1Digest(3) User Contributed Perl Documentation Digest(3)
2
6 File::RsyncP::Digest - Perl interface to rsync message digest
7 algorithms
8
10 use File::RsyncP::Digest;
11 $rsDigest = new File::RsyncP::Digest;
13 # specify rsync protocol version (default is <= 26 -> buggy digests).
15 $rsDigest->protocol(version);
16 # file MD4 digests
18 $rsDigest->reset();
19 $rsDigest->add(LIST);
20 $rsDigest->addfile(HANDLE);
21 $digest = $rsDigest->digest();
23 $string = $rsDigest->hexdigest();
24 # Return 32 byte pair of digests (protocol <= 26 and >= 27).
26 $digestPair = $rsDigest->digest2();
27 $digest = File::RsyncP::Digest->hash(SCALAR);
29 $string = File::RsyncP::Digest->hexhash(SCALAR);
30 # block digests
32 $digests = $rsDigest->blockDigest($data, $blockSize, $md4DigestLen,
33 $checksumSeed);
34 $digests = $rsDigest->blockDigestUpdate($state, $blockSize,
36 $blockLastLen, $md4DigestLen, $checksumSeed);
37 $digests2 = $rsDigest->blockDigestExtract($digests16, $md4DigestLen);
39
41 The File::RsyncP::Digest module allows you to compute rsync digests,
42 including the RSA Data Security Inc. MD4 Message Digest algorithm, and
43 Adler32 checksums from within Perl programs.
44 Rsync Digests
46 Rsync uses two main digests (or checksums), for checking with very high
47 probability that the underlying data is identical, without the need to
48 exchange the underlying data.
49 The server (remote) side of rsync generates a checksumSeed (usually
51 unix time()) that is exchanged during the protocol startup. This seed
52 is used in both the file and MD4 checksum calculations. This causes
53 the block and file checksums to change every time Rsync is run.
54 File Digest
56 This is an MD4 digest of the checksum seed, followed by the entire
57 file's contents. This digest is 128 bits long. The file digest is
58 sent at the end of a file's deltas to ensure that the reconstructed
59 file is correct. This digest is also optionally computed and sent
60 as part of the file list if the --checksum option is specified to
61 rsync.
62 Block digest
64 Each file is divided into blocks of default length 700 bytes. The
65 digest of each block is formed by computing the Adler32 checksum of
66 the block, and also the MD4 digest of the block followed by the
67 checksum seed. During phase 1, just the first two bytes of the MD4
68 digest are sent, meaning the total digest is 6 bytes or 48 bits (4
69 bytes for Adler32 and the first 2 bytes of the MD4 digest). During
70 phase 2 (which is necessary for received files that have an
71 incorrect file digest), the entire MD4 checksum is used (128 bits)
72 meaning the block digest is 20 bytes or 160 bits. (Prior to rsync
73 protocol XXX, the full 20 byte digest was sent every time and there
74 was only a single phase.)
75 This module contains routines for computing file and block digests in a
77 manner that is identical to rsync.
78 Incidentally, rsync contains two bugs in its implementation of MD4 (up
80 to and including rsync protocol version 26):
81 • MD4Final() is not called when the data size (ie: file or block size
83 plus 4 bytes for the checksum seed) is a multiple of 64.
84 • MD4 is not correct for total data sizes greater than 512MB (2^32
86 bits). Rsync's MD4 only maintains the data size using a 32 bit
87 counter, so it overflows for file sizes bigger than 512MB.
88 The effects of these bugs are benign: the MD4 digest should not be
90 cryptographically weakened and both sides are consistent.
91 This module implements both versions of the MD4 digest: the buggy
93 version for protocol versions <= 26 and the correct version for
94 protocol versions >= 27. The default mode is the buggy version
95 (protocol versions <= 26).
96 You can specify the rsync protocol version to determine which MD4
98 version is used:
99 # specify rsync protocol version (default is <= 26 -> buggy digests).
101 $rsDigest->protocol(version);
102 Also, you can get both digests in a single call. The result is
104 returned as a single 32 byte scalar: the first 16 bytes is the buggy
105 digest and the second 16 bytes is the correct digest:
106 # Return 32 byte pair of digests (protocol <= 26 and >= 27).
108 $digestPair = $rsDigest->digest2();
109 Usage
111 A new rsync digest context object is created with the new operation.
112 Multiple simultaneous digest contexts can be maintained, if desired.
113 Computing Block Digests
115 After a context is created, the function to compute block checksums is:
116 $digests = $rsDigest->blockDigest($data, $blockSize, $md4DigestLen,
118 $checksumSeed)
119 The first argument is the data, which can contain as much raw data as
121 you wish (ie: multiple blocks). Both the Adler32 checksum and the MD4
122 checksum are computed for each block in data. The partial end block
123 (if present) is also processed. The 4 bytes of the integer
124 checksumSeed is added at the end of each block digest calculation if it
125 is non-zero. The blockSize is specified in the second argument
126 (default is 700). The third argument, md4DigestLen, specifies how many
127 bytes of the MD4 digest are included in the returned data. Rsync uses
128 a value of 2 for the first pass (meaning 6 bytes of total digests are
129 returned per block), and all 16 bytes for the second pass (meaning 20
130 bytes of total digests are returned per block). The returned number of
131 bytes is the number of bytes in each digest (Alder32 + partial/compete
132 MD4) times the number of blocks:
133 (4 + md4DigestLen) * ceil(length(data) / blockSize);
135 To allow block checksums to be cached (when checksumSeed is unknown),
137 and then quickly updated with the known checksumSeed, the checksum data
138 should be first computed with a digest length of -1 and a checksumSeed
139 of 0:
140 $state = $rsDigest->blockDigest($data, $blockSize, -1, 0);
142 The returned $state should be saved for later retrieval, together with
144 the length of the last partial block (eg: length($data) % $blockSize).
145 The length of $state depends upon the number of blocks and the block
146 size. In addition to the 16 bytes of MD4 state, up to 63 bytes of
147 unprocessed data per block also is saved in $state. For each block,
148 16 + ($blockSize % 64)
150 bytes are saved in $state, so $state is most compact when $blockSize is
152 a multiple of 64. (The last, partial, block might have a smaller block
153 size, requiring up to 63 bytes of state even if $blockSize is a
154 multiple of 64.)
155 Once the checksumSeed is known the updated checksums can then be
157 computed using:
158 $digests = $rsDigest->blockDigestUpdate($state, $blockSize,
160 $blockLastLen, $md4DigestLen, $checksumSeed);
161 The first argument is the cached checksums from blockDigest. The third
163 argument is the length of the (partial) last block.
164 Alternatively, I hope to add a --checksum-seed=n option to rsync that
166 allows the checksum seed to be set to 0. This causes the checksum seed
167 to be omitted from the MD4 calculation and it makes caching the
168 checksums much easier. A zero checksum seed does not weaken the block
169 digest. I'm not sure whether or not it weakens the file digest (the
170 checksum seed is applied at the start of the file digest and end of the
171 block digest). In this case, the full 16 byte checksums should be
172 computed using:
173 $digests16 = $rsDigest->blockDigest($data, $blockSize, 16, 0);
175 and for phase 1 the 2 byte MD4 substrings can be extracted with:
177 $digests2 = $rsDigest->blockDigestExtract($digests16, 2);
179 The original $digests16 does not need any additional processing for
181 phase 2.
182 Computing File Digests
184 In addition, functions identical to Digest::MD4 are provided that allow
185 rsync's MD4 file digest to be computed. The checksum seed, if non-
186 zero, is included at the start of the data, before the file's contents
187 are added.
188 The context is updated with the add operation which adds the strings
190 contained in the LIST parameter. Note, however, that "add('foo',
191 'bar')", "add('foo')" followed by "add('bar')" and "add('foobar')"
192 should all give the same result.
193 The final MD4 message digest value is returned by the digest operation
195 as a 16-byte binary string. This operation delivers the result of add
196 operations since the last new or reset operation. Note that the digest
197 operation is effectively a destructive, read-once operation. Once it
198 has been performed, the context must be reset before being used to
199 calculate another digest value.
200 Several convenience functions are also provided. The addfile operation
202 takes an open file-handle and reads it until end-of file in 1024 byte
203 blocks adding the contents to the context. The file-handle can either
204 be specified by name or passed as a type-glob reference, as shown in
205 the examples below. The hexdigest operation calls digest and returns
206 the result as a printable string of hexdecimal digits. This is exactly
207 the same operation as performed by the unpack operation in the examples
208 below.
209 The hash operation can act as either a static member function (ie you
211 invoke it on the MD4 class as in the synopsis above) or as a normal
212 virtual function. In both cases it performs the complete MD4 cycle
213 (reset, add, digest) on the supplied scalar value. This is convenient
214 for handling small quantities of data. When invoked on the class a
215 temporary context is created. When invoked through an already created
216 context object, this context is used. The latter form is slightly more
217 efficient. The hexhash operation is analogous to hexdigest.
218
220 use File::RsyncP::Digest;
221 my $rsDigest = new File::RsyncP::Digest;
223 $rsDigest->add('foo', 'bar');
224 $rsDigest->add('baz');
225 my $digest = $rsDigest->digest();
226 print("Rsync MD4 Digest is " . unpack("H*", $digest) . "\n");
228 The above example would print out the message
230 Rsync MD4 Digest is 6df23dc03f9b54cc38a0fc1483df6e21
232 To compute the rsync phase 1 block checksums (4 + 2 = 6 bytes per
234 block) for a 2000 byte file containing 700 a's, 700 b's and 600 c's,
235 with a checksum seed of 0x12345678:
236 use File::RsyncP::Digest;
238 my $rsDigest = new File::RsyncP::Digest;
240 my $data = ("a" x 700) . ("b" x 700) . ("c" x 600);
241 my $digest = $rsDigest->rsyncChecksum($data, 700, 2, 0x12345678);
242 print("Rsync block checksums are " . unpack("H*", $digest) . "\n");
244 This will print:
246 Rsync block checksums are 3c09a624641bf80b0ce3abd208e8645d5b49
248 The same result can be achieved in two steps by saving the state, and
250 then finishing the calculation:
251 my $state = $rsDigest->blockDigest($data, 700, -1, 0);
253 my $digest = $rsDigest->blockDigestUpdate($state, 700,
255 length($data) % 700, 2, 0x12345678);
256 or by computing full-length MD4 digests, and extracting the 2 byte
258 version:
259 my $digest16 = $rsDigest->blockDigest($data, 700, 16, 0x12345678);
261 my $digest = $rsDigest->blockDigestExtract($digest16, 2);
262
264 This program is free software: you can redistribute it and/or modify it
265 under the terms of the GNU General Public License as published by the
266 Free Software Foundation, either version 3 of the License, or (at your
267 option) any later version.
268 This program is distributed in the hope that it will be useful, but
270 WITHOUT ANY WARRANTY; without even the implied warranty of
271 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
272 General Public License for more details.
273 You should have received a copy of the GNU General Public License along
275 with this program. If not, see <http://www.gnu.org/licenses/>.
276 The MD4 algorithm is defined in RFC1320. The basic C code implementing
278 the algorithm is derived from that in the RFC and is covered by the
279 following copyright:
280 MD4 is Copyright (C) 1990-2, RSA Data Security, Inc. All rights
282 reserved.
283 License to copy and use this software is granted provided that it
285 is identified as the "RSA Data Security, Inc. MD4 Message-Digest
286 Algorithm" in all material mentioning or referencing this software
287 or this function.
288 License is also granted to make and use derivative works provided
290 that such works are identified as "derived from the RSA Data
291 Security, Inc. MD4 Message-Digest Algorithm" in all material
292 mentioning or referencing the derived work.
293 RSA Data Security, Inc. makes no representations concerning either
295 the merchantability of this software or the suitability of this
296 software for any particular purpose. It is provided "as is"
297 without express or implied warranty of any kind.
298 These notices must be retained in any copies of any part of this
300 documentation and/or software.
301 This copyright does not prohibit distribution of any version of Perl
303 containing this extension under the terms of the GNU or Artistic
304 licences.
305
307 File::RsyncP::Digest was written by Craig Barratt
308 <cbarratt@users.sourceforge.net> based on Digest::MD4 and the Adler32
309 implementation was based on rsync 2.5.5.
310 Digest::MD4 was adapted by Mike McCauley ("mikem@open.com.au"), based
312 entirely on MD5-1.7, written by Neil Winton
313 ("N.Winton@axion.bt.co.uk").
314 Rsync was written by Andrew Tridgell <tridge@samba.org> and Paul
316 Mackerras. It is available under a GPL license. See
317 <http://rsync.samba.org>.
318
320 See <http://perlrsync.sourceforge.net> for File::RsyncP's SourceForge
321 home page.
322 See File::RsyncP, File::RsyncP::FileIO and File::RsyncP::FileList.
324perl v5.36.0 2022-07-22 Digest(3)