android 数据安全中常用的加密算法 1 Base64

Base64实现转换原理

    它是用64个可打印字符表示二进制所有数据方法。由于2的6次方等于64，所以可以用每6个位元为一个单元，对应某个可打印字符。我们知道三个字节有24个位元，就可以刚好对应于4个Base64单元，即3个字节需要用4个Base64的可打印字符来表示。在Base64中的可打印字符包括字母A-Z、a-z、数字0-9 ，这样共有62个字符，此外两个可打印符号在不同的系统中一般有所不同。但是，我们经常所说的Base64另外2个字符是：“+/”。这64个字符，所对应表如下。

转换的时候，将三个byte的数据，先后放入一个24bit的缓冲区中，先来的byte占高位。数据不足3byte的话，于缓冲区中剩下的bit用0补足。然后，每次取出6个bit，按照其值选择ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/中的字符作为编码后的输出。不断进行，直到全部输入数据转换完成。 如果最后剩下两个输入数据，在编码结果后加1个“=”；如果最后剩下一个输入数据，编码结果后加2个“=”；如果没有剩下任何数据，就什么都不要加，这样才可以保证资料还原的正确性。

Base64代码实现如下：

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.zip.Deflater;
import java.util.zip.Inflater;public class Base64 {private static final byte[] encodingTable = { (byte) 'A', (byte) 'B',(byte) 'C', (byte) 'D', (byte) 'E', (byte) 'F', (byte) 'G', (byte) 'H',(byte) 'I', (byte) 'J', (byte) 'K', (byte) 'L', (byte) 'M', (byte) 'N',(byte) 'O', (byte) 'P', (byte) 'Q', (byte) 'R', (byte) 'S', (byte) 'T',(byte) 'U', (byte) 'V', (byte) 'W', (byte) 'X', (byte) 'Y', (byte) 'Z',(byte) 'a', (byte) 'b', (byte) 'c', (byte) 'd', (byte) 'e', (byte) 'f',(byte) 'g', (byte) 'h', (byte) 'i', (byte) 'j', (byte) 'k', (byte) 'l',(byte) 'm', (byte) 'n', (byte) 'o', (byte) 'p', (byte) 'q', (byte) 'r',(byte) 's', (byte) 't', (byte) 'u', (byte) 'v', (byte) 'w', (byte) 'x',(byte) 'y', (byte) 'z', (byte) '0', (byte) '1', (byte) '2', (byte) '3',(byte) '4', (byte) '5', (byte) '6', (byte) '7', (byte) '8', (byte) '9',(byte) '+', (byte) '/' };private static final byte[] decodingTable;static {decodingTable = new byte[128];for (int i = 0; i < 128; i++) {decodingTable[i] = (byte) -1;}for (int i = 'A'; i <= 'Z'; i++) {decodingTable[i] = (byte) (i - 'A');}for (int i = 'a'; i <= 'z'; i++) {decodingTable[i] = (byte) (i - 'a' + 26);}for (int i = '0'; i <= '9'; i++) {decodingTable[i] = (byte) (i - '0' + 52);}decodingTable['+'] = 62;decodingTable['/'] = 63;}public static String encodeS(byte[] data) {return new String(encode(data));}public static byte[] encode(byte[] data) {byte[] bytes;int modulus = data.length % 3;if (modulus == 0) {bytes = new byte[(4 * data.length) / 3];} else {bytes = new byte[4 * ((data.length / 3) + 1)];}int dataLength = (data.length - modulus);int a1;int a2;int a3;for (int i = 0, j = 0; i < dataLength; i += 3, j += 4) {a1 = data[i] & 0xff;a2 = data[i + 1] & 0xff;a3 = data[i + 2] & 0xff;bytes[j] = encodingTable[(a1 >>> 2) & 0x3f];bytes[j + 1] = encodingTable[((a1 << 4) | (a2 >>> 4)) & 0x3f];bytes[j + 2] = encodingTable[((a2 << 2) | (a3 >>> 6)) & 0x3f];bytes[j + 3] = encodingTable[a3 & 0x3f];}int b1;int b2;int b3;int d1;int d2;switch (modulus) {case 0: /* nothing left to do */break;case 1:d1 = data[data.length - 1] & 0xff;b1 = (d1 >>> 2) & 0x3f;b2 = (d1 << 4) & 0x3f;bytes[bytes.length - 4] = encodingTable[b1];bytes[bytes.length - 3] = encodingTable[b2];bytes[bytes.length - 2] = (byte) '=';bytes[bytes.length - 1] = (byte) '=';break;case 2:d1 = data[data.length - 2] & 0xff;d2 = data[data.length - 1] & 0xff;b1 = (d1 >>> 2) & 0x3f;b2 = ((d1 << 4) | (d2 >>> 4)) & 0x3f;b3 = (d2 << 2) & 0x3f;bytes[bytes.length - 4] = encodingTable[b1];bytes[bytes.length - 3] = encodingTable[b2];bytes[bytes.length - 2] = encodingTable[b3];bytes[bytes.length - 1] = (byte) '=';break;}return bytes;}public static byte[] decode(byte[] data) {byte[] bytes;byte b1;byte b2;byte b3;byte b4;data = discardNonBase64Bytes(data);if (data[data.length - 2] == '=') {bytes = new byte[(((data.length / 4) - 1) * 3) + 1];} else if (data[data.length - 1] == '=') {bytes = new byte[(((data.length / 4) - 1) * 3) + 2];} else {bytes = new byte[((data.length / 4) * 3)];}for (int i = 0, j = 0; i < (data.length - 4); i += 4, j += 3) {b1 = decodingTable[data[i]];b2 = decodingTable[data[i + 1]];b3 = decodingTable[data[i + 2]];b4 = decodingTable[data[i + 3]];bytes[j] = (byte) ((b1 << 2) | (b2 >> 4));bytes[j + 1] = (byte) ((b2 << 4) | (b3 >> 2));bytes[j + 2] = (byte) ((b3 << 6) | b4);}if (data[data.length - 2] == '=') {b1 = decodingTable[data[data.length - 4]];b2 = decodingTable[data[data.length - 3]];bytes[bytes.length - 1] = (byte) ((b1 << 2) | (b2 >> 4));} else if (data[data.length - 1] == '=') {b1 = decodingTable[data[data.length - 4]];b2 = decodingTable[data[data.length - 3]];b3 = decodingTable[data[data.length - 2]];bytes[bytes.length - 2] = (byte) ((b1 << 2) | (b2 >> 4));bytes[bytes.length - 1] = (byte) ((b2 << 4) | (b3 >> 2));} else {b1 = decodingTable[data[data.length - 4]];b2 = decodingTable[data[data.length - 3]];b3 = decodingTable[data[data.length - 2]];b4 = decodingTable[data[data.length - 1]];bytes[bytes.length - 3] = (byte) ((b1 << 2) | (b2 >> 4));bytes[bytes.length - 2] = (byte) ((b2 << 4) | (b3 >> 2));bytes[bytes.length - 1] = (byte) ((b3 << 6) | b4);}return bytes;}public static byte[] decode(String data) {byte[] bytes;byte b1;byte b2;byte b3;byte b4;data = discardNonBase64Chars(data);if (data.charAt(data.length() - 2) == '=') {bytes = new byte[(((data.length() / 4) - 1) * 3) + 1];} else if (data.charAt(data.length() - 1) == '=') {bytes = new byte[(((data.length() / 4) - 1) * 3) + 2];} else {bytes = new byte[((data.length() / 4) * 3)];}for (int i = 0, j = 0; i < (data.length() - 4); i += 4, j += 3) {b1 = decodingTable[data.charAt(i)];b2 = decodingTable[data.charAt(i + 1)];b3 = decodingTable[data.charAt(i + 2)];b4 = decodingTable[data.charAt(i + 3)];bytes[j] = (byte) ((b1 << 2) | (b2 >> 4));bytes[j + 1] = (byte) ((b2 << 4) | (b3 >> 2));bytes[j + 2] = (byte) ((b3 << 6) | b4);}if (data.charAt(data.length() - 2) == '=') {b1 = decodingTable[data.charAt(data.length() - 4)];b2 = decodingTable[data.charAt(data.length() - 3)];bytes[bytes.length - 1] = (byte) ((b1 << 2) | (b2 >> 4));} else if (data.charAt(data.length() - 1) == '=') {b1 = decodingTable[data.charAt(data.length() - 4)];b2 = decodingTable[data.charAt(data.length() - 3)];b3 = decodingTable[data.charAt(data.length() - 2)];bytes[bytes.length - 2] = (byte) ((b1 << 2) | (b2 >> 4));bytes[bytes.length - 1] = (byte) ((b2 << 4) | (b3 >> 2));} else {b1 = decodingTable[data.charAt(data.length() - 4)];b2 = decodingTable[data.charAt(data.length() - 3)];b3 = decodingTable[data.charAt(data.length() - 2)];b4 = decodingTable[data.charAt(data.length() - 1)];bytes[bytes.length - 3] = (byte) ((b1 << 2) | (b2 >> 4));bytes[bytes.length - 2] = (byte) ((b2 << 4) | (b3 >> 2));bytes[bytes.length - 1] = (byte) ((b3 << 6) | b4);}return bytes;}private static byte[] discardNonBase64Bytes(byte[] data) {byte[] temp = new byte[data.length];int bytesCopied = 0;for (int i = 0; i < data.length; i++) {if (isValidBase64Byte(data[i])) {temp[bytesCopied++] = data[i];}}byte[] newData = new byte[bytesCopied];System.arraycopy(temp, 0, newData, 0, bytesCopied);return newData;}private static String discardNonBase64Chars(String data) {StringBuffer sb = new StringBuffer();int length = data.length();for (int i = 0; i < length; i++) {if (isValidBase64Byte((byte) (data.charAt(i)))) {sb.append(data.charAt(i));}}return sb.toString();}private static boolean isValidBase64Byte(byte b) {if (b == '=') {return true;} else if ((b < 0) || (b >= 128)) {return false;} else if (decodingTable[b] == -1) {return false;}return true;}public static byte[] compressBytes(byte input[]) {int cachesize = 1024;Deflater compresser = new Deflater();compresser.reset();compresser.setInput(input);compresser.finish();byte output[] = new byte[0];ByteArrayOutputStream o = new ByteArrayOutputStream(input.length);try {byte[] buf = new byte[cachesize];int got;while (!compresser.finished()) {got = compresser.deflate(buf);o.write(buf, 0, got);}output = o.toByteArray();} finally {try {o.close();} catch (IOException e) {e.printStackTrace();}}return output;}public static byte[] decompressBytes(byte input[]) {int cachesize = 1024;Inflater decompresser = new Inflater();byte output[] = new byte[0];decompresser.reset();decompresser.setInput(input);ByteArrayOutputStream o = new ByteArrayOutputStream(input.length);try {byte[] buf = new byte[cachesize];int got;while (!decompresser.finished()) {got = decompresser.inflate(buf);o.write(buf, 0, got);}output = o.toByteArray();} catch (Exception e) {e.printStackTrace();} finally {try {o.close();} catch (IOException e) {e.printStackTrace();}}return output;}
}