  Non-repudiation protocols have an important role in many areas where secured
transactions with proofs of participation are necessary. Formal methods are
clever and without error, therefore using them for verifying such protocols is
crucial. In this purpose, we show how to partially represent non-repudiation as
a combination of authentications on the Fair Zhou-Gollmann protocol. After
discussing its limits, we define a new method based on the handling of the
knowledge of protocol participants. This method is very general and is of
natural use, as it consists in adding simple annotations, like for
authentication problems. The method is very easy to implement in tools able to
handle participants knowledge. We have implemented it in the AVISPA Tool and
analyzed the optimistic Cederquist-Corin- Dashti protocol, discovering two
unknown attacks. This extension of the AVISPA Tool for handling non-repudiation
opens a highway to the specification of many other properties, without any more
change in the tool itself.
