diff --git a/2020/day25/ex1-c.c b/2020/day25/ex1-c.c
new file mode 100644
index 0000000..92eb481
--- /dev/null
+++ b/2020/day25/ex1-c.c
@@ -0,0 +1,31 @@
+/* ex1-c: Advent2020, day 25/part 1
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+int main(ac, av)
+    int ac;
+    char **av;
+{
+    ulong pub[] = {1, 1}, enc[] = {1, 1}, key[2], res;
+
+    scanf("%lu", key);
+    scanf("%lu", key+1);
+    while (1) {
+        pub[0] = pub[0] * 7 % 20201227;
+        pub[1] = pub[1] * 7 % 20201227;
+        enc[0] = enc[0] * key[0] %20201227;
+        enc[1] = enc[1] * key[1] %20201227;
+        if (pub[0] == key[0]) {
+            res=enc[1];
+            break;
+        } else if (pub[1] == key[1]) {
+            res=enc[0];
+            break;
+        }
+    }
+    printf("%s : res=%lu\n", *av, res);
+    exit (0);
+}