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/*
* authdecrypt - routine to decrypt a packet to see if this guy knows our key.
*/
#include "ntp_stdlib.h"
/*
* For our purposes an NTP packet looks like:
*
* a variable amount of unencrypted data, multiple of 8 bytes, followed by:
* NOCRYPT_OCTETS worth of unencrypted data, followed by:
* BLOCK_OCTETS worth of ciphered checksum.
*/
#define NOCRYPT_OCTETS 4
#define BLOCK_OCTETS 8
#define NOCRYPT_LONGS ((NOCRYPT_OCTETS)/sizeof(U_LONG))
#define BLOCK_LONGS ((BLOCK_OCTETS)/sizeof(U_LONG))
/*
* Imported from the key data base module
*/
extern u_long cache_keyid; /* cached key ID */
extern u_char DEScache_dkeys[]; /* cached decryption keys */
extern u_char DESzerodkeys[]; /* zero key decryption keys */
/*
* Stat counters, imported from data base module
*/
extern U_LONG authdecryptions;
extern U_LONG authkeyuncached;
int
DESauthdecrypt(keyno, pkt, length)
u_long keyno;
const U_LONG *pkt;
int length; /* length of variable data in octets */
{
register const U_LONG *pd;
register int i;
register u_char *keys;
register int longlen;
U_LONG work[2];
authdecryptions++;
if (keyno == 0)
keys = DESzerodkeys;
else {
if (keyno != cache_keyid) {
authkeyuncached++;
if (!authhavekey(keyno))
return 0;
}
keys = DEScache_dkeys;
}
/*
* Get encryption block data in host byte order and decrypt it.
*/
longlen = length / sizeof(U_LONG);
pd = pkt + longlen; /* points at NOCRYPT area */
work[0] = *(pd + NOCRYPT_LONGS);
work[1] = *(pd + NOCRYPT_LONGS + 1);
if (longlen & 0x1) {
DESauth_des(work, keys);
work[0] ^= *(--pd);
}
for (i = longlen/2; i > 0; i--) {
DESauth_des(work, keys);
work[1] ^= *(--pd);
work[0] ^= *(--pd);
}
/*
* Success if the encryption data is zero
*/
if ((work[0] == 0) && (work[1] == 0))
return 1;
return 0;
}
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