commit 4c0887edfbdfe9df288e2292223ae734505b9601
Author: Connor <cloningdonor@gmail.com>
Date:   Mon Aug 21 04:22:57 2017 -0700

diff --git a/sckey.c b/sckey.c
new file mode 100644
index 0000000..e93b1d6
--- /dev/null
+++ b/sckey.c
@@ -0,0 +1,34 @@
+int validate(const char *key) {
+    int magic = 3;
+    int count = 0;
+    for (char c; (c = *key); key++) {
+        if (c < '0' || c > '9') continue;
+        int v = c - '0';
+        if (++count == 13) {
+            // final character.
+            return magic % 10 == v;
+        } else {
+            v ^= magic * 2;
+            magic += v;
+        }
+    }
+    return 0;
+}
+
+int main(int argc, char **argv) {
+    if (argc == 1) {
+        // self-test.
+        if (!validate("0000-00000-0003")) return -1;
+        if (!validate("1234-56789-0123")) return -1;
+        if (!validate("1337-42069-0008")) return -1;
+        if (!validate("1998-00000-1997")) return -1;
+        if (!validate("3333-33333-3333")) return -1;
+        return 0;
+    }
+
+    int ret = 0;
+    for (int i = 1; i < argc; i++) {
+        if (!validate(argv[i])) ret++;
+    }
+    return ret;
+}