Method for encryption of information

• US 5,987,139 A
• Filed: 02/18/1997
• Issued: 11/16/1999
• Est. Priority Date: 09/27/1995
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Bodin, Roland
• Primary Examiner(s): Swann, Tod R.
• Assistant Examiner(s): SAYADIAN, HRAYR
• Application Number:
  US08/765,269
• Time in Patent Office: 1,001 Days
• Field of Search: 380/29, 380/44, 380/46, 380/49, 380/43, 455/410, 455/411
• US Class Current: 380/44
• CPC Class Codes: H04L 2209/80 Wireless network architectures or network communication protocols for wireless network security H04W12/00
• CPC Class Codes: H04L 9/0656 Pseudorandom key sequence combined element-for-element with data sequence, e.g. one-time-pad [OTP] or Vernam's cipher
• CPC Class Codes: H04W 12/037 of the control plane, e.g. signalling traffic

Abstract

The method involves modifying an encryption key (Kc) in accordance with a given algorithm and in dependence on the ordinal number of a time slot to obtain a modified encryption key. A modified pseudo-random sequence is formed from the resultant modified encryption key. The modification is carried out in accordance with the aid of an encryption algorithm. A logical operation is performed on the modified pseudo-random sequence and for each block of the non-encrypted information. Preferably the operation is performed on the information block that belongs to the time slot whose ordinal number has been used to form the modified encryption key. As an additional option, the frame number can also be modified in accordance with a given algorithm and in dependence on the ordinal number of the relevant time slot. The method provides reliable encryption in TDMA mobile radio systems in which two or more time slots are used for one and the same transmission without requiring any substantial changes to signaling protocol and/or system equipment.

Claims

1. A method of encrypting information transmitted between a base station and a mobile station in a time division multiple access (TDMA) mobile radio system wherein said information is divided into at least two blocks and transmitted in separate time slots in each frame in a frame sequence, said method of encrypting information comprising the steps of:

a) obtaining a first encryption key;

b) for each time slot used for transmission of said information, modifying said first encryption key in dependence on an ordinal number of the time slot so as to obtain a modified encryption key;

c) for each time slot, forming a pseudo-random sequence from its modified encryption key and from an ordinal number of a frame in which the information is transmitted in accordance with an encryption algorithm; and

d) for each frame and time slot, performing a logic operation between its corresponding pseudo-random sequence and the information to be transmitted.

2. The method according to claim 1, wherein the logic operation of step d) is performed on each information block that belongs to the time slot whose ordinal number has been used to form said modified encryption key.