encryption.mht

一种使用动态公匙加密方式的加密算法

            does not use it.</P>
            <P>After the key is imported or generated, it is now time to =
perform=20
            the encryption or decryption. The encryption and decryption =
depends=20
            on a session key, which is based on the key we just imported =
or=20
            generated previously.</P><PRE><SPAN =
class=3Dcpp-keyword>void</SPAN> Encrypt()
{
    <SPAN class=3Dcpp-keyword>unsigned</SPAN> <SPAN =
class=3Dcpp-keyword>long</SPAN> length =3D m_clear.GetLength() +<SPAN =
class=3Dcpp-literal>1</SPAN>;
    <SPAN class=3Dcpp-keyword>unsigned</SPAN> <SPAN =
class=3Dcpp-keyword>char</SPAN> * cipherBlock =3D (<SPAN =
class=3Dcpp-keyword>unsigned</SPAN> <SPAN =
class=3Dcpp-keyword>char</SPAN>*)malloc(length);
    memset(cipherBlock, <SPAN class=3Dcpp-literal>0</SPAN>, length);
    memcpy(cipherBlock, m_clear, length -<SPAN =
class=3Dcpp-literal>1</SPAN>);&nbsp;

    <SPAN class=3Dcpp-keyword>if</SPAN> (!CryptEncrypt(hSessionKey, =
<SPAN class=3Dcpp-literal>0</SPAN>, TRUE, <SPAN =
class=3Dcpp-literal>0</SPAN>, cipherBlock, &amp;length, length))
    {
        dwResult =3D GetLastError();
        MessageBox(<SPAN class=3Dcpp-string>"Error CryptEncrypt() =
failed."</SPAN>, <SPAN class=3Dcpp-string>"Information"</SPAN>, MB_OK);
        <SPAN class=3Dcpp-keyword>return</SPAN>;
    }

    m_cipher =3D cipherBlock;
    m_clear =3D <SPAN class=3Dcpp-string>""</SPAN>;
    free(cipherBlock);
}</PRE>
            <P>The decryption does not vary much from the encryption. =
Like with=20
            the encryption it is done with just one invocation and the =
cipher=20
            text is then decrypted.</P><PRE><SPAN =
class=3Dcpp-keyword>void</SPAN> Decrypt()
{
&nbsp;   <SPAN class=3Dcpp-keyword>unsigned</SPAN> <SPAN =
class=3Dcpp-keyword>long</SPAN> length =3D m_cipher.GetLength() +<SPAN =
class=3Dcpp-literal>1</SPAN>;
    <SPAN class=3Dcpp-keyword>unsigned</SPAN> <SPAN =
class=3Dcpp-keyword>char</SPAN> * cipherBlock =3D (<SPAN =
class=3Dcpp-keyword>unsigned</SPAN> <SPAN =
class=3Dcpp-keyword>char</SPAN>*)malloc(length);
&nbsp;   memset(cipherBlock, <SPAN class=3Dcpp-literal>0</SPAN>, =
length);
&nbsp;   memcpy(cipherBlock, m_cipher, length -<SPAN =
class=3Dcpp-literal>1</SPAN>);&nbsp;

&nbsp;   <SPAN class=3Dcpp-keyword>if</SPAN> (!CryptDecrypt(hSessionKey, =
<SPAN class=3Dcpp-literal>0</SPAN>, TRUE, <SPAN =
class=3Dcpp-literal>0</SPAN>, cipherBlock, &amp;length))
&nbsp;   {
        dwResult =3D GetLastError();
        MessageBox(<SPAN class=3Dcpp-string>"Error CryptDecrypt() =
failed."</SPAN>,=20
                            <SPAN =
class=3Dcpp-string>"Information"</SPAN>, MB_OK);
&nbsp;&nbsp;      <SPAN class=3Dcpp-keyword>return</SPAN>;
&nbsp;   }

   &nbsp;m_clear =3D cipherBlock;
&nbsp;   m_cipher =3D <SPAN class=3Dcpp-string>""</SPAN>;

   &nbsp;free(cipherBlock);
}</PRE>
            <H2>Points of interest</H2>
            <P>The clear text / data is typically a string containing =
any text.=20
            Before performing the encryption it is a good idea to copy =
the data=20
            into an unsigned char pointer to avoid future problems with =
casts=20
            when the encryption is performed.</P>
            <P>Finally, when using the Win32 Crypto API, please notice =
that it=20
            might not necessary compile as is. This is because a =
pre-processor=20
            definition is missing in the project.</P><PRE><SPAN =
class=3Dcpp-preprocessor>#define _WIN32_WINNT 0x0400</SPAN></PRE>
            <P>When defining the above globally to the project, it will =
be=20
            possible to compile code that is using the Crypto API. My =
suggestion=20
            is to place the definition in the <I>StdAfx.h</I> =
file.</P><!-- Article Ends --></DIV></SPAN>
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