diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index fb68a1c..cf7ad4e 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -4030,6 +4030,7 @@
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> send alert message" ) );
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "send alert level=%u message=%u", level, message ));
 
     ssl->out_msgtype = MBEDTLS_SSL_MSG_ALERT;
     ssl->out_msglen = 2;
@@ -4041,7 +4042,6 @@
         MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_write_record", ret );
         return( ret );
     }
-
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= send alert message" ) );
 
     return( 0 );
@@ -4262,6 +4262,8 @@
 
     if( ( ret = mbedtls_ssl_read_record( ssl ) ) != 0 )
     {
+        /* mbedtls_ssl_read_record may have sent an alert already. We
+           let it decide whether to alert. */
         MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_read_record", ret );
         return( ret );
     }
@@ -4283,6 +4285,9 @@
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "SSLv3 client has no certificate" ) );
 
+            /* The client was asked for a certificate but didn't send
+               one. The client should know what's going on, so we
+               don't send an alert. */
             ssl->session_negotiate->verify_result = MBEDTLS_X509_BADCERT_MISSING;
             if( authmode == MBEDTLS_SSL_VERIFY_OPTIONAL )
                 return( 0 );
@@ -4304,6 +4309,9 @@
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "TLSv1 client has no certificate" ) );
 
+            /* The client was asked for a certificate but didn't send
+               one. The client should know what's going on, so we
+               don't send an alert. */
             ssl->session_negotiate->verify_result = MBEDTLS_X509_BADCERT_MISSING;
             if( authmode == MBEDTLS_SSL_VERIFY_OPTIONAL )
                 return( 0 );
@@ -4318,6 +4326,8 @@
     if( ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
         return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
     }
 
@@ -4325,6 +4335,8 @@
         ssl->in_hslen < mbedtls_ssl_hs_hdr_len( ssl ) + 3 + 3 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
     }
 
@@ -4339,6 +4351,8 @@
         ssl->in_hslen != n + 3 + mbedtls_ssl_hs_hdr_len( ssl ) )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
     }
 
@@ -4354,6 +4368,8 @@
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc(%d bytes) failed",
                        sizeof( mbedtls_x509_crt ) ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
         return( MBEDTLS_ERR_SSL_ALLOC_FAILED );
     }
 
@@ -4366,6 +4382,8 @@
         if( ssl->in_msg[i] != 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
             return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
         }
 
@@ -4376,13 +4394,35 @@
         if( n < 128 || i + n > ssl->in_hslen )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
             return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
         }
 
         ret = mbedtls_x509_crt_parse_der( ssl->session_negotiate->peer_cert,
                                   ssl->in_msg + i, n );
-        if( 0 != ret && ( MBEDTLS_ERR_X509_UNKNOWN_SIG_ALG + MBEDTLS_ERR_OID_NOT_FOUND ) != ret )
+        switch( ret )
         {
+        uint8_t alert;
+
+        case 0: /*ok*/
+        case MBEDTLS_ERR_X509_UNKNOWN_SIG_ALG + MBEDTLS_ERR_OID_NOT_FOUND:
+            /* Ignore certificate with an unknown algorithm: maybe a
+               prior certificate was already trusted. */
+            break;
+
+        case MBEDTLS_ERR_X509_ALLOC_FAILED:
+            alert = MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR;
+            goto crt_parse_der_failed;
+
+        case MBEDTLS_ERR_X509_UNKNOWN_VERSION:
+            alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            goto crt_parse_der_failed;
+
+        default:
+            alert = MBEDTLS_SSL_ALERT_MSG_BAD_CERT;
+        crt_parse_der_failed:
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL, alert );
             MBEDTLS_SSL_DEBUG_RET( 1, " mbedtls_x509_crt_parse_der", ret );
             return( ret );
         }
@@ -4403,6 +4443,8 @@
         if( ssl->session->peer_cert == NULL )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "new server cert during renegotiation" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED );
             return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
         }
 
@@ -4413,6 +4455,8 @@
                     ssl->session->peer_cert->raw.len ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "server cert changed during renegotiation" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED );
             return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
         }
     }
@@ -4439,6 +4483,8 @@
         if( ca_chain == NULL )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "got no CA chain" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_BAD_CERT );
             return( MBEDTLS_ERR_SSL_CA_CHAIN_REQUIRED );
         }
 
@@ -4489,6 +4535,36 @@
 
         if( authmode == MBEDTLS_SSL_VERIFY_OPTIONAL )
             ret = 0;
+
+        if( ret != 0 )
+        {
+            /* The certificate may have been rejected for several reasons.
+               Pick one and send the corresponding alert. Which alert to send
+               may be a subject of debate in some cases. */
+            uint8_t alert = MBEDTLS_SSL_ALERT_MSG_CERT_UNKNOWN;
+            if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_OTHER )
+                alert = MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_CN_MISMATCH )
+                alert = MBEDTLS_SSL_ALERT_MSG_BAD_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_KEY_USAGE )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_EXT_KEY_USAGE )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_NS_CERT_TYPE )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_BAD_PK )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_BAD_KEY )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_EXPIRED )
+                alert = MBEDTLS_SSL_ALERT_MSG_CERT_EXPIRED;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_REVOKED )
+                alert = MBEDTLS_SSL_ALERT_MSG_CERT_REVOKED;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_NOT_TRUSTED )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNKNOWN_CA;
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            alert );
+        }
     }
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= parse certificate" ) );
@@ -4541,12 +4617,16 @@
     if( ssl->in_msgtype != MBEDTLS_SSL_MSG_CHANGE_CIPHER_SPEC )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad change cipher spec message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
         return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
     }
 
     if( ssl->in_msglen != 1 || ssl->in_msg[0] != 1 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad change cipher spec message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CHANGE_CIPHER_SPEC );
     }
 
@@ -4569,6 +4649,8 @@
         if( ++ssl->in_epoch == 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "DTLS epoch would wrap" ) );
+            /* This is highly unlikely to happen for legitimate reasons, so
+               treat it as an attack and don't send an alert. */
             return( MBEDTLS_ERR_SSL_COUNTER_WRAPPING );
         }
     }
@@ -4593,6 +4675,8 @@
         if( ( ret = mbedtls_ssl_hw_record_activate( ssl, MBEDTLS_SSL_CHANNEL_INBOUND ) ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_hw_record_activate", ret );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
             return( MBEDTLS_ERR_SSL_HW_ACCEL_FAILED );
         }
     }
@@ -5171,6 +5255,8 @@
     if( ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad finished message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
         return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
     }
 
@@ -5186,6 +5272,8 @@
         ssl->in_hslen  != mbedtls_ssl_hs_hdr_len( ssl ) + hash_len )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad finished message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_FINISHED );
     }
 
@@ -5193,6 +5281,8 @@
                       buf, hash_len ) != 0 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad finished message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_FINISHED );
     }