一、概述
1.Base64是什么：
Base64是網絡上最常見的用于傳輸8Bit字節代碼的編碼方式之一，大家可以查看RFC2045～RFC2049，上面有MIME的詳細規范。Base64編碼可用于在HTTP環境下傳遞較長的標識信息。例如，在Java Persistence系統Hibernate中，就采用了Base64來將一個較長的唯一標識符（一般為128-bit的UUID）編碼為一個字符串，用作HTTP表單和HTTP GET URL中的參數。在其他應用程序中，也常常需要把二進制數據編碼為適合放在URL（包括隱藏表單域）中的形式。此時，采用Base64編碼不僅比較簡短，同時也具有不可讀性，即所編碼的數據不會被人用肉眼所直接看到
2.簡介：
標準的Base64并不適合直接放在URL里傳輸，因為URL編碼器會把標準Base64中的“/”和“+”字符變為形如“%XX”的形式，而這些“%”號在存入數據庫時還需要再進行轉換，因為ANSI SQL中已將“%”號用作通配符。
為解決此問題，可采用一種用于URL的改進Base64編碼，它不在末尾填充'='號，并將標準Base64中的“+”和“/”分別改成了“*”和“-”，這樣就免去了在URL編解碼和數據庫存儲時所要作的轉換，避免了編碼信息長度在此過程中的增加，并統一了數據庫、表單等處對象標識符的格式。
另有一種用于正則表達式的改進Base64變種，它將“+”和“/”改成了“!”和“-”，因為“+”,“*”以及前面在IRCu中用到的“[”和“]”在正則表達式中都可能具有特殊含義。
此外還有一些變種，它們將“+/”改為“_-”或“._”（用作編程語言中的標識符名稱）或“.-”（用于XML中的Nmtoken）甚至“_:”（用于XML中的Name）。
Base64要求把每三個8Bit的字節轉換為四個6Bit的字節（3*8 = 4*6 = 24），然后把6Bit再添兩位高位0，組成四個8Bit的字節，也就是說，轉換后的字符串理論上將要比原來的長1/3。
3.規則：
關于這個編碼的規則：
①.把3個字符變成4個字符..
②.每76個字符加一個換行符..
③.最后的結束符也要處理..
這樣說會不會太抽象了？不用著急，我們來看一個例子：
轉換前： aaaaaabb ccccdddd eeffffff
轉換后： 00aaaaaa 00bbcccc 00ddddee 00ffffff
應該很清楚了吧？上面的三個字節是原文，下面的四個字節是轉換后的Base64編碼，其前兩位均為0。
轉換后，我們用一個碼表來得到我們想要的字符串（也就是最終的Base64編碼）
二、java實現代碼示例：
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public final class Base64 { static private final int BASELENGTH = 255; static private final int LOOKUPLENGTH = 64; static private final int TWENTYFOURBITGROUP = 24; static private final int EIGHTBIT = 8; static private final int SIXTEENBIT = 16; static private final int SIXBIT = 6; static private final int FOURBYTE = 4; static private final int SIGN = -128; static private final char PAD = '='; static private final boolean fDebug = false; static final private byte[] base64Alphabet = new byte[BASELENGTH]; static final private char[] lookUpBase64Alphabet = new char[LOOKUPLENGTH]; static { for (int i = 0; i < BASELENGTH; i++) { base64Alphabet[i] = -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] = (char) ('A' + i); for (int i = 26, j = 0; i <= 51; i++, j++) lookUpBase64Alphabet[i] = (char) ('a' + j); for (int i = 52, j = 0; i <= 61; i++, j++) lookUpBase64Alphabet[i] = (char) ('0' + j); lookUpBase64Alphabet[62] = (char) '+'; lookUpBase64Alphabet[63] = (char) '/'; } protected static boolean isWhiteSpace(char octect) { return (octect == 0x20 || octect == 0xd || octect == 0xa || octect == 0x9); } protected static boolean isPad(char octect) { return (octect == PAD); } protected static boolean isData(char octect) { return (base64Alphabet[octect] != -1); } protected static boolean isBase64(char octect) { return (isWhiteSpace(octect) || isPad(octect) || isData(octect)); } /** * Encodes hex octects into Base64 * * @param binaryData * Array containing binaryData * @return Encoded Base64 array */ public static String encode(byte[] binaryData) { if (binaryData == null) return null; int lengthDataBits = binaryData.length * EIGHTBIT; if (lengthDataBits == 0) { return ""; } int fewerThan24bits = lengthDataBits % TWENTYFOURBITGROUP; int numberTriplets = lengthDataBits / TWENTYFOURBITGROUP; int numberQuartet = fewerThan24bits != 0 ? numberTriplets + 1 : numberTriplets; int numberLines = (numberQuartet - 1) / 19 + 1; char encodedData[] = null; encodedData = new char[numberQuartet * 4 + numberLines]; byte k = 0, l = 0, b1 = 0, b2 = 0, b3 = 0; int encodedIndex = 0; int dataIndex = 0; int i = 0; if (fDebug) { System.out.println("number of triplets = " + numberTriplets); } for (int line = 0; line < numberLines - 1; line++) { for (int quartet = 0; quartet < 19; quartet++) { b1 = binaryData[dataIndex++]; b2 = binaryData[dataIndex++]; b3 = binaryData[dataIndex++]; if (fDebug) { System.out.println("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); if (fDebug) { System.out.println("val2 = " + val2); System.out.println("k4 = " + (k << 4)); System.out.println("vak = " + (val2 | (k << 4))); } encodedData[encodedIndex++] = lookUpBase64Alphabet[val1]; encodedData[encodedIndex++] = lookUpBase64Alphabet[val2 | (k << 4)]; encodedData[encodedIndex++] = lookUpBase64Alphabet[(l << 2) | val3]; encodedData[encodedIndex++] = lookUpBase64Alphabet[b3 & 0x3f]; i++; } encodedData[encodedIndex++] = 0xa; } for (; i < numberTriplets; i++) { b1 = binaryData[dataIndex++]; b2 = binaryData[dataIndex++]; b3 = binaryData[dataIndex++]; if (fDebug) { System.out.println("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); if (fDebug) { System.out.println("val2 = " + val2); System.out.println("k4 = " + (k << 4)); System.out.println("vak = " + (val2 | (k << 4))); } encodedData[encodedIndex++] = lookUpBase64Alphabet[val1]; encodedData[encodedIndex++] = lookUpBase64Alphabet[val2 | (k << 4)]; encodedData[encodedIndex++] = lookUpBase64Alphabet[(l << 2) | val3]; encodedData[encodedIndex++] = lookUpBase64Alphabet[b3 & 0x3f]; } // form integral number of 6-bit groups if (fewerThan24bits == EIGHTBIT) { b1 = binaryData[dataIndex]; k = (byte) (b1 & 0x03); if (fDebug) { System.out.println("b1=" + b1); System.out.println("b1<<2 = " + (b1 >> 2)); } byte val1 = ((b1 & SIGN) == 0) ? (byte) (b1 >> 2) : (byte) ((b1) >> 2 ^ 0xc0); encodedData[encodedIndex++] = lookUpBase64Alphabet[val1]; encodedData[encodedIndex++] = lookUpBase64Alphabet[k << 4]; encodedData[encodedIndex++] = PAD; encodedData[encodedIndex++] = 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++] = lookUpBase64Alphabet[val2 | (k << 4)]; encodedData[encodedIndex++] = lookUpBase64Alphabet[l << 2]; encodedData[encodedIndex++] = PAD; } encodedData[encodedIndex] = 0xa; return new String(encodedData); } /** * Decodes Base64 data into octects * * @param binaryData * Byte array containing Base64 data * @return Array containind decoded data. */ public static byte[] decode(String encoded) { if (encoded == null) return null; char[] base64Data = encoded.toCharArray(); // remove white spaces int len = removeWhiteSpace(base64Data); if (len % FOURBYTE != 0) { return null;// should be divisible by four } int numberQuadruple = (len / FOURBYTE); if (numberQuadruple == 0) return new byte[0]; byte decodedData[] = null; byte b1 = 0, b2 = 0, b3 = 0, b4 = 0, marker0 = 0, marker1 = 0; char d1 = 0, d2 = 0, d3 = 0, d4 = 0; int i = 0; int encodedIndex = 0; int dataIndex = 0; decodedData = new byte[(numberQuadruple) * 3]; for (; i < numberQuadruple - 1; i++) { if (!isData((d1 = base64Data[dataIndex++])) || !isData((d2 = base64Data[dataIndex++])) || !isData((d3 = base64Data[dataIndex++])) || !isData((d4 = base64Data[dataIndex++]))) return null;// if found "no data" just return null b1 = base64Alphabet[d1]; b2 = base64Alphabet[d2]; b3 = base64Alphabet[d3]; b4 = base64Alphabet[d4]; decodedData[encodedIndex++] = (byte) (b1 << 2 | b2 >> 4); decodedData[encodedIndex++] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf)); decodedData[encodedIndex++] = (byte) (b3 << 6 | b4); } if (!isData((d1 = base64Data[dataIndex++])) || !isData((d2 = base64Data[dataIndex++]))) { return null;// if found "no data" just return null } b1 = base64Alphabet[d1]; b2 = base64Alphabet[d2]; d3 = base64Data[dataIndex++]; d4 = base64Data[dataIndex++]; if (!isData((d3)) || !isData((d4))) {// Check if they are PAD characters if (isPad(d3) && isPad(d4)) { // Two PAD e.g. 3c[Pad][Pad] if ((b2 & 0xf) != 0)// last 4 bits should be zero return null; byte[] tmp = new byte[i * 3 + 1]; System.arraycopy(decodedData, 0, tmp, 0, i * 3); tmp[encodedIndex] = (byte) (b1 << 2 | b2 >> 4); return tmp; } else if (!isPad(d3) && isPad(d4)) { // One PAD e.g. 3cQ[Pad] b3 = base64Alphabet[d3]; if ((b3 & 0x3) != 0)// last 2 bits should be zero return null; byte[] tmp = new byte[i * 3 + 2]; System.arraycopy(decodedData, 0, tmp, 0, i * 3); tmp[encodedIndex++] = (byte) (b1 << 2 | b2 >> 4); tmp[encodedIndex] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf)); return tmp; } else { return null;// an error like "3c[Pad]r", "3cdX", "3cXd", "3cXX" // where X is non data } } else { // No PAD e.g 3cQl b3 = base64Alphabet[d3]; b4 = base64Alphabet[d4]; decodedData[encodedIndex++] = (byte) (b1 << 2 | b2 >> 4); decodedData[encodedIndex++] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf)); decodedData[encodedIndex++] = (byte) (b3 << 6 | b4); } return decodedData; } /** * remove WhiteSpace from MIME containing encoded Base64 data. * * @param data * the byte array of base64 data (with WS) * @return the new length */ protected static int removeWhiteSpace(char[] data) { if (data == null) return 0; // count characters that's not whitespace int newSize = 0; int len = data.length; for (int i = 0; i < len; i++) { if (!isWhiteSpace(data[i])) data[newSize++] = data[i]; } return newSize; } public static void main(String[] args) { System.out.println(encode("中華人民共和國".getBytes())); }} |
