Constant time base64

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

3 files changed, 82 insertions, 20 deletions

diff --git a/app/src/main/java/com/wireguard/config/Interface.java b/app/src/main/java/com/wireguard/config/Interface.java
index 35185cf9..9f74bab0 100644
--- a/app/src/main/java/com/wireguard/config/Interface.java
+++ b/app/src/main/java/com/wireguard/config/Interface.java
@@ -6,6 +6,7 @@ import android.databinding.Observable;
 
 import com.wireguard.android.BR;
 import com.wireguard.crypto.Keypair;
+import com.wireguard.crypto.KeyEncoding;
 
 /**
  * Represents the configuration for a WireGuard interface (an [Interface] block).
@@ -90,7 +91,7 @@ public class Interface extends BaseObservable implements Observable {
 
     public void setPrivateKey(String privateKey) {
         // Avoid exceptions from Keypair while the user is typing.
-        if (privateKey.length() != Keypair.KEY_STRING_LENGTH)
+        if (privateKey.length() != KeyEncoding.WG_KEY_LEN_BASE64)
             return;
         keypair = new Keypair(privateKey);
         notifyPropertyChanged(BR.privateKey);
diff --git a/app/src/main/java/com/wireguard/crypto/KeyEncoding.java b/app/src/main/java/com/wireguard/crypto/KeyEncoding.java
new file mode 100644
index 00000000..c62657cb
--- /dev/null
+++ b/app/src/main/java/com/wireguard/crypto/KeyEncoding.java
@@ -0,0 +1,75 @@
+/* Copyright (C) 2015-2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ *
+ * This is a specialized constant-time base64 implementation that resists side-channel attacks.
+ */
+
+package com.wireguard.crypto;
+
+public class KeyEncoding {
+	public static final int WG_KEY_LEN = 32;
+	public static final int WG_KEY_LEN_BASE64 = 44;
+
+	private static void encodeBase64(char dest[], final int dest_offset, final byte src[], final int src_offset) {
+		final byte input[] = { (byte)((src[0 + src_offset] >>> 2) & 63),
+				       (byte)(((src[0 + src_offset] << 4) | ((src[1 + src_offset] & 0xff) >>> 4)) & 63),
+				       (byte)(((src[1 + src_offset] << 2) | ((src[2 + src_offset] & 0xff) >>> 6)) & 63),
+				       (byte)(src[2 + src_offset] & 63) };
+		for (int i = 0; i < 4; ++i)
+			dest[i + dest_offset] = (char)(input[i] + 'A'
+						+ (((25 - input[i]) >>> 8) & 6)
+						- (((51 - input[i]) >>> 8) & 75)
+						- (((61 - input[i]) >>> 8) & 15)
+						+ (((62 - input[i]) >>> 8) & 3));
+	}
+
+	public static String keyToBase64(final byte key[]) {
+		if (key.length != WG_KEY_LEN)
+			throw new IllegalArgumentException("WireGuard keys must be 32 bytes");
+		final char base64[] = new char[WG_KEY_LEN_BASE64];
+		int i;
+		for (i = 0; i < WG_KEY_LEN / 3; ++i)
+			encodeBase64(base64, i * 4, key, i * 3);
+		final byte endSegment[] = { key[i * 3 + 0], key[i * 3 + 1], 0 };
+		encodeBase64(base64, i * 4, endSegment, 0);
+		base64[WG_KEY_LEN_BASE64 - 1] = '=';
+		return new String(base64);
+	}
+
+	private static int decodeBase64(final char src[], int src_offset) {
+		int val = 0;
+		for (int i = 0; i < 4; ++i)
+			val |= (-1
+				    + ((((('A' - 1) - src[i + src_offset]) & (src[i + src_offset] - ('Z' + 1))) >>> 8) & (src[i + src_offset] - 64))
+				    + ((((('a' - 1) - src[i + src_offset]) & (src[i + src_offset] - ('z' + 1))) >>> 8) & (src[i + src_offset] - 70))
+				    + ((((('0' - 1) - src[i + src_offset]) & (src[i + src_offset] - ('9' + 1))) >>> 8) & (src[i + src_offset] + 5))
+				    + ((((('+' - 1) - src[i + src_offset]) & (src[i + src_offset] - ('+' + 1))) >>> 8) & 63)
+				    + ((((('/' - 1) - src[i + src_offset]) & (src[i + src_offset] - ('/' + 1))) >>> 8) & 64)
+				) << (18 - 6 * i);
+		return val;
+	}
+
+	public static byte[] keyFromBase64(final String input) {
+		final char base64[] = input.toCharArray();
+		if (base64.length != WG_KEY_LEN_BASE64 || base64[WG_KEY_LEN_BASE64 - 1] != '=')
+			throw new IllegalArgumentException("WireGuard base64 keys must be 44 characters encoding 32 bytes");
+		final byte key[] = new byte[WG_KEY_LEN];
+		int i;
+		int val;
+
+		for (i = 0; i < WG_KEY_LEN / 3; ++i) {
+			val = decodeBase64(base64, i * 4);
+			if (val < 0)
+				throw new IllegalArgumentException("WireGuard base64 keys must be 44 characters encoding 32 bytes");
+			key[i * 3 + 0] = (byte)((val >>> 16) & 0xff);
+			key[i * 3 + 1] = (byte)((val >>> 8) & 0xff);
+			key[i * 3 + 2] = (byte)(val & 0xff);
+		}
+		final char endSegment[] = { base64[i * 4 + 0], base64[i * 4 + 1], base64[i * 4 + 2], 'A' };
+		val = decodeBase64(endSegment, 0);
+		if (val < 0 || (val & 0xff) != 0)
+			throw new IllegalArgumentException("WireGuard base64 keys must be 44 characters encoding 32 bytes");
+		key[i * 3 + 0] = (byte)((val >>> 16) & 0xff);
+		key[i * 3 + 1] = (byte)((val >>> 8) & 0xff);
+		return key;
+	}
+}
diff --git a/app/src/main/java/com/wireguard/crypto/Keypair.java b/app/src/main/java/com/wireguard/crypto/Keypair.java
index ef2a4b31..f98b5e21 100644
--- a/app/src/main/java/com/wireguard/crypto/Keypair.java
+++ b/app/src/main/java/com/wireguard/crypto/Keypair.java
@@ -9,12 +9,9 @@ import java.security.SecureRandom;
  */
 
 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];
+        final byte privateKey[] = new byte[KeyEncoding.WG_KEY_LEN];
         secureRandom.nextBytes(privateKey);
         privateKey[0] &= 248;
         privateKey[31] &= 127;
@@ -23,22 +20,11 @@ public class Keypair {
     }
 
     private static byte[] generatePublicKey(byte privateKey[]) {
-        final byte publicKey[] = new byte[KEY_LENGTH];
+        final byte publicKey[] = new byte[KeyEncoding.WG_KEY_LEN];
         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[];
 
@@ -52,14 +38,14 @@ public class Keypair {
     }
 
     public Keypair(String privateKey) {
-        this(parseKey(privateKey));
+        this(KeyEncoding.keyFromBase64(privateKey));
     }
 
     public String getPrivateKey() {
-        return unParseKey(privateKey);
+        return KeyEncoding.keyToBase64(privateKey);
     }
 
     public String getPublicKey() {
-        return unParseKey(publicKey);
+        return KeyEncoding.keyToBase64(publicKey);
     }
 }