applied cryptography - protocols, algorithms, and source code in c

The plaintext is being xored with a keyword to

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Unformatted text preview: 0 a•b•b=a The simple-XOR algorithm is really an embarrassment; it’s nothing more than a Vigenère polyalphabetic cipher. It’s here only because of its prevalence in commercial software packages, at least those in the MS-DOS and Macintosh worlds [1502,1387]. Unfortunately, if a software security program proclaims that it has a “proprietary” encryption algorithm—significantly faster than DES—the odds are that it is some variant of this. /* Usage: crypto key input_file output_file */ void main (int argc, char *argv) { FILE *fi, *fo; char *cp; int c; if ((cp = argv[1]) && *cp!='\0') { if ((fi = fopen(argv[2], “rb”)) != NULL) { if ((fo = fopen(argv[3], “wb”)) != NULL) while ((c = getc(fi)) != EOF) { if (!*cp) cp = argv[1]; c ^= *(cp++); putc(c,fo); } fclose(fo); } fclose(fi); } } } This is a symmetric algorithm. The plaintext is being XORed with a keyword to generate the ciphertext. Since XORing the same value twice restores the original, encryption and decryption use exactly the same program: P•K=C C•K=P There’s no real security here. This kind of encryption is trivial to break, even without computers [587,1475]. It will only take a few seconds with a computer. Assume the plaintext is English. Furthermore, assume the key length is any small number of bytes. Here’s how to break it: { 1. Discover the length of the key by a procedure known as counting coincidences [577]. XOR the ciphertext against itself shifted various numbers of bytes, and count those bytes that are equal. If the displacement is a multiple of the key length, then something over 6 percent of the bytes will be equal. If it is not, then less than 0.4 percent will be equal (assuming a random key encrypting normal ASCII text; other plaintext will have different numbers). This is called the index of coincidence. The smallest displacement that indicates a multiple of the key length is the length of the key. 2. Shift the ciphertext by that length and XOR it with itself. This removes the key and leaves you with plaintext XO...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.

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