Abstract: 2. For use with a cryptographic teletype transmission system having a relay for transferring teletype characters to an encyphering transmitter, a decyphering receiver and a teletype printer, a security system comprising a generator for producing an auxiliary teletype character, circuit means responsive to the failure of transfer of a valid teletype character to said transmitter to activate said generator to supply an auxiliary character to said transmitter, a clamp circuit responsive to activation of said generator to inhibit transfer of a valid character to said transmitter until said auxiliary character has been completed, a transfer system for transferring the decyphered characters from the receiver to the printer, storage means for storing a plurality of successive teletype characters, a recognition circuit responsive to the presence of said auxiliary teletype character in said storage means for inhibiting printing of said auxiliary teletype character.