Prevent the creation of 'weak' PINs.

Simple checks to prevent the same number, or sequentially
increasing/decreasing PINs. e.g. 1111, 1234, 54321, etc.

app/src/test/java/org/thoughtcrime/securesms/registration/v2/PinValidityChecker_validity_Test.java

@@ -0,0 +1,38 @@
+package org.thoughtcrime.securesms.registration.v2;
+
+import org.junit.Test;
+import org.thoughtcrime.securesms.registration.v2.testdata.PinValidityVector;
+import org.thoughtcrime.securesms.util.Util;
+import org.whispersystems.signalservice.internal.registrationpin.PinValidityChecker;
+import org.whispersystems.signalservice.internal.util.JsonUtil;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public final class PinValidityChecker_validity_Test {
+
+  @Test
+  public void vectors_valid() throws IOException {
+    for (PinValidityVector vector : getKbsPinValidityTestVectorList()) {
+      boolean valid = PinValidityChecker.valid(vector.getPin());
+
+      assertEquals(String.format("%s [%s]", vector.getName(), vector.getPin()),
+                   vector.isValid(),
+                   valid);
+    }
+  }
+
+  private static PinValidityVector[] getKbsPinValidityTestVectorList() throws IOException {
+    try (InputStream resourceAsStream = ClassLoader.getSystemClassLoader().getResourceAsStream("data/kbs_pin_validity_vectors.json")) {
+
+      PinValidityVector[] data = JsonUtil.fromJson(Util.readFullyAsString(resourceAsStream), PinValidityVector[].class);
+
+      assertTrue(data.length > 0);
+
+      return data;
+    }
+  }
+}

app/src/test/java/org/thoughtcrime/securesms/registration/v2/testdata/PinValidityVector.java

@@ -0,0 +1,27 @@
+package org.thoughtcrime.securesms.registration.v2.testdata;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+public class PinValidityVector {
+
+  @JsonProperty("name")
+  private String name;
+
+  @JsonProperty("pin")
+  private String pin;
+
+  @JsonProperty("valid")
+  private boolean valid;
+
+  public String getName() {
+    return name;
+  }
+
+  public String getPin() {
+    return pin;
+  }
+
+  public boolean isValid() {
+    return valid;
+  }
+}

app/src/test/resources/data/kbs_pin_validity_vectors.json

@@ -0,0 +1,62 @@
+[
+  {
+    "name": "Empty",
+    "pin": "",
+    "valid": false
+  },
+  {
+    "name": "Alpha",
+    "pin": "abcd",
+    "valid": true
+  },
+  {
+    "name": "Sequential",
+    "pin": "1234",
+    "valid": false
+  },
+  {
+    "name": "Non-sequential",
+    "pin": "6485",
+    "valid": true
+  },
+  {
+    "name": "Sequential descending",
+    "pin": "43210",
+    "valid": false
+  },
+  {
+    "name": "Sequential with space",
+    "pin": "1234 ",
+    "valid": false
+  },
+  {
+    "name": "Non-sequential with space",
+    "pin": "1236 ",
+    "valid": true
+  },
+  {
+    "name": "Sequential Non-arabic digits",
+    "pin": "١٢٣٤٥",
+    "valid": false
+  },
+  {
+    "name": "Sequential descending Non-arabic digits",
+    "pin": "٥٤٣٢١",
+    "valid": false
+  },
+  {
+    "name": "Non-sequential Non-arabic digits",
+    "pin": "١٢٣٥٤",
+    "valid": true
+  },
+  {
+    "name": "All digits the same",
+    "pin": "9999",
+    "valid": false
+  },
+  {
+    "name": "All Non-arabic digits the same",
+    "pin": "٢٢٢٢",
+    "valid": false
+  }
+]