此代码是一个 BCB 的单元，非常简单，提供了四个函数， 要改成 Delphi 或其它 C/C++ 也很容易，有需要的自已改吧。此代码经过测试，结果正确。 

    下面是头文件： 

//---------------------------------------------------------------------------
//  MIME & BASE64 Encode/Decode unit. (H)
//  Copyright (c) 2000 Mental Studio - http://mentals.126.com
//  Author : Raptor - raptorz@126.com
//---------------------------------------------------------------------------
#ifndef mimeb64H
#define mimeb64H
//---------------------------------------------------------------------------

AnsiString MimeEncode( AnsiString s );
AnsiString MimeDecode( AnsiString s );
AnsiString Base64Encode( AnsiString s );
AnsiString Base64Decode( AnsiString s );
//---------------------------------------------------------------------------
#endif
    

    下面是 CPP 文件： 

//---------------------------------------------------------------------------
//  MIME & BASE64 Encode/Decode unit. (CPP)
//  Copyright (c) 2000 Mental Studio - http://mentals.126.com
//  Author : Raptor - raptorz@126.com
//---------------------------------------------------------------------------

#include 
#pragma hdrstop

#include "mimeb64.h"

//---------------------------------------------------------------------------
#pragma package(smart_init)
//---------------------------------------------------------------------------
//  4bit binary to char 0-F
char Hex2Chr( Byte n )
{
    n &= 0xF;
    if ( n < 10 )
        return ( char )( n + '0' );
    else
        return ( char )( n - 10 + 'A' );
}
//---------------------------------------------------------------------------
//  char 0-F to 4bit binary
Byte Chr2Hex( char c )
{
    c = toupper( c );
    if ( c >= '0' && c <= '9' )
        return ( int )( c - '0' );
    else if ( c >= 'A' && c <= 'F' )
        return ( int )( c - 'A' + 10 );
    else
        return -1;
}
//---------------------------------------------------------------------------
//  Base64 code table
//  0-63 : A-Z(25) a-z(51), 0-9(61), +(62), /(63)
char __fastcall Base2Chr( Byte n )
{
    n &= 0x3F;
    if ( n < 26 )
        return ( char )( n + 'A' );
    else if ( n < 52 )
        return ( char )( n - 26 + 'a' );
    else if ( n < 62 )
        return ( char )( n - 52 + '0' );
    else if ( n == 62 )
        return '+';
    else
        return '/';
}
//---------------------------------------------------------------------------
Byte Chr2Base( char c )
{
    if ( c >= 'A' && c <= 'Z' )
        return ( Byte )( c - 'A' );
    else if ( c >= 'a' && c <= 'z' )
        return ( Byte )( c - 'a' + 26 );
    else if ( c >= '0' && c <= '9' )
        return ( Byte )( c - '0' + 52 );
    else if ( c == '+' )
        return 62;
    else
        return 63;
}
//---------------------------------------------------------------------------
//
AnsiString MimeEncode( AnsiString s )
{
    AnsiString asTemp = "";
    int i = 1, n = s.Length( );

    while ( i <= n )
    {
        asTemp += '=';
        asTemp += Hex2Chr( ( Byte )( s[i] >> 4 ) );
        asTemp += Hex2Chr( ( Byte )( s[i] ) );
        i++;
    }
    return asTemp;
}
//---------------------------------------------------------------------------
//
AnsiString MimeDecode( AnsiString s )
{
    AnsiString asTemp = "";
    int i = 1, n = s.Length( );
    Byte ch, cl;    //  因为有些软件(e.g.Foxmail)对一些 < 0x7f 的字符不编码，此函数作了一些修改

    while ( i <= n )
    {
        if ( ( s[i] == '=' ) && ( i + 2 <= n ) )
        {
            ch = Chr2Hex( s[i + 1] );
            cl = Chr2Hex( s[i + 2] );
            if ( ( ch == ( Byte )-1 ) || ( cl == ( Byte )-1 ) )
                asTemp += s[i]; // 未编码的'='
            else
            {
                asTemp += ( char )( ( ch << 4 ) | cl );
                i += 2;
            }
        }
        else
            asTemp += s[i]; // 未编码的字符
        i++;
    }
    return asTemp;
}
//---------------------------------------------------------------------------
//
AnsiString Base64Encode( AnsiString s )
{
    int n = s.Length( );
    Byte c, t;
    AnsiString asTemp = "";

    for ( int i = 1; i <= n; i++ )
    {
        c = s[i];
        if ( i % 3 == 1 )
        {
            asTemp += Base2Chr( c >> 2 );
            t = ( c << 4 ) & 0x3F;
        }
        else if ( i % 3 == 2 )
        {
            asTemp += Base2Chr( t | ( c >> 4 ) );
            t = ( c << 2 ) & 0x3F;
        }
        else
        {
            asTemp += Base2Chr( t | ( c >> 6 ) );
            asTemp += Base2Chr( c );
        }
    }
    if ( n % 3 != 0 )
    {
        asTemp += Base2Chr( t );
        if ( n % 3 == 1 )
            asTemp += "==";
        else
            asTemp += "=";
    }
    return asTemp;
}
//---------------------------------------------------------------------------
//
AnsiString Base64Decode( AnsiString s )
{
    int n = s.Length( );
    Byte c, t;
    AnsiString asTemp = "";

    for ( int i = 1; i <= n; i++ )
    {
        if ( s[i] == '=' )
            break;
        c = Chr2Base( s[i] );
        if ( i % 4 == 1 )
            t = c << 2;
        else if ( i % 4 == 2 )
        {
            asTemp += ( char )( t | ( c >> 4 ) );
            t = ( Byte )( c << 4 );
        }
        else if ( i % 4 == 3 )
        {
            asTemp += ( char )( t | ( c >> 2 ) );
            t = ( Byte )( c << 6 );
        }
        else
            asTemp += ( char )( t | c );
    }
    return asTemp;
}
//---------------------------------------------------------------------------