An interesting fact reported this week by Quentin de la Bedoyere on lying in the Catechism of the Catholic Church. As he notes, the explanation of the obligation was changed between editions:
Version 1: a lie is 'to speak or act against the truth in order to lead into error someone who has the right to know the truth.' The 'Justice' version.
Version 2: a lie is 'to speak or act against the truth in order to lead someone into error' - with an added proviso that one should use discreet language. The 'Natural Law' version.
The fact that Version one appeared in the CCC is sufficient to show that it is a permissible position in Catholic theology. It is less restrictive, and would be a lot easier to use in hard cases: one may tell untruths in order to lead into error, when one's interlocutor is engaged in some wicked enterprise for which the information one has is vital. It is also clearly what St Robert Southwell acted on, as famously explained at his trial. I'll try to put his dialogue with the prosecutor onto this blog.