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Block Ack Security

Block Ack Security

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Block Ack Security

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  1. Block Ack Security Authors: Date: 2008-03-19 Matthew Fischer, Broadcom

  2. Abstract An introduction to some security issues with the existing BA mechanism and a proposed set of modifications to reduce and/or eliminate the security problems. Matthew Fischer, Broadcom

  3. Block Ack Security problems • The following security problems exist: • The SN values of data packets are not protected – yet, SN values of data packets can be used to adjust the RX Buffer LE value. A single forged SN value can cause the recipient to move the LE value too far forward, thereby causing the recipient to discard frames below the new LE that should not have been discarded. Data is lost at the recipient. • A single forged SN value in a data packet can also cause the recipient to place the received frames in an incorrect order, which can cause problems both when the security layer examines the sequence of PN values in the MAC SN-ordered frames and when the frames are passed to the next layer for processing. • A single forged SN value in a data packet can cause RX scorecard information to be updated, and a subsequent transmission of a BA frame in response to a legitimate AMPDU can include this bogus scorecard information. • A captured and replayed packet cannot be detected except by replay detection in the security layer. If the RX buffer reordering is performed before this check, then the SN in that replayed packet can cause incorrect RX Buffer LE movement. • The BAR frame is not protected – yet the BAR frame SSN value is used to adjust the RX Buffer LE value. A single forged SN value can cause the recipient to move the LE value too far forward, thereby causing the recipient to discard frames below the new LE that should not have been discarded. Data is lost at the recipient. • The BA frame is not protected – yet the BA frame SSN value is used to adjust the originator’s TX scorecard LE value. Forged BA frames can cause false adjustments to the LE value that result in some data packets not being transmitted to the recipient, since they now have SN values below the new LE value. Data is lost. • Forged BA frames can suppress retransmission of frames that were not successfully received (even without moving LE at TX)‏ Matthew Fischer, Broadcom

  4. BA security solution • A solution to this security problem is: • Protect the SN value of the MAC header of the data packets by including the SN value as part of the PN value • Either include the FRAG bits in PN, or disallow fragmentation • Modify the setup of the security agreement that is negotiated between the originator and the recipient so that the originator and recipient can negotiate the use of the new PN scheme • Use a new protected form of the BAR frame to convey BAR information, and allow this protected BAR frame to cause RX Buffer LE movement while forbidding unprotected BAR frames from making RX Buffer LE changes • Use a new protected form of the BA frame to convey BA information, disallow acceptance of unprotected BA frame, except optionally, after SIFS • Allow alternative architectural ordering of Block Ack Reordering AFTER MPDU decryption, and include a new Block Ack Replay Detection function just before the Block Ack Reordering but preserve existing ordering option as well for legacy implementation Matthew Fischer, Broadcom

  5. Secure Block Ack Rules Part 1 • *1 – unencrypted BAR is not used to shift recipient LE • Encrypted BAR can shift recipient LE • Unencrypted BAR can still be used to cause a BA to be generated • *2 – unsolicited unencrypted BA is not used to shift originator LE • SIFS-response unencrypted BA may be used to shift originator LE • Encrypted BA may be used to shift originator LE at any time • 3 – PN is modified to include SN and is tracked per TID • RX Buffer reorder based on protected SN • *4 - recipient sends BA using partial state • BA of SSN is based on immediately preceding MPDU(s) • SIFS separation requires JAM for attacker to succeed • BA SSN mismatch to AMPDU SNs can be used to detect weak attack • Some of these precautions can be followed by STA not using a protected SN, such steps are marked as * Matthew Fischer, Broadcom

  6. Secure Block Ack Rules Part 2 • 5a - Recipients that are incapable of verifying SN values before crafting a BA (i.e. within SIFS) • employ “ephemeral” state operation • I.e. WINSTART_R and scorecard information is deleted after BA transmission • Incorrect RX scoreboard information that is due to rogue MPDU SN values will not survive • This is less stateful than partial state operation • Shall not send an encrypted BA after SIFS • Because encrypted information is based on potentially rogue SN info • WINSTART_R moves do not matter • E.g. those generated by rogue BAR or rogue MPDU SN • WINSTART_B moves are based on protected SN values • Implies reversal of reordering and decryption steps Matthew Fischer, Broadcom

  7. Secure Block Ack Rules Part 3 • 5b - Recipients that are capable of verifying SN values before crafting a BA • extract the SN from the IV • Either examine one of the FRAG bits or use TA lookup result • may use any of partial-partial state, regular partial state or full state • WINSTART_R moves are based on protected SN values • WINSTART_B moves are based on protected SN values • *5c - Recipients that are incapable of verifying SN values at all (i.e. not participating in the new protocol) • employ ephemeral state operation • incorrect RX scoreboard information that is due to rogue MPDU SN values will not survive • WINSTART_R moves do not matter • WINSTART_B moves will be based on potentially faulty SN values, but at least relative ordering can be established by examining PN values • 6 – Only the new protected MGMT frame can be used to perform BAR-style RX BUFFER pointer moves • *7 – use only AMPDU, even if wishing to send a single MPDU • i.e. do not use non-AMPDU for single frame transmission, since it will elicit ACK which can be jammed Matthew Fischer, Broadcom

  8. New PN value generation – Per TID UpperBits MAC SN FRAG • Each stream is identified by {TA,RA,TID} • Non-QOS frames need not apply • PN is assigned per TID • The TID within the frame is protected by the Nonce • Use MAC SN and FRAG for the low order bits of the new PN value • Ensures that the PN values appear in increasing sequence at the recipient, otherwise the replay detection in the security processing section would declare a replay error. • Some values skipped if FRAG does not use all 16 values. • The new PN is treated as a single value, within each TID • I.e. When the MAC SN value reaches 0xFFF, then the next value of SN will be 0x000 again. When SN wrap occurs, the UpperBits field of the new PN is incremented by one. • If any one of the TID PN spaces uses all 2^48 PN values (minus any initial offset), then a new key must be used for this RA/TA pair • I.e. for the existing security protocol, once the PN number space has been exhausted, a new key must be used before the PN sequence can be restarted. Each TID has its own PN space, so when any is exahausted, all must start over. • SN values can continue where they left off (some loss of PN space is incurred, but it is quite small) – STA can optionally reset the SN • Each TID for a given RA/TA pair has a PN space to use that is equivalent in size to the original PN • If any one of the TID PN spaces runs through all possible 2^48 PN values which belong to that TID, then the RA/TA key must be replaced. • MGMT Type frames get their own PN space 32 bits 12 bits 4 bits Matthew Fischer, Broadcom

  9. AAD construction • AAD construction is unchanged • But PN construction is modified • SN value is used in the PN construction • This protects the SN Matthew Fischer, Broadcom

  10. Management frame change • Modified PN will not be recognized by legacy STA • Existing devices will not function properly if the modified PN value is used • Therefore, it is necessary to negotiate the use of the new protocol through a modified management exchange • New bits added to the Extended Capabilities information element • To signal capability • To signal requirements Matthew Fischer, Broadcom

  11. Extended Capabilities Element changes HT Info Exch Sp On-Demand Beacon Ext Chnl Switching WAVE Indication Resv EBAC UNBAI USBAI EBRC UBRI PSNC PSNR Resv ANA • EBAC – Encrypted BA Capable • UNBAI – Unencrypted Non-SIFS BA Ignored • USBAI – Unencrypted SIFS BA Ignored • EBRC – Encrypted BAR Capable • UBRI – Unencrypted BAR Ignored • PSNC – Protected SN Capable • PSNR – Protected SN Required B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B14 B15 Matthew Fischer, Broadcom

  12. Encrypted BAR frame • New Action frame • Category = Block Ack • Action = BAR • Body = BAR Control, BAR Information (see TGn draft) • Multi-TID version allowed • Uncompressed? • Encrypted according to TGw Matthew Fischer, Broadcom

  13. Encrypted BA frame • New Action frame • Category = Block Ack • Action = BA • Body = BAR Control, BAR Information (see TGn draft) • Multi-TID version allowed • Uncompressed? • Optionally includes recipient RX Buffer LE value • To allow originator to synch its TX Buffer with RX Buffer • Encrypted according to TGw Matthew Fischer, Broadcom

  14. Specification change for order of operations • Allow alternative ordering of Block Ack Reordering AFTER A new Block AckReplay Detection function, but preserve existing ordering option as well for legacy implementations. Add a BlockAck Replay Detection function here Move MPDU Decryption and Integrity Function to here Matthew Fischer, Broadcom

  15. Add Replay Detection within Block Ack Reordering block • Add new parameter WinPN_B at recipient, per TID, WinPN_B and WinStart_B operations are as follows: • Initialize WinPN_B = SSN from ADDBA • WinPN_B = next higher value of PN that has SN portion that matches ADDBA-SSN from current WinPN_B value • If RX DATA packet fails decryption • Discard • If RX PN < WinPN_B • Discard • If RX PN = WinPN_B + 1 • WinPN_B = highest PN in RX Buffer in sequence starting with WinPN_B+1 • Note that “false holes” may exist in the sequence when the protected MORE FRAGS bit appears in MPDUs which have a FRAG bit value of less than 0xF • “Next in sequence” determination skips these holes • Move WinStart_B to equal the SN portion of adjusted WinPN_B • Accept the packet • If RX PN > WinPN_B + WinSize_B * 16 • WinPN_B = WinStart_B = PN – WinSize_B * 16 + 1 • Accept the packet • If WinPN_B + 1 < RX PN <= WinPN_B + WinSize_B * 16 • No change to WinStart_B or WinPN_B • Accept the packet • If secure BAR passes decryption • WinStart_B = WinPN_B = SSN from secure BAR • Note that WinSize_B is expressed in terms of MSDUs, but PN values track fragments, hence the factor of 16 Matthew Fischer, Broadcom

  16. References Matthew Fischer, Broadcom