I posted this to sci.crypt, I would also like the opinion of the people on this list: --------------------------------------- I have posted about this before, but this time I decided to whip up some code so you can all see what I'm talking about and all the details. This encryption scheme uses XOR to encrypt data 1 character at a time. Because of this it is suitable for protocols where you need to send single bytes (or less) at a time. Examples of this would be interactive ascii. Also it is easily applicable to crippled lines like 7 bit lines. This is because if the plaintext is 7 bits you can send just the 7 bits after the XOR. Even more general you can use it to encrypt any size at a time, down to single bits if you just have 1 bit to send immediately and dont want to wait for more data to become applicable. Ok. So what are its disavantages? Syncronization, as soon as synchronization is gone thats it, its all over. Each block of 'pad' is generated from previous plaintext, if you cant figure out the previous plaintext you are lost. Notice it uses a hash() function, this could be anything, ie DES, or perhaps a one way function (no unhash() is ever needed). The algorithm is fairly simple, the beginning is a bit wierd and could have been done several ways. The first 8 bits in this implementation are used for synchronization. prog -e key <file >file2 prog -d key <file2 >file3 ok.. here it is.. poke holes in it. What are the weaknesses, is it totally unsecure? Is it as secure as the hash function (ie. if DES was used, is strength equivalent to the strength of DES?) -------------cut here--------------------------------------------- #define SIZE 8 #ifdef MEMCPY #define bcopy(s,d,l) memcpy(d,s,l) #endif usage(s) char *s; { printf("%s [-e|e|-d|d] key\n",s); exit(1); } main(argc,argv) char **argv; int argc; { char data[SIZE],pad[SIZE],c[1],c1[1],*p,key[SIZE]; int cnt,encrypt=0; if(argc!=3) usage(argv[0]); for(cnt=0;cnt<SIZE;cnt++) /* fill in extra characters in key */ key[cnt]=0xaa; strncpy(key,argv[2],SIZE); p=argv[1]; if(*p=='-') p++; if(*p=='e') encrypt++; else if(*p!='d') usage(argv[0]); if(encrypt) { /* to start off encryption, we send a bunch of random bits */ /* then we hash those bits and use them as pad */ blkrand(data,SIZE); write(1,data,SIZE); hash(data,key); } else { /* to start off decryption, we receive the random bits and */ /* hash them (just like the encryptor did) and we should */ /* have the same starting pad as the encryptor */ read(0,data,SIZE); hash(data,key); } bcopy(data,pad,SIZE); cnt=0; while(read(0,c,1)>0) { /* read characters, send them with our pad */ /* every time we run out of pad, we make new pad */ /* by hashing old data */ *c1 = *c ^ pad[cnt]; if(encrypt) /* data is the plain text always */ data[cnt]=*c; else data[cnt]=*c1; write(1,c1,1); if(++cnt==SIZE) { cnt=0; bcopy(data,pad,SIZE); hash(pad,key); } } } /* provide a block of random bits */ blkrand(block,len) int len; char *block; { srand(time(0)); while(len--) *block++ = (rand() >>7)&0xff; } /* hash a block, could be DES, or even a one way function */ hash(block,key) char *block,*key; { char b2[SIZE]; int i=SIZE; while(i--) b2[i] = *block++ ^ *key++; bcopy(b2,block,SIZE); }
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Timothy Newsham