makedes.html
来自「java类库详细讲解」· HTML 代码 · 共 195 行
HTML
195 行
<HTML>
<HEAD>
<META http-equiv="Content-Type" content="text/html; charset=UTF-8">
<TITLE>Converting a 56-bit Value to a DES Key
(Java Developers Almanac Example)
</TITLE>
<META CONTENT="Patrick Chan" NAME="AUTHOR">
<META CONTENT="Code Examples from The Java Developers Almanac 1.4" NAME="DESCRIPTION">
<META CONTENT="Addison-Wesley/Patrick Chan" NAME="OWNER">
<META CONTENT="3/20/02" NAME="revision">
<STYLE TYPE="text/css">
<!-- BODY CODE {font-family: Courier, Monospace; font-size: 11pt} TABLE, BODY {font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 10pt} PRE {font-family: Courier, Monospace; font-size: 10pt} H3 {font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 11pt} A.eglink {text-decoration: none} A:hover.eglink {text-decoration: underline} -->
</STYLE>
</HEAD>
<BODY>
<TABLE CELLSPACING="0" CELLPADDING="0" BORDER="0">
<TR>
<TD rowspan="3"><A HREF="/?l=ex"><IMG BORDER="0" ALIGN="BOTTOM" HSPACE="10" SRC="/egs/almanac14a.jpg"></A></TD><TD VALIGN="top"><font face="Times" size="6"><b>The Java Developers Almanac 1.4</b></font>
<br>
Order this book from <a href="/cgi-bin/scripts/redirect.pl?l=ex&url=http://www.amazon.com/exec/obidos/ASIN/0201752808/xeo">Amazon</a>.
</TD>
</TR>
<TR>
<TD align="right" valign="bottom">
<FORM method="get" action="/cgi-bin/search/find.pl">
<INPUT size="25" name="words" type="text"><INPUT value="Search" type="submit">
</FORM>
</TD>
</TR>
</TABLE>
<HR color="#6666cc">
<DIV ALIGN="LEFT">
<A HREF="/">Home</A>
>
<A HREF="../index.html">List of Packages</A>
>
<B><A HREF="../javax.crypto/pkg.html">javax.crypto</A></B><font color="#666666" SIZE="-2">
[14 examples]
</font>
>
<B><A HREF="../javax.crypto/pkg.html#Encrypting%20and%20Decrypting">Encrypting and Decrypting</A></B><font color="#666666" SIZE="-2">
[6 examples]
</font>
</DIV><P>
<h3>
e465.
Converting a 56-bit Value to a DES Key</h3>
The DES encrypter/decrypter requires a 64-bit key where only 56-bit
are significant. The other 8-bit are parity bits used to ensure that
the key has not been corrupted. To make the 64-bit key, the 56-bit
value is broken up into 7-bit chunks. Each 7-bit chunk is moved into
an 8-bit slot taking up the most significant bit positions. The least
significant bit (the parity bit) is set to either 1 or 0 in order to
make the quantity of 1 bits in the byte an odd number.
<P> This example implements a method to convert a 56-bit value into
a valid DES key. Such a method could be used to convert a 7-character
string password to a valid DES key.
<P> See also <a href="../javax.crypto/DesString.html" class="eglink"><font size="-1"><b>e462</b> Encrypting a String with DES</font></a>.
<pre>
// Takes a 7-byte quantity and returns a valid 8-byte DES key.
// The input and output bytes are big-endian, where the most significant
// byte is in element 0.
public static byte[] addParity(byte[] in) {
byte[] result = new byte[8];
// Keeps track of the bit position in the result
int resultIx = 1;
// Used to keep track of the number of 1 bits in each 7-bit chunk
int bitCount = 0;
// Process each of the 56 bits
for (int i=0; i<56; i++) {
// Get the bit at bit position i
boolean bit = (in[6-i/8]&(1<<(i%8))) > 0;
// If set, set the corresponding bit in the result
if (bit) {
result[7-resultIx/8] |= (1<<(resultIx%8))&0xFF;
bitCount++;
}
// Set the parity bit after every 7 bits
if ((i+1) % 7 == 0) {
if (bitCount % 2 == 0) {
// Set low-order bit (parity bit) if bit count is even
result[7-resultIx/8] |= 1;
}
resultIx++;
bitCount = 0;
}
resultIx++;
}
return result;
}
</pre>
<pre>
// Get the 56-bit value
byte[] raw = new byte[]{<font color="#0066ff"><i>0x01, 0x72, 0x43, 0x3E, 0x1C, 0x7A, 0x55</i></font>};
byte[] keyBytes = addParity(raw);
// You can check that the parity has been set properly
try {
boolean b = DESKeySpec.isParityAdjusted(keyBytes, 0);
} catch (java.security.InvalidKeyException e) {
// The DES is invalid
}
// Convert the bytes into a SecretKey suitable for use by Cipher
SecretKey key = new SecretKeySpec(keyBytes, "DES");
</pre>
<P><table width="600" CELLSPACING="0" CELLPADDING="2" BORDER="0">
<tr>
<td bgcolor="#6666cc" align="center"><font color="#ffffff">
Related Examples
</font></td>
</tr>
</table>
e461. <a class="eglink" href="ListCipher.html?l=rel">
Listing All Available Encryption and Decryption Algorithms
</a>
<br>
e462. <a class="eglink" href="DesString.html?l=rel">
Encrypting a String with DES
</a>
<br>
e463. <a class="eglink" href="DesFile.html?l=rel">
Encrypting a File or Stream with DES
</a>
<br>
e464. <a class="eglink" href="PassKey.html?l=rel">
Encrypting with DES Using a Pass Phrase
</a>
<br>
e466. <a class="eglink" href="EncryptObject.html?l=rel">
Encrypting an Object with DES
</a>
<br>
<table width="600" CELLSPACING="0" CELLPADDING="2" BORDER="0">
<tr>
<td align="left">
<br>
See also:
<a class="eglink" href="/egs/javax.crypto/pkg.html?l=rel#Key%20Agreement">
Key Agreement
</a>
<a class="eglink" href="/egs/javax.crypto/pkg.html?l=rel#MAC">
MAC
</a>
<a class="eglink" href="/egs/javax.crypto/pkg.html?l=rel#Symmetric%20Keys">
Symmetric Keys
</a>
</td>
</tr>
</table>
<br>
<br>
<FONT FACE="Verdana, Arial, Helvetica, sans-serif" SIZE="0">
© 2002 Addison-Wesley.
</FONT>
</BODY>
</HTML>
⌨️ 快捷键说明
复制代码Ctrl + C
搜索代码Ctrl + F
全屏模式F11
增大字号Ctrl + =
减小字号Ctrl + -
显示快捷键?