Base64編碼的原理及實現

Base64索引表：

Value	Char		Value	Char		Value	Char		Value	Char
0	A		16	Q		32	g		48	w
1	B		17	R		33	h		49	x
2	C		18	S		34	i		50	y
3	D		19	T		35	j		51	z
4	E		20	U		36	k		52	0
5	F		21	V		37	l		53	1
6	G		22	W		38	m		54	2
7	H		23	X		39	n		55	3
8	I		24	Y		40	o		56	4
9	J		25	Z		41	p		57	5
10	K		26	a		42	q		58	6
11	L		27	b		43	r		59	7
12	M		28	c		44	s		60	8
13	N		29	d		45	t		61	9
14	O		30	e		46	u		62	+
15	P		31	f		47	v		63	/

  編碼“Man”

文本	M								a								n
ASCII編碼	77								97								110
二進制位	0	1	0	0	1	1	0	1	0	1	1	0	0	0	0	1	0	1	1	0	1	1	1	0
索引	19						22						5						46
Base64編碼	T						W						F						u

在此例中，Base64算法將三個字符編碼爲4個字符

 代碼實現方式：

    private static char[] base64EncodeChars = new char[]{
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
        'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
        'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
        'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
        'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
        'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
        'w', 'x', 'y', 'z', '0', '1', '2', '3',
        '4', '5', '6', '7', '8', '9', '+', '/'};
   private static byte[] base64DecodeChars = new byte[]{
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,
        -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
        15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
        -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
        41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1};

public static String encode(byte[] data) {
    StringBuffer sb = new StringBuffer();
    int len = data.length;
    int i = 0;
    int b1, b2, b3;
    while (i < len) {
        b1 = data[i++] & 0xff;
        if (i == len) {
            sb.append(base64EncodeChars[b1 >>> 2]);
            sb.append(base64EncodeChars[(b1 & 0x3) << 4]);
            sb.append("==");
            break;
        }
        b2 = data[i++] & 0xff;
        if (i == len) {
            sb.append(base64EncodeChars[b1 >>> 2]);
            sb.append(base64EncodeChars[((b1 & 0x03) << 4) | ((b2 & 0xf0) >>> 4)]);
            sb.append(base64EncodeChars[(b2 & 0x0f) << 2]);
            sb.append("=");
            break;
        }
        b3 = data[i++] & 0xff;
        sb.append(base64EncodeChars[b1 >>> 2]);
        sb.append(base64EncodeChars[((b1 & 0x03) << 4) | ((b2 & 0xf0) >>> 4)]);
        sb.append(base64EncodeChars[((b2 & 0x0f) << 2) | ((b3 & 0xc0) >>> 6)]);
        sb.append(base64EncodeChars[b3 & 0x3f]);
    }
    return sb.toString();
}

public static byte[] decode(String str) throws UnsupportedEncodingException {
    StringBuffer sb = new StringBuffer();
    byte[] data = str.getBytes("US-ASCII");
    int len = data.length;
    int i = 0;
    int b1, b2, b3, b4;
    while (i < len) {
       
        do {
            b1 = base64DecodeChars[data[i++]];
        } while (i < len && b1 == -1);
        if (b1 == -1) break;
       
        do {
            b2 = base64DecodeChars
                    [data[i++]];
        } while (i < len && b2 == -1);
        if (b2 == -1) break;
        sb.append((char) ((b1 << 2) | ((b2 & 0x30) >>> 4)));
       
        do {
            b3 = data[i++];
            if (b3 == 61) return sb.toString().getBytes("iso8859-1");
            b3 = base64DecodeChars[b3];
        } while (i < len && b3 == -1);
        if (b3 == -1) break;
        sb.append((char) (((b2 & 0x0f) << 4) | ((b3 & 0x3c) >>> 2)));
       
        do {
            b4 = data[i++];
            if (b4 == 61) return sb.toString().getBytes("iso8859-1"); //iso8859-1
            b4 = base64DecodeChars[b4];
        } while (i < len && b4 == -1);
        if (b4 == -1) break;
        sb.append((char) (((b3 & 0x03) << 6) | b4));
    }
    return sb.toString().getBytes("iso8859-1");
}