modexp: Implement simple mp_modexp function.

[u/mdw/catacomb] / mp.h

diff --git a/mp.h b/mp.h
index aa9bbac..7f96746 100644 (file)
--- a/mp.h
+++ b/mp.h
@@ -968,10 +968,25 @@ extern int mp_jacobi(mp */*a*/, mp */*n*/);
  *             work if %$p$% is composite: you must factor the modulus, take
  *             a square root mod each factor, and recombine the results
  *             using the Chinese Remainder Theorem.
+ *
+ *             We guarantee that the square root returned is the smallest
+ *             one (i.e., the `positive' square root).
  */
 
 extern mp *mp_modsqrt(mp */*d*/, mp */*a*/, mp */*p*/);
 
+/* --- @mp_modexp@ --- *
+ *
+ * Arguments:  @mp *d@ = fake destination
+ *             @mp *x@ = base of exponentiation
+ *             @mp *e@ = exponent
+ *             @mp *n@ = modulus (must be positive)
+ *
+ * Returns:    The value %$x^e \bmod n$%.
+ */
+
+extern mp *mp_modexp(mp */*d*/, mp */*x*/, mp */*e*/, mp */*n*/);
+
 /*----- Test harness support ----------------------------------------------*/
 
 #include <mLib/testrig.h>