diff --git a/README.md b/README.md
index 88e76292..60aa447f 100644
--- a/README.md
+++ b/README.md
@@ -266,6 +266,10 @@ insightful technical information you may find helpful.
     * `crypto_sign()`
     * `crypto_sign_open()`
 * PECL Libsodium Features
+    * `crypto_aead_aegis128l_encrypt()`
+    * `crypto_aead_aegis128l_decrypt()`
+    * `crypto_aead_aegis256_encrypt()`
+    * `crypto_aead_aegis256_decrypt()`
     * `crypto_aead_aes256gcm_encrypt()`
     * `crypto_aead_aes256gcm_decrypt()`
     * `crypto_aead_chacha20poly1305_encrypt()`
@@ -343,13 +347,13 @@ insightful technical information you may find helpful.
 
 ### Features Excluded from this Polyfill
 
-* `\Sodium\memzero()` - Although we expose this API endpoint, we can't reliably
+* `sodium_memzero()` - Although we expose this API endpoint, we can't reliably
   zero buffers from PHP.
   
   If you have the PHP extension installed, sodium_compat
   will use the native implementation to zero out the string provided. Otherwise
   it will throw a `SodiumException`.
-* `\Sodium\crypto_pwhash()` - It's not feasible to polyfill scrypt or Argon2
+* `sodium_crypto_pwhash()` - It's not feasible to polyfill scrypt or Argon2
   into PHP and get reasonable performance. Users would feel motivated to select
   parameters that downgrade security to avoid denial of service (DoS) attacks.
   
@@ -361,6 +365,8 @@ insightful technical information you may find helpful.
   To detect support for Argon2i at runtime, use
   `ParagonIE_Sodium_Compat::crypto_pwhash_is_available()`, which returns a
    boolean value (`TRUE` or `FALSE`).
+* Libsodium's HKDF API (`crypto_kdf_hkdf_*()`) is not included because PHP has
+  its own [HMAC features](https://php.met/hash_hmac) amd it was not deemed necessary.
 
 ### PHPCompatibility Ruleset
 
diff --git a/src/Core/AEGIS/State128L.php b/src/Core/AEGIS/State128L.php
index 0bbf99d2..9decd2b4 100644
--- a/src/Core/AEGIS/State128L.php
+++ b/src/Core/AEGIS/State128L.php
@@ -1,4 +1,9 @@
 <?php
+
+if (class_exists('ParagonIE_Sodium_Core_AEGIS_State128L', false)) {
+    return;
+}
+
 if (!defined('SODIUM_COMPAT_AEGIS_C0')) {
     define('SODIUM_COMPAT_AEGIS_C0', "\x00\x01\x01\x02\x03\x05\x08\x0d\x15\x22\x37\x59\x90\xe9\x79\x62");
 }
@@ -237,20 +242,24 @@ public function update($m0, $m1)
            S'6 = AESRound(S5, S6)
            S'7 = AESRound(S6, S7)
          */
-        $s_0 = ParagonIE_Sodium_Core_AES::aesRound(
-            $this->state[7],
-            $this->state[0] ^ $m0
+        list($s_0, $s_1) = ParagonIE_Sodium_Core_AES::doubleRound(
+            $this->state[7], $this->state[0] ^ $m0,
+            $this->state[0], $this->state[1]
+        );
+
+        list($s_2, $s_3) = ParagonIE_Sodium_Core_AES::doubleRound(
+            $this->state[1], $this->state[2],
+            $this->state[2], $this->state[3]
+        );
+
+        list($s_4, $s_5) = ParagonIE_Sodium_Core_AES::doubleRound(
+            $this->state[3], $this->state[4] ^ $m1,
+            $this->state[4], $this->state[5]
         );
-        $s_1 = ParagonIE_Sodium_Core_AES::aesRound($this->state[0], $this->state[1]);
-        $s_2 = ParagonIE_Sodium_Core_AES::aesRound($this->state[1], $this->state[2]);
-        $s_3 = ParagonIE_Sodium_Core_AES::aesRound($this->state[2], $this->state[3]);
-        $s_4 = ParagonIE_Sodium_Core_AES::aesRound(
-            $this->state[3],
-            $this->state[4] ^ $m1
+        list($s_6, $s_7) = ParagonIE_Sodium_Core_AES::doubleRound(
+            $this->state[5], $this->state[6],
+            $this->state[6], $this->state[7]
         );
-        $s_5 = ParagonIE_Sodium_Core_AES::aesRound($this->state[4], $this->state[5]);
-        $s_6 = ParagonIE_Sodium_Core_AES::aesRound($this->state[5], $this->state[6]);
-        $s_7 = ParagonIE_Sodium_Core_AES::aesRound($this->state[6], $this->state[7]);
 
         /*
            S0  = S'0
diff --git a/src/Core/AEGIS/State256.php b/src/Core/AEGIS/State256.php
index 7c11cf73..6f88b828 100644
--- a/src/Core/AEGIS/State256.php
+++ b/src/Core/AEGIS/State256.php
@@ -1,5 +1,9 @@
 <?php
 
+if (class_exists('ParagonIE_Sodium_Core_AEGIS_State256', false)) {
+    return;
+}
+
 if (!defined('SODIUM_COMPAT_AEGIS_C0')) {
     define('SODIUM_COMPAT_AEGIS_C0', "\x00\x01\x01\x02\x03\x05\x08\x0d\x15\x22\x37\x59\x90\xe9\x79\x62");
 }
@@ -203,15 +207,19 @@ public function update($m)
             S'4 = AESRound(S3, S4)
             S'5 = AESRound(S4, S5)
          */
-        $s_0 = ParagonIE_Sodium_Core_AES::aesRound(
-            $this->state[5],
-            $this->state[0] ^ $m
+        list($s_0, $s_1) = ParagonIE_Sodium_Core_AES::doubleRound(
+            $this->state[5],$this->state[0] ^ $m,
+            $this->state[0], $this->state[1]
+        );
+
+        list($s_2, $s_3) = ParagonIE_Sodium_Core_AES::doubleRound(
+            $this->state[1], $this->state[2],
+            $this->state[2], $this->state[3]
+        );
+        list($s_4, $s_5) = ParagonIE_Sodium_Core_AES::doubleRound(
+            $this->state[3], $this->state[4],
+            $this->state[4], $this->state[5]
         );
-        $s_1 = ParagonIE_Sodium_Core_AES::aesRound($this->state[0], $this->state[1]);
-        $s_2 = ParagonIE_Sodium_Core_AES::aesRound($this->state[1], $this->state[2]);
-        $s_3 = ParagonIE_Sodium_Core_AES::aesRound($this->state[2], $this->state[3]);
-        $s_4 = ParagonIE_Sodium_Core_AES::aesRound($this->state[3], $this->state[4]);
-        $s_5 = ParagonIE_Sodium_Core_AES::aesRound($this->state[4], $this->state[5]);
 
         /*
             S0  = S'0
diff --git a/src/Core/AES.php b/src/Core/AES.php
index a7b75499..d86cff1a 100644
--- a/src/Core/AES.php
+++ b/src/Core/AES.php
@@ -1,5 +1,9 @@
 <?php
 
+if (class_exists('ParagonIE_Sodium_Core_AES', false)) {
+    return;
+}
+
 /**
  * Bitsliced implementation of the AES block cipher.
  *
@@ -441,6 +445,56 @@ public static function aesRound($x, $y)
             self::store32_le($q[6]);
     }
 
+    /**
+     * Process two AES blocks in one shot.
+     *
+     * @param string $b0  First AES block
+     * @param string $rk0 First round key
+     * @param string $b1  Second AES block
+     * @param string $rk1 Second round key
+     * @return string[]
+     */
+    public static function doubleRound($b0, $rk0, $b1, $rk1)
+    {
+        $q = ParagonIE_Sodium_Core_AES_Block::init();
+        // First block
+        $q[0] = self::load_4(self::substr($b0, 0, 4));
+        $q[2] = self::load_4(self::substr($b0, 4, 4));
+        $q[4] = self::load_4(self::substr($b0, 8, 4));
+        $q[6] = self::load_4(self::substr($b0, 12, 4));
+        // Second block
+        $q[1] = self::load_4(self::substr($b1, 0, 4));
+        $q[3] = self::load_4(self::substr($b1, 4, 4));
+        $q[5] = self::load_4(self::substr($b1, 8, 4));
+        $q[7] = self::load_4(self::substr($b1, 12, 4));;
+
+        $rk = ParagonIE_Sodium_Core_AES_Block::init();
+        // First round key
+        $rk[0] = self::load_4(self::substr($rk0, 0, 4));
+        $rk[2] = self::load_4(self::substr($rk0, 4, 4));
+        $rk[4] = self::load_4(self::substr($rk0, 8, 4));
+        $rk[6] = self::load_4(self::substr($rk0, 12, 4));
+        // Second round key
+        $rk[1] = self::load_4(self::substr($rk1, 0, 4));
+        $rk[3] = self::load_4(self::substr($rk1, 4, 4));
+        $rk[5] = self::load_4(self::substr($rk1, 8, 4));
+        $rk[7] = self::load_4(self::substr($rk1, 12, 4));
+
+        $q->orthogonalize();
+        self::sbox($q);
+        $q->shiftRows();
+        $q->mixColumns();
+        $q->orthogonalize();
+        // add round key without key schedule:
+        for ($i = 0; $i < 8; ++$i) {
+            $q[$i] ^= $rk[$i];
+        }
+        return array(
+            self::store32_le($q[0]) . self::store32_le($q[2]) . self::store32_le($q[4]) . self::store32_le($q[6]),
+            self::store32_le($q[1]) . self::store32_le($q[3]) . self::store32_le($q[5]) . self::store32_le($q[7]),
+        );
+    }
+
     /**
      * @param ParagonIE_Sodium_Core_AES_Expanded $skey
      * @param ParagonIE_Sodium_Core_AES_Block $q
diff --git a/src/Core/AES/Expanded.php b/src/Core/AES/Expanded.php
index e8b84924..84a6a476 100644
--- a/src/Core/AES/Expanded.php
+++ b/src/Core/AES/Expanded.php
@@ -1,5 +1,9 @@
 <?php
 
+if (class_exists('ParagonIE_Sodium_Core_AES_Expanded', false)) {
+    return;
+}
+
 /**
  * @internal This should only be used by sodium_compat
  */
diff --git a/src/Core/AES/KeySchedule.php b/src/Core/AES/KeySchedule.php
index 1a5bf304..6b0df18a 100644
--- a/src/Core/AES/KeySchedule.php
+++ b/src/Core/AES/KeySchedule.php
@@ -1,5 +1,9 @@
 <?php
 
+if (class_exists('ParagonIE_Sodium_Core_AES_KeySchedule', false)) {
+    return;
+}
+
 /**
  * @internal This should only be used by sodium_compat
  */
diff --git a/tests/unit/AESTest.php b/tests/unit/AESTest.php
index 37fd027b..4371d790 100644
--- a/tests/unit/AESTest.php
+++ b/tests/unit/AESTest.php
@@ -331,6 +331,36 @@ public function testAesRound()
         );
     }
 
+    public function testAesDoubleRound()
+    {
+        $in = ParagonIE_Sodium_Core_Util::hex2bin('000102030405060708090a0b0c0d0e0f');
+        $rk = ParagonIE_Sodium_Core_Util::hex2bin('101112131415161718191a1b1c1d1e1f');
+        $this->assertSame(
+            '7a7b4e5638782546a8c0477a3b813f437a7b4e5638782546a8c0477a3b813f43',
+            ParagonIE_Sodium_Core_Util::bin2hex(
+                implode('', ParagonIE_Sodium_Core_AES::doubleRound($in, $rk, $in, $rk))
+            )
+        );
+
+        // Let's randomize this to test equivalence.
+        $in0 = random_bytes(16);
+        $in1 = random_bytes(16);
+        $rk0 = random_bytes(16);
+        $rk1 = random_bytes(16);
+
+        $c0 = ParagonIE_Sodium_Core_AES::aesRound($in0, $rk0);
+        $c1 = ParagonIE_Sodium_Core_AES::aesRound($in1, $rk1);
+        list($c2, $c3) = ParagonIE_Sodium_Core_AES::doubleRound($in0, $rk0, $in1, $rk1);
+        $this->assertSame(
+            ParagonIE_Sodium_Core_Util::bin2hex($c0),
+            ParagonIE_Sodium_Core_Util::bin2hex($c2)
+        );
+        $this->assertSame(
+            ParagonIE_Sodium_Core_Util::bin2hex($c1),
+            ParagonIE_Sodium_Core_Util::bin2hex($c3)
+        );
+    }
+
     /**
      * @dataProvider aes128ecbProvider
      * @covers ParagonIE_Sodium_Core_AES::encryptBlockECB