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Why is PDCP SDU size maximum kept as 8188 octets?

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Why is PDCP SDU size maximum kept as 8188 octets?
posted Dec 22, 2013 by anonymous

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1 Answer

+4 votes

The ZUC algorithm is the core of the standardized 3GPP Confidentiality and Integrity algorithms.

ZUC is a word-oriented stream cipher. It takes a 128-bit initial key and a 128-bit initial vector (IV) as input, and outputs a keystream of 32-bit words (where each 32-bit word is hence called a key-word). This keystream can be used for encryption/decryption.

The execution of ZUC has two stages: initialization stage and working stage. In the first stage, a key/IV initialization is performed, i.e., the cipher is clocked without producing output. The second stage is a working stage. In this stage, with every clock pulse, it produces a 32-bit word of output.

Let ZUC generate keystream of L words: To encrypt a message of LENGTH bits, it is required that L=LENGTH/32.

So maximum length of key-stream generated from a single key is 32 x 2047 = 65504 bits (or 8188 bytes).

It can not be set to 32 x 2048 because in this case it will exceed from 65536. That is not possible. This is the only case where we can set the the maximum supported size of a PDCP SDU i.e. 8188 octets.

Within  the  security  architecture  of  the  LTE  system  there  are  standardized  algorithms  for 
confidentiality and integrity. . Two sets of algorithms 128‐EEA1/128‐EIA1 and 128‐EEA2/128‐EIA2 
have been specified as:

  • The Confidentiality algorithm 128‐EEA3 is a stream cipher that is used to encrypt/decrypt blocks 
    of data using a confidentiality key CK. The block of data may be between 1 and 65504 bits long. 
    The algorithm uses ZUC as a keystream generator. 

The confidentiality algorithm 128‐EEA3 is a stream cipher that is used to encrypt/decrypt blocks 
of data under a confidentiality key. The block of data can be between 1 and 65504 bits in length.

  • The Integrity algorithm 128‐EIA3 computes a 32‐bit MAC (Message Authentication Code) of a 
    given input message using an integrity key IK. The core algorithms adopted by the MAC are a 
    universal hash and ZUC. 

The integrity algorithm 128‐EIA3 is a message authentication code (MAC) function that is used to 
compute the MAC of an input message using an integrity key (IK). The message can be between 
and 65504
 bits in length. 

Reference 1: 3GPP Confidentiality and Integrity Algorithms 128-EEA3 & 128-EIA3. Document 1: 128-EEA3 and 128-EIA3 Specification

Reference 2: 3GPP Confidentiality and Integrity Algorithms 128-EEA3 & 128-EIA3. Document 2: ZUC Specification

answer Apr 2, 2014 by Hiteshwar Thakur
32 x 2047 = 65504
how this 32 and 2047 are derived
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