At the request of the inquisitors, Pope Innocent IV in 1254, laid down the rule that because of the attendant dangers involved, the names of the witnesses against alleged heretics were to be withheld…The reasons for such procedure were pressing and insistent. The policy thus adopted by Innocent IV was confirmed and continued by his successors. Pope Boniface VIII modified this prescription on the extent that, once the grave danger had passed on account of which the names had been kept secret, they must be revealed to the accused, and he saw to it that this restriction was not abused.
It is clear, then, that the inquisitorial tribunal forbade the revelation of the names of witnesses to the accused. The conditions that necessitated this measure still obtained in the following century, for we find Bernard Gui citing the same decrees of the pontiffs and repeating teh same reasons…However, through all of this the depositions themselves were always taken down in writing, and the names of the witnesses were made known to the inquisitor and the various councillors and jurisprudentes who assisted him. Moreover, the depositions of the witnesses were made known to the accused, so that he could refute the charges alleged against him.
Albert Shannon, “The Secrecy of Witnesses in Inquisitorial Tribunals and in Contemporary Secular Criminal Trials”, Essays in Medieval Life and Thought: Presented in Honor of Austin Patterson Evans (1965)