If you need to display readonly data in Umbraco the datatype you are looking for is the Label datatype. With this datatype it is not possible to input a value for an editor in the back-office of Umbraco, but you can get & set this value through the ContentService API:

A friendly heads up: by default this datatype won’t handle large string values, so if you are trying to set a larger string of some sort (for instance some base64 encoded data) you might get an exception saying: “The data was truncated while converting from one data type to another.”

To fix this, go to the Datatype > Label setting in the Umbraco back-office and change it’s configuration value type from "String" to "Long string".

Sometimes it's that easy.
Cheers friends! ❤️