Dr. Dobb's Journal - October 2008 - (Page 49)

D10deja_p5db 8/15/08 10:05 AM Page 49 good because the main problem with the Book cipher is the need for a long book. To encode a 5000-character plaintext, you need at least a 5000-word book. In fact, it should be much longer because you cannot expect the frequency of all letters in the text to match the frequency of initial letters of words in the book. Due to the disproportion in frequencies of all letters and initial letters, the coding process may become impossible, and you could run out of some letters even before you start coding. For example, there are practically no English words beginning with the letter “X”—in the Harry Potter book, there is only one such “word” in the phrase “a fiery X appeared on the door.” In other books, the only “words” beginning with “X” are chapter numbers written as Roman numerals. In the second Beale cipher (the only one that was successfully decoded), every “X” was coded as 1005, probably pointing to the word “sexes” in the Declaration of Independence, which was used as a key (“sexes” obviously does not start with “x,” but does sound like it). Some books also contain zero words beginning with the letter “Z” unless they contain the , word “zero” itself. Therefore, it seems like a good idea to investigate the frequencies of first, second, third, or even last letters in words, to find out which of them matches the frequency of all letters. We analyzed 14 books, from Pride and Prejudice to The Da Vinci Code, and found out that the distribution of frequencies for the third letter in the word has the best correlation with frequencies of all letters. A possible solution is to let users choose which letter in the word to use, by setting an appropriate parameter. To get the system rolling, you also have to agree with the correspondent about the books you will use to encode the messages. Using “real” books isn’t very practical (OCRing them on both sides introduces different errors). So you will probably use some texts available on the Internet, and you might even consider promoting some of your own essays into “books” for additional security. (If we were running the NSA, we would make sure we had a machine-readable copy of the Listing Two // bkcode #include #include #include #include #define N_CAPITAL_LETTERS 26 #define LTRTOIDX(x) (x-'A') int main(int argc, char *argv[]) { char plain[_MAX_PATH], codfile[_MAX_PATH]; char cipher[_MAX_PATH], positions[_MAX_PATH]; char drive[_MAX_DRIVE]; char dir[_MAX_DIR]; char fname[_MAX_FNAME]; char ext[_MAX_EXT]; FILE *fp_plain, *fp_cod, *fp_cipher, *fp_pos; long pos[N_CAPITAL_LETTERS]; int ch_plain, ch_cod; // 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); _makepath(positions, drive, dir, fname, "pos"); _splitpath(argv[2], drive, dir, fname, ext ); if (strlen(ext) == 0) strcpy(ext, "txt"); _makepath(plain, drive, dir, fname, ext); _makepath(cipher, drive, dir, fname, "cry"); // File opening if ((fp_plain = fopen(plain, "r")) == NULL) goto error3; if ((fp_cod = fopen(codfile, "r")) == NULL) goto error3; if ((fp_cipher = fopen(cipher, "w")) == NULL) goto error3; fp_pos = fopen(positions, "rb+"); // Position array intialization if (fp_pos == NULL) memset(pos, 0, sizeof(pos)); else { fread(pos, sizeof(long), N_CAPITAL_LETTERS, fp_pos); fclose(fp_pos); } // Main loop do { ch_plain = toupper(getc(fp_plain)); if (isalpha(ch_plain)) { fseek(fp_cod, pos[LTRTOIDX(ch_plain)], SEEK_SET); do { ch_cod = getc(fp_cod); } while (!(ch_cod == ch_plain || ch_cod == EOF)); if (ch_cod == ch_plain) { pos[LTRTOIDX(ch_plain)] = ftell(fp_cod); fprintf (fp_cipher, "%1ld ", pos[LTRTOIDX(ch_plain)]); } else goto error2; } } while (ch_plain != EOF); // Termination fp_pos = fopen(positions, "wb"); fwrite (pos, sizeof(long), N_CAPITAL_LETTERS, fp_pos); fclose(fp_plain); fclose(fp_cod); fclose(fp_cipher); fclose(fp_pos); return 0; // Error handling error1: printf ("USAGE: bkcode codfile message\n"); return 1; error2: printf ("Run out of letters %c!\n", ch_plain); fclose(fp_plain); fclose(fp_cod); fclose(fp_cipher); remove(cipher); return 1; error3: printf ("Can not open files\n"); return 1; } October 2008 l www.ddj.com l Dr. Dobb’s Journal 49 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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