<<A5 algorithm key length.rtf>> Jørgen Bo Madsen Fra: James Moran [james.moran@gsm.org] Sendt: 3. december 1999 12:02 Til: Jørgen Erik Bo Madsen Emne: A5 algorithm key length Prioritet: Høj Jorgen, Below you will find the answers to the questions posed in your letter dated 12th October. 1. Is the implementation of European GSM encryption algorithm A5/1 reduced in strength to 54 bits? The key length of Kc used in the GSM encryption is 64 bits but 10 of the bits are set to 0. Therefore the effective key length is 54 bits. 2. Is the implementation of the A5/1 encryption algorithm used in one or more GSM systems in Denmark reduced in strength to 54 bits? I can confirm that A5 must be used by all GSM operators and that all GSM operators in Denmark currently use the standard algorithm which has the 54 bit effective key length. 3. Why is the A5/1 algorithm reduced in strength? The key length is determined by control regulations that exist in many countries regarding the use of encryption. As algorithms are treated as dual use goods, similar to munitions, their movement and use is regulated and certain countries place a limit on their strength. As GSM was designed and developed to be used throughout Europe the design of the algorithm had to take the restrictions of various countries into account. 4. Who ordered the reduction of the A5 algorithm? The algorithm specifications were written by ETSI SAGE and this group would have decided on the key length. If you require anything further do not hesitate to contact me. Regards, James ================================================================ This e-mail is intended only for the addressee(s) named above. As this e-mail may contain confidential or privileged information, if you are not a named addressee, or the person responsible for delivering the message to the named addressee, please telephone the Association immediately on the number below. The contents should not be disclosed to any other person nor copies taken. James Moran Fraud and Security Director GSM Association Headquarters Avoca Court, Temple Road, Blackrock, Co. Dublin, IRELAND. Phone: +353 1 2091827; Fax: +353 1 2695958 GSM: +353 86 8565124 Email: james.moran@gsm.org Web: http://www.gsmworld.com/