Dr. Dobb's Journal - October 2008 - (Page 50)

D10deja_p5db 8/15/08 10:13 AM Page 50 Core Technology THE BOOK CIPHER ALGORITHM Library of Congress in a form suitable for fast searching.) Implementation The Book cipher algorithm uses letters of subsequent words in some text or book as a key to encode a message Listing Three // bkdecode #include #include #include int main(int argc, char *argv[]) { char codfile[_MAX_PATH]; char cipher[_MAX_PATH], decoded[_MAX_PATH]; char drive[_MAX_DRIVE]; char dir[_MAX_DIR]; char fname[_MAX_FNAME]; char ext[_MAX_EXT]; FILE *fp_cipher, *fp_cod, *fp_decoded; long position, cod_size; // Argument processing if (argc!=3) goto error1; _splitpath(argv[1], drive, dir, fname, ext ); if (strlen(ext) == 0) strcpy(ext, "cod"); _makepath(codfile, drive, dir, fname, ext); _splitpath(argv[2], drive, dir, fname, ext ); _makepath(cipher, drive, dir, fname, "cry"); _makepath(decoded, drive, dir, fname, "txt"); // if if if File opening ((fp_cipher = fopen(cipher, "r")) == NULL) goto error3; ((fp_cod = fopen(codfile, "r")) == NULL) goto error3; ((fp_decoded = fopen(decoded, "w")) == NULL) goto error3; // Determine codfile size fseek(fp_cod, 0, SEEK_END); cod_size = ftell(fp_cod); // Main loop while ((fscanf(fp_cipher, "%ld", &position) != EOF)) { if (--position <= cod_size) { fseek(fp_cod, position, SEEK_SET); putc(getc(fp_cod), fp_decoded); } else goto error2; } // Termination fclose(fp_cipher); fclose(fp_cod); fclose(fp_decoded); return 0; // Error handling error1: printf ("USAGE: bkdecode codfile cipher \n"); return 1; error2: printf ("Invalid ciphertext\n"); fclose(fp_cipher); fclose(fp_cod); fclose(fp_decoded); remove (decoded); return 1; error3: printf ("Can not open files\n"); return 1; } To illustrate the algorithm, we wrote three programs in C—bkadd (Listing One), bkcode (Listing Two), and bkdecode (Listing Three). They use Standard C, except for processing command-line arguments with the Microsoft-specific Visual C _splitpath and _makepath functions. File processing is via the getc and putc functions, and can be improved by using buffers to increase efficiency. The idea is to create a “tank” of letters from books for each of the correspondents. Using the command bkadd allice mybook.txt 1, Bob creates the file allice.cod with the initial letters of each word in mybook.txt. The last parameter determines which letter of each word should be used—positive values 1, 2, 3, stand for the first, second, third, respectively, and setc letter, while negative values mean that letters should be counted from the last position in the word. If this parameter is omitted, the first letters are taken as the default. Program bkcode is used to encode a message: bkcode allice message1.txt transforms message1.txt into message1.cry using letters from allice.cod. At the successful completion of the process, the program generates the file allice.pos with pointers to the positions of last used letters a, b, c, d… in allice.cod. When the next message is encoded (bkcode allice message2.txt), the search for the letters automatically continues from the previously memorized positions; no part of the (transformed) book is used twice. If the process fails (that is, if the Figure 2: Encoding the message ”DRDOBBS” using the Book cipher. 50 Dr. Dobb’s Journal l www.ddj.com l October 2008 http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - October 2008

Dr. Dobb's Journal - October 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Developer’s Notebook Is Your Next Language COBOL? Conversations Safe Coding Practices Code Signing in Adobe AIR OpenID Single Sign-On The Book Cipher Algorithm Indexing and Searching Image files Extending Continuous Integration Into ALM The Agile Edge Effective Concurrency Swaine’s Flames

Dr. Dobb's Journal - October 2008

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