WPA-PSK 詳解：定義、運作原理及其安全風險

預共用金鑰 (PSK)

一個長度為 8 到 63 個字元的靜態密碼，由無線存取點與所有用戶端設備共享，用作產生加密金鑰的種子材料。

小型企業和家用網路中的主要漏洞。只要有一人知道 PSK，整個網路就有可能遭到入侵，且撤銷存取權限需要變更整個網路的密碼。

Pairwise Master Key (PMK)

使用 PBKDF2 雜湊演算法（執行 4,096 次）從 PSK 和網路 SSID 衍生出的 256 位元金鑰。

PMK 是 WPA 架構中的頂級金鑰。由於它結合了 SSID，因此變更網路名稱需要重新計算所有設備上的 PMK。

4-Way Handshake

無線存取點與用戶端交換隨機數（nonce）的密碼學交換程序，以便在不透過空中傳輸主金鑰的情況下，獨立計算出工作階段加密金鑰。

這是發生離線字典攻擊的關鍵階段。如果攻擊者擷取到此握手程序，即可嘗試在完全離線的情況下破解 PSK，不需要與網路進行任何互動。

Pairwise Transient Key (PTK)

在 4-Way Handshake 期間產生的臨時單一工作階段加密金鑰，用於加密特定用戶端與無線存取點之間的單播數據流量。

確保即使所有使用者共享相同的 PSK，也無法輕易解密彼此的單播流量 - 不過如果 PSK 被破解，此保護就會失效。

Message Integrity Code (MIC)

在握手期間傳輸的密碼學總和檢查碼，用以證明傳送方擁有正確的 PMK 並已成功計算出 PTK。

MIC 是離線字典攻擊的目標。攻擊者擷取 MIC 並使用暴力破解工具產生相符的 MIC，從而找出原始的 PSK。

WPA-Enterprise / 802.1X

一項用於基於連接埠之網路存取控制的 IEEE 標準，提供一種認證機制，要求每個使用者或設備使用 EAP 方法個別對 RADIUS 伺服器進行認證。

企業停用 WPA-PSK 的必要升級路徑。提供針對每位使用者的身分識別、即時撤銷以及完整稽核軌跡。

Captive Portal

公共存取網路使用者在獲准存取網路前必須進行互動的網頁，通常用於擷取身分、強制執行服務條款並套用存取策略。

向訪客提供靜態 PSK 的現代替代方案。支援身分擷取、符合 GDPR 的同意書收集、頻寬管理以及行銷分析整合。

Deauthentication Attack

一種阻斷服務攻擊，藉由傳送偽造的 802.11 管理訊框，強制用戶端與無線存取點斷開連接，使其重新連接並執行新的 4-Way Handshake。

攻擊者用來主動產生握手流量以便進行擷取的手段。偵測此攻擊需要無線入侵偵測系統（WIDS）來監控異常的取消認證訊框數量。

RADIUS (Remote Authentication Dial-In User Service)

一種網路協定，為連接到網路服務的使用者提供集中化的認證、授權和計費（AAA）管理。

部署 WPA-Enterprise 所需的核心基礎架構元件。可以託管於雲端或部署於現地，並可與 Active Directory、Azure AD 或 Okta 等身分識別提供者整合。

Why is WPA-PSK insecure for business and enterprise networks?

WPA-PSK relies on a single static passphrase shared among all network users and devices. Because the master key is distributed universally, former employees and visitors retain network access until the passphrase is manually changed on every access point and endpoint. Furthermore, any client transmitting traffic allows an attacker with a packet capture tool to intercept the 4-way handshake and execute offline dictionary attacks without ever triggering network lockouts.

How do attackers capture the WPA 4-way handshake to crack passphrases?

Adversaries operate wireless adapters in monitor mode to capture authentication nonces (ANonce and SNonce) and the Message Integrity Code (MIC) exchanged during the 4-way handshake. Attackers frequently transmit spoofed 802.11 deauthentication frames to force connected devices to reconnect and re-initiate the handshake. Once captured, the MIC is tested against dictionary wordlists offline using high-performance GPU clusters.

Can clients on a WPA-PSK network decrypt other users' unicast data traffic?

Yes. In WPA-PSK, unicast session encryption keys (Pairwise Transient Keys, PTKs) are derived from the shared Pairwise Master Key (PMK), the access point BSSID, the client MAC address, and transmitted nonces. If an attacker knows the pre-shared key and captures the initial 4-way handshake when a specific device connects, they can compute that device's PTK and decrypt all subsequent over-the-air unicast traffic.

What is the difference between WPA-PSK and 802.1X Enterprise authentication?

WPA-PSK uses a static shared secret with zero per-user identity or individual revocation. In contrast, IEEE 802.1X Enterprise delegates authentication to a central RADIUS server. Each user or device authenticates using distinct credentials or digital certificates (EAP-TLS). Compromised devices or terminated employees can be revoked instantly from the central directory (Entra ID, Okta, Active Directory) without re-keying any other hardware.

How do Private Pre-Shared Keys (xPSK / PPSK) solve the headless IoT dilemma?

Many legacy IoT devices, such as barcode scanners, badge printers, and smart displays, lack 802.1X supplicants and cannot support certificate-based authentication. Private Pre-Shared Keys (xPSK / PPSK) assign a unique passphrase to each individual device or device group on a single SSID. When a device connects, Cloud RADIUS maps its distinct PSK to a dedicated isolated VLAN and firewall policy, enabling granular revocation without affecting other IoT devices.

How does Purple help enterprises migrate away from legacy WPA-PSK networks?

Purple provides Cloud RADIUS, automated 802.1X device onboarding with digital certificates (EAP-TLS), central xPSK pooling for headless IoT devices, and branded captive portals for visitors. This eliminates shared static keys, automates identity compliance, and provides real-time visibility and footfall analytics across distributed venues.