StreetPass is a feature that allows your 3DS to connect with other 3DS consoles using WiFi in standby mode. It can be used to share Mii(s) on Mii Plaza for example. Applications' StreetPass data are stored in the CECD module's NAND savegame, applications can move received StreetPass data to an arbitrary savegame. Wifi infrastructure with APs are used to communicate where the data-frames are encrypted with WPA2 CCMP, like UDS/Download Play.
 WiFi Probe Request Frame
Whenever the 3DS is woken from sleep (or turned on), and WiFi is enabled, it sends out a 802.11 Probe Request which include all saved access points, as well a probe to an SSID containing an arbitrary string of data, such as "ic[kSvm9s@*cYD>/~IEVj\(fGG;qDo8j". This string changes at least daily, and most likely every time the device is woken up.
The MAC address used for these probes is the static MAC address found in the System Settings application. Unlike the StreetPass MAC address, it will not change over time. This MAC address OUI also differs from the one used in StreetPass.
 CCMP Key
The StreetPass local-WLAN CCMP data-encryption key is generated by the StreetPass CECD module, where the CCMP key is the 16-byte output from encrypting an all-zero block with AES-CTR via PS:EncryptDecryptAes, with keytype6. The CTR is the first 0x10-bytes from a SHA1-HMAC hash. The SHA1-HMAC key is a 17-byte text string including the NULL-terminator, a seperate HMAC key is used for retail/dev-units, this is determined via UNITINFO bit0. The data hashed with SHA1-HMAC is a 0x1C-byte buffer, which is described below.
 Hash Block
|0x0||0x8||8-byte StreetPass consoleID for the host, from the probe frames.|
|0x8||0x8||8-byte StreetPass consoleID for the client, from the probe frames.|
|0x10||0x6||MAC address for host.|
|0x16||0x6||MAC address for client.|
 StreetPass Exchange
While StreetPass is enabled, the 3DS constantly sends out Probe Requests with an SSID of "Nintendo_3DS_continuous_scan_000". Unlike beacons, which are actively advertising the device's presence, the 3DS is essentially actively looking for other 3DSes. This design is likely to limit impact to non-3DS WiFi capable devices. Each Probe Request contains basic information about that 3DS, including an identifier, and active StreetPass services. If another 3DS is in range, the second 3DS (#2) will respond with a Probe Response, to which the original 3DS (#1), and of the receiving device with every frame thereafter, will respond with an 802.11 Acknowledgement. 3DS(#1) then sends an 802.11 Action frame, as well as an additional Probe Request. The second 3DS sends back another Probe Response that begins the encrypted exchange between the two devices, no authentication/association is done here.
The MAC address used in sleep-mode seems to change every time there's a StreetPass hit, as well as the last 8-bytes(StreetPass consoleID) of the Nintendo tag data. The MAC address + 8-byte StreetPass consoleID is seen to change every time the user enters and exits and Settings application if they have not had a StreetPass in an observed time period of 24 hours. It is uncertain how the 3DS determines when it can do a StreetPass again with another 3DS, or what information is actually used to track that. It may be related to how long that 3DS was in range constantly/out of range. 3DSes that are constantly in range of each other in sleep-mode, usually do StreetPass every 11 hours.
 Probe Request Frame
Using Wireshark tool with a WiFi card in monitor mode allow you to see the data used to scan for other 3DS in the range. The below is a broadcast probe request from an 3DS while in standby mode, with SSID "Nintendo_3DS_continuous_scan_000". This SSID remains consistent across all 3DS units. This frame also contains a custom variable length Nintendo tag, which contains information regarding the offered StreetPass services. The sequence numbers for these probe request increment by 3 for every probe, until another 3DS responds with a probe response.
0000 00 00 1a 00 2f 48 00 00 19 7d 19 de 2a 00 00 00 ..../H...}..*... 0010 12 16 9e 09 a0 00 c9 02 00 00 40 00 00 00 ff ff ..........@..... 0020 ff ff ff ff da 6b f7 22 f3 77 ff ff ff ff ff ff .....k.".w...... 0030 40 77 00 20 4e 69 6e 74 65 6e 64 6f 5f 33 44 53 @w. Nintendo_3DS 0040 5f 63 6f 6e 74 69 6e 75 6f 75 73 5f 73 63 61 6e _continuous_scan 0050 5f 30 30 30 01 08 82 84 8b 0c 12 96 18 24 32 04 _000.........$2. 0060 30 48 60 6c dd 15 00 1f 32 01 11 05 00 02 08 00 0H`l....2....... 0070 00 f0 08 c8 34 6e 05 0f c9 c6 80 5b 6f bc 5a ....4n.....[o.Z
 Nintendo Tag Format
The offsets, in bytes, mentioned in the table below start at the beginning of the Nintendo tag ID, which is variable in length, and can be found right after the Vendor Specific OUI type of the 802.11 frame, which is often seen as a byte of "01". Each one of the elements are discussed in more detail after the table. Note that this table represents a current theory on what each of the fields represent, with the argument stated in the corresponding sections.
|0x00||0x01||Protocol Identification||May be for protocol identification. All captures thus far show this value at 17, hexadecimal 11.||11|
|0x01||0x01||StreetPass Service Length||Length in bytes of only the StreetPass Services field.||05|
|0x02||0x05||StreetPass Services||Starting at the 0x02 offset, it appears to be a list of StreetPass services, each in length of 5 bytes. This continues on depending on the number of services the user has enabled at the time.||00 02 08 00 00|
|varies||0x02||Unknown||At the end of the StreetPass Services field is a two byte field that is the same among all devices thus far. Its purpose is unknown.||f0 08|
|-0x08||0x08||StreetPass consoleID||Seen to change when the Settings app is used if there has not been a StreetPass tag recently. Also may change after each StreetPass hit and system power-off?||c8 34 6e 05 0f c9 c6 80|
 Protocol Version
Appears to represent a protocol version, or device identification. This field remains consistent on all devices, despite variable enabled StreetPass services or length of the tag. Could also represent region.
 StreetPass Service Length
This field is used to indicate the length of the StreetPass Services field. Removing and adding services has shown this field to increment and decrement in 5 bytes, or 11 bytes depending on the game. The StreetPass Services field has then expanded or reduced accordingly.
 StreetPass Services
The third field in the protocol header appears to be an arbitrary length list of StreetPass services enabled on the device. Each StreetPass service seems to be identified by a 5-byte ID. If you enable or disable services, the number of 5-byte IDs grows and shrinks within this list. Observed service IDs include:
Mii Plaza: 00 02 08 00 00 Ridge Racer: 00 03 58 00 30 Sims 3: 00 03 65 00 30 Street Fighter: 00 03 05 00 02 (FF FF FF FF FF FF)
Some services have a 6-byte field preceding or succeeding the StreetPass service that is just FF bytes (e.g. FF FF FF FF FF FF). The purpose of these is unknown, although may be used as data for a service, or as separator of some sort for different types of StreetPass services.
 Unknown 2-byte Field
The purpose of this field is not known yet. It has remained the same across all devices thus far. The value has always been observed as f008.
 StreetPass consoleID
When there's a StreetPass hit, and no StreetPass data changed on either of the 3DSes, no data is transferred besides probes? After turning off power, then powering on and entering sleepmode, the MAC doesn't change from prior to power off but the last 8-bytes of the Nintendo tag changes. This tag has been seen to not be sequential over time. After one of the new StreetPass content is handled, (running one of the StreetPass titles etc) this 8-byte StreetPass consoleID changes?
 Initial Probe Response Frame
If a 3DS (#2) receives another device's probe request and has not yet tagged that device in an arbitrary amount of time (~12 hours), the receiving 3DS (#2) will respond with a Probe Response frame. The destination MAC address is the StreetPass MAC address of the 3DS (#1) that was transmitting the probe request, while the receiving device sets its StreetPass MAC address as the source address. This is important to note because further exchanges may cease using destination and/or source addresses.
In the probe response, the 3DS (#2) appears to offer a channel of 1, 6, or 11. Different channels have been seen offered between the same set of 3DS for each StreetPass. Offered channels, and channel range most likely varies by region.
The StreetPass Probe Response frame contains the same Nintendo tag in Probe Requests of the device that is transmitting the Probe Response frame.
 Subsequent Probe Request and Response Frames
The 3DS (#1) that the Initial Probe Response is directed to will send an 802.11 Action frame back to the device. The sequence numbers at this point stop stepping up by 3, and instead increase by one based from each originating device's SN. It will then send another Probe Request, this time sent directly to the responding 3DS (#2) by specifying its MAC address in the destination field, and setting its own MAC address in the source address field. It also does not have a SSID specified in the frame, except the frame will contain a BSSID with the value of the 3DS (#2) that responded to the initial Probe, and thus acts as the master in the 802.11 exchange.
 StreetPass Spoofing
A streetpass "AP" was spoofed with hostapd by setting the SSID to "Nintendo_3DS_continuous_scan_000", with the extra Nintendo tag from another 3DS' probe request. Like 3DS<>3DS communications, the 3DS didn't authenticate or associate with the host. Streetpass communications use CCMP encryption. Eventually the 3DS stops communicating with the host since the host doesn't reply to any of the data frames, then sends a 802.11 "Action" frame, with category ID 0x7f and Nintendo's vendor ID: 00 1f 32.(However the 3DS keeps communicating with the above process repeatedly) Communication with two 3DSes are the same as above except there's actual encrypted data sent to/from both consoles, unlike the fake host.