原由

项目中经常需要使用base64进行处理，通过base64可以将特殊字符转化为普通可见字符，便于网络传输，代价是增长了传输长度。

base64将每3个byte转化为4个6bit位，然后高位补两个零。这意味着，base64编码后长度会变长。

1. 当源文长度不是3的倍数时，需要补零。编码后以等号“=”表示。至多有1至2个补零的情况出现，则结尾最多一至两个等号。
2. 编码后长度可预测4*（len+2)/3。
3. base64是个可逆编码。
4. 源文长度预估如下：末尾等号个数为p（此时p小于等于2），编码长度l (此时l为4的倍数)，则源文长度 ( l * 3/4 - p)

源文	源长度	base编码后	编码后长度
abc	3	YWJj	4
abca	4	YWJjYQ==	8
abcab	5	YWJjYWI=	8
abcabc	6	YWJjYWJj	8

 

 

 

源码请见：

C语言实现

base64的源码其实没有几行，但是在项目代码中添加base64源码总有造轮子的嫌疑。这里使用libopenssl库。

1 #include 
     
       2 #include 
      
        3 #include 
       
         4 #include 
        
          5 #include 
         
           6 #include 
          
            7 8 int Base64Encode(const char* message, char** buffer) { //Encodes a string to base64 9 BIO *bio, *b64;10 FILE* stream;11 int encodedSize = 4*ceil((double)strlen(message)/3);12 *buffer = (char *)malloc(encodedSize+1);13 14 stream = fmemopen(*buffer, encodedSize+1, "w");15 b64 = BIO_new(BIO_f_base64());16 bio = BIO_new_fp(stream, BIO_NOCLOSE);17 bio = BIO_push(b64, bio);18 BIO_set_flags(bio, BIO_FLAGS_BASE64_NO_NL); //Ignore newlines - write everything in one line19 BIO_write(bio, message, strlen(message));20 BIO_flush(bio);21 BIO_free_all(bio);22 fclose(stream);23 24 return (0); //success25 }26 int calcDecodeLength(const char* b64input) { //Calculates the length of a decoded base64 string27 int len = strlen(b64input);28 int padding = 0;29 30 if (b64input[len-1] == '=' && b64input[len-2] == '=') //last two chars are =31 padding = 2;32 else if (b64input[len-1] == '=') //last char is =33 padding = 1;34 35 return (int)len*0.75 - padding;36 }37 38 int Base64Decode(char* b64message, char** buffer) { //Decodes a base64 encoded string39 BIO *bio, *b64;40 int decodeLen = calcDecodeLength(b64message),41 len = 0;42 *buffer = (char*)malloc(decodeLen+1);43 FILE* stream = fmemopen(b64message, strlen(b64message), "r");44 45 b64 = BIO_new(BIO_f_base64());46 bio = BIO_new_fp(stream, BIO_NOCLOSE);47 bio = BIO_push(b64, bio);48 BIO_set_flags(bio, BIO_FLAGS_BASE64_NO_NL); //Do not use newlines to flush buffer49 len = BIO_read(bio, *buffer, strlen(b64message));50 //Can test here if len == decodeLen - if not, then return an error51 (*buffer)[len] = '\0';52 53 BIO_free_all(bio);54 fclose(stream);55 56 return (0); //success57 }58 59 int main() {60 //Encode To Base6461 char* base64EncodeOutput;62 char* pPlainText = "Hi, I'm fpzeng";63 printf("Input plain text: %s\n", pPlainText);64 Base64Encode(pPlainText, &base64EncodeOutput);65 printf("Output (base64): %s\n", base64EncodeOutput);66 67 //Decode From Base6468 char* base64DecodeOutput;69 Base64Decode(base64EncodeOutput, &base64DecodeOutput);70 printf("Output: %s\n", base64DecodeOutput);71 72 return(0);73 }
          
         
        
       
      
     

 

Makefile 

.PHONY : cleanOPT=-O0DEBUG= -gCFLAGS=-Wall -fPIC $(XCFLAGS) $(INC) $(OPT) $(SO_DEF) $(DEBUG) $(DEF) INCLUDES = -I./ SRCS = base64.c OBJS = $(SRCS:.c=.o)LIBS = -lssl -lcrypto -lm MAIN = base64 all:    $(MAIN)	@echo  $(MAIN) has been compiled$(MAIN): $(OBJS) 	$(CC) $(CFLAGS) $(INCLUDES) -o $(MAIN) $(OBJS) $(LFLAGS) $(LIBS)	@echo output: $(OBJS).c.o:	$(CC) $(CFLAGS) $(INCLUDES) -c $<  -o $@ clean: 		@echo clean $(MAIN) $(OBJS)	rm -fr $(MAIN) $(OBJS)

　　

Python实现