Bluetooth Core Spec经典蓝牙安全机制导读

关键词：Bluetooth Classic Security 经典蓝牙安全机制 Pairing Authentication Encryption

本文基于Bluetooth Core Spec v5.3，梳理了Core Spec中BT Classic Security相关内容。

根据Vol 1, Part C: Core Specification Change History, 经典蓝牙Security相关修改历史如下：

v1.1 TO v1.2
- Deprecate the use of Unit Key for security
v2.0+EDR TO v2.1+EDR
- Introduce Encryption Pause and Resume
- Introduce Secure Simple Paring
- Introduce Security Mode 4
v4.0 TO v4.1
- Introduce BR/EDR secure connections

Vol 1, Part A, 5 SECURITY OVERVIEW非常重要，一定要细读。该章说明了Bluetooth security机制演变历史和设计目标，重要的名词术语，包括

Bluetooth security model包含五种security features:
- Pairing: create secret keys
- Bonding: store secret keys
- Device authentication: verify the same keys
- Encryption
- Message integrity
经典蓝牙的三种security mechanism，依照出现时间先后，依次为：
- BR/EDR Legacy Paring：use short variable PIN or fixed PIN
- Secure Simple Pairing (SSP): four association models
- Secure Connections: upgrades Secure Simple Pairing
Secure Simple Paring (SSP)根据设备的IO capabilites可以使用四种association models:
- Just Works
- Numeric Comparison
- Passkey Entry
- Out-Of-Band

关于Encryption，BR/EDR和BLE都是在Controller完成的。
关于Key generation，BR/EDR是由Controller完成的，但是BLE是由Host完成的。

Vol 2, Part C 3.2 FEATURE DEFINITIONS 列出了LMP中security相关feature及其定义，3.5 FEATURE REQUIREMENTS列出了features之间的依赖/互斥关系，包括

Broadcast encryption - Mutually exclusive feature: Coarse clock adjustment
Encryption - Mandatory
Pause Encryption
Secure Connections (Controller Support) - Require features: Pause encryption and Ping
Secure Connections (Host Support) - Require features: Secure Simple Pairing (Host Support) and Secure Connections (Controller Support)
Secure Simple Pairing (Controller Support) - Mandatory
Secure Simple Pairing (Host Support)

Vol 2, Part C, 4.2 SECURITY 详细说明了以下功能涉及到的PDU、Error Code和Procedure Sequence。如果你没有看之前的Security Overview，很可能陷入细节当中，而不能理解Security的整体流程和这些功能的相互关系。

Authentication
- Legacy authentication:
  - Performed when at least one device does not support both the Secure Connections(Controller Support) and Secure Connections(Host Support) features.
  - A mutual authentication is not mandatory.
- Secure authentication:
  - Performed when both devices support the Secure Connections(Controller Support) and Secure Connections(Host Support) features.
  - Always a mutual authentication.
Pairing
- (Legacy) pairing - Section 4.2.2, Only when at least one of the two sides is based on the very old Core Spec v2.0 and below.¹
- Secure Simple Pairing - Section 4.2.7, Mandatory between two devices both supporting Core Spec v2.1 and above.¹
Change link key
Change current link key type
Encryption
- E0 encryption(legacy) mechanism - At least one device does not support both the Secure Connections(Controller Support) and Secure Connections(Host Support) features.
- AES-CCM encryption mechanism - Both devices support Secure Connections(Controller Support) and Secure Connection(Host Support) features.
Request supported encryption key size

Vol 2, Part H Security Spec规定了Encryption、Authentication和Key Generation schemes的实现细节，比如详细的steps和algorithms。为了便于理解，读者有必要了解对称加密和非对称加密的基本概念²。该章包含了以下内容：

Random number generation
Key management
Encryption (E0)
Authentication
The authentication and key-generating functions
Secure Simple Pairing
AES-CCM encryption for BR/EDR

读者可以阅读综述性文献³，加深对Core Spec中security mechanism的理解。

Reference

Legacy pairing and fixed PIN code or Passkey ↩︎ ↩︎
理解公钥与私钥 ↩︎
Guide to Bluetooth Security ↩︎

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