Эмулятор процессора Intel 8086 версия 4.05 (ENG) / asm_files / encrypt_subrotine.asm

Assembler Source Code a:link {text-decoration: none} a:visited {text-decoration: none} a:hover {text-decoration: none; color:#ff0000}     ; encrypt_subrotine.asm ; 8086 subroutine to encrypt only lower ; case alphabets of a string using xlat ; and also decrypt the same. ; the main principle behind this algorithm is because ; tables are precoded manually to decode one another. ; for example if first table sets letter 'a' to be 'k', ; the second table does the reverse, it sets letter ; 'k' to be 'a'... etc.... ; only lower case letters are encoded! name "crypt" org 100h jmp start ; string has '$' in the end: string1 db 'hello world!', 0Dh,0Ah, '$' ; in ascii table the code of 'a' is 97 (counted from 0) ; first table is used to encrypt, second ; table is used to decrypt: ; how encoding works: 'abcdefghijklmnopqrstvuwxyz' table1 db 97 dup (' '), 'klmnxyzabcopqrstvuwdefghij' table2 db 97 dup (' '), 'hijtuvwxyzabcdklmnoprqsefg' start: ; encrypt: lea bx, table1 lea si, string1 call parse ; show result: lea dx, string1 ; output of a string at ds:dx mov ah, 09 int 21h ; decrypt: lea bx, table2 lea si, string1 call parse ; show result: lea dx, string1 ; output of a string at ds:dx mov ah, 09 int 21h ; wait for any key... mov ah, 0 int 16h ret ; exit to operating system. ; subroutine to encrypt/decrypt ; parameters: ; si - address of string to encrypt ; bx - table to use. parse proc near next_char: cmp [si], '$' ; end of string? je end_of_string mov al, [si] cmp al, 'a' jb skip cmp al, 'z' ja skip ; xlat algorithm: al = ds:[bx + unsigned al] xlatb ; encrypt using table2. mov [si], al skip: inc si jmp next_char end_of_string: ret parse endp end

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