Abstract: A cryptographic key recovery system for generating a cryptographic key for use by a pair of communicating parties while simultaneously providing for its recovery using one or more key recover agents. A plurality of m-bit shared key parts (P, Q) are generated which are shared with respective key recovery agents, while an n-bit nonshared key part (R) is generated that is not shared with any key recovery agent. The shared key parts (P, Q) are combined to generate an m-bit value which is concatenated with the nonshared key part (R) to generate an (m+n)-bit value from which an encryption key is generated. The cryptographic system has the effective work factor of an n-bit key to all of the key recovery agents acting in concert, but has the effective work factor of an (m+n)-bit to any other combination of third parties.

Abstract: A system for encrypting and decrypting digital data wherein the data is divided in packets of N blocks X(1) . . . X(N) of 2.sup.m bits, comprises an encryption device and a decryption device. The encryption device reverses the sequence of the blocks X(1) . . . X(N) before a XOR operation and next an encryption operation by means of an encryption algorithm E is carried out on each block of a packet. Thereby the following encrypted blocks Y(1) . . . Y(N) are formed: Y(1)=E ?X(N)+IV!, Y(i)=E ?X(N-i+1)+Y(i-1)! for i>1 and i.ltoreq.N. The encrypted blocks Y(1) . . . Y(N) are transferred by a sender in reversed sequence Y(N) . . . Y(1) to a receiver. The decryption device at the receiver obtains the original blocks X(1) . . . X(N) by carrying out a decryption operation by means of a decryption algorithm D and next a XOR operation on each block Y(N) . . . Y(l) received. Thereby the original blocks are obtained as follows: X(i)=D ?Y(N-i+1)!+Y(N-i) for i=1, 2, . . . , N-1; X(N)=D ?Y(1)!+IV.

Abstract: A cryptographic key recovery system that is interoperable with existing systems for establishing keys between communicating parties. The sender uses a reversible key inversion function to generate key recovery values P, Q and (optionally) R as a function of a session key and public information, so that the session key may be regenerated from the key recovery values P, Q and (if generated) R. Key recovery values P and Q are encrypted using the respective public recovery keys of a pair of key recovery agents. The encrypted P and Q values are included along with other recovery information in a session header accompanying an encrypted message sent from the sender to the receiver. The key recovery agents may recover the P and Q values for a law enforcement agent by decrypting the encrypted P and Q values in the session header, using their respective private recovery keys corresponding to the public keys.

Abstract: An apparatus and the method of using the apparatus to form a wall. Initially, a pair of vertical wall guides are mounted upright at selected locations. The guides are made in two parts, a front guide and a rear guide, so the guides can be moved toward and away from each other to compensate for uneven walls and thicknesses of stone. Staves or forming boards are mounted horizontally between the wall guides by threaded bolts, to form a guide for the stones. The stones are set sequentially in a horizontally disposed course using the staves as a guide to lay the wall straight and level. Additional staves are fixed to the guides as necessary. Once, it is determined that a particular row of stones has set, its restraining stave can be removed. This action usually starts at the bottom course and moves upward until all staves are removed.

Abstract: A roller door assembly has means for tensioning same wherein a load is applied to a bottom edge of the roller and means for counterbalancing the weight of the door wherein a load is applied to the roller, the loads exerted by both said means being dependent on the amount of door off the roller.

Abstract: A roller door assembly comprises a roller door having on its vertical edges spaced restraining members, and guides at each edge of the door including opposing members forming a gap therebetween. The restraining members are on one side of the gap with the door on the other side thereof, whereby upto a desired pressure or force on the door the restraining members cannot pass through said gaps but at said desired pressure or force the restraining members are compressed and/or the gap expends for the restraining members to pass through said gaps to release the door.