Jørgen Erik 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

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James Moran
Fraud and Security Director
GSM Association Headquarters
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Email: james.moran@gsm.org  Web: http://www.gsmworld.com/