GSM encryption. Reduction of algorithm. Interesting doc from GSM Org.

Bo Elkjær bo.elkjaer at eb.dk
Tue Jan 9 00:27:13 PST 2001


 <<A5 algorithm key length.rtf>> 
Jørgen Bo Madsen
Fra:	James Moran [james.moran at 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
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GSM: +353 86 8565124
Email: james.moran at gsm.org  Web: http://www.gsmworld.com/

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