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。 ## 一、编码规则 Base64编码的思想是是采用64个基本的ASCII码字符对数据进行重新编码。它将需要编码的数据拆分成字节数组。以3个字节为一组。按顺序排列24 位数据，再把这24位数据分成4组，即每组6位。再在每组的的最高位前补两个0凑足一个字节。这样就把一个3字节为一组的数据重新编码成了4个字节。当所要编码的数据的字节数不是3的整倍数，也就是说在分组时最后一组不够3个字节。这时在最后一组填充1到2个0字节。并在最后编码完成后在结尾添加1到2个 “=”。 例：将对ABC进行BASE64编码： 1、首先取ABC对应的ASCII码值。A（65）B（66）C（67）； 2、再取二进制值A（01000001）B（01000010）C（01000011）； 3、然后把这三个字节的二进制码接起来（010000010100001001000011）； 4、 再以6位为单位分成4个数据块,并在最高位填充两个0后形成4个字节的编码后的值，（00010000）（00010100）（00001001）（00000011），其中蓝色部分为真实数据； 5、再把这四个字节数据转化成10进制数得（16）（20）（9）（3）； 6、最后根据BASE64给出的64个基本字符表，查出对应的ASCII码字符（Q）（U）（J）（D），这里的值实际就是数据在字符表中的索引。 注：BASE64字符表：ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/ ## 二、解码规则 解码过程就是把4个字节再还原成3个字节再根据不同的数据形式把字节数组重新整理成数据。 ## 三、java程序 ~~~ import java.io.ByteArrayOutputStream; import java.io.IOException; public class Base64 { // Create arrays to hold the base64 characters and a // lookup for base64 chars private static byte[] base64Alphabet; /** * The base length. */ static final int BASELENGTH = 255; /** * Chunk separator per RFC 2045 section 2.1. * * @see RFC 2045 section 2.1 */ static final byte[] CHUNK_SEPARATOR = "\r\n".getBytes(); /** * Chunk size per RFC 2045 section 6.8. * *

* The {@value} character limit does not count the trailing CRLF, but counts * all other characters, including any equal signs. *

* * @see RFC 2045 section 6.8 */ static final int CHUNK_SIZE = 76; /** * Used to calculate the number of bits in a byte. */ static final int EIGHTBIT = 8; /** * Used to get the number of Quadruples. */ static final int FOURBYTE = 4; private static byte[] lookUpBase64Alphabet; /** * Lookup length. */ static final int LOOKUPLENGTH = 64; /** * Byte used to pad output. */ static final byte PAD = (byte) '='; /** * Used to test the sign of a byte. */ static final int SIGN = -128; /** * Used when encoding something which has fewer than 24 bits. */ static final int SIXTEENBIT = 16; /** * Used to determine how many bits data contains. */ static final int TWENTYFOURBITGROUP = 24; // Populating the lookup and character arrays static { lookUpBase64Alphabet = new byte[LOOKUPLENGTH]; base64Alphabet = new byte[BASELENGTH]; for (int i = 0; i < BASELENGTH; i++) { base64Alphabet[i] = (byte) -1; } for (int i = 'Z'; i >= 'A'; i--) { base64Alphabet[i] = (byte) (i - 'A'); } for (int i = 'z'; i >= 'a'; i--) { base64Alphabet[i] = (byte) (i - 'a' + 26); } for (int i = '9'; i >= '0'; i--) { base64Alphabet[i] = (byte) (i - '0' + 52); } base64Alphabet['+'] = 62; base64Alphabet['/'] = 63; for (int i = 0; i <= 25; i++) { lookUpBase64Alphabet[i] = (byte) ('A' + i); } for (int i = 26, j = 0; i <= 51; i++, j++) { lookUpBase64Alphabet[i] = (byte) ('a' + j); } for (int i = 52, j = 0; i <= 61; i++, j++) { lookUpBase64Alphabet[i] = (byte) ('0' + j); } lookUpBase64Alphabet[62] = (byte) '+'; lookUpBase64Alphabet[63] = (byte) '/'; } /** * Decodes Base64 data into octects * * @param base64Data * Byte array containing Base64 data * @return Array containing decoded data. */ public static byte[] decodeBase64(byte[] base64Data) { // RFC 2045 requires that we discard ALL non-Base64 characters base64Data = discardNonBase64(base64Data); // handle the edge case, so we don't have to worry about it // later if (base64Data.length == 0) { return new byte[0]; } int numberQuadruple = base64Data.length / FOURBYTE; byte decodedData[] = null; byte b1 = 0, b2 = 0, b3 = 0, b4 = 0, marker0 = 0, marker1 = 0; // Throw away anything not in base64Data int encodedIndex = 0; int dataIndex = 0; { // this sizes the output array properly - rlw int lastData = base64Data.length; // ignore the '=' padding while (base64Data[lastData - 1] == PAD) { if (--lastData == 0) { return new byte[0]; } } decodedData = new byte[lastData - numberQuadruple]; } for (int i = 0; i < numberQuadruple; i++) { dataIndex = i * 4; marker0 = base64Data[dataIndex + 2]; marker1 = base64Data[dataIndex + 3]; b1 = base64Alphabet[base64Data[dataIndex]]; b2 = base64Alphabet[base64Data[dataIndex + 1]]; if (marker0 != PAD && marker1 != PAD) { // No PAD e.g 3cQl b3 = base64Alphabet[marker0]; b4 = base64Alphabet[marker1]; decodedData[encodedIndex] = (byte) (b1 << 2 | b2 >> 4); decodedData[encodedIndex + 1] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf)); decodedData[encodedIndex + 2] = (byte) (b3 << 6 | b4); } else if (marker0 == PAD) { // Two PAD e.g. 3c[Pad][Pad] decodedData[encodedIndex] = (byte) (b1 << 2 | b2 >> 4); } else if (marker1 == PAD) { // One PAD e.g. 3cQ[Pad] b3 = base64Alphabet[marker0]; decodedData[encodedIndex] = (byte) (b1 << 2 | b2 >> 4); decodedData[encodedIndex + 1] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf)); } encodedIndex += 3; } return decodedData; } /** * Discards any characters outside of the base64 alphabet, per the * requirements on page 25 of RFC 2045 - "Any characters outside of the * base64 alphabet are to be ignored in base64 encoded data." * * @param data * The base-64 encoded data to groom * @return The data, less non-base64 characters (see RFC 2045). */ static byte[] discardNonBase64(byte[] data) { byte groomedData[] = new byte[data.length]; int bytesCopied = 0; for (int i = 0; i < data.length; i++) { if (isBase64(data[i])) { groomedData[bytesCopied++] = data[i]; } } byte packedData[] = new byte[bytesCopied]; System.arraycopy(groomedData, 0, packedData, 0, bytesCopied); return packedData; } /** * Discards any whitespace from a base-64 encoded block. * * @param data * The base-64 encoded data to discard the whitespace from. * @return The data, less whitespace (see RFC 2045). */ static byte[] discardWhitespace(byte[] data) { byte groomedData[] = new byte[data.length]; int bytesCopied = 0; for (int i = 0; i < data.length; i++) { switch (data[i]) { case (byte) ' ': case (byte) '\n': case (byte) '\r': case (byte) '\t': break; default: groomedData[bytesCopied++] = data[i]; } } byte packedData[] = new byte[bytesCopied]; System.arraycopy(groomedData, 0, packedData, 0, bytesCopied); return packedData; } /** * Encodes binary data using the base64 algorithm but does not chunk the * output. * * @param binaryData * binary data to encode * @return Base64 characters */ public static byte[] encodeBase64(byte[] binaryData) { return encodeBase64(binaryData, false); } /** * Encodes binary data using the base64 algorithm, optionally chunking the * output into 76 character blocks. * * @param binaryData * Array containing binary data to encode. * @param isChunked * if isChunked is true this encoder will chunk the base64 output * into 76 character blocks * @return Base64-encoded data. */ public static byte[] encodeBase64(byte[] binaryData, boolean isChunked) { int lengthDataBits = binaryData.length * EIGHTBIT; int fewerThan24bits = lengthDataBits % TWENTYFOURBITGROUP; int numberTriplets = lengthDataBits / TWENTYFOURBITGROUP; byte encodedData[] = null; int encodedDataLength = 0; int nbrChunks = 0; if (fewerThan24bits != 0) { // data not divisible by 24 bit encodedDataLength = (numberTriplets + 1) * 4; } else { // 16 or 8 bit encodedDataLength = numberTriplets * 4; } // If the output is to be "chunked" into 76 character // sections, // for compliance with RFC 2045 MIME, then it is important to // allow for extra length to account for the separator(s) if (isChunked) { nbrChunks = (CHUNK_SEPARATOR.length == 0 ? 0 : (int) Math .ceil((float) encodedDataLength / CHUNK_SIZE)); encodedDataLength += nbrChunks * CHUNK_SEPARATOR.length; } encodedData = new byte[encodedDataLength]; byte k = 0, l = 0, b1 = 0, b2 = 0, b3 = 0; int encodedIndex = 0; int dataIndex = 0; int i = 0; int nextSeparatorIndex = CHUNK_SIZE; int chunksSoFar = 0; // log.debug("number of triplets = " + numberTriplets); for (i = 0; i < numberTriplets; i++) { dataIndex = i * 3; b1 = binaryData[dataIndex]; b2 = binaryData[dataIndex + 1]; b3 = binaryData[dataIndex + 2]; // log.debug("b1= " + b1 +", b2= " + b2 + ", b3= " + b3); l = (byte) (b2 & 0x0f); k = (byte) (b1 & 0x03); byte val1 = ((b1 & SIGN) == 0) ? (byte) (b1 >> 2) : (byte) ((b1) >> 2 ^ 0xc0); byte val2 = ((b2 & SIGN) == 0) ? (byte) (b2 >> 4) : (byte) ((b2) >> 4 ^ 0xf0); byte val3 = ((b3 & SIGN) == 0) ? (byte) (b3 >> 6) : (byte) ((b3) >> 6 ^ 0xfc); encodedData[encodedIndex] = lookUpBase64Alphabet[val1]; // log.debug( "val2 = " + val2 ); // log.debug( "k4 = " + (k<<4) ); // log.debug( "vak = " + (val2 | (k<<4)) ); encodedData[encodedIndex + 1] = lookUpBase64Alphabet[val2 | (k << 4)]; encodedData[encodedIndex + 2] = lookUpBase64Alphabet[(l << 2) | val3]; encodedData[encodedIndex + 3] = lookUpBase64Alphabet[b3 & 0x3f]; encodedIndex += 4; // If we are chunking, let's put a chunk separator down. if (isChunked) { // this assumes that CHUNK_SIZE % 4 == 0 if (encodedIndex == nextSeparatorIndex) { System.arraycopy(CHUNK_SEPARATOR, 0, encodedData, encodedIndex, CHUNK_SEPARATOR.length); chunksSoFar++; nextSeparatorIndex = (CHUNK_SIZE * (chunksSoFar + 1)) + (chunksSoFar * CHUNK_SEPARATOR.length); encodedIndex += CHUNK_SEPARATOR.length; } } } // form integral number of 6-bit groups dataIndex = i * 3; if (fewerThan24bits == EIGHTBIT) { b1 = binaryData[dataIndex]; k = (byte) (b1 & 0x03); // log.debug("b1=" + b1); // log.debug("b1<<2 = " + (b1>>2) ); byte val1 = ((b1 & SIGN) == 0) ? (byte) (b1 >> 2) : (byte) ((b1) >> 2 ^ 0xc0); encodedData[encodedIndex] = lookUpBase64Alphabet[val1]; encodedData[encodedIndex + 1] = lookUpBase64Alphabet[k << 4]; encodedData[encodedIndex + 2] = PAD; encodedData[encodedIndex + 3] = PAD; } else if (fewerThan24bits == SIXTEENBIT) { b1 = binaryData[dataIndex]; b2 = binaryData[dataIndex + 1]; l = (byte) (b2 & 0x0f); k = (byte) (b1 & 0x03); byte val1 = ((b1 & SIGN) == 0) ? (byte) (b1 >> 2) : (byte) ((b1) >> 2 ^ 0xc0); byte val2 = ((b2 & SIGN) == 0) ? (byte) (b2 >> 4) : (byte) ((b2) >> 4 ^ 0xf0); encodedData[encodedIndex] = lookUpBase64Alphabet[val1]; encodedData[encodedIndex + 1] = lookUpBase64Alphabet[val2 | (k << 4)]; encodedData[encodedIndex + 2] = lookUpBase64Alphabet[l << 2]; encodedData[encodedIndex + 3] = PAD; } if (isChunked) { // we also add a separator to the end of the final chunk. if (chunksSoFar < nbrChunks) { System.arraycopy(CHUNK_SEPARATOR, 0, encodedData, encodedDataLength - CHUNK_SEPARATOR.length, CHUNK_SEPARATOR.length); } } return encodedData; } /** * Encodes binary data using the base64 algorithm and chunks the encoded * output into 76 character blocks * * @param binaryData * binary data to encode * @return Base64 characters chunked in 76 character blocks */ public static byte[] encodeBase64Chunked(byte[] binaryData) { return encodeBase64(binaryData, true); } /** * Tests a given byte array to see if it contains only valid characters * within the Base64 alphabet. * * @param arrayOctect * byte array to test * @return true if all bytes are valid characters in the Base64 alphabet or * if the byte array is empty; false, otherwise */ public static boolean isArrayByteBase64(byte[] arrayOctect) { arrayOctect = discardWhitespace(arrayOctect); int length = arrayOctect.length; if (length == 0) { // shouldn't a 0 length array be valid base64 data? // return false; return true; } for (int i = 0; i < length; i++) { if (!isBase64(arrayOctect[i])) { return false; } } return true; } private static boolean isBase64(byte octect) { if (octect == PAD) { return true; } else if (base64Alphabet[0xff & octect] == -1) { return false; } else { return true; } } /** * Decodes a byte[] containing containing characters in the Base64 alphabet. * * @param pArray * A byte array containing Base64 character data * @return a byte array containing binary data */ public byte[] decode(byte[] pArray) { return decodeBase64(pArray); } /** * Encodes a byte[] containing binary data, into a byte[] containing * characters in the Base64 alphabet. * * @param pArray * a byte array containing binary data * @return A byte array containing only Base64 character data */ public byte[] encode(byte[] pArray) { return encodeBase64(pArray, false); } public static byte[] encodeXml(byte src[]) { ByteArrayOutputStream baos = new ByteArrayOutputStream(); ByteArrayOutputStream text_os = new ByteArrayOutputStream(); byte tag_left = (byte) '<'; byte tag_right = (byte) '>'; boolean begin = false; boolean in_tag = false; for (int i = 0; i < src.length; i++) { byte b = src[i]; if (begin == false && b == tag_left) begin = true; if (b == tag_left) in_tag = true; else if (b == tag_right) in_tag = false; if (!begin) continue; if (in_tag || b == tag_right) { if (b == tag_left) { byte[] text = text_os.toByteArray(); try { baos.write(encodeBase64(text)); } catch (IOException e) { e.printStackTrace(); } text_os.reset(); } baos.write(b); } else { text_os.write(b); } } return baos.toByteArray(); } } ~~~