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After a while of working with this program, it keeps stopping after it creates an output file. I am using Visual Basic 2010 and am still a beginner at this. The homework question is this:

Description (Symmetric Encryption):

  1. Encoding

    • Ask the user to type some text
    • Ask the user to type a private key within this range [1-255]. Perform the range validity checking.
    • Encrypt the input text using the provided private key, put the cipher text in a file named by the user.

  2. Decoding

    • Ask the user to specify the file to decode.
    • Load the cipher text from that file and try to descrypt it without assuming the private key is the same one used in encoding.
    • Put all the trial results in a seperated file named by the user.
    • d. Figure out what the most reasonable result (or original plaintext) is.

I can figure out how to encrypt the text, but have trouble creating the output file with the libraries located at this textbook address: http://www.kipirvine.com/asm/examples/index.htm

I will include my code below and show through the commented material how many tries I have had at this. The book does not explain very well to me so if I could see what it is trying to say it would be very helpful!

Thanks in advance!

INCLUDE Irvine32.inc

    BUFMAX = 128                    ; maximum buffer size
    KEYMAX = 128                    ; maximum buffer size
    BUFFER_SIZE = 5000

.data

    sPrompt BYTE        "Enter some text message:       ", 0
    keyPrompt   BYTE        "Enter a private key [1-255]:       ", 0
    cFile   BYTE        "Enter a filename for cypher text: ", 0
    sEncrypt    BYTE        "Cypher text                    ", 0
    sDecrypt    BYTE        "Decrypted:                 ", 0
    error   BYTE        "The key must be within 1 - 255!    ", 0
    buffer  BYTE         BUFMAX + 1 DUP(0)
    bufSize DWORD    ?
    keyStr  BYTE         KEYMAX + 1 DUP(0)
    keySize DWORD    ?
    key     DWORD    ?
    filename    BYTE        "newfile.txt                    ", 0
    fileHdl DWORD   ?
    bufFile BYTE        BUFFER_SIZE DUP (?)

.code
main PROC

    call InputTheString             ; input the plain text
    call InputTheKey                ; input the security key
    call CypherFile             ; input a cypher filename
    ;call TranslateBuffer           ; encrypt the buffer
    ;mov edx, OFFSET sEncrypt           ; display encrypted message
    ;call DisplayMessage
    ;call TranslateBuffer           ; decrypt the buffer
    ;mov edx, OFFSET sDecrypt           ; display decrypted message
    ;call DisplayMessage
    exit
main ENDP

InputTheKey PROC

    pushad                      ; save 32-bit registers
LK: mov edx, OFFSET keyPrompt       ; display a prompt
    call WriteString                ; Enter a private key [1-255]
    call Crlf                       ; start a new line
    call ReadInt                    ; read int into system
    mov key, eax                    ; store int into keyStr
    cmp eax, 255                    ; compare newly read int
    ja LC                       ; jump if above 255 to LC
    cmp eax, 1                  ; compare newly read int
    jb LC                       ; jump if below 1 to LC
    jmp LR                      ; if between range jump to LR
LC: mov edx, OFFSET error           ; The key must be within 1 - 255!
    call WriteString                ; Display the error
    call Crlf                       ; start a new line
    loop LK                     ; loop back to enter the security key
LR: popad                       ; restore the registers
    ret
InputTheKey ENDP

CypherFile PROC
    pushad
    mov edx, OFFSET cFile           ; "Enter a filename for cypher text
    call WriteString                ; Enter a name for encrypted file
    call Crlf
    call ReadString             ; Store the filename in eax
    ;mov filename, eax
    mov edx, OFFSET filename
    ;push eax
    ;mov eax, fileHdl
    ;mov edx, OFFSET bufFile
    ;mov ecx, BUFFER_SIZE
    ;mov edx, "C:\outputtext.txt"
    call CreateOutputFile
    ;mov edx, OFFSET filename
    ;mov ecx, SIZEOF filename
    ;push eax
    ;mov eax, bufSize
    call WriteToFile
    pop eax
    ;call CloseFile
    ret
CypherFile ENDP
InputTheString PROC

    pushad                      ; save 32-bit registers
    mov edx, OFFSET sPrompt         ; display a prompt
    call WriteString                ; "Enter some text message"
    call Crlf                       ; start a new line
    mov ecx, BUFMAX             ; maximum character count
    mov edx, OFFSET buffer          ; point to the buffer
    call ReadString             ; input the string
    mov bufSize, eax                ; save the length
    popad
    ret
InputTheString ENDP
    COMMENT !
DisplayMessage PROC

    pushad
    call WriteString
    mov edx, OFFSET buffer          ; display the buffer
    call WriteString
    call Crlf
    call Crlf
    popad
    ret
DisplayMessage ENDP

TranslateBuffer PROC

    pushad
    mov ecx, bufSize                ; loop counter
    mov esi, 0                  ; index 0 in buffer
    mov edi, 0                  ; index 0 in the key
L1:
    mov al, keyStr[edi]             ; get a character from encryption key
    xor buffer[esi], al             ; translate a byte
    inc esi                     ; point to next byte
    inc edi                     ; go to next position in key
    cmp edi, keySize                ; compare if equal to size of the key
    jb L2
    mov edi, 0                  ; reset to beginning of the key
L2: loop L1
    popad
    ret
TranslateBuffer ENDP
!
END main
rkhb
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user1848703
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1 Answers1

2

Where to begin?

Aren't you missing a few parameters to ReadString? Perhaps a pointer to where to store the entered file name? The size of the buffer to receive the file name?

CypherFile you push all registers onto the stack with pushad but at the end, you only pop eax. Big problem right there, that should be popad

As it stands, it writes nothing to the output file since the parameters to WriteToFile are commented out.

EDIT - Going against my "just giving code" You need to tell ReadString where to save the entered filename and size of buffer. Then pass this on to CreateOutputFile - In CyperFile proc -

mov     edx, offset buffer      ; you are missing a buffer for filename
mov     ecx, BUFMAX             ; buffer size
call    ReadString              

mov     edx, offset buffer      ; Pass this to CreateOutputFile
call    CreateOutputFile

Now reading the source for CreateOutputFile, it says it returns a file handle on success, save this somewhere. You use this with CloseFile when you are done writing to the file. If it does not create the file successfully, it will return INVALID_HANDLE_VALUE

Gunner
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  • Ok, the popad kept it from crashing. What I do not understand is creating the file and naming it with a name the user types in. What steps are needed to do this? Does the buffer go into the edx? – user1848703 Nov 24 '12 at 01:10
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    Open irvine32.asm and you will see all the procs, and comments as to what each proc receives and returns. – Gunner Nov 24 '12 at 01:14
  • I have read them, from what I understand it needs to use edx for something. I am sorry, I am a big beginner at this and do not get what the book and instructor says about this. Do I: mov edx, BUFFER bufFile then mov ecx, filename ? – user1848703 Nov 24 '12 at 01:29
  • This is what CreateOutputFile says: Creates a new file and opens it in output mode. ; Receives: EDX points to the filename. ; Returns: If the file was created successfully, EAX ; contains a valid file handle. Otherwise, EAX ; equals INVALID_HANDLE_VALUE. – user1848703 Nov 24 '12 at 01:31
  • Tried joining the chat, but it wont let me since I am 10 points shy to use it – user1848703 Nov 24 '12 at 01:41
  • I am working on getting my numbers up as we speak. What did you mean by parameters? – user1848703 Nov 24 '12 at 01:48
  • wow! that worked! I can't believe it, I have been stressing over this for a week now! As soon as I get the points to vote this up I will. It created the file no trouble. Now I have to work on pushing the text to that file – user1848703 Nov 24 '12 at 02:00
  • It creates the file, so does the same process work for writing to the same file? – user1848703 Nov 24 '12 at 02:55