non-repudiation with lowest effort

Dear all,

First of all I am happy that someone reads this message.
what is in your opinion the simplest way to relalise a web service that ensures non-repudiation for a point-to-point communication with no intermediaries.
Is the transport layer (ssl/tls) able to handle this or is message-security required.
Wich standard would you prefere to solve this problem with the lowest effort
Is XML signature enough?

kind regards,

Jan