;; base64.asm: Copyright (C) 2009 Brian Raiter <breadbox@muppetlabs.com>;; Licensed under the terms of the GNU General Public License, either;; version 2 or (at your option) any later version.;;;; To build:;;	nasm -f bin -o base64 base64.asm && chmod +x base64;;;; Usage: base64 [FILE];; base64 reads base64-encoded text from FILE, or from standard input;; if FILE is omitted, and writes the decoded bytes to stdout.;; The exit status is zero on success, or an errno code on error.BITS 32;; Maximum size of the input buffer.%define BUFSIZ 8192;; The executable's ELF header and program header table, which overlap;; each other slightly. The program header table defines a single;; segment of memory, with both write and execute permissions set,;; which is loaded with contents of the entire file plus enough space;; after to hold the program's I/O buffers. The entry point for the;; program appears within the program header table. The initial code;; sets eax to 5, edi to 256, removes argc, argv[0], and argv[1] from;; the stack, and sets the zero flag if argc is one.		org	0x5B05B000		db	0x7F, "ELF", 1, 1, 1, 0	; e_ident		dd	0		dd	0		dw	2			; e_type		dw	3			; e_machine		dd	1			; e_version		dd	_start			; e_entry		dd	phdr - $$		; e_phoff		dd	0			; e_shoff		dd	0			; e_flags		dw	0x34			; e_ehsize		dw	0x20			; e_phentsizephdr:		dw	1			; e_phnum	; p_type		dw	0			; e_shentsize		dw	0			; e_shnum	; p_offset		dw	0			; e_shstrndx		db	0					; p_vaddr_start:		mov	al, 5		pop	ebx		dec	ebx					; p_paddr		pop	ebx		pop	ebx		mov	edi, 256				; p_filesz		jmp	short setup				; p_memsz		dw	0		dd	7					; p_flags		dd	0x1000					; p_align;; The program jumps here to reenter the inner loop after the input;; buffer has been refilled. The buffer size count is moved to ebx,;; while eax has its high bytes cleared.resume:		xchg	eax, ebx;; The program's inner loop. Successive bytes are retrieved from the;; input buffer and examined. Byte values less than '+' (e.g.;; whitespace characters) are ignored. The remaining values are looked;; up in hextable and the bit pattern is appended to edi. When four;; such values have been accumulated, the three bytes so formed are;; added to the output buffer, and edi is cleared. (Note that the;; hextable entry for '=' has the value 0xFF, which causes the overall;; size of the output buffer to be decremented.) The loop continues;; until the input buffer's content is exhausted.byteloop:		mov	al, [ecx]		inc	ecx		cmp	al, '+'		jl	short nextchar		mov	al, [hextable + eax]		cmp	al, 0xFE		adc	ebp, byte -1		or	edi, eax		shl	edi, 6		jnc	short nextchar		bswap	edi		mov	[edx], edi		lea	edx, [byte edx + ebp + 3]		pop	edi		push	edinextchar:	inc	ebx		jnz	short byteloop;; ebx is incremented to one, indicating stdout, and the contents of;; the output buffer are passed to the write system call. write is;; called repeatedly until the entire buffer has been output (or until;; an error is returned, in which case the program exits).		inc	ebx		sub	edx, ecxwriteloop:	lea	eax, [byte ebx + 3]		int	0x80		neg	eax		jns	short errorout		sub	ecx, eax		add	edx, eax		jnz	short writeloop;; ecx is pointed to the start of the input buffer and the read system;; call is invoked. If a positive value is returned, then edx is;; pointed to output buffer, located just past the input, and the;; program jumps into the inner loop.entry:		lea	eax, [byte edx + 3]		mov	ebx, esi		mov	ecx, buffer		mov	dh, BUFSIZ / 256		int	0x80		lea	edx, [ecx + eax]		neg	eax		jl	short resume;; The program comes here on error (or when stdin returns EOF, in;; which case eax will be zero). The value in eax is passed to the;; exit system call.errorout:	xchg	eax, ebx		xor	eax, eax		inc	eax		int	0x80;; The initialization code continues here. edi's starting value is;; stored on the stack so it can be retrieved when needed. If an;; argv[1] is present, the program passes it to the open system call;; to get a file descriptor, which is stored in esi. (If no arguments;; are present, then esi will just be zero, the file descriptor for;; stdin.) The program then enters the main loop.setup:		push	edi		jle	short entry		int	0x80		xchg	eax, esi		sub	eax, esi		jg	short errorout		jmp	short entry;; The base64 translation table. Only the ASCII values between 43 and;; 127 inclusive are given in this table -- other values are filtered;; out before lookup occurs. Each entry provides a bit pattern which;; is stored in the upper six bits. The bottom two bits of each entry;; must be cleared. The exception to this rule is the entry for the;; equal sign -- this is set to 0xFF to indicate that this value does;; not contribute to the program's final output. Note that the entries;; for plus and slash are duplicated by the dash and underscore;; entries; this supports a variant of the standard base64 encoding;; created for use with filenames.hextable equ $ - '+'		db	0xF8, 0, 0xF8, 0, 0xFC		db	0xD0, 0xD4, 0xD8, 0xDC, 0xE0		db	0xE4, 0xE8, 0xEC, 0xF0, 0xF4		db	0, 0, 0, 0xFF, 0, 0, 0		db	0x00, 0x04, 0x08, 0x0C, 0x10, 0x14, 0x18, 0x1C, 0x20		db	0x24, 0x28, 0x2C, 0x30, 0x34, 0x38, 0x3C, 0x40, 0x44		db	0x48, 0x4C, 0x50, 0x54, 0x58, 0x5C, 0x60, 0x64		db	0, 0, 0, 0, 0xFC, 0		db	0x68, 0x6C, 0x70, 0x74, 0x78, 0x7C, 0x80, 0x84, 0x88		db	0x8C, 0x90, 0x94, 0x98, 0x9C, 0xA0, 0xA4, 0xA8, 0xAC		db	0xB0, 0xB4, 0xB8, 0xBC, 0xC0, 0xC4, 0xC8, 0xCC;; The file size is padded out to exactly 256 bytes. (Note that the;; last few entries of the table extend past this point into the;; bss section of the program's memory image.)ALIGN 256, db 0;; The input buffer and output buffer are located here.buffer equ $$ + 0x0200