Common Procedures for Extended Encryption and Message Integrity

2.6.11.4 Security Mode Command Message on f-csch

• Set C_SIG_ENCRYPT_MODE_s to C_SIG_ENCRYPT_MODE_r.

• If ENC_KEY_SIZE_r is included, the mobile station shall set ENC_KEY_SIZE_s to ENC_KEY_SIZE_r.

• If MSG_INTEGRITY_SUP_s is equal to ‘0’, C_SIG_ENCRYPT_MODE_r is not equal to ‘000’ and the mobile sent a Security Mode Request Message with the NEW_SSEQ_H_INCL field equal to ‘1’ prior to receiving this message when C_SIG_ENCRYPT_MODE_s was not equal to ‘000’, the mobile station shall perform the following:

• 
  Set TX_EXT_SSEQ[0][KEY_ID], TX_EXT_SSEQ[1][KEY_ID], RX_EXT_SSEQ[0][KEY_ID], and RX_EXT_SSEQ[1][KEY_ID] to 1 + 256  NEW_SSEQ_H included in the Security Mode Request Message.
  

 If CHANGE_KEYS_r is equal to ‘0’, the mobile station shall set RESTORE_KEY to ‘0’.

 If CHANGE_KEYS_r is equal to ‘1’, the mobile station shall perform the following:

+ Set KEY_ID to SDU_KEY_ID provided by the LAC Layer (see [4]).

+ If KEY_ID is equal to ‘00’ or ‘01’, the mobile station shall perform the following:

 Set TX_EXT_SSEQ[0][KEY_ID], TX_EXT_SSEQ[1][KEY_ID], RX_EXT_SSEQ[0][KEY_ID], and RX_EXT_SSEQ[1][KEY_ID] to 1 + 256  NEW_SSEQ_H included in the Security Mode Request Message.

 Form a 128-bit pattern by concatenating the CMEAKEY with a copy of itself (the most recently generated CMEAKEY associated with the AUTHR of the Registration Message, Origination Message, Page Response Message or the CMEAKEY associated with the AUTHU generated during Unique Challenge-Response procedure as described in 2.3.12.1.4).

 Set ENC_KEY[KEY_ID] to the 128-bit pattern.

 Set INT_KEY[KEY_ID] to the 128-bit pattern.

 Set LAST_2G_KEY_ID_s to KEY_ID.

 Set TX_EXT_SSEQ[0][KEY_ID], TX_EXT_SSEQ[1][KEY_ID], RX_EXT_SSEQ[0][KEY_ID], and RX_EXT_SSEQ[1][KEY_ID] to 1 + 256  NEW_SSEQ_H included in the Authentication Response Message, or Security Mode Request Message.

 Set ENC_KEY[KEY_ID] to the CK generated by AKA.

 Set INT_KEY[KEY_ID] to the IK generated by AKA.

 Set LAST_3G_KEY_ID_s to KEY_ID.

 If the mobile station supports R-UIM, then the mobile station shall set USE_UAK_s to USE_UAK_r; otherwise, the mobile station shall perform the following:

à If USE_UAK_r is equal to ‘1’, then the mobile station shall send a Mobile Station Reject Order with ORDQ equal to ‘00010100’ (UAK not supported).

+ Set RESTORE_KEY to ‘0’.

+ Perform the key strength reduction algorithm procedures to reduce the key strength of ENC_KEY[KEY_ID] according to ENC_KEY_SIZE_r as described in 2.3.12.5.3.

+ Send a Security Mode Completion Order.

+ Disable the key setup timer if it is running.

• 
  Set INTEGRITY_MODE_s to the SDU_INTEGRITY_ALGO delivered by the LAC Layer.
  

• If ENC_KEY_SIZE_r is included and not set to reserved value and if current key strength is greater than the desired key strength specified by ENC_KEY_SIZE_r according to table 3.7.4.5-2, mobile station shall perform the key strength reduction algorithm procedures to reduce the key strength of ENC_KEY[KEY_ID] according to ENC_KEY_SIZE_r as described in 2.3.12.5.4. The current key strength is 64 bit if KEY_ID is equal to ‘00’ or ‘01’ and is 128 bit if KEY_ID is equal to ‘10’ or ‘11’.

2.6.11.5 Base Station Reject Order on f-csch and f-dsch.

The mobile station shall perform the following procedures in the order listed below.

• If ORDQ_r is equal to ‘00000000’ or if ORDQ_r is equal to ‘00000010’, REJECT_REASON_r = ‘0011’, and REJECTED_L3_MSG_TYPE_r indicates the rejected message is an Origination Message, the mobile station shall set ENC_KEY[i] and INT_KEY[i] to NULL, where i ranges from ‘00’ to ‘11’. The mobile station shall set C_SIG_ENCRYPT_MODE_s to ‘000’. The mobile station shall re-originate by sending a new Origination Message.

• If ORDQ_r is equal to ‘00000001’ or if ORDQ_r is equal to ‘00000010’ and REJECTED_L3_MSG_TYPE_r indicates the rejected message is not an Origination Message, the mobile station shall send a Security Mode Request Message. If MSG_INTEGRITY_SUP_s is equal to ‘0’, the mobile station shall select a 24-bit number and include this number in the NEW_SSEQ_H field in the Security Mode Request Message; otherwise, the mobile station shall select a 24-bit number and deliver this number to the LAC Layer along with the Security Mode Request Message. If the mobile receives two Base Station Reject Orders without successfully decrypting any encrypted message or without successfully validating the MACI of any message between the two orders, the mobile station shall set REG_SECURITY_RESYNC to YES and enter the System Determination Substate with an encryption/message integrity failure indication.

2.6.11.6 Mobile Station processing when decryption or MACI check failed

Whenever the mobile station cannot decrypt an encrypted message or validate the MACI of a message that requires MACI validation, the mobile station may send an un-encrypted Mobile Station Reject Order indicating the failure condition to the base station.

If the failure to decrypt or to validate MACI persists, the mobile station may attempt to resynchronize the crypto-sync with the base station by sending a Security Mode Request Message to the base station as follows:

• If MSG_INTEGRITY_SUP_s is equal to ‘0’, the mobile station shall select a 24-bit number and include this number in the NEW_SSEQ_H⁴⁴ field in the Security Mode Request Message; otherwise, the mobile station shall select a 24-bit number and deliver this number to the LAC Layer along with the Security Mode Request Message.

The mobile station shall set REG_SECURITY_RESYNC to YES and enter the System Determination Substate with an encryption/message integrity failure indication if either of the following conditions are true:

• The mobile station chooses not to perform resynchronization procedure as described above.

• The mobile station still cannot decrypt message or validate the MACI of message from the base station after successfully receiving and processing Security Mode Command Message in response to Security Mode Request Message sent by mobile station in resynchronization procedure as specified in 2.6.11.4 or 2.6.4.1.14.

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