Add benchmark for AES-XEX

This commit adds benchmark based on what has already been done with
AES-CBC mode.
diff --git a/programs/test/benchmark.c b/programs/test/benchmark.c
index 07298c1..ec5794e 100644
--- a/programs/test/benchmark.c
+++ b/programs/test/benchmark.c
@@ -99,7 +99,7 @@
 #define OPTIONS                                                         \
     "md4, md5, ripemd160, sha1, sha256, sha512,\n"                      \
     "arc4, des3, des, camellia, blowfish,\n"                            \
-    "aes_cbc, aes_gcm, aes_ccm, aes_cmac, des3_cmac,\n"                 \
+    "aes_cbc, aes_gcm, aes_ccm, aes_cmac, aes_xex, des3_cmac,\n"        \
     "havege, ctr_drbg, hmac_drbg\n"                                     \
     "rsa, dhm, ecdsa, ecdh.\n"
 
@@ -233,7 +233,7 @@
 typedef struct {
     char md4, md5, ripemd160, sha1, sha256, sha512,
          arc4, des3, des,
-         aes_cbc, aes_gcm, aes_ccm, aes_cmac, des3_cmac,
+         aes_cbc, aes_gcm, aes_ccm, aes_cmac, aes_xex, des3_cmac,
          aria, camellia, blowfish,
          havege, ctr_drbg, hmac_drbg,
          rsa, dhm, ecdsa, ecdh;
@@ -279,6 +279,8 @@
                 todo.des = 1;
             else if( strcmp( argv[i], "aes_cbc" ) == 0 )
                 todo.aes_cbc = 1;
+            else if( strcmp( argv[i], "aes_xex" ) == 0 )
+                todo.aes_xex = 1;
             else if( strcmp( argv[i], "aes_gcm" ) == 0 )
                 todo.aes_gcm = 1;
             else if( strcmp( argv[i], "aes_ccm" ) == 0 )
@@ -426,6 +428,29 @@
         mbedtls_aes_free( &aes );
     }
 #endif
+#if defined(MBEDTLS_CIPHER_MODE_XEX)
+    if( todo.aes_xex )
+    {
+        int keysize;
+        mbedtls_aes_context crypt_ctx, tweak_ctx;
+        mbedtls_aes_init( &crypt_ctx );
+        mbedtls_aes_init( &tweak_ctx );
+        for( keysize = 128; keysize <= 256; keysize += 64 )
+        {
+            mbedtls_snprintf( title, sizeof( title ), "AES-XEX-%d", keysize );
+
+            memset( buf, 0, sizeof( buf ) );
+            memset( tmp, 0, sizeof( tmp ) );
+            mbedtls_aes_setkey_enc( &crypt_ctx, tmp, keysize );
+            mbedtls_aes_setkey_enc( &tweak_ctx, tmp, keysize );
+
+            TIME_AND_TSC( title,
+                mbedtls_aes_crypt_xex( &crypt_ctx, &tweak_ctx, MBEDTLS_AES_ENCRYPT, BUFSIZE, tmp, buf, buf ) );
+        }
+        mbedtls_aes_free( &crypt_ctx );
+        mbedtls_aes_free( &tweak_ctx );
+    }
+#endif
 #if defined(MBEDTLS_GCM_C)
     if( todo.aes_gcm )
     {