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-rw-r--r--plugin/auth_ed25519/ref10/fe_isnegative.c16
1 files changed, 16 insertions, 0 deletions
diff --git a/plugin/auth_ed25519/ref10/fe_isnegative.c b/plugin/auth_ed25519/ref10/fe_isnegative.c
new file mode 100644
index 00000000..3b2c8b8d
--- /dev/null
+++ b/plugin/auth_ed25519/ref10/fe_isnegative.c
@@ -0,0 +1,16 @@
+#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;
+}