Introduction

How do you send a binary file to somebody with a plain MS-DOS or Windows based install, assuming the person can receive via email only text files less than 5 kilobytes without attachments? This is a bootstrapping problem, and using the wonderful utility DEBUG.COM present on all these systems, it can be accomplished with the following simple program. After sending this program once, you can sent arbitrary binary files in little plain text pieces.

The program decodes text files encoded in base64 encoding and is RFC 1341 compliant. The program errs on the side of caution, decoding files even if they contain illegal characters or the number of legal characters is not an exact multiple of 4.

User's Guide

To compile the following program save it to a file named decode.asm and type in a DOS prompt:

   debug < decode.asm

To run the program use stdin and stdout, for example to convert a base64 encoded file in.txt to the binary out.jpg type:

   decode < in.txt > out.jpg

Credits

Thanks go to Jan Aleman (Professional Data Management) for the great idea to use DEBUG to do the programming.
Also useful were the cool DEBUG tutorial by Fran Golden at http://www.datainstitute.com/debug1.htm, and the assembly reference at http://members.tripod.com/~Vitaly_Filatov/ng/asm/.

The Source Code

The source (decode.asm) is short and simple, and can be mailed as a plain text file which is less than 2 kilobytes:

a 100
jmp 34e
DB 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/$'
DB 'Error.' 0a 0d
DW 0

a 34e
mov ah,44
mov al,0
mov bx,0
int 21
mov ah,44
mov al,1
or dx,20
int 21
mov di,0
mov si,0
call 371
jnc 367
call 41c
int 20
mov bx,0
call 3c7
jc 3ac
or bl,al
mov cl,6
shl bx,cl
call 3c7
jc 3ac
or bl,al
mov cl,4
shl bx,cl
call 3e3
shl bx,1
shl bx,1 
call 3c7
jc 3ac
or bl,al
mov cl,6
shl bx,cl
call 3e3
call 3c7
jc 3ac
or bl,al
mov bh,bl
call 3e3
clc
ret
push ax
mov bx,0
mov ah,[103+bx]
cmp ah,24
jz 3c1
cmp ah,al
jz 3c4
inc bx
jmp 3b1
stc
jmp 3c5
clc
pop ax
ret
push bx
cmp di,[14c]
jne 3d3
call 3f2
jc 3e1
mov al,[14e+di]
inc di
call 3ad
jnc 3df
jmp 3c8
mov al,bl
pop bx
ret
cmp si,100
jnz 3ec
call 41c
mov [si+24e],bh
inc si
ret
push ax
push bx
push cx
push dx
mov ah,3f
mov bx,0
mov cx,100
mov dx,14e
int 21
jnc 408
call 43d
mov [14c],ax
cmp ax,0
jne 413
stc
jmp 414
clc
mov di,0
pop dx
pop cx
pop bx
pop ax
ret
push ax
push bx
push cx
push dx
mov ah,40
mov bx,1
mov cx,si
mov dx,24e
int 21
jnc 431
call 43d
sub si,cx
je 438
call 43d
pop dx
pop cx
pop bx
pop ax
ret
mov ah,40
mov bx,2
mov cx,8
mov dx,14c
int 21
int 20

n decode.com
r cx
34c
w
q