsync protocol version (default is <= 26 -> buggy digests).
           $rsDigest->protocol(version);

       Also, you can get both digests in a single call.  The result is returned as a single 32
       byte scalar: the first 16 bytes is the buggy digest and the second 16 bytes is the correct
       digest:

           # Return 32 byte pair of digests (protocol <= 26 and >= 27).
           $digestPair = $rsDigest->digest2();

   Usage
       A new rsync digest context object is created with the new operation.  Multiple
       simultaneous digest contexts can be maintained, if desired.

   Computing Block Digests
       After a context is created, the function to compute block checksums is:

           $digests = $rsDigest->blockDigest($data, $blockSize, $md4DigestLen,
                                             $checksumSeed)

       The first argument is the data, which can contain as much raw data as you wish (ie:
       multiple blocks).  Both the Adler32 checksum and the MD4 checksum are computed for each
       block in data.  The partial end block (if present) is also processed.  The 4 bytes of the
       integer checksumSeed is added at the end of each block digest calculation if it is non-
       zero.  The blockSize is specified in the second argument (default is 700).  The third
       argument, md4DigestLen, specifies how many bytes of the MD4 digest are included in the
       returned data.  Rsync uses a value of 2 for the first pass (meaning 6 bytes of total
       digests are returned per block), and all 16 bytes for the second pass (meaning 20 bytes of
       total digests are returned per block).  The returned number of bytes is the number of
       bytes in each digest (Alder32 + partial/compete MD4) times the number of blocks:

           (4 + md4DigestLen) * ceil(length(data) / blockSize);

       To allow block checksums to be cached (when checksumSeed is unknown), and then quickly
       updated with the known checksumSeed, the checksum data should be first computed with a
       digest length of -1 and a checksumSeed of 0:

           $state = $rsDigest->blockDigest($data, $blockSize, -1, 0);

       The returned $state should be saved for later retrieval, together with the length of the
       last partial block (eg: length($data) % $blockSize).  The length of $state depends upon
       the number of blocks and the block size.  In addition to the 16 bytes of MD4 state, up to
       63 bytes of unprocessed data per block also is saved in $state.  For each block,

           16 + ($blockSize % 64)

       bytes are saved in $state, so $state is most compact when $blockSize is a multiple of 64.
       (The last, partial, block might have a smaller block size, requiring up to 63 bytes of
       state even if $blockSize is a multiple of 64.)

       Once the checksumSeed is known the updated checksums can then be computed using:

           $digests = $rsDigest->blockDigestUpdate($state, $blockSize,
                                       $blockLastLen, $md4DigestLen, $checksumSeed);

       The first argument is the cached checksums from blockDigest.  The third argument is the
       length of the (partial) last block.

       Alternatively, I hope to add a --checksum-seed=n option to rsync that allows the checksum
       seed to be set to 0.  This causes the checksum seed to be omitted from the MD4 calculation
       and it makes caching the checksums much easier.  A zero checksum seed does not weaken the
       block digest.  I'm not sure whether or not it weakens the file digest (the checksum seed
       is applied at the start of the file digest and end of the block digest).  In this case,
       the full 16 byte checksums should be computed using:

           $digests16 = $rsDigest->blockDigest($data, $blockSize, 16, 0);

       and for phase 1 the 2 byte MD4 substrings can be extracted with:

           $digests2  = $rsDigest->blockDigestExtract($digests16, 2);

       The original $digests16 does not need any additional processing for phase 2.

   Computing File Digests
       In addition, functions identical to Digest::MD4 are provided that allow rsync's MD4 file
       digest to be computed.  The checksum seed, if non-zero, is included at the start of the
       data, before the file's contents are added.

       The context is updated with the add operation which adds the strings contained in the LIST
       parameter. Note, however, that "add('foo', 'bar')", "add('foo')" followed by "add('bar')"
       and "add('foobar')" should all give the same result.

       The final MD4 message digest value is returned by the digest operation as a 16-byte binary
       string. This operation delivers the result of add operations since the last new or reset
       operation. Note that the digest operation is effectively a destructive, read-once
       operation. Once it has been performed, the context must be reset before being used to
       calculate another digest value.

       Several convenience functions are also provided. The addfile operation takes an open file-
       handle and reads it until end-of file in 1024 byte blocks adding the contents to the
       context. The file-handle can either be specified by name or passed as a type-glob
       reference, as shown in the examples below. The hexdigest operation calls digest and
       returns the result as a printable string of hexdecimal digits. This is exactly the same
       operation as performed by the unpack operation in the examples below.

       The hash operation can act as either a static member function (ie you invoke it on the MD4
       class as in the synopsis above) or as a normal virtual function. In both cases it performs
       the complete MD4 cycle (reset, add, digest) on the supplied scalar value. This is
       convenient for handling small quantities of data. When invoked on the class a temporary
       context is created. When invoked through an already created context object, this context
       is used. The latter form is slightly more efficient. The hexhash operation is analogous to
       hexdigest.

EXAMPLES

           use File::RsyncP::Digest;

           my $rsDigest = new File::RsyncP::Digest;
           $rsDigest->add('foo', 'bar');
           $rsDigest->add('baz');
           my $digest = $rsDigest->digest();

           print("Rsync MD4 Digest is " . unpack("H*", $digest) . "\n");

       The above example would print out the message

           Rsync MD4 Digest is 6df23dc03f9b54cc38a0fc1483df6e21

       To compute the rsync phase 1 block checksums (4 + 2 = 6 bytes per block) for a 2000 byte
       file containing 700 a's, 700 b's and 600 c's, with a checksum seed of 0x12345678:

           use File::RsyncP::Digest;

           my $rsDigest = new File::RsyncP::Digest;
           my $data = ("a" x 700) . ("b" x 700) . ("c" x 600);
           my $digest = $rsDigest->rsyncChecksum($data, 700, 2, 0x12345678);

           print("Rsync block checksums are " . unpack("H*", $digest) . "\n");

       This will print:

           Rsync block checksums are 3c09a624641bf80b0ce3abd208e8645d5b49

       The same result can be achieved in two steps by saving the state, and then finishing the
       calculation:

           my $state = $rsDigest->blockDigest($data, 700, -1, 0);

           my $digest = $rsDigest->blockDigestUpdate($state, 700,
                                           length($data) % 700, 2, 0x12345678);

       or by computing full-length MD4 digests, and extracting the 2 byte version:

           my $digest16 = $rsDigest->blockDigest($data, 700, 16, 0x12345678);
           my $digest   = $rsDigest->blockDigestExtract($digest16, 2);

LICENSE

This program is free software; you can redistribute it and/or modify it under the terms of
the GNU General Public License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License in the LICENSE file
along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple
Place, Suite 330, Boston, MA 02111-1307 USA.

The MD4 algorithm is defined in RFC1320. The basic C code implementing the algorithm is
derived from that in the RFC and is covered by the following copyright:

License to copy and use this software is granted provided that it
is identified as the "RSA Data Security, Inc. MD4 Message-Digest
Algorithm" in all material mentioning or referencing this software
or this function.

License is also granted to make and use derivative works provided
that such works are identified as "derived from the RSA Data
Security, Inc. MD4 Message-Digest Algorithm" in all material
mentioning or referencing the derived work.

RSA Data Security, Inc. makes no representations concerning either
the merchantability of this software or the suitability of this
software for any particular purpose. It is provided "as is"
without express or implied warranty of any kind.

These notices must be retained in any copies of any part of this
documentation and/or software.

This copyright does not prohibit distribution of any version of Perl containing this
extension under the terms of the GNU or Artistic licences.

AUTHOR