Base64 encoding and decoding in C Language

Base64 Is based on 64 Printable characters to represent the representation of binary data. Because 26 = 64 , So every 6 It is a unit that corresponds to a printable character. Three bytes in total 24 Bit, corresponding 4 Items Base64 Unit, that is 3 Bytes 4 Printable characters. It is often used as the transmission code of emails. In Base64 The printable characters include uppercase letters.A-Z , Lowercase letters A-z Arabic numerals 0-9 . 62 Characters, and the two printable symbols are different in different systems. Generally, the plus sign ( + ) And forward slash ( / ). Plus the "completion symbol", usually with the equal sign ( = ).

complete base64 definition visibility RFC 1421 and RFC 2045 . The encoded data is slightly longer than the original data, which is the original 4/3 . In the email, according to RFC 822 , each 76 character must contain a carriage return and line break. It can be estimated that the length of the encoded data is about 135.1% .

Base64When encoding, put three pieces of data into one24In the buffer, the first-come buffer occupies a high position. Insufficient data3Bytes, the remaining bits in the buffer are used0Complement. Then, retrieve 6 at a time (because26 = 64) Bit, select according to its valueAbcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789 +/As the encoded output. Continue until all input data is converted. If the last two inputs are left, add one"="; If the last input data is left, the encoding result is followed by two"="; If there is no data left, do not add anything. This ensures the correctness of data restoration.

CLanguageSource codeAs follows:

 /* ** Base64 encoding and decoding implementation * C language sourceCode** Ye Jianfei ** instructions for use: * command line parameter description: if the "-d" parameter exists, it is base64 decoded; otherwise, it is base64 encoded. * The input comes from the standard input stdin, and the output is the standard output stdout. You can redirect input and output streams. ** Base64 encoding: enter any binary stream and read it until the file is fully read (the keyboard input ends with a file Terminator ). * Outputs the base64 encoding of plain text. ** Base64 decoding: base64 encoding of the input plain text. It can be read until the file is fully read (the keyboard input ends with a file Terminator ). * Output the original binary stream. *  */  # Include <Stdio. h> # Include <Stdlib. h> # Include <String. h> Const   Char * Base64char = " Abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789 +/  "  ;  Char * Base64_encode ( Const Unsigned Char * Bindata, Char * Base64, Int  Binlength ){  Int  I, J; unsigned  Char  Current;  For (I =0 , J = 0 ; I <binlength; I + = 3  ) {Current = (Bindata [I]> 2  ); Current & = (Unsigned Char ) 0x3f  ; Base64 [J ++] = Base64char [( Int  ) Current]; Current = (Unsigned Char ) (Bindata [I] <4 ) & (Unsigned Char ) 0x30  );  If (I + 1 > = Binlength) {base64 [J ++] = Base64char [( Int  ) Current]; base64 [J ++] = '  =  '  ; Base64 [J ++] ='  =  '  ;  Break  ;} Current | = (Unsigned Char ) (Bindata [I + 1 ]> 4 ) & (Unsigned Char ) 0x0f  ); Base64 [J ++] = Base64char [( Int  ) Current]; Current = (Unsigned Char ) (Bindata [I + 1 ] < 2 ) & (Unsigned Char ) 0x3c  );  If (I + 2 > = Binlength) {base64 [J ++] = Base64char [( Int  ) Current]; base64 [J ++] = '  = '  ;  Break  ;} Current | = (Unsigned Char ) (Bindata [I + 2 ]> 6 ) & (Unsigned Char ) 0x03  ); Base64 [J ++] = Base64char [( Int  ) Current]; Current = (Unsigned Char ) Bindata [I +2 ]) & (Unsigned Char ) 0x3f  ); Base64 [J ++] = Base64char [( Int  ) Current];} base64 [J] = '  \ 0  '  ;  Return  Base64 ;}  Int Base64_decode ( Const   Char * Base64, unsigned Char * Bindata ){  Int  I, J; unsigned  Char  K; unsigned  Char Temp [ 4  ];  For (I = 0 , J = 0 ; Base64 [I]! = '  \ 0  ' ; I + = 4  ) {Memset (temp,  0xff , Sizeof  (Temp ));  For (K = 0 ; K < 64 ; K ++ ){  If (Base64char [k] = Base64 [I]) temp [  0 ] = K ;} For (K = 0 ; K < 64 ; K ++ ){  If (Base64char [k] = base64 [I + 1  ]) Temp [  1 ] = K ;}  For (K = 0 ; K < 64 ; K ++ ){ If (Base64char [k] = base64 [I + 2  ]) Temp [  2 ] = K ;}  For (K = 0 ; K < 64 ; K ++ ){  If (Base64char [k] = base64 [I + 3  ]) Temp [  3 ] =K;} bindata [J ++] = (Unsigned Char ) (Unsigned Char ) (Temp [ 0 ] < 2 ))& 0xfc ) | (Unsigned  Char ) (Unsigned Char ) (Temp [ 1 ]> 4 )& 0x03  )); If (Base64 [I + 2 ] = '  =  '  )  Break  ; Bindata [J ++] = (Unsigned Char ) (Unsigned Char ) (Temp [ 1 ] < 4 ))& 0xf0 ) | (Unsigned Char ) (Unsigned Char ) (Temp [ 2 ]> 2 )& 0x0f  ));  If (Base64 [I + 3 ] = '  =  '  )  Break  ; Bindata [J ++] = (UnsignedChar ) (Unsigned Char ) (Temp [ 2 ] < 6 ))& 0xf0 ) | (Unsigned  Char ) (Temp [ 3 ] & 0x3f  ));}  Return  J ;}  Int Main ( Int Argc,Char * Argv []) {  Int  I; unsigned  Char Bindata [ 2050  ];  Char Base64 [ 4096  ]; Size_t bytes;  If (Argc = 1  ){  //  Encode         While (! Feof (stdin) {bytes = Fread (bindata, 1 , 2049  , Stdin); base64_encode (bindata, base64, bytes); printf (  "  % S  "  , Base64 );}}  Else   If (Argc = 2 &&! Strcmp (argv [ 1 ],"  -D  "  )){  //  Decode          While (! Feof (stdin )){  For (I = 0 ; I < 2048 ; I ++ ) {Base64 [I] = Getchar ();  If (Base64 [I] =EOF)  Break  ;  Else   If (Base64 [I] = '  \ N  ' | Base64 [I] = '  \ R  '  ) I -- ;} Bytes = Base64_decode (base64, bindata); fwrite (bindata, bytes, 1  , Stdout );}}  Else  {Fprintf (stderr,  "  Usage: % s [-D] \ n \ t-d \ tdecode data \ n  " , Argv [ 0  ]);  Return  Exit_failure ;}  Return  Exit_success ;}