;this program makes random passwords
;$ fasm randpw.asm randpw
;Usage: $ ./randpw <length> <number>

format ELF64 executable 3
use64
entry _start

segment readable writeable
length: dd 8 ;default password length
number: dd 1 ;generate one password by default
filename: db '/dev/random', 0
bad_args: db 'PARAMETER ERROR', 10
bad_args_len = $ - bad_args
array: db '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
max = $ - array
newline: db 10
filedes: dd ?
rand: dd ?

segment readable executable
_start:

;first argument
    mov rsi, qword [rsp+16]
    test rsi, rsi ;check for end of arguments
    jz .main_program
    call atoi
    mov [length], ebx

;second argument
    mov rsi, qword [rsp+24]
    test rsi, rsi ;check for end of arguments
    jz .main_program
    call atoi
    mov [number], ebx

    xor r14d, r14d ;counter for number of passwords

.main_program:
    mov eax, 2 ;syscall number for SYS_open
    mov rdi, filename
    mov esi, 0 ;O_RDONLY
    mov edx, 0
    syscall
    mov [filedes], eax ;save file descriptor for later

.next_pw:
    mov r15d, [length]
.main_loop:
    test r15d, r15d
    jz .endline
    dec r15d

    mov eax, 0 ;syscall number for SYS_read
    mov edi, [filedes] ;file descriptor
    mov rsi, rand
    mov edx, 4
    syscall

;index = rand % max
    mov eax, [rand]
    cqo
    mov ebx, max
    div ebx

    mov eax, 1 ;syscall number for SYS_write
    mov edi, 1 ;STDOUT_FILENO
    mov rsi, array ;characters
    add rsi, rdx ;the offset, modulus result from previous computation
    mov edx, 1 ;count
    syscall

    jmp .main_loop

.endline:
    mov eax, 1 ;syscall number for SYS_write
    mov edi, 1 ;STDOUT_FILENO
    mov rsi, newline
    mov edx, 1 ;count
    syscall

    inc r14d
    cmp r14d, [number] ;number of passwords to generate
    jne .next_pw

    mov eax, 60 ;syscall number for SYS_exit
    xor edi, edi ;success exit value
    syscall

atoi: ;takes string pointer in rsi, returns integer in ebx
    push rbp
    mov rbp, rsp

    xor ebx, ebx ;return value

.loop:
    lodsb ;rsi is incremented automatically
    test al, al ;end of string, leave loop
    jz .end

;check whether the numbers are within bounds
    cmp al, '0'
    jb error
    cmp al, '9'
    ja error

;multiply total by 10 and add the current digit converted to decimal
    movzx eax, al
    sub eax, 48 ;convert current digit from ASCII to decimal
    lea ebx, [ebx * 4 + ebx] ;multiply total by 5
    lea ebx, [ebx * 2 + eax] ;multiply total again by 2 and add the current digit
    jmp .loop

.end:
    leave
    ret

error:
    mov eax, 1 ;syscall number for SYS_write
    mov edi, 2 ;STDERR_FILENO
    mov rsi, bad_args ;pointer to string
    mov edx, bad_args_len ;length of string
    syscall

    mov eax, 60 ;syscall number for SYS_exit
    mov edi, 1 ;failure exit value
    syscall

; vim: set ts=4 sw=4 sts=4 syn=fasm:
    

    lea rdi, [filename]    

mov rdi, filename    

lea rdi, [filename]    

mov edi, filename
lea edi, [filename]    

_start:
mov ecx,[rsp+8*0] ; argc count of input arguments