Fast Pair

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Google Fast Pair Service (GFPS), or Fast Pair (FP)

Accessory 一型一密：Model ID, Anti-Spoofing Public/Private Key Pair

Seeker (phone) <-> Provider (accessory)

Provider: - 广播 Model ID - 支持存储 128-bit Account Keys，至少5个key slots。每个Account Key标识一个用户账号。Account Key太多，影响adv长度。 - 支持RPA。

advertisement用bloom filter: - 对每个Account Key (K)，结合 salt，计算 H = sha256( K || salt ) - 再将H压缩到8个4 bytes的集合 X = { X_0, .., X_7 } - 将X映射到bloom filter的8个bit上 - salt随RPA更新

Key-based Pairing

基于Anit-Spoofing Key Pair： - Seeker隐式确认 Provider拥有其广播的Model ID所对应的Anit-Spoofing Private Key

基于Account Key: - Seeker & Provider 确认双方拥有相同的Account Key

pairing

Seeker -> Provider: Encrypted Request (含seeker addr, provider addr, salt), Public Key (optional)

Provider: - 如果含Public Key，则provider计算 K = SHA256(ECDH(Anit-Spoofing Private Key, Public Key))[0 .. 15]，解密Encrypted Request - 如果不含Public Key，则provider尝试使用Accout Key List中的Account Key作为K，尝试解密Encrypted Request

Provider -> Seeker: Encrypted Response (含provider addr, salt)

Seeker: - 选择Numeric Comparison模式 - ble stack生成6-digit passkey

Seeker -> Provider: Encrypted passkey block (含passkey, salt)

Provider: - 检查passkey是否是expected （依赖accessory形态）

Provider -> Seeker: Encrypted passkey block (含passkey, salt)

Seeker & Provider : 成功pairing

Seeker -> Provider : Account key (encrypted)

Characteristic: Additional Data

AES-CTR & HMAC-SHA256

ukey2

应用层kex, AES_256_CBC-HMAC_SHA256

ukey2

M_1 : client -> server : [ { ciphersuite, client pubkey commit } ]

M_2 : server -> client : selected ciphersuite, server pubkey

client -> server : client pubkey of the selected ciphersuite ( matched with the client pubkey commit )

DHS = DH(client privkey, server pubkey) = DH(server privkey, client pubkey)

PRK_AUTH = HKDF-Extract(“UKEY2 v1 auth”, DHS)

PRK_NEXT = HKDF-Extract(“UKEY2 v1 next”, DHS)

AUTH_STRING = HKDF-Expand(PRK_AUTH, M_1|M_2, L_auth)

NEXT_SECRET = HKDF-Expand(PRK_NEXT, M_1|M_2, L_next)