clone53421's Journal: Base64 decoder (80x86)

Decodes standard MIME-encoded base64 data. (Non-printable characters won't be visible, but they are echoed. E.g. the bell character, 07, should cause the terminal to beep.)

Every 4 characters typed will be decoded to the corresponding 3 characters. (Nothing will be echoed until 4 input characters have been typed.) Any input characters outside the base64 character table will be ignored and will not count toward the 4-character sequence.

The = character is a special character in base64 encoding: it is only used to pad the end of base64-encoded data when encoding the last 1 or 2 bytes (instead of the 3 bytes needed to have a full 24-bit chunk). Although the base64 encoding standard does not allow = symbols in the middle of base64-encoded output, the code does not attempt to verify this. The = symbol is treated as if it was an A (index 0) and it adjusts the number of output characters that are printed: If the 4-character sequence contains one = symbol, only the high 16 bits will be printed (2 characters). If it contains two = symbols, only the high 8 bits will be printed (1 character). Although sequences containing more than two = characters are not permitted according to the base64-encoding standard, the code will still treat them like A's (index 0) and print only the high 8 bits.

See also: Base64 encoder (80x86)

Paste in Notepad, save as base64de_asm.bat, run.

@echo off
goto batch
 
a
;Base64 decoder
;Accepts input from STDIN and prints to STDOUT
;Input bytes are buffered and decoded at intervals of 4 bytes
;Esc or Ctrl-Z exit, any buffered input will be discarded
;_input
        mov si,01cc ;_outbuffer
        mov bx,0003 ;loop counter
        mov dx,0002 ;counts output chars
;_inloop
        mov di,018c ;_charset
        mov ah,08
        int 21 ;get a key
        cmp al,1b ;check for esc
        jnz 013a ;_continue
;_quit
        mov ah,03
        xor bh,bh
        int 10 ;get cursor position
        mov ah,08
        int 10 ;preserve character under cursor
        mov ah,02
        mov bl,al
        mov cx,dx
        mov dl,1a
        int 21 ;send ^z
        mov dx,cx
        int 10 ;restore cursor position
        mov ah,0a
        mov al,bl
        mov cx,1
        int 10 ;restore character under cursor
        mov ax,4c01
        int 21 ;quit to dos
;_continue
        cmp al,1a ;check for ^z
        jz 0114 ;_quit
        cmp al,3d ;check for = (padding)
        jnz 0147 ;_process
        dec dx ;subtract an output character
        xor al,al
        jmp 0150 ;_skip
;_process
        mov cx,0040 ;64 = number of valid chars
        repnz
        scasb ;find al
        jnz 0109 ;_inloop
        mov al,cl
;_skip
        shl al,1
        shl al,1
        mov ah,[si]
        mov cl,06
        rol ax,cl
        mov [si],ah
        shl al,1
        shl al,1
        mov ah,[si+1]
        rol ax,cl
        mov [si+1],ah
        shl al,1
        shl al,1
        mov ah,[si+2]
        shl ax,cl
        mov [si+2],ah
        dec bx
        jns 0109 ;_inloop
;_output
        mov bx,0002
        sub bx,dx
        add si,bx
        mov bx,dx
        mov ah,02
;_printloop
        mov dl,[bx+si] ;get the next character to be printed
        int 21
        dec bx
        jns 0182 ;_printloop
        jmp 0100 ;_input
;_charset
        db "/[phone]zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA"
;_outbuffer
        db 0,0,0
;ASCIIZ
db "base64de.com",0
;compile
MOV AX,CS
MOV DS,AX
MOV AH,3C
XOR CX,CX
MOV DX,01cf ;ASCIIZ
INT 21
MOV BX,AX
MOV CX,01cf ;ASCIIZ
MOV DX,0100 ;where the program starts
SUB CX,DX
MOV AH,40
INT 21
MOV AH,3E
INT 21
MOV AX,4C00
INT 21
 
r ip
1dc
g
q
 
:exists
echo You don't need to run this. Run BASE64DE.COM instead.
goto run
 
:batch
if exist base64de.com goto exists
 
if exist %0 goto extension
debug < %0.bat
goto run
 
:extension
debug < %0
 
:run
pause
cls
base64de.com