Documentation has identical description for messageToDisplay for both SmartID and MobileID. It says:
Text displayed in addition to ServiceName and before asking signing PIN. Maximum length is 40 bytes. In case of Latin letters, this means also a 40 character long text, but Cyrillic characters may be encoded by two bytes and you will not be able to send more than 20 symbols.
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In reality Smart-ID uses UTF-8 for this text and every Smart-ID app is capable of displaying up to 60 characters (no matter Cyryllic or not).
Documentation has identical description for messageToDisplay for both SmartID and MobileID. It says:
Text displayed in addition to ServiceName and before asking signing PIN. Maximum length is 40 bytes. In case of Latin letters, this means also a 40 character long text, but Cyrillic characters may be encoded by two bytes and you will not be able to send more than 20 symbols.
=== In reality Smart-ID uses UTF-8 for this text and every Smart-ID app is capable of displaying up to 60 characters (no matter Cyryllic or not).