PC-Online 1998 February/PCOnline_02_1998.iso/filesbbs/win95/cryptext.exe/decrypt.c

C/C++ Source or Header | 1997-03-04 | 5KB | 228 lines

#include <windows.h>
#include <stdlib.h>
#include <stdio.h>
#include "sha.h"
#include "rc4.h"

#define BUFFERSIZE 16384
#define KEYSIZE 20

union SHAKEY
{
    DWORD    keyW [KEYSIZE / sizeof (DWORD)];
    BYTE    keyB [KEYSIZE];
};

typedef struct tagIV
{
    FILETIME    fileTime;
    int            rand;
} IV, *LPIV;

union unionIV
{
    IV        iv;
    BYTE    byteIV [sizeof (IV)];
};


// functions
void    LoadIV (HANDLE, LPBYTE);
BOOL    CheckKeyIsValid (LPSTR, LPBYTE);
void    CreateUniqueKey (LPDWORD, LPBYTE, LPIV);

int main (int argc, char *argv[])
{
    union unionIV iv;
    union SHAKEY key, uniqueKey;
    rc4Key         streamKey;
    DWORD        nBytesSoFar;
    DWORD        nBytesRead, nBytesWritten;
    HANDLE         hFile;
    char        szFile [_MAX_PATH];
    char        szNewFile [_MAX_PATH];
    char        szPass [_MAX_PATH];
    BYTE        lpFileBuffer [BUFFERSIZE];

    // check parameters
    // must be two parameters
    if (argc != 3)
    {
        printf ("\nInvalid parameters. Command line must be:"
            "\n\tDECRYPT \"<passphrase>\" <filename>"
            "\n e.g.\tDECRYPT \"goggomobile\", C:\\TEMP\\TEST.TXT.$#!\n");
        return 1;
    }
    // get passphrase and filename
    lstrcpy (szPass, argv[1]);
    lstrcpy (szFile, argv[2]);

    // check for validity of passphrase
    if (!CheckKeyIsValid (szPass, key.keyB))
        return 1;

    // open the file
    hFile = CreateFile (szFile,
        GENERIC_READ | GENERIC_WRITE, 0, NULL,
        OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

    if (hFile == INVALID_HANDLE_VALUE)
    {
        printf ("\nError opening %s\n", szFile);
           return 1;
    }

    // Load the iv
    LoadIV (hFile, iv.byteIV);

    // use the iv to create a unique key for this file
    CreateUniqueKey (uniqueKey.keyW, key.keyB, &(iv.iv));

    // do RC4 setup
    rc4Setup (uniqueKey.keyB, KEYSIZE, &streamKey);

    // decrypt the file
    nBytesSoFar = 0;
    do
    {
        // fill the buffer from the file
        ReadFile (hFile, lpFileBuffer, BUFFERSIZE, &nBytesRead, NULL);
        if (nBytesRead)
        {
            // encrypt or decrypt as required
            rc4 (lpFileBuffer, nBytesRead, &streamKey);
            // reset the file pointer for the write
            SetFilePointer (hFile,
                nBytesSoFar,        // offset into file
                NULL,                // only using files < 2^32 in size
                FILE_BEGIN);        // starting point for offset
            // write the buffer
            WriteFile (hFile, lpFileBuffer, nBytesRead,
                &nBytesWritten, NULL);
            // increment byte count
            nBytesSoFar += nBytesRead;
        }
    } while (nBytesRead);

    // close the file
    CloseHandle (hFile);
    // rename the file to remove the encryption extension
    lstrcpyn (szNewFile, szFile, lstrlen (szFile) - 3);
    MoveFile (szFile, szNewFile);
    // finish
    return 0;
}


void
LoadIV (HANDLE hFile, LPBYTE ivAsBytes)
{
    DWORD    nBytesRead;

    // set the pointer to the beginning of the iv
    SetFilePointer (hFile, 0-(sizeof(IV)+KEYSIZE), NULL, FILE_END);

    // read the iv
    ReadFile (hFile, ivAsBytes, sizeof (IV), &nBytesRead, NULL);

    // set the pointer to the last real byte in the file
    SetFilePointer (hFile, 0-(sizeof(IV)+KEYSIZE), NULL, FILE_END);

    // set the new EOF
    SetEndOfFile (hFile);

    // reset file pointer to the beginning of the file
    SetFilePointer (hFile, 0, NULL, FILE_BEGIN);

    return;
}


BOOL
CheckKeyIsValid (LPSTR szPass, LPBYTE lpKey)
{
    LONG    dwErrorCode;
    DWORD    dwSize;
    HKEY    hKey;
    union SHAKEY hashedKey, testKey;
    char    szKey [KEYSIZE*2+1];
    char    szTemp [_MAX_PATH+KEYSIZE+1];

    // open the app key
    dwErrorCode = RegOpenKeyEx (HKEY_CURRENT_USER,
        "Software\\Cryptext\\Configuration",
        0,
        KEY_ALL_ACCESS,
        &hKey);

    // succeeded?
    if (dwErrorCode != ERROR_SUCCESS)
    {
        printf ("\nPassword checksum not found in registry\n");
        return FALSE;
    }
    else
    {
        // get the hashed encryption key from the registry
        dwSize = KEYSIZE;
        dwErrorCode = RegQueryValueEx (hKey,
            "HashedKey",
            NULL,
            NULL,
            hashedKey.keyB,
            &dwSize);

        // close the registry key
        RegCloseKey (hKey);
    }

    // encryption key not found or key not 160 bits long
    if (dwErrorCode != ERROR_SUCCESS || dwSize != KEYSIZE)
    {
        printf ("\nPassword checksum not found or invalid\n");
        return FALSE;
    }

    sha_memory ((LPBYTE)szPass, lstrlen (szPass), testKey.keyW);

    // copy the key
    CopyMemory (lpKey, testKey.keyB, KEYSIZE);

    // convert it to ASCII representation
    wsprintf (szKey, "%08lx%08lx%08lx%08lx%08lx",
        testKey.keyW[0], testKey.keyW[1], testKey.keyW[2], testKey.keyW[3], testKey.keyW[4]);

    // add the hash to the password
    wsprintf (szTemp, "%s%s", szPass, szKey);

    // hash again
    sha_memory ((LPBYTE)szTemp, lstrlen (szTemp), testKey.keyW);

    // now verify against the stored hashed key
    if (hashedKey.keyW[0] != testKey.keyW[0] ||
        hashedKey.keyW[1] != testKey.keyW[1] ||
        hashedKey.keyW[2] != testKey.keyW[2] ||
        hashedKey.keyW[3] != testKey.keyW[3] ||
        hashedKey.keyW[4] != testKey.keyW[4])
    {
        printf ("\nInvalid passphrase\n");
        return FALSE;
    }

    return TRUE;
}


void
CreateUniqueKey (LPDWORD uniqueKey, LPBYTE lpKey, LPIV lpiv)
{
    BYTE    buffer [KEYSIZE + sizeof (IV)];

    // copy hash of passphrase
    CopyMemory (buffer, lpKey, KEYSIZE);
    // copy date/time
    CopyMemory (buffer+KEYSIZE, lpiv, sizeof (IV));
    // hash to create key for rc4
    sha_memory (buffer, KEYSIZE + sizeof (IV), uniqueKey);
}