j"19HxigV4QyBv3tHpQVcUEQyq1pzZVdoAutLûI would think it's simply leveraging the same signing process as Bitcoin uses for making payments... You sign with the keys. Don't have the keys? You can't sign validly. You can verify without the private key in hand.
Money Button would do that, too.
text/plaintext twetch_twtext_1590522179240.txt|"1PuQa7K62MiKCtssSLKy1kh56WWU7MtUR5SET twdata_jsonnullurlnullcommentnullmb_user622reply@ce831762a8b04b104f2c37ee9c1bb08a48d4316cf0bf2b4d02b48fa49df68324typepost timestampnullapptwetchinvoice$c6706cf4-24f8-4744-a810-e6e2c3be28e5|"15PciHG22SNLQJXMoSUaWVi7WSqc7hCfva
BITCOIN_ECDSA"1Eyk15KEeYX9JwH8e98pqpX3kTUuY9d43CLXIOu0GgtujTu8rvEKldqtZber+Ony5f6Vk9BRbATy5yVgaLu3NafUJPnM07ekq1ZKtoD8CStInVYORqDN9MyLo8k=
https://whatsonchain.com/tx/eedac794b5d118db2f0d49482d55bb92def26c27153db5865eb20c9c547ac0aa