Arduino_libraries/AsyncMQTT_ESP32/docs/4.-Limitations-and-known-issues.md

Limitations and known issues

• The library is spec compliant with one limitation. In case of power loss the following is not honored:

Must be kept in memory:

• All messages in a QoS 1 or 2 flow, which are not confirmed by the broker
• All received QoS 2 messages, which are not yet confirmed to the broker

This means retransmission is not honored in case of a power failure. This behaviour is like explained in point 4.1.1 of the MQTT specification v3.1.1

• You cannot send payload larger that what can fit on RAM.

SSL limitations

• SSL requires the build flag -DASYNC_TCP_SSL_ENABLED=1
• SSL only supports fingerprints for server validation.
• If you do not specify one or more acceptable server fingerprints, the SSL connection will be vulnerable to man-in-the-middle attacks.
• Some server certificate signature algorithms do not work. SHA1, SHA224, SHA256, and MD5 are working. SHA384, and SHA512 will cause a crash.
• TLS1.2 is not supported.