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。

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。
编码后长度可预测4*（len+2)/3。
base64是个可逆编码。
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：https://github.com/fpzeng/aes128

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。

 #include <stdio.h>

 #include <string.h>

 #include <openssl/bio.h>

 #include <openssl/evp.h>

 #include <stdio.h>

 #include <math.h>

 int Base64Encode(const char* message, char** buffer) { //Encodes a string to base64

   BIO *bio, *b64;

   FILE* stream;

   int encodedSize = *ceil((double)strlen(message)/);

   *buffer = (char *)malloc(encodedSize+);

   stream = fmemopen(*buffer, encodedSize+, "w");

   b64 = BIO_new(BIO_f_base64());

   bio = BIO_new_fp(stream, BIO_NOCLOSE);

   bio = BIO_push(b64, bio);

   BIO_set_flags(bio, BIO_FLAGS_BASE64_NO_NL); //Ignore newlines - write everything in one line

   BIO_write(bio, message, strlen(message));

   BIO_flush(bio);

   BIO_free_all(bio);

   fclose(stream);

   return (); //success

 }

 int calcDecodeLength(const char* b64input) { //Calculates the length of a decoded base64 string

   int len = strlen(b64input);

   int padding = ;

   if (b64input[len-] == '=' && b64input[len-] == '=') //last two chars are =

     padding = ;

   else if (b64input[len-] == '=') //last char is =

     padding = ;

   return (int)len*0.75 - padding;

 }

 int Base64Decode(char* b64message, char** buffer) { //Decodes a base64 encoded string

   BIO *bio, *b64;

   int decodeLen = calcDecodeLength(b64message),

       len = ;

   *buffer = (char*)malloc(decodeLen+);

   FILE* stream = fmemopen(b64message, strlen(b64message), "r");

   b64 = BIO_new(BIO_f_base64());

   bio = BIO_new_fp(stream, BIO_NOCLOSE);

   bio = BIO_push(b64, bio);

   BIO_set_flags(bio, BIO_FLAGS_BASE64_NO_NL); //Do not use newlines to flush buffer

   len = BIO_read(bio, *buffer, strlen(b64message));

     //Can test here if len == decodeLen - if not, then return an error

   (*buffer)[len] = '\0';

   BIO_free_all(bio);

   fclose(stream);

   return (); //success

 }

 int main() {

   //Encode To Base64

   char* base64EncodeOutput;

   char* pPlainText = "Hi, I'm fpzeng";

   printf("Input plain text: %s\n", pPlainText);

   Base64Encode(pPlainText, &base64EncodeOutput);

   printf("Output (base64): %s\n", base64EncodeOutput);

   //Decode From Base64

   char* base64DecodeOutput;

   Base64Decode(base64EncodeOutput, &base64DecodeOutput);

   printf("Output: %s\n", base64DecodeOutput);

   return();

 }

Makefile

.PHONY : clean

OPT=-O0

DEBUG= -g

CFLAGS=-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)

4的

BASE64的实现

原由

C语言实现

Makefile

Python实现