add randommod.[ch]

janmojzis · Dec 28, 2021 · 1645cd3 · 1645cd3
1 parent 1696201
commit 1645cd3
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diff --git a/randommod.c b/randommod.c
@@ -0,0 +1,17 @@
+/* taken from public-domain nacl-20110221, from curvecp/randommod.c */
+#include "randombytes.h"
+#include "randommod.h"
+
+/* XXX: current implementation is limited to n<2^55 */
+
+long long randommod(long long n) {
+    long long result = 0;
+    long long j;
+    unsigned char r[32];
+    if (n <= 1) return 0;
+    randombytes(r, 32);
+    for (j = 0; j < 32; ++j) {
+        result = (result * 256 + (unsigned long long) r[j]) % n;
+    }
+    return result;
+}
diff --git a/randommod.h b/randommod.h
@@ -0,0 +1,6 @@
+#ifndef _RANDOMMOD_H____
+#define _RANDOMMOD_H____
+
+extern long long randommod(long long);
+
+#endif