编解码

发表于 2018/02/23 更新于 2025/02/02

作者 Hacket Zeng

39 分钟阅读

编解码

BASE64

Base64 是网络上最常见的用于传输 8Bit 字节代码的编码方式之一，大家可以查看 RFC2045～RFC2049，上面有 MIME 的详细规范。Base64 编码可用于在 HTTP 环境下传递较长的标识信息。例如，在 Java Persistence 系统 Hibernate 中，就采用了 Base64 来将一个较长的唯一标识符（一般为 128-bit 的 UUID）编码为一个字符串，用作 HTTP 表单和 HTTP GET URL 中的参数。在其他应用程序中，也常常需要把二进制数据编码为适合放在 URL（包括隐藏表单域）中的形式。此时，采用 Base64 编码不仅比较简短，同时也具有不可读性，即所编码的数据不会被人用肉眼所直接看到。

RFC2045 还规定每行位 76 个字符，每行末尾需添加一个回车换行符，即便是最后一行不够 76 个字符，也要加换行符。

实现原理

Base64 是一种用 64 个字符来表示任意二进制数据的方法。

每 3 个 8 位二进制码位一组，转换为 4 个 6 位二进制码为一组（不足 6 位时地位补 0）。3 个 8 位二进制码和 4 个 6 位二进制码长度都是 24 位。
对获得的 4 个 6 位二进制码补位，每个 6 位二进制码添加两位高位 0，组成 4 个 8 位二进制码。
将获得的 4 个 8 位二进制码转换为 4 个十进制码。
将获得的十进制码转换为 Base64 字符表中对应的字符。

案例1： 字符串 “A”，进行 Base64 编码，如下所示：

过程	数据
字符	A
ASCII 码	65
二进制码	01000001
4 个 6 位二进制码	010000 010000
4 个 8 位二进制码	00010000 00010000
十进制码	16 16
字符表映射码	Q Q = =

字符串 A 经过 Base64 编码后得到字符串 QQ==。

结果出现了两个等号。很显然，当原文的二进制码长度不足 24 位，最终转换为十进制时也不足 4 项，这时就需要用等号补位。

将 Base64 编码后的字符串最多会有 2 个等号，这时因为：

余数 = 原文字节数 MOD 3。

案例2： 字符串 “ 密 “，对其使用 UTF-8 编码等到 Byte 数组{-27，-81，-122}，

过程	数据
字符	密
UTF-8 编码	-27 -81 -122
二进制码	11100101 10101111 10000110
4 个 6 位二进制码	111001 011010 111110 000110
4 个 8 位二进制码	00111001 00011010 00111110 00000110
十进制码	57 26 62 6
字符表映射码	5 a + G
字符串 “ 密 “ 经过 Base64 编码后得到字符串	5a+G

案例3：

过程	数据
数字 1 查 ASCII 码表	49
1 的二进制分成 3 个 8bit：	00110001 00000000 00000000
分成 4 个 bit:	001100 010000 000000 000000
前面补 00：	00001100 00010000 00000000 00000000
查 BASE64 表对应的十进制值：	12 16 = =
最终值	MQ==

案例4： 字符 Man

过程
M a n
77 97 110
01001101 01100001 01101110
010011 010110 000101 101110
19 22 5 46
T W F u

BASE64 码表

ACII编码表

BASE64 实现

1、com.sun.misc API

1、com.sun.misc 包是 Sun 公司提供给内部使用的专用 API，在 java API 文档中我们看不到任何有关 BASE64 影子，不建议使用。

2、Apache 的实现：（建议使用这种方式，当然，自己实现也可以）

参考 org.apache.commons.codec.binary.Base64
下载地址： http://commons.apache.org/codec/download_codec.cgi
Apache 还提供了，非标准的实现方式：
1.不再添加回车符。
2.Url Base64,也就是将 “+” 和 “" 换成了 “-“ 和 “_” 符号，且不适用补位。

3、Android 提供的 Base64

4、工具类，见下

参考：Base64笔记 - 阮一峰的网络日志

Base64 工具类

Base64Util

  
package com.baidu.encryptdemo;

import java.io.UnsupportedEncodingException;

/**
 * A Base64 Encoder/Decoder.
 * <p/>
 * This class is used to encode and decode data in Base64 format as described in RFC 1521.
 * <p/>
 * This is "Open Source" software and released under the
 * <a href="http://www.gnu.org/licenses/lgpl.html">GNU/LGPL </a>
 * license. <br>
 * It is provided "as is" without warranty of any kind. <br>
 * Copyright 2003: Christian d'Heureuse, Inventec Informatik AG, Switzerland.
 * <br>
 * Home page: <a href="http://www.source-code.biz">www.source-code.biz </a> <br>
 * <p/>
 * Original name <b>Base64Coder.java </b>
 * <p/>
 * Version history: <br>
 * 2003-07-22 Christian d'Heureuse (chdh): Module created. <br>
 * 2005-08-11 chdh: Lincense changed from GPL to LGPL. <br>
 * 2006-11-21 chdh: <br>
 * &nbsp; Method encode(String) renamed to encodeString(String). <br>
 * &nbsp; Method decode(String) renamed to decodeString(String). <br>
 * &nbsp; New method encode(byte[],int) added. <br>
 * &nbsp; New method decode(String) added. <br>
 * 2007-04-30 francis.naoum: Added to sfp_lib. <br>
 * &nbsp; byte[] encode(byte[]) changed to return a String. <br>
 *
 * @author Francis Naoum
 * @since 1.0.0
 */
public final class Base64Util {

    /**
     * Mapping table from 6-bit nibbles to Base64 characters.
     */
    private static final char[] MAP1 = new char[64];

    static {
        int i = 0;

        for (char c = 'A'; c <= 'Z'; c++) {
            MAP1[i++] = c;
        }

        for (char c = 'a'; c <= 'z'; c++) {
            MAP1[i++] = c;
        }

        for (char c = '0'; c <= '9'; c++) {
            MAP1[i++] = c;
        }

        MAP1[i++] = '+';
        MAP1[i++] = '/';
    }

    /**
     * Mapping table from Base64 characters to 6-bit nibbles.
     */
    private static final byte[] MAP2 = new byte[128];

    static {
        for (int i = 0; i < MAP2.length; i++) {
            MAP2[i] = -1;
        }

        for (int i = 0; i < 64; i++) {
            MAP2[MAP1[i]] = (byte) i;
        }
    }

    /**
     * Encodes a string into Base64 format. No blanks or line breaks are inserted.
     *
     * @param string a String to be encoded.
     * @return A String with the Base64 encoded data.
     */
    public static String encodeString(final String string) {
        String encodedString = null;

        try {
            encodedString = encodeString(string, "UTF-8");
        } catch (UnsupportedEncodingException uue) {
            // Should never happen, java has to support "UTF-8".
        }

        return encodedString;
    }

    /**
     * Encodes a string into Base64 format. No blanks or line breaks are inserted.
     *
     * @param string   a String to be encoded.
     * @param encoding The character encoding of the string.
     * @return A String with the Base64 encoded data.
     * @throws UnsupportedEncodingException if the java runtime does not support <code>encoding</code>.
     */
    public static String encodeString(final String string, final String encoding) throws
            UnsupportedEncodingException {
        byte[] stringBytes = string.getBytes(encoding);
        return encode(stringBytes);
    }

    /**
     * Encodes a byte array into Base64 format. No blanks or line breaks are inserted.
     * <p/>
     * This method has been modified to return a string not a char[].
     *
     * @param input an array containing the data bytes to be encoded.
     * @return A character array with the Base64 encoded data.
     */
    public static String encode(final byte[] input) {
        char[] chars = encode(input, input.length);
        return new String(chars);
    }

    /**
     * Encodes a byte array into Base64 format. No blanks or line breaks are inserted.
     *
     * @param input an array containing the data bytes to be encoded.
     * @param iLen  number of bytes to process in <code>in</code>.
     * @return A character array with the Base64 encoded data.
     */
    public static char[] encode(final byte[] input, final int iLen) {
        int oDataLen = (iLen * 4 + 2) / 3; // output length without padding
        int oLen = ((iLen + 2) / 3) * 4; // output length including padding
        char[] out = new char[oLen];
        int ip = 0;
        int op = 0;

        while (ip < iLen) {
            int i0 = input[ip++] & 0xff;
            int i1 = ip < iLen ? input[ip++] & 0xff : 0;
            int i2 = ip < iLen ? input[ip++] & 0xff : 0;
            int o0 = i0 >>> 2;
            int o1 = ((i0 & 3) << 4) | (i1 >>> 4);
            int o2 = ((i1 & 0xf) << 2) | (i2 >>> 6);
            int o3 = i2 & 0x3F;
            out[op++] = MAP1[o0];
            out[op++] = MAP1[o1];
            out[op] = op < oDataLen ? MAP1[o2] : '=';
            op++;
            out[op] = op < oDataLen ? MAP1[o3] : '=';
            op++;
        }

        return out;
    }

    /**
     * Decodes a string from Base64 format.
     *
     * @param string a Base64 String to be decoded.
     * @return A String containing the decoded data.
     * @throws IllegalArgumentException if the input is not valid Base64 encoded data.
     */
    public static String decodeString(final String string) {
        String decodedString = null;

        try {
            byte[] decodedBytes = decode(string);
            decodedString = new String(decodedBytes, "UTF-8");
        } catch (UnsupportedEncodingException uue) {
            // Should never happen, java has to support "UTF-8".
        }

        return decodedString;
    }

    /**
     * Decodes a byte array from Base64 format.
     *
     * @param string a Base64 String to be decoded.
     * @return An array containing the decoded data bytes.
     * @throws IllegalArgumentException if the input is not valid Base64 encoded data.
     */
    public static byte[] decode(final String string) {
        return decode(string.toCharArray());
    }

    /**
     * Decodes a byte array from Base64 format. No blanks or line breaks are allowed within the Base64 encoded data.
     *
     * @param input a character array containing the Base64 encoded data.
     * @return An array containing the decoded data bytes.
     * @throws IllegalArgumentException if the input is not valid Base64 encoded data.
     */
    public static byte[] decode(final char[] input) {
        int iLen = input.length;

        if (iLen % 4 != 0) {
            throw new IllegalArgumentException(
                    "Length of Base64 encoded input string is not a multiple of 4.");
        }

        while (iLen > 0 && input[iLen - 1] == '=') {
            iLen--;
        }

        int oLen = (iLen * 3) / 4;
        byte[] out = new byte[oLen];
        int ip = 0;
        int op = 0;

        while (ip < iLen) {
            int i0 = input[ip++];
            int i1 = input[ip++];
            int i2 = ip < iLen ? input[ip++] : 'A';
            int i3 = ip < iLen ? input[ip++] : 'A';

            if (i0 > 127 || i1 > 127 || i2 > 127 || i3 > 127) {
                throw new IllegalArgumentException("Illegal character in Base64 encoded data.");
            }

            int b0 = MAP2[i0];
            int b1 = MAP2[i1];
            int b2 = MAP2[i2];
            int b3 = MAP2[i3];

            if (b0 < 0 || b1 < 0 || b2 < 0 || b3 < 0) {
                throw new IllegalArgumentException("Illegal character in Base64 encoded data.");
            }

            int o0 = (b0 << 2) | (b1 >>> 4);
            int o1 = ((b1 & 0xf) << 4) | (b2 >>> 2);
            int o2 = ((b2 & 3) << 6) | b3;
            out[op++] = (byte) o0;

            if (op < oLen) {
                out[op++] = (byte) o1;
            }

            if (op < oLen) {
                out[op++] = (byte) o2;
            }
        }

        return out;
    }

    /**
     * Dummy constructor.
     */
    private Base64Util() {
    }
}

BdBase64Util

  
package com.baidu.encryptdemo;

import java.io.UnsupportedEncodingException;

/***
 * Utilities for encoding and decoding the Base64 representation of binary data. See RFCs <a
 * href="http://www.ietf.org/rfc/rfc2045.txt">2045</a> and <a href="http://www.ietf.org/rfc/rfc3548.txt">3548</a>.
 */
public class BdBase64Util {
    /***
     * Default values for encoder/decoder flags.
     */
    public static final int DEFAULT = 0;

    /***
     * Encoder flag bit to omit the padding '=' characters at the end of the output (if any).
     */
    public static final int NO_PADDING = 1;

    /***
     * Encoder flag bit to omit all line terminators (i.e., the output will be on one long line).
     */
    public static final int NO_WRAP = 2;

    /***
     * Encoder flag bit to indicate lines should be terminated with a CRLF pair instead of just an LF. Has no effect if
     * {@code NO_WRAP} is specified as well.
     */
    public static final int CRLF = 4;

    /***
     * Encoder/decoder flag bit to indicate using the "URL and filename safe" variant of Base64 (see RFC 3548 section 4)
     * where {@code -} and {@code _} are used in place of {@code +} and {@code /}.
     */
    public static final int URL_SAFE = 8;

    /***
     * Flag to pass to {@link Base64OutputStream} to indicate that it should not close the output stream it is wrapping
     * when it itself is closed.
     */
    public static final int NO_CLOSE = 16;

    static abstract class Coder {
        public byte[] output;
        public int op;

        /***
         * Encode/decode another block of input data. this.output is provided by the caller, and must be big enough to
         * hold all the coded data. On exit, this.opwill be set to the length of the coded data.
         *
         * @param finish true if this is the final call to process for this object. Will finalize the coder state and
         *               include any final bytes in the output.
         * @return true if the input so far is good; false if some error has been detected in the input stream..
         */
        public abstract boolean process(byte[] input, int offset, int len, boolean finish);

        /***
         * @return the maximum number of bytes a call to process() could produce for the given number of input bytes.
         * This may be an overestimate.
         */
        public abstract int maxOutputSize(int len);
    }

    /***
     * Decode the Base64-encoded data in input and return the data in a new byte array.
     * <p/>
     * <p/>
     * The padding '=' characters at the end are considered optional, but if any are present, there must be the correct
     * number of them.
     *
     * @param str the input String to decode, which is converted to bytes using the default charset
     * @throws IllegalArgumentException if the input contains incorrect padding
     */
    public static byte[] decode(String str) {
        return decode(str.getBytes(), DEFAULT);
    }

    /***
     * Decode the Base64-encoded data in input and return the data in a new byte array.
     * <p/>
     * <p/>
     * The padding '=' characters at the end are considered optional, but if any are present, there must be the correct
     * number of them.
     *
     * @param str   the input String to decode, which is converted to bytes using the default charset
     * @param flags controls certain features of the decoded output. Pass {@code DEFAULT} to decode standard Base64.
     * @throws IllegalArgumentException if the input contains incorrect padding
     */
    public static byte[] decode(String str, int flags) {
        return decode(str.getBytes(), flags);
    }

    /***
     * Decode the Base64-encoded data in input and return the data in a new byte array.
     * <p/>
     * <p/>
     * The padding '=' characters at the end are considered optional, but if any are present, there must be the correct
     * number of them.
     *
     * @param input the input array to decode
     * @throws IllegalArgumentException if the input contains incorrect padding
     */
    public static byte[] decode(byte[] input) {
        return decode(input, 0, input.length, DEFAULT);
    }

    /***
     * Decode the Base64-encoded data in input and return the data in a new byte array.
     * <p/>
     * <p/>
     * The padding '=' characters at the end are considered optional, but if any are present, there must be the correct
     * number of them.
     *
     * @param input the input array to decode
     * @param flags controls certain features of the decoded output. Pass {@code DEFAULT} to decode standard Base64.
     * @throws IllegalArgumentException if the input contains incorrect padding
     */
    public static byte[] decode(byte[] input, int flags) {
        return decode(input, 0, input.length, flags);
    }

    /***
     * Decode the Base64-encoded data in input and return the data in a new byte array.
     * <p/>
     * <p/>
     * The padding '=' characters at the end are considered optional, but if any are present, there must be the correct
     * number of them.
     *
     * @param input  the data to decode
     * @param offset the position within the input array at which to start
     * @param len    the number of bytes of input to decode
     * @param flags  controls certain features of the decoded output. Pass {@code DEFAULT} to decode standard Base64.
     * @throws IllegalArgumentException if the input contains incorrect padding
     */
    public static byte[] decode(byte[] input, int offset, int len, int flags) {
        // Allocate space for the most data the input could represent.
        // (It could contain less if it contains whitespace, etc.)
        Decoder decoder = new Decoder(flags, new byte[len * 3 / 4]);

        if (!decoder.process(input, offset, len, true)) {
            throw new IllegalArgumentException("bad base-64");
        }

        // Maybe we got lucky and allocated exactly enough output space.
        if (decoder.op == decoder.output.length) {
            return decoder.output;
        }

        // Need to shorten the array, so allocate a new one of the
        // right size and copy.
        byte[] temp = new byte[decoder.op];
        System.arraycopy(decoder.output, 0, temp, 0, decoder.op);
        return temp;
    }

    static class Decoder extends Coder {
        /***
         * Lookup table for turning bytes into their position in the Base64 alphabet.
         */
        private static final int DECODE[] = {-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, -2, -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, -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, -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, -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,};

        /***
         * Decode lookup table for the "web safe" variant (RFC 3548 sec. 4) where - and _ replace + and /.
         */
        private static final int DECODE_WEBSAFE[] = {-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, -1, -1, 62, -1, -1, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -2, -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, 63,
                -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, -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, -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, -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,};

        /***
         * Non-data values in the DECODE arrays.
         */
        private static final int SKIP = -1;
        private static final int EQUALS = -2;

        /***
         * States 0-3 are reading through the next input tuple. State 4 is having read one '=' and expecting exactly one
         * more. State 5 is expecting no more data or padding characters in the input. State 6 is the error state; an
         * error has been detected in the input and no future input can "fix" it.
         */
        private int state; // state number (0 to 6)
        private int value;

        final private int[] alphabet;

        public Decoder(int flags, byte[] output) {
            this.output = output;

            alphabet = ((flags & URL_SAFE) == 0) ? DECODE : DECODE_WEBSAFE;
            state = 0;
            value = 0;
        }

        /***
         * @return an overestimate for the number of bytes {@code len} bytes could decode to.
         */
        public int maxOutputSize(int len) {
            return len * 3 / 4 + 10;
        }

        /***
         * Decode another block of input data.
         *
         * @return true if the state machine is still healthy. false if bad base-64 data has been detected in the input
         * stream.
         */
        public boolean process(byte[] input, int offset, int len, boolean finish) {
            if (this.state == 6)
                return false;

            int p = offset;
            len += offset;

            // Using local variables makes the decoder about 12%
            // faster than if we manipulate the member variables in
            // the loop. (Even alphabet makes a measurable
            // difference, which is somewhat surprising to me since
            // the member variable is final.)
            int state = this.state;
            int value = this.value;
            int op = 0;
            final byte[] output = this.output;
            final int[] alphabet = this.alphabet;

            while (p < len) {
                // Try the fast path: we're starting a new tuple and the
                // next four bytes of the input stream are all data
                // bytes. This corresponds to going through states
                // 0-1-2-3-0. We expect to use this method for most of
                // the data.
                //
                // If any of the next four bytes of input are non-data
                // (whitespace, etc.), value will end up negative. (All
                // the non-data values in decode are small negative
                // numbers, so shifting any of them up and or'ing them
                // together will result in a value with its top bit set.)
                //
                // You can remove this whole block and the output should
                // be the same, just slower.
                if (state == 0) {
                    while (p + 4 <= len
                            && (value =
                            ((alphabet[input[p] & 0xff] << 18) | (alphabet[input[p + 1] & 0xff] << 12)
                                    | (alphabet[input[p + 2] & 0xff] << 6) | (alphabet[input[p + 3] & 0xff]))) >= 0) {
                        output[op + 2] = (byte) value;
                        output[op + 1] = (byte) (value >> 8);
                        output[op] = (byte) (value >> 16);
                        op += 3;
                        p += 4;
                    }
                    if (p >= len)
                        break;
                }

                // The fast path isn't available -- either we've read a
                // partial tuple, or the next four input bytes aren't all
                // data, or whatever. Fall back to the slower state
                // machine implementation.

                int d = alphabet[input[p++] & 0xff];

                switch (state) {
                    case 0:
                        if (d >= 0) {
                            value = d;
                            ++state;
                        } else if (d != SKIP) {
                            this.state = 6;
                            return false;
                        }
                        break;

                    case 1:
                        if (d >= 0) {
                            value = (value << 6) | d;
                            ++state;
                        } else if (d != SKIP) {
                            this.state = 6;
                            return false;
                        }
                        break;

                    case 2:
                        if (d >= 0) {
                            value = (value << 6) | d;
                            ++state;
                        } else if (d == EQUALS) {
                            // Emit the last (partial) output tuple;
                            // expect exactly one more padding character.
                            output[op++] = (byte) (value >> 4);
                            state = 4;
                        } else if (d != SKIP) {
                            this.state = 6;
                            return false;
                        }
                        break;

                    case 3:
                        if (d >= 0) {
                            // Emit the output triple and return to state 0.
                            value = (value << 6) | d;
                            output[op + 2] = (byte) value;
                            output[op + 1] = (byte) (value >> 8);
                            output[op] = (byte) (value >> 16);
                            op += 3;
                            state = 0;
                        } else if (d == EQUALS) {
                            // Emit the last (partial) output tuple;
                            // expect no further data or padding characters.
                            output[op + 1] = (byte) (value >> 2);
                            output[op] = (byte) (value >> 10);
                            op += 2;
                            state = 5;
                        } else if (d != SKIP) {
                            this.state = 6;
                            return false;
                        }
                        break;

                    case 4:
                        if (d == EQUALS) {
                            ++state;
                        } else if (d != SKIP) {
                            this.state = 6;
                            return false;
                        }
                        break;

                    case 5:
                        if (d != SKIP) {
                            this.state = 6;
                            return false;
                        }
                        break;
                }
            }

            if (!finish) {
                // We're out of input, but a future call could provide
                // more.
                this.state = state;
                this.value = value;
                this.op = op;
                return true;
            }

            // Done reading input. Now figure out where we are left in
            // the state machine and finish up.

            switch (state) {
                case 0:
                    // Output length is a multiple of three. Fine.
                    break;
                case 1:
                    // Read one extra input byte, which isn't enough to
                    // make another output byte. Illegal.
                    this.state = 6;
                    return false;
                case 2:
                    // Read two extra input bytes, enough to emit 1 more
                    // output byte. Fine.
                    output[op++] = (byte) (value >> 4);
                    break;
                case 3:
                    // Read three extra input bytes, enough to emit 2 more
                    // output bytes. Fine.
                    output[op++] = (byte) (value >> 10);
                    output[op++] = (byte) (value >> 2);
                    break;
                case 4:
                    // Read one padding '=' when we expected 2. Illegal.
                    this.state = 6;
                    return false;
                case 5:
                    // Read all the padding '='s we expected and no more.
                    // Fine.
                    break;
            }

            this.state = state;
            this.op = op;
            return true;
        }
    }

    /***
     * Base64-encode the given data and return a newly allocated String with the result.
     *
     * @param input the data to encode
     */
    public static String encodeToString(byte[] input) {
        try {
            return new String(encode(input, DEFAULT), "US-ASCII");
        } catch (UnsupportedEncodingException e) {
            // US-ASCII is guaranteed to be available.
            throw new AssertionError(e);
        }
    }

    /***
     * Base64-encode the given data and return a newly allocated String with the result.
     *
     * @param input the data to encode
     * @param flags controls certain features of the encoded output. Passing {@code DEFAULT} results in output that
     *              adheres to RFC 2045.
     */
    public static String encodeToString(byte[] input, int flags) {
        try {
            return new String(encode(input, flags), "US-ASCII");
        } catch (UnsupportedEncodingException e) {
            // US-ASCII is guaranteed to be available.
            throw new AssertionError(e);
        }
    }

    /***
     * Base64-encode the given data and return a newly allocated String with the result.
     *
     * @param input  the data to encode
     * @param offset the position within the input array at which to start
     * @param len    the number of bytes of input to encode
     * @param flags  controls certain features of the encoded output. Passing {@code DEFAULT} results in output that
     *               adheres to RFC 2045.
     */
    public static String encodeToString(byte[] input, int offset, int len, int flags) {
        try {
            return new String(encode(input, offset, len, flags), "US-ASCII");
        } catch (UnsupportedEncodingException e) {
            // US-ASCII is guaranteed to be available.
            throw new AssertionError(e);
        }
    }

    /***
     * Base64-encode the given data and return a newly allocated byte[] with the result.
     *
     * @param input the data to encode
     */
    public static byte[] encode(byte[] input) {
        return encode(input, 0, input.length, DEFAULT);
    }

    /***
     * Base64-encode the given data and return a newly allocated byte[] with the result.
     *
     * @param input the data to encode
     * @param flags controls certain features of the encoded output. Passing {@code DEFAULT} results in output that
     *              adheres to RFC 2045.
     */
    public static byte[] encode(byte[] input, int flags) {
        return encode(input, 0, input.length, flags);
    }

    /***
     * Base64-encode the given data and return a newly allocated byte[] with the result.
     *
     * @param input  the data to encode
     * @param offset the position within the input array at which to start
     * @param len    the number of bytes of input to encode
     * @param flags  controls certain features of the encoded output. Passing {@code DEFAULT} results in output that
     *               adheres to RFC 2045.
     */
    public static byte[] encode(byte[] input, int offset, int len, int flags) {
        Encoder encoder = new Encoder(flags, null);

        // Compute the exact length of the array we will produce.
        int output_len = len / 3 * 4;

        // Account for the tail of the data and the padding bytes, if any.
        if (encoder.do_padding) {
            if (len % 3 > 0) {
                output_len += 4;
            }
        } else {
            switch (len % 3) {
                case 0:
                    break;
                case 1:
                    output_len += 2;
                    break;
                case 2:
                    output_len += 3;
                    break;
            }
        }

        // Account for the newlines, if any.
        if (encoder.do_newline && len > 0) {
            output_len += (((len - 1) / (3 * Encoder.LINE_GROUPS)) + 1) * (encoder.do_cr ? 2 : 1);
        }

        encoder.output = new byte[output_len];
        encoder.process(input, offset, len, true);

        assert encoder.op == output_len;

        return encoder.output;
    }

    static class Encoder extends Coder {
        /***
         * Emit a new line every this many output tuples. Corresponds to a 76-character line length (the maximum
         * allowable according to <a href="http://www.ietf.org/rfc/rfc2045.txt">RFC 2045</a>).
         */
        public static final int LINE_GROUPS = 19;

        /***
         * Lookup table for turning Base64 alphabet positions (6 bits) into output bytes.
         */
        private static final byte ENCODE[] = {'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', '+', '/',};

        /***
         * Lookup table for turning Base64 alphabet positions (6 bits) into output bytes.
         */
        private static final byte ENCODE_WEBSAFE[] = {'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', '-', '_',};

        final private byte[] tail;
        int tailLen;
        private int count;

        final public boolean do_padding;
        final public boolean do_newline;
        final public boolean do_cr;
        final private byte[] alphabet;

        public Encoder(int flags, byte[] output) {
            this.output = output;

            do_padding = (flags & NO_PADDING) == 0;
            do_newline = (flags & NO_WRAP) == 0;
            do_cr = (flags & CRLF) != 0;
            alphabet = ((flags & URL_SAFE) == 0) ? ENCODE : ENCODE_WEBSAFE;

            tail = new byte[2];
            tailLen = 0;

            count = do_newline ? LINE_GROUPS : -1;
        }

        /***
         * @return an overestimate for the number of bytes {@code len} bytes could encode to.
         */
        public int maxOutputSize(int len) {
            return len * 8 / 5 + 10;
        }

        public boolean process(byte[] input, int offset, int len, boolean finish) {
            // Using local variables makes the encoder about 9% faster.
            final byte[] alphabet = this.alphabet;
            final byte[] output = this.output;
            int op = 0;
            int count = this.count;

            int p = offset;
            len += offset;
            int v = -1;

            // First we need to concatenate the tail of the previous call
            // with any input bytes available now and see if we can empty
            // the tail.

            switch (tailLen) {
                case 0:
                    // There was no tail.
                    break;

                case 1:
                    if (p + 2 <= len) {
                        // A 1-byte tail with at least 2 bytes of
                        // input available now.
                        v = ((tail[0] & 0xff) << 16) | ((input[p++] & 0xff) << 8) | (input[p++] & 0xff);
                        tailLen = 0;
                    }
                    ;
                    break;

                case 2:
                    if (p + 1 <= len) {
                        // A 2-byte tail with at least 1 byte of input.
                        v = ((tail[0] & 0xff) << 16) | ((tail[1] & 0xff) << 8) | (input[p++] & 0xff);
                        tailLen = 0;
                    }
                    break;
            }

            if (v != -1) {
                output[op++] = alphabet[(v >> 18) & 0x3f];
                output[op++] = alphabet[(v >> 12) & 0x3f];
                output[op++] = alphabet[(v >> 6) & 0x3f];
                output[op++] = alphabet[v & 0x3f];
                if (--count == 0) {
                    if (do_cr)
                        output[op++] = '\r';
                    output[op++] = '\n';
                    count = LINE_GROUPS;
                }
            }

            // At this point either there is no tail, or there are fewer
            // than 3 bytes of input available.

            // The main loop, turning 3 input bytes into 4 output bytes on
            // each iteration.
            while (p + 3 <= len) {
                v = ((input[p] & 0xff) << 16) | ((input[p + 1] & 0xff) << 8) | (input[p + 2] & 0xff);
                output[op] = alphabet[(v >> 18) & 0x3f];
                output[op + 1] = alphabet[(v >> 12) & 0x3f];
                output[op + 2] = alphabet[(v >> 6) & 0x3f];
                output[op + 3] = alphabet[v & 0x3f];
                p += 3;
                op += 4;
                if (--count == 0) {
                    if (do_cr)
                        output[op++] = '\r';
                    output[op++] = '\n';
                    count = LINE_GROUPS;
                }
            }

            if (finish) {
                // Finish up the tail of the input. Note that we need to
                // consume any bytes in tail before any bytes
                // remaining in input; there should be at most two bytes
                // total.

                if (p - tailLen == len - 1) {
                    int t = 0;
                    v = ((tailLen > 0 ? tail[t++] : input[p++]) & 0xff) << 4;
                    tailLen -= t;
                    output[op++] = alphabet[(v >> 6) & 0x3f];
                    output[op++] = alphabet[v & 0x3f];
                    if (do_padding) {
                        output[op++] = '=';
                        output[op++] = '=';
                    }
                    if (do_newline) {
                        if (do_cr)
                            output[op++] = '\r';
                        output[op++] = '\n';
                    }
                } else if (p - tailLen == len - 2) {
                    int t = 0;
                    v =
                            (((tailLen > 1 ? tail[t++] : input[p++]) & 0xff) << 10)
                                    | (((tailLen > 0 ? tail[t++] : input[p++]) & 0xff) << 2);
                    tailLen -= t;
                    output[op++] = alphabet[(v >> 12) & 0x3f];
                    output[op++] = alphabet[(v >> 6) & 0x3f];
                    output[op++] = alphabet[v & 0x3f];
                    if (do_padding) {
                        output[op++] = '=';
                    }
                    if (do_newline) {
                        if (do_cr)
                            output[op++] = '\r';
                        output[op++] = '\n';
                    }
                } else if (do_newline && op > 0 && count != LINE_GROUPS) {
                    if (do_cr)
                        output[op++] = '\r';
                    output[op++] = '\n';
                }

                assert tailLen == 0;
                assert p == len;
            } else {
                // Save the leftovers in tail to be consumed on the next
                // call to encodeInternal.

                if (p == len - 1) {
                    tail[tailLen++] = input[p];
                } else if (p == len - 2) {
                    tail[tailLen++] = input[p];
                    tail[tailLen++] = input[p + 1];
                }
            }

            this.op = op;
            this.count = count;

            return true;
        }
    }

    private BdBase64Util() {
    } // don't instantiate
}

JDK8.x 和 common codec 中对 Base64 算法的实现

  
public class Base64Util {

    public static String jdkBase64Encode(String src) {
        return Base64.getEncoder().encodeToString(src.getBytes());
    }

    public static String jdkBase64Decode(String src) {
        return new String(Base64.getDecoder().decode(src));
    }

    public static String codecBase64Encode(String src) {
        return org.apache.commons.codec.binary.Base64.encodeBase64String(src.getBytes());
    }

    public static String codecBase64Decode(String src) {
        return new String(org.apache.commons.codec.binary.Base64.decodeBase64(src));
    }
}

计算机基础, 加解密

加解密安全

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