18. Where any person having a right to institute a suit or make an application has, by means of fraud, been kept from the knowledge of such right or of the title on which it is founded, or where any document necessary to establish such right has been fraudulently concealed from him, the time limited for instituting a suit or making an application- (a) against the person guilty of the fraud or accessory thereto, or (b) against any person claiming through him otherwise than in good faith and for a valuable consideration, shall be computed from the time when the fraud first became known to the person injuriously affected thereby, or, in the case of the concealed document, when he first had the means of producing it or compelling its production.