Keypair: Create class for generating/storing keys

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/java/com/wireguard/crypto/Keypair.java

@@ -0,0 +1,65 @@
+package com.wireguard.crypto;
+
+import android.util.Base64;
+
+import java.security.SecureRandom;
+
+/**
+ * Represents a Curve25519 keypair as used by WireGuard.
+ */
+
+public class Keypair {
+    private static final int KEY_LENGTH = 32;
+    public static final int KEY_STRING_LENGTH = 44;
+
+    private static byte[] generatePrivateKey() {
+        final SecureRandom secureRandom = new SecureRandom();
+        final byte privateKey[] = new byte[KEY_LENGTH];
+        secureRandom.nextBytes(privateKey);
+        privateKey[0] &= 248;
+        privateKey[31] &= 127;
+        privateKey[31] |= 64;
+        return privateKey;
+    }
+
+    private static byte[] generatePublicKey(byte privateKey[]) {
+        final byte publicKey[] = new byte[KEY_LENGTH];
+        Curve25519.eval(publicKey, 0, privateKey, null);
+        return publicKey;
+    }
+
+    private static byte[] parseKey(String key) {
+        final byte keyBytes[] = Base64.decode(key, Base64.NO_WRAP);
+        if (keyBytes.length != KEY_LENGTH)
+            throw new IndexOutOfBoundsException("Key is not the correct length");
+        return keyBytes;
+    }
+
+    private static String unParseKey(byte keyBytes[]) {
+        return Base64.encodeToString(keyBytes, Base64.NO_WRAP);
+    }
+
+    private final byte privateKey[];
+    private final byte publicKey[];
+
+    public Keypair() {
+        this(generatePrivateKey());
+    }
+
+    private Keypair(byte privateKey[]) {
+        this.privateKey = privateKey;
+        publicKey = generatePublicKey(privateKey);
+    }
+
+    public Keypair(String privateKey) {
+        this(parseKey(privateKey));
+    }
+
+    public String getPrivateKey() {
+        return unParseKey(privateKey);
+    }
+
+    public String getPublicKey() {
+        return unParseKey(publicKey);
+    }
+}