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Java > Open Source Codes > de > schlichtherle > io > archive > zip > raes > SafeZip32RaesDriver


1 /*
2  * Copyright 2006 Schlichtherle IT Services
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  * http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16
17 package de.schlichtherle.io.archive.zip.raes;
18
19 import de.schlichtherle.io.archive.zip.*;
20 import de.schlichtherle.io.rof.*;
21
22 import java.io.*;
23 import java.util.zip.*;
24
25 /**
26  * A safe archive driver which builds RAES encrypted ZIP32 archives.
27  * For input archive files up to 512 KB, the cipher text gets authenticated
28  * using the RAES provided Message Authentication Code (MAC) <em>before</em>
29  * the archive can be accessed by a client application.
30  * For larger input archive files, the MAC is not used, but instead the
31  * CRC-32 value of the decrypted and deflated archive entries is checked
32  * when the archive entry stream is <em>closed</em> by the client application,
33  * resulting in some {@link IOException}.
34  * <p>
35  * This operation mode is considered to be safe:
36  * Although a formal prove is missing, it should be computationally
37  * infeasible to modify an archive file so that <em>after</em> decryption
38  * of the archive and <em>after</em> inflation (decompression) of an
39  * entry's data its CRC-32 value still matches!
40  * This should hold true even though CRC-32 is not at all a good cryptographic
41  * hash function because of its frequent collisions, its linear output and
42  * small output size.
43  * It's the ZIP inflation algorithm which actually comes to our rescue!
44  * <p>
45  * Instances of this class are immutable.
46  *
47  * @see UnsafeZip32RaesDriver
48  * @see ParanoidZip32RaesDriver
49  * @author Christian Schlichtherle
50  * @version @version@
51  * @since TrueZIP 6.0
52  */
53 public class SafeZip32RaesDriver extends AbstractZip32RaesDriver {
54
55     /**
56      * The default trigger for authentication.
57      * Input archive files smaller than or equal to this size in bytes
58      * get authenticated using the RAES Message Authentication Code (MAC)
59      * before they are accessed.
60      * This is 512 KB.
61      */
62     private static final long AUTHENTICATION_TRIGGER = 512 * 1024;
63
64     public SafeZip32RaesDriver() {
65         super(AUTHENTICATION_TRIGGER);
66     }
67
68     protected Zip32InputArchive createZip32InputArchive(
69             ReadOnlyFile rof,
70             String JavaDoc encoding,
71             boolean preambled,
72             boolean postambled)
73     throws NullPointerException JavaDoc,
74             UnsupportedEncodingException,
75             FileNotFoundException,
76             ZipException,
77             IOException {
78         // Optimization: If the read-only file is smaller than the
79 // authentication trigger, then its entire cipher text has already
80 // been authenticated by
81 // {@link AbstractZip32RaesDriver#createInputArchive}.
82 // Hence, checking the CRC-32 value of the plain text ZIP32 file is
83 // redundant.
84 return rof.length() > getAuthenticationTrigger()
85                 ? new CheckedZip32InputArchive(rof, encoding, preambled, postambled)
86                 : new Zip32InputArchive(rof, encoding, preambled, postambled);
87     }
88
89     protected Zip32OutputArchive createZip32OutputArchive(
90             final OutputStream out,
91             final String JavaDoc encoding,
92             final Zip32InputArchive source)
93     throws NullPointerException JavaDoc,
94             UnsupportedEncodingException,
95             IOException {
96         return new Zip32OutputArchive(out, encoding, source);
97     }
98 }
99
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