[Xen-devel] Side channel attack

[Zakirasafi <zakirasafi@xxxxxxxxx>]

Dear All

The following code is for side channel attack on xen hypevisor. In this code I am having problem in understanding the highlighted red line. In the line what ".byte 15, byte 49" do???

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
 #include <openssl/aes.h>
unsigned int timestamp(void)
 {
unsigned int bottom;
unsigned int top;
asm volatile(".byte 15;.byte 49" : "=a"(bottom),"=d"(top)); return bottom;
 }
unsigned char key[16];
AES_KEY expanded;
unsigned char zero[16];
unsigned char scrambledzero[16];

void handle(char out[40],char in[],int len)
 {
 unsigned char workarea[len * 3];
int i;

for (i = 0;i < 40;++i) out[i] = 0;
 *(unsigned int *) (out + 32) = timestamp();
if (len < 16) return; for (i = 0;i < 16;++i) out[i] = in[i];
for (i = 16;i < len;++i) workarea[i] = in[i];

AES_encrypt(in,workarea,&expanded);

 /* a real server would now check AES-based authenticator, */
/* process legitimate packets, and generate useful output */
for (i = 0;i < 16;++i) out[16 + i] = scrambledzero[i];
 *(unsigned int *) (out + 36) = timestamp();
}
struct sockaddr_in server;
 struct sockaddr_in client;
 socklen_t clientlen;
int s;
char in[1537];
int r;
char out[40];
main(int argc,char **argv)
 { if (read(0,key,sizeof key) < sizeof key) return 111;
 AES_set_encrypt_key(key,128,&expanded);
AES_encrypt(zero,scrambledzero,&expanded);
if (!argv[1]) return 100;
if (!inet_aton(argv[1],&server.sin_addr)) return 100;
 server.sin_family = AF_INET;
 server.sin_port = htons(10000);
s = socket(AF_INET,SOCK_DGRAM,0);
 if (s == -1) return 111;
if (bind(s,(struct sockaddr *) &server,sizeof server) == -1) return 111;
for (;;)
 { clientlen = sizeof client;
 r = recvfrom(s,in,sizeof in,0
 ,(struct sockaddr *) &client,&clientlen);
if (r < 16) continue;
 if (r >= sizeof in) continue;
handle(out,in,r);
sendto(s,out,40,0,(struct sockaddr *) &client,clientlen); }
}

Thanks and Regards,

Zakira Inayat

Ph.D Scholar in University of Malaya, Malaysia

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