diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index def20db..1a80640 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -120,7 +120,8 @@
 #define MBEDTLS_ERR_SSL_NON_FATAL                         -0x6680  /**< The alert message received indicates a non-fatal error. */
 #define MBEDTLS_ERR_SSL_INVALID_VERIFY_HASH               -0x6600  /**< Couldn't set the hash for verifying CertificateVerify */
 #define MBEDTLS_ERR_SSL_CONTINUE_PROCESSING               -0x6580  /**< Internal-only message signaling that further message-processing should be done */
-#define MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS                -0x6500  /**< A cryptographic operation is in progress. Try again later. */
+#define MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS                 -0x6500  /**< The asynchronous operation is not completed yet. */
+#define MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS                -0x6480  /**< A cryptographic operation is in progress. Try again later. */
 
 /*
  * Various constants
@@ -537,7 +538,6 @@
  */
 typedef int mbedtls_ssl_get_timer_t( void * ctx );
 
-
 /* Defined below */
 typedef struct mbedtls_ssl_session mbedtls_ssl_session;
 typedef struct mbedtls_ssl_context mbedtls_ssl_context;
@@ -554,6 +554,218 @@
 typedef struct mbedtls_ssl_flight_item mbedtls_ssl_flight_item;
 #endif
 
+#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
+#if defined(MBEDTLS_X509_CRT_PARSE_C)
+/**
+ * \brief           Callback type: start external signature operation.
+ *
+ *                  This callback is called during an SSL handshake to start
+ *                  a signature decryption operation using an
+ *                  external processor. The parameter \p cert contains
+ *                  the public key; it is up to the callback function to
+ *                  determine how to access the associated private key.
+ *
+ *                  This function typically sends or enqueues a request, and
+ *                  does not wait for the operation to complete. This allows
+ *                  the handshake step to be non-blocking.
+ *
+ *                  The parameters \p ssl and \p cert are guaranteed to remain
+ *                  valid throughout the handshake. On the other hand, this
+ *                  function must save the contents of \p hash if the value
+ *                  is needed for later processing, because the \p hash buffer
+ *                  is no longer valid after this function returns.
+ *
+ *                  This function may call mbedtls_ssl_set_async_operation_data()
+ *                  to store an operation context for later retrieval
+ *                  by the resume or cancel callback.
+ *
+ * \note            For RSA signatures, this function must produce output
+ *                  that is consistent with PKCS#1 v1.5 in the same way as
+ *                  mbedtls_rsa_pkcs1_sign(). Before the private key operation,
+ *                  apply the padding steps described in RFC 8017, section 9.2
+ *                  "EMSA-PKCS1-v1_5" as follows.
+ *                  - If \p md_alg is #MBEDTLS_MD_NONE, apply the PKCS#1 v1.5
+ *                    encoding, treating \p hash as the DigestInfo to be
+ *                    padded. In other words, apply EMSA-PKCS1-v1_5 starting
+ *                    from step 3, with `T = hash` and `tLen = hash_len`.
+ *                  - If `md_alg != MBEDTLS_MD_NONE`, apply the PKCS#1 v1.5
+ *                    encoding, treating \p hash as the hash to be encoded and
+ *                    padded. In other words, apply EMSA-PKCS1-v1_5 starting
+ *                    from step 2, with `digestAlgorithm` obtained by calling
+ *                    mbedtls_oid_get_oid_by_md() on \p md_alg.
+ *
+ * \note            For ECDSA signatures, the output format is the DER encoding
+ *                  `Ecdsa-Sig-Value` defined in
+ *                  [RFC 4492 section 5.4](https://tools.ietf.org/html/rfc4492#section-5.4).
+ *
+ * \param ssl             The SSL connection instance. It should not be
+ *                        modified other than via
+ *                        mbedtls_ssl_set_async_operation_data().
+ * \param cert            Certificate containing the public key.
+ *                        In simple cases, this is one of the pointers passed to
+ *                        mbedtls_ssl_conf_own_cert() when configuring the SSL
+ *                        connection. However, if other callbacks are used, this
+ *                        property may not hold. For example, if an SNI callback
+ *                        is registered with mbedtls_ssl_conf_sni(), then
+ *                        this callback determines what certificate is used.
+ * \param md_alg          Hash algorithm.
+ * \param hash            Buffer containing the hash. This buffer is
+ *                        no longer valid when the function returns.
+ * \param hash_len        Size of the \c hash buffer in bytes.
+ *
+ * \return          0 if the operation was started successfully and the SSL
+ *                  stack should call the resume callback immediately.
+ * \return          #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS if the operation
+ *                  was started successfully and the SSL stack should return
+ *                  immediately without calling the resume callback yet.
+ * \return          #MBEDTLS_ERR_SSL_HW_ACCEL_FALLTHROUGH if the external
+ *                  processor does not support this key. The SSL stack will
+ *                  use the private key object instead.
+ * \return          Any other error indicates a fatal failure and is
+ *                  propagated up the call chain. The callback should
+ *                  use \c MBEDTLS_ERR_PK_xxx error codes, and <b>must not</b>
+ *                  use \c MBEDTLS_ERR_SSL_xxx error codes except as
+ *                  directed in the documentation of this callback.
+ */
+typedef int mbedtls_ssl_async_sign_t( mbedtls_ssl_context *ssl,
+                                      mbedtls_x509_crt *cert,
+                                      mbedtls_md_type_t md_alg,
+                                      const unsigned char *hash,
+                                      size_t hash_len );
+
+/**
+ * \brief           Callback type: start external decryption operation.
+ *
+ *                  This callback is called during an SSL handshake to start
+ *                  an RSA decryption operation using an
+ *                  external processor. The parameter \p cert contains
+ *                  the public key; it is up to the callback function to
+ *                  determine how to access the associated private key.
+ *
+ *                  This function typically sends or enqueues a request, and
+ *                  does not wait for the operation to complete. This allows
+ *                  the handshake step to be non-blocking.
+ *
+ *                  The parameters \p ssl and \p cert are guaranteed to remain
+ *                  valid throughout the handshake. On the other hand, this
+ *                  function must save the contents of \p input if the value
+ *                  is needed for later processing, because the \p input buffer
+ *                  is no longer valid after this function returns.
+ *
+ *                  This function may call mbedtls_ssl_set_async_operation_data()
+ *                  to store an operation context for later retrieval
+ *                  by the resume or cancel callback.
+ *
+ * \warning         RSA decryption as used in TLS is subject to a potential
+ *                  timing side channel attack first discovered by Bleichenbacher
+ *                  in 1998. This attack can be remotely exploitable
+ *                  in practice. To avoid this attack, you must ensure that
+ *                  if the callback performs an RSA decryption, the time it
+ *                  takes to execute and return the result does not depend
+ *                  on whether the RSA decryption succeeded or reported
+ *                  invalid padding.
+ *
+ * \param ssl             The SSL connection instance. It should not be
+ *                        modified other than via
+ *                        mbedtls_ssl_set_async_operation_data().
+ * \param cert            Certificate containing the public key.
+ *                        In simple cases, this is one of the pointers passed to
+ *                        mbedtls_ssl_conf_own_cert() when configuring the SSL
+ *                        connection. However, if other callbacks are used, this
+ *                        property may not hold. For example, if an SNI callback
+ *                        is registered with mbedtls_ssl_conf_sni(), then
+ *                        this callback determines what certificate is used.
+ * \param input           Buffer containing the input ciphertext. This buffer
+ *                        is no longer valid when the function returns.
+ * \param input_len       Size of the \p input buffer in bytes.
+ *
+ * \return          0 if the operation was started successfully and the SSL
+ *                  stack should call the resume callback immediately.
+ * \return          #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS if the operation
+ *                  was started successfully and the SSL stack should return
+ *                  immediately without calling the resume callback yet.
+ * \return          #MBEDTLS_ERR_SSL_HW_ACCEL_FALLTHROUGH if the external
+ *                  processor does not support this key. The SSL stack will
+ *                  use the private key object instead.
+ * \return          Any other error indicates a fatal failure and is
+ *                  propagated up the call chain. The callback should
+ *                  use \c MBEDTLS_ERR_PK_xxx error codes, and <b>must not</b>
+ *                  use \c MBEDTLS_ERR_SSL_xxx error codes except as
+ *                  directed in the documentation of this callback.
+ */
+typedef int mbedtls_ssl_async_decrypt_t( mbedtls_ssl_context *ssl,
+                                         mbedtls_x509_crt *cert,
+                                         const unsigned char *input,
+                                         size_t input_len );
+#endif /* MBEDTLS_X509_CRT_PARSE_C */
+
+/**
+ * \brief           Callback type: resume external operation.
+ *
+ *                  This callback is called during an SSL handshake to resume
+ *                  an external operation started by the
+ *                  ::mbedtls_ssl_async_sign_t or
+ *                  ::mbedtls_ssl_async_decrypt_t callback.
+ *
+ *                  This function typically checks the status of a pending
+ *                  request or causes the request queue to make progress, and
+ *                  does not wait for the operation to complete. This allows
+ *                  the handshake step to be non-blocking.
+ *
+ *                  This function may call mbedtls_ssl_get_async_operation_data()
+ *                  to retrieve an operation context set by the start callback.
+ *                  It may call mbedtls_ssl_set_async_operation_data() to modify
+ *                  this context.
+ *
+ *                  Note that when this function returns a status other than
+ *                  #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS, it must free any
+ *                  resources associated with the operation.
+ *
+ * \param ssl             The SSL connection instance. It should not be
+ *                        modified other than via
+ *                        mbedtls_ssl_set_async_operation_data().
+ * \param output          Buffer containing the output (signature or decrypted
+ *                        data) on success.
+ * \param output_len      On success, number of bytes written to \p output.
+ * \param output_size     Size of the \p output buffer in bytes.
+ *
+ * \return          0 if output of the operation is available in the
+ *                  \p output buffer.
+ * \return          #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS if the operation
+ *                  is still in progress. Subsequent requests for progress
+ *                  on the SSL connection will call the resume callback
+ *                  again.
+ * \return          Any other error means that the operation is aborted.
+ *                  The SSL handshake is aborted. The callback should
+ *                  use \c MBEDTLS_ERR_PK_xxx error codes, and <b>must not</b>
+ *                  use \c MBEDTLS_ERR_SSL_xxx error codes except as
+ *                  directed in the documentation of this callback.
+ */
+typedef int mbedtls_ssl_async_resume_t( mbedtls_ssl_context *ssl,
+                                        unsigned char *output,
+                                        size_t *output_len,
+                                        size_t output_size );
+
+/**
+ * \brief           Callback type: cancel external operation.
+ *
+ *                  This callback is called if an SSL connection is closed
+ *                  while an asynchronous operation is in progress. Note that
+ *                  this callback is not called if the
+ *                  ::mbedtls_ssl_async_resume_t callback has run and has
+ *                  returned a value other than
+ *                  #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS, since in that case
+ *                  the asynchronous operation has already completed.
+ *
+ *                  This function may call mbedtls_ssl_get_async_operation_data()
+ *                  to retrieve an operation context set by the start callback.
+ *
+ * \param ssl             The SSL connection instance. It should not be
+ *                        modified.
+ */
+typedef void mbedtls_ssl_async_cancel_t( mbedtls_ssl_context *ssl );
+#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
+
 /*
  * This structure is used for storing current session data.
  */
@@ -670,6 +882,16 @@
     mbedtls_x509_crl *ca_crl;       /*!< trusted CAs CRLs                   */
 #endif /* MBEDTLS_X509_CRT_PARSE_C */
 
+#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
+#if defined(MBEDTLS_X509_CRT_PARSE_C)
+    mbedtls_ssl_async_sign_t *f_async_sign_start; /*!< start asynchronous signature operation */
+    mbedtls_ssl_async_decrypt_t *f_async_decrypt_start; /*!< start asynchronous decryption operation */
+#endif /* MBEDTLS_X509_CRT_PARSE_C */
+    mbedtls_ssl_async_resume_t *f_async_resume; /*!< resume asynchronous operation */
+    mbedtls_ssl_async_cancel_t *f_async_cancel; /*!< cancel asynchronous operation */
+    void *p_async_config_data; /*!< Configuration data set by mbedtls_ssl_conf_async_private_cb(). */
+#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
+
 #if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
     const int *sig_hashes;          /*!< allowed signature hashes           */
 #endif
@@ -1308,6 +1530,85 @@
         void *p_export_keys );
 #endif /* MBEDTLS_SSL_EXPORT_KEYS */
 
+#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
+/**
+ * \brief           Configure asynchronous private key operation callbacks.
+ *
+ * \param conf              SSL configuration context
+ * \param f_async_sign      Callback to start a signature operation. See
+ *                          the description of ::mbedtls_ssl_async_sign_t
+ *                          for more information. This may be \c NULL if the
+ *                          external processor does not support any signature
+ *                          operation; in this case the private key object
+ *                          associated with the certificate will be used.
+ * \param f_async_decrypt   Callback to start a decryption operation. See
+ *                          the description of ::mbedtls_ssl_async_decrypt_t
+ *                          for more information. This may be \c NULL if the
+ *                          external processor does not support any decryption
+ *                          operation; in this case the private key object
+ *                          associated with the certificate will be used.
+ * \param f_async_resume    Callback to resume an asynchronous operation. See
+ *                          the description of ::mbedtls_ssl_async_resume_t
+ *                          for more information. This may not be \c NULL unless
+ *                          \p f_async_sign and \p f_async_decrypt are both
+ *                          \c NULL.
+ * \param f_async_cancel    Callback to cancel an asynchronous operation. See
+ *                          the description of ::mbedtls_ssl_async_cancel_t
+ *                          for more information. This may be \c NULL if
+ *                          no cleanup is needed.
+ * \param config_data       A pointer to configuration data which can be
+ *                          retrieved with
+ *                          mbedtls_ssl_conf_get_async_config_data(). The
+ *                          library stores this value without dereferencing it.
+ */
+void mbedtls_ssl_conf_async_private_cb( mbedtls_ssl_config *conf,
+                                        mbedtls_ssl_async_sign_t *f_async_sign,
+                                        mbedtls_ssl_async_decrypt_t *f_async_decrypt,
+                                        mbedtls_ssl_async_resume_t *f_async_resume,
+                                        mbedtls_ssl_async_cancel_t *f_async_cancel,
+                                        void *config_data );
+
+/**
+ * \brief           Retrieve the configuration data set by
+ *                  mbedtls_ssl_conf_async_private_cb().
+ *
+ * \param conf      SSL configuration context
+ * \return          The configuration data set by
+ *                  mbedtls_ssl_conf_async_private_cb().
+ */
+void *mbedtls_ssl_conf_get_async_config_data( const mbedtls_ssl_config *conf );
+
+/**
+ * \brief           Retrieve the asynchronous operation user context.
+ *
+ * \note            This function may only be called while a handshake
+ *                  is in progress.
+ *
+ * \param ssl       The SSL context to access.
+ *
+ * \return          The asynchronous operation user context that was last
+ *                  set during the current handshake. If
+ *                  mbedtls_ssl_set_async_operation_data() has not yet been
+ *                  called during the current handshake, this function returns
+ *                  \c NULL.
+ */
+void *mbedtls_ssl_get_async_operation_data( const mbedtls_ssl_context *ssl );
+
+/**
+ * \brief           Retrieve the asynchronous operation user context.
+ *
+ * \note            This function may only be called while a handshake
+ *                  is in progress.
+ *
+ * \param ssl       The SSL context to access.
+ * \param ctx       The new value of the asynchronous operation user context.
+ *                  Call mbedtls_ssl_get_async_operation_data() later during the
+ *                  same handshake to retrieve this value.
+ */
+void mbedtls_ssl_set_async_operation_data( mbedtls_ssl_context *ssl,
+                                 void *ctx );
+#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
+
 /**
  * \brief          Callback type: generate a cookie
  *