Problem - Kevin Tafuro

A more portable but less accurate way of getting timing information on Unix-based
platforms is to ask the operating system for the clock using the gettimeofday( ) function.
The resolution varies depending on your underlying hardware, but it’s usually
very good. It can be implemented using RDTSC but might have additional overhead.
Nonetheless, on most operating systems, gettimeofday( ) is very accurate.
Other Ways to Get the Time
On many machines, there are other ways to get the time. One way is to use the POSIX
times( ) function, which has the advantage that you can separate the time your process
spends in the kernel from the time spent in user space running your code. While times(
) is obsoleted on many systems, getrusage( ) does the same thing.
Another alternative is the ISO C89 standard function, clock( ). However, other timers
we discuss generally provide resolution that is as good as or better than this function.
Here’s a macro that will use gettimeofday( ) to put the number of microseconds
since January 1, 1970 into an unsigned 64-bit integer (if your compiler does not support
a 64-bit integer type, you’ll have to store the two 32-bit values separately and
diff them properly; see below).
#include
#define current_time_as_int64(a) { \
struct timeval t; \
gettimeofday(&t, 0); \
*a = (spc_uint64_t)((t.tv_sec * 1000000) + t.tv_usec); \
}
Attackers can often force the worst-case performance for functionality with well-chosen
inputs. Therefore, you should always be sure to determine the worst-case performance
characteristics of whatever it is you’re doing, and plan accordingly.
The gettimeofday( )-based macro does not compute the same thing
the RDTSC version does! The former returns the number of microseconds
elapsed, while the latter returns the number of cycles elapsed.
You’ll usually be interested in the number of seconds elapsed. Therefore, you’ll need
to convert the result of the gettimeofday( ) call to a number of cycles. To perform
this conversion, divide by the clock speed, represented as a floating-point number in
gigahertz.
Because you care about elapsed time, you’ll want to subtract the starting time from
the ending time to come up with elapsed time. You can transform a per-second representation
to a per-cycle representation once you’ve calculated the total running
Timing Cryptographic Primitives | 153
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