Mobility Solutions BCMCS Key Derivation Procedure Harmonization with IETF SRTP

Proprietary information - Lucent Technologies 2 SRTP Keying hierarchy Using BCMCS terms, SRTP keying starts at BAK –SRTP considers Key Management done by external protocols SRTP has two levels of key hierarchy: –Session-level Master key (equivalent to BAK) Delivered somehow by external means –Traffic level keys SK encrypt, SK auth, SK salt Derived locally from BAK and packet index Used to encrypt RTP, encrypt and authenticate RTCP

Proprietary information - Lucent Technologies 3 Comparison between BCMCS and SRTP BCMCS 1.Longer Term Session key (BAK) 2. One key (SK) for traffic protection Encryption only Key Derived from BAK 3.SK Manager (server) tells when to change SK 4.SK derivation is defined by 3GPP2 SRTP 1.Longer Term Session Key (Master Key) 2. Three keys for traffic protection SK encrypt, SK auth, SK salt Key Derived from MK 3. SK’s are changed by an internal counter - SRTP session parameter key_derivation_rate 4. SRTP RFC mandates its own key derivation algorithm BAK is equivalent to SRTP Master key both are allowed attributes (lifetime, key_id, etc) both are at most session-long, but can be changed sooner

Proprietary information - Lucent Technologies 4 Proposed solution – Preferred Modify proposed BCMCS SK derivation algorithm –Make it compliant with SRTP draft Advantages –Similar Key Hierarchy with current BAK (BAK->SK) –Compatibility with SRTP out-of-box –Need to deliver only BAK and parameters to SRTP and BCMCS BAK_id, BAK lifetime, SK derivation rate –Ability to secure both RTP and RTCP RTCP mandates authentication, so one SK not enough Disadvantages: –Requires change to BCMCS Security Framework document

Proprietary information - Lucent Technologies 5 Proposed solution – Alternative Use BCMCS SK as Master key (BAK equiv) in SRTP Advantages: –Avoids changing the current BCMCS Security Framework Disadvantages: –SK has no attributes (new work – add items) –SK is likely to change much more frequently than either BAK or SRTP Master key, so efficiency may suffer –Introduces unnecessary extra key level in the hierarchy Secondary short-term session keys