Matrix (matrix.org) SDK written in dart.
For E2EE, libolm must be provided.
Additionally, OpenSSL (libcrypto) must be provided on native platforms for E2EE.
For flutter apps you can easily import it with the flutter_olm and the flutter_openssl_crypto packages.
flutter pub add matrix
flutter pub add flutter_olm
flutter pub add flutter_openssl_crypto
See the API documentation for details: