session-android/libaxolotl/jni/ed25519/fe_isnegative.c
Moxie Marlinspike 2ed8d333d9 Add ed25519
2014-10-20 12:14:18 -07:00

17 lines
266 B
C

#include "fe.h"
/*
return 1 if f is in {1,3,5,...,q-2}
return 0 if f is in {0,2,4,...,q-1}
Preconditions:
|f| bounded by 1.1*2^26,1.1*2^25,1.1*2^26,1.1*2^25,etc.
*/
int fe_isnegative(const fe f)
{
unsigned char s[32];
fe_tobytes(s,f);
return s[0] & 1;
}