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138 lines
5.3 KiB
Plaintext
138 lines
5.3 KiB
Plaintext
Introduction
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============
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This document describes the various stages of the authentication / association
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process for WPA/WPA2 Networks. It is intended to give the reader a 'map' of
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what pieces come into play and when.
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For a more broad overview of authentication procedures, refer to Section
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4.10.3 of 802.11.
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WPA is an earlier stop-gap solution to WEP security vulnerabilities.
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Essentially it is a subset of draft 802.11i. WPA2 refers to the released
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802.11i specification. WPA2 is more of a marketing term; term used inside
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802.11 is RSN (Robust Security Network). Typically WPA uses TKIP with RC4
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(also known as ARC4) stream cipher. WPA2 uses CCMP with AES stream cipher
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which must always be supported. TKIP with ARC4 is also a supported mode of
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operation.
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Overview of Getting Connected
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=============================
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To get connected to a typical WPA2 network the user must first know the
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Pre-Shared-Key or PSK. This key can be generated based on a passphrase
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instead of being entered in hex form by the user. The passphrase generation
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is specified in 802.11, Annex M.4. Since the PSK should be long-lived, it
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should not be exposed over-the-air. For this purpose the PSK is not used
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directly; instead a session key is derived. The process for derivation of
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this key is called the 4-Way Handshake.
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For the purposes of the 4-Way Handshake below, PSK becomes the PMK (Pairwise
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Master Key). The PMK is also obtained from an EAP exchange, but that is a
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different topic.
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4-Way Handshake generates a key referred to as the PTK (Pairwise Transient
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Key). PTK is generated by feeding the PMK, AP Nonce (ANonce), STA Nonce
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(SNonce), AP MAC Address and the STA MAC Address through a hashing function.
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The SNonce and ANonce are randomly generated numbers and are exchanged by the
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STA and the AP during the initial two steps of the Four-Way Handshake. The
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third step of the handshake establishes the GTK (Group Temporal Key). The
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fourth step serves as the acknowledgement step.
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The Four-Way Handshake is accomplished by exchanging EAPoL-Key protocol frames
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after performing OPEN_SYSTEM Association to the target AP.
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Key Derivation
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==============
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The PTK obtained from the 4-Way handshake is divided into 3 separate keys:
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- 16 byte KCK (Confirmation Key). The KCK is used in the computation of the
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MIC field used by EAPoL-Key frames. The KCK is used as input to HMAC-MD5,
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HMAC-SHA1 or CMAC-AES depending on what hashing function is used.
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- 16 byte KEK (Encryption Key). The KEK is used to encrypt data in message 3
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if it contains the GTK.
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- 16 byte TK (Temporal Key). Used to encrypt / decrypt normal packets flowing
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between the STA and the AP.
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If TKIP used, then the following elements are also derived from the same PTK:
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- 8 byte Michael MIC Authenticator TX Key
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- 8 byte Michael MIC Authenticator RX Key
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For more details on the Four-Way Handshake, refer to 802.11 Section 11.6.2
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Overall Process
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===============
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- The user selects a network to connect to. This network can be known
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implicitly or via a scan. A Probe request or a Beacon must first be
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received and the RSN IE must be stored in order to connect to the AP. The
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RSN element contains the security parameters supported by the BSS. This RSN
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element must also be compared against the RSN element received from the AP
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in message 3 of the 4-Way Handshake.
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- OPEN_SYSTEM Authentication and Association requests are sent to the AP. In
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the case of the Association request, the RSN element with the selected
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encryption parameters must be included.
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- Once the OPEN_SYSTEM Association is successful, the AP shall send Message 1
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of the 4-Way Handshake. This comes in the form of EAPoL-Key messages and
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require special privileges / setup in order to be read from the netdev
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device.
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- STA records all necessary parameters from Message 1 (ANonce, Key Replay
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Counter). STA Generates the PTK, constructs Message 2 and sends it to the
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AP.
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- Once Message 3 is received from the AP, the STA sanity checks certain
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parameters (Key Replay Counter, ANonce, MIC). The RSN received from the AP
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is also checked against the one received in Message 3. If everything checks
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out, then Message 4 is constructed and sent to the AP.
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- Based on TK and GTK, the keys are set into the hardware. The Re-Key offload
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is enabled as well (if supported?)
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Here's an example of how this looks like:
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< Request: New Key (0x0b) len 68 [ack]
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Key Data: len 16
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3c 7d 08 8b 94 10 0f 21 06 6d 7b 18 a1 7e e0 ad
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Key Cipher: CCMP (0x000fac04)
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Key Sequence: len 6
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00 00 00 00 00 00
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MAC Address: 24:A2:E1:EC:17:04
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Key Index: 0 (0x00)
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Interface Index: 3 (0x00000003)
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< Request: Set Key (0x0a) len 28 [ack]
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Key Index: 0 (0x00)
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Interface Index: 3 (0x00000003)
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Key Default: true
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Key Default Types: len 4
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Unicast: true
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< Request: New Key (0x0b) len 64 [ack]
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Key Data: len 16
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2e 15 6e 7c 4e 3d f1 37 09 13 ed bd 62 8e 25 65
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Key Cipher: CCMP (0x000fac04)
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Key Sequence: len 6
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00 00 00 00 00 00
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Key Default Types: len 4
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Multicast: true
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Key Index: 2 (0x02)
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Interface Index: 3
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< Request: Set Rekey Offload (0x4f) len 64 [ack]
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Interface Index: 3 (0x00000003)
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Rekey Data: len 52
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14 00 01 00 24 7a 90 94 e5 43 de 1b 1e 3d d4 a0
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5c d0 e7 76 14 00 02 00 28 4d b5 b1 cf bc e4 25
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f4 f5 00 47 64 83 87 bc 0c 00 03 00 00 00 00
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00 00 00 01
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