As Prof Bill Buchanan OBE on LinkedIn points out, the Brexit Trade Agreement [1] on Page 921 goes:<p><i>The underlying certificate used by the s/MIME mechanism has to be in compliance with X.509
standard. In order to ensure common standards and procedures with other Prüm applications, the
processing rules for s/MIME encryption operations or to be applied under various Commercial Product of the Shelves (COTS) environments, are as follows:<p>– the sequence of the operations is: first encryption and then signing,<p>– the encryption algorithm AES (Advanced Encryption Standard) with 256 bit key length and
RSA with 1024 bit key length shall be applied for symmetric and asymmetric encryption
respectively,<p>– the hash algorithm SHA-1 shall be applied.<p>s/MIME functionality is built into the vast majority of modern e-mail software packages including
Outlook, Mozilla Mail as well as Netscape Communicator 4.x and inter-operates among all major email software packages.</i><p>[1]<a href="https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/948104/EU-UK_Trade_and_Cooperation_Agreement_24.12.2020.pdf" rel="nofollow">https://assets.publishing.service.gov.uk/government/uploads/...</a>