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-rw-r--r--libtommath/bn_mp_dr_is_modulus.c39
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diff --git a/libtommath/bn_mp_dr_is_modulus.c b/libtommath/bn_mp_dr_is_modulus.c
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+#include <tommath.h>
+#ifdef BN_MP_DR_IS_MODULUS_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis
+ *
+ * LibTomMath is a library that provides multiple-precision
+ * integer arithmetic as well as number theoretic functionality.
+ *
+ * The library was designed directly after the MPI library by
+ * Michael Fromberger but has been written from scratch with
+ * additional optimizations in place.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
+ */
+
+/* determines if a number is a valid DR modulus */
+int mp_dr_is_modulus(mp_int *a)
+{
+ int ix;
+
+ /* must be at least two digits */
+ if (a->used < 2) {
+ return 0;
+ }
+
+ /* must be of the form b**k - a [a <= b] so all
+ * but the first digit must be equal to -1 (mod b).
+ */
+ for (ix = 1; ix < a->used; ix++) {
+ if (a->dp[ix] != MP_MASK) {
+ return 0;
+ }
+ }
+ return 1;
+}
+
+#endif