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5 <TITLE>7z Format</TITLE>
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13 <P><B>7z</B> is a new archive format, providing a high compression ratio.</P>
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15 <P>The main features of the <B>7z</B> format:</P>
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17 <LI>Open architecture
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18 <LI>High compression ratio
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19 <LI>Strong AES-256 encryption
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20 <LI>Ability to use any compression, conversion or encryption method
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21 <LI>Supports files with sizes up to 16000000000 GB
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22 <LI>Unicode file names
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23 <LI>Solid compression
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24 <LI>Archive headers compression
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27 <P><B>7z</B> has an open architecture, so it can support any new compression methods.</P>
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28 The following methods currently are integrated into <B>7z</B>:<P>
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30 <TABLE cellspacing ="2" cellpadding ="4">
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31 <TR> <TH class="Title" width="60">Method</TH> <TH class="Title">Description</TH> </TR>
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32 <TR> <TD class="Item">LZMA</TD> <TD class="Item">Improved and optimized version of LZ77 algorithm</TD></TR>
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33 <TR> <TD class="Item">PPMD</TD> <TD class="Item">Dmitry Shkarin's PPMdH with small changes</TD></TR>
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34 <TR> <TD class="Item">BCJ</TD> <TD class="Item">Converter for 32-bit x86 executables</TD></TR>
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35 <TR> <TD class="Item">BCJ2</TD> <TD class="Item">Converter for 32-bit x86 executables</TD></TR>
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36 <TR> <TD class="Item">BZip2</TD> <TD class="Item">Standard BWT algorithm</TD></TR>
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37 <TR> <TD class="Item">Deflate</TD> <TD class="Item">Standard LZ77-based algorithm</TD></TR>
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40 <P><B>LZMA</B> is the default and general compression method of <B>7z</B> format.
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41 The main features of the <B>LZMA</B> method:</P>
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43 <LI>High compression ratio
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44 <LI>Variable dictionary size (up to 4 GB)
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45 <LI>Compression speed: about 1 MB/s on 2 GHz CPU
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46 <LI>Decompression speed: about 10-20 MB/s on 2 GHz CPU
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47 <LI>Small memory requirement for decompression (depends from dictionary size)
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48 <LI>Small code size for decompression: about 5 KB
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49 <LI>Supports multi-threading and P4's hyper-threading
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52 <P>The <B>LZMA</B> compression algorithm is very suitable for embedded applications.
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53 If you want to use <B>LZMA</B> code, you can ask for consultation, custom code programming,
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54 and required developer licenses at
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55 <P><A href="http://www.7-zip.org/support.html" target="_blank">www.7-zip.org/support.html</A></P>
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58 <P>7-Zip also supports encryption with the AES-256 algorithm.
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59 This algorithm uses a cipher key with length of 256 bits. To create the key, 7-Zip
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60 uses a derivation function based on an SHA-256 hash algorithm.
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61 A key derivation function produces a derived key from a text password defined by the user.
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62 To increase the cost of an exhaustive search for passwords, 7-Zip uses a big number
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63 of iterations to produce the cipher key from the text password.</P>
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65 <H2>Tips for selecting password length</H2>
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67 <P>Here is an estimate of the time required for an exhaustive
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68 password search attack, when the password is a random
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69 sequence of lowercase Latin letters.</P>
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71 <P>We suppose that one user can check 10 passwords per second and an
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72 organization with a budget of about $1 billion can check 10 billion
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73 passwords per second. We also
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74 suppose that the processor in use doubles its performance every two years;
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75 so, each additional Latin letter of a long password adds about
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76 9 years to an exhaustive key search attack.</P>
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78 <P>The result is this estimate of the time to succeed in an attack:</P>
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82 <TH>Password Length</TH>
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83 <TH>Single User Attack</TH>
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84 <TH>Organization Attack</TH>
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