How to implement encryption for kotlin data class fields using decorator pattern

Question

I am a investigating the decorator pattern in kotlin and would like to encrypt/decrypt data class fields with a cryptography decorator.

One thing I do not understand is that the decorator approach requires an instance of the class being decorated. which means the data class fields I wish to encrypt will already containing plaintext before the decorator has an opportunity to encrypt any values.

I cannot find any examples of how to implement encryption/decryption using the decorator pattern related to class constructor fields. is it possible to achieve my desired implementation.

I am attempting to create this cryptography decorator to allow me to persist plaintext data as encrypted strings in my current android application room local database.

I have achieved the desired result using a room type converter for my custom data type field as shown below, however I would also like an alternative approach of employing a decorator class combined with kotlin delegation.

class EncryptedStringConverter : KoinComponent {

    private val cryptographer: Cryptographer by inject()

    @TypeConverter
    fun fromEncryptedStringToString(encryptedString: EncryptedString): String = encrypt(encryptedString.plaintText)

    @TypeConverter
    fun fromPlaintextToEncryptedString(plaintext: String): EncryptedString = decrypt(plaintext)

    private fun encrypt(plaintext: String): String = cryptographer.conceal(plainText = plaintext)

    private fun decrypt(encryptedString: String): EncryptedString = EncryptedString(cryptographer.reveal(encryptedString))

}

data class EncryptedString(val plaintText: String)

@Entity(
    tableName = "user_table"
)
data class UserDO(

    var firstName: EncryptedString,

    var lastName: EncryptedString,

    var email: EncryptedString,

    var companyName: EncryptedString,

)