A Java-based solution

mask.sh

@@ -1,4 +1,3 @@
 #!/bin/sh
 
-# Call your program here instead of cat.
-cat
+java -cp solution/classes luhnybin.Filter

solution/src/luhnybin/Filter.java

@@ -0,0 +1,73 @@
+package luhnybin;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.util.ArrayList;
+import java.util.List;
+
+public class Filter {
+
+  InputStream in;
+  OutputStream out;
+  List<Integer> buffer = new ArrayList<Integer>();
+  List<Integer> digits = new ArrayList<Integer>();
+
+  public static void main(String[] args) throws IOException {
+    new Filter(System.in, System.out).process();
+  }
+
+  public Filter(InputStream in, OutputStream out) {
+    this.in = in;
+    this.out = out;
+  }
+
+  public void process() throws IOException {
+    for (int b; (b = in.read()) != -1;) {
+      if (!isCreditCardSymbol(b)) {
+        flush();
+        out.write(b);
+        continue;
+      }
+      buffer.add(b);
+      if (isDigit(b)) digits.add(b);
+      if (digits.size() < 14) continue;
+      int count = findLongestTrailingCreditCardNumber();
+      maskTrailingDigitsInBuffer(count);
+    }
+    flush();
+  }
+
+  void flush() throws IOException {
+    for (Integer b : buffer) out.write(b);
+    buffer.clear();
+    digits.clear();
+  }
+
+  void maskTrailingDigitsInBuffer(int count) {
+    for (int i = buffer.size() - 1; i >= 0 && count > 0; i--)
+      if (isDigit(buffer.get(i))) {
+        buffer.set(i, (int)'X');
+        count--;
+      }
+  }
+
+  int findLongestTrailingCreditCardNumber() {
+    int result = 0, sum = 0, limit = Math.min(digits.size(), 16);
+    for (int i = 1; i <= limit; i++) {
+      int digit = digits.get(digits.size() - i) - 48;
+      if (i % 2 == 0) digit *= 2;
+      sum += digit / 10 + digit % 10;
+      if (i >= 14 && sum % 10 == 0) result = i;
+    }
+    return result;
+  }
+
+  boolean isCreditCardSymbol(int b) {
+    return isDigit(b) || b == 32 || b == 45;
+  }
+
+  boolean isDigit(int b) {
+    return b >= 48 && b <= 57;
+  }
+}