feat(app): implement Google Play App Licensing

Signed-off-by: YuutaW <17158086+Trumeet@users.noreply.github.com>

1 files changed, 94 insertions, 0 deletions

diff --git a/app/src/main/java/moe/yuuta/gplicense/AESObfuscator.java b/app/src/main/java/moe/yuuta/gplicense/AESObfuscator.java
new file mode 100644
index 0000000..7370f58
--- /dev/null
+++ b/app/src/main/java/moe/yuuta/gplicense/AESObfuscator.java
@@ -0,0 +1,94 @@
+package moe.yuuta.gplicense;
+
+import java.io.UnsupportedEncodingException;
+import java.security.GeneralSecurityException;
+import java.security.spec.KeySpec;
+
+import javax.crypto.BadPaddingException;
+import javax.crypto.Cipher;
+import javax.crypto.IllegalBlockSizeException;
+import javax.crypto.SecretKey;
+import javax.crypto.SecretKeyFactory;
+import javax.crypto.spec.IvParameterSpec;
+import javax.crypto.spec.PBEKeySpec;
+import javax.crypto.spec.SecretKeySpec;
+
+import moe.yuuta.gplicense.util.Base64;
+import moe.yuuta.gplicense.util.Base64DecoderException;
+
+/**
+ * An Obfuscator that uses AES to encrypt data.
+ */
+public class AESObfuscator implements Obfuscator {
+    private static final String UTF8 = "UTF-8";
+    private static final String KEYGEN_ALGORITHM = "PBEWITHSHAAND256BITAES-CBC-BC";
+    private static final String CIPHER_ALGORITHM = "AES/CBC/PKCS5Padding";
+    private static final byte[] IV =
+            {16, 74, 71, -80, 32, 101, -47, 72, 117, -14, 0, -29, 70, 65, -12, 74};
+    private static final String header = "moe.yuuta.workmode.AO-1|";
+
+    private Cipher mEncryptor;
+    private Cipher mDecryptor;
+
+    /**
+     * @param salt          an array of random bytes to use for each (un)obfuscation
+     * @param applicationId application identifier, e.g. the package name
+     * @param deviceId      device identifier. Use as many sources as possible to
+     *                      create this unique identifier.
+     */
+    public AESObfuscator(byte[] salt, String applicationId, String deviceId) {
+        try {
+            SecretKeyFactory factory = SecretKeyFactory.getInstance(KEYGEN_ALGORITHM);
+            KeySpec keySpec =
+                    new PBEKeySpec((applicationId + deviceId).toCharArray(), salt, 1024, 256);
+            SecretKey tmp = factory.generateSecret(keySpec);
+            SecretKey secret = new SecretKeySpec(tmp.getEncoded(), "AES");
+            mEncryptor = Cipher.getInstance(CIPHER_ALGORITHM);
+            mEncryptor.init(Cipher.ENCRYPT_MODE, secret, new IvParameterSpec(IV));
+            mDecryptor = Cipher.getInstance(CIPHER_ALGORITHM);
+            mDecryptor.init(Cipher.DECRYPT_MODE, secret, new IvParameterSpec(IV));
+        } catch (GeneralSecurityException e) {
+            // This can't happen on a compatible Android device.
+            throw new RuntimeException("Invalid environment", e);
+        }
+    }
+
+    public String obfuscate(String original, String key) {
+        if (original == null) {
+            return null;
+        }
+        try {
+            // Header is appended as an integrity check
+            return Base64.encode(mEncryptor.doFinal((header + key + original).getBytes(UTF8)));
+        } catch (UnsupportedEncodingException e) {
+            throw new RuntimeException("Invalid environment", e);
+        } catch (GeneralSecurityException e) {
+            throw new RuntimeException("Invalid environment", e);
+        }
+    }
+
+    public String unobfuscate(String obfuscated, String key) throws ValidationException {
+        if (obfuscated == null) {
+            return null;
+        }
+        try {
+            String result = new String(mDecryptor.doFinal(Base64.decode(obfuscated)), UTF8);
+            // Check for presence of header. This serves as a final integrity check, for cases
+            // where the block size is correct during decryption.
+            int headerIndex = result.indexOf(header + key);
+            if (headerIndex != 0) {
+                throw new ValidationException("Header not found (invalid data or key)" + ":" +
+                        obfuscated);
+            }
+            return result.substring(header.length() + key.length(), result.length());
+        } catch (Base64DecoderException e) {
+            throw new ValidationException(e.getMessage() + ":" + obfuscated);
+        } catch (IllegalBlockSizeException e) {
+            throw new ValidationException(e.getMessage() + ":" + obfuscated);
+        } catch (BadPaddingException e) {
+            throw new ValidationException(e.getMessage() + ":" + obfuscated);
+        } catch (UnsupportedEncodingException e) {
+            throw new RuntimeException("Invalid environment", e);
+        }
+    }
+}
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