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STATUS_BTH_ATT_INSUFFICIENT_ENCRYPTION_KEY_SIZE
Encryption exists, but not at the strength required by the attribute
STATUS_BTH_ATT_INSUFFICIENT_ENCRYPTION_KEY_SIZE is ATT error 0x0C. The server is not saying that the link is completely unencrypted; it is saying that the encryption key size in use is insufficient for this access rule.
This is why it must be kept separate from Insufficient Encryption and Insufficient Authentication. The correct diagnosis involves the negotiated security properties of the connection and the profile's required minimum, not the characteristic payload or handle layout.
What to check
- Preserve the pairing method, negotiated key-size information where the platform exposes it, bond state, current encryption state, and the attribute requirement.
- Re-establish security using a pairing flow and peer capabilities that can satisfy the required key strength; repeated GATT requests alone cannot increase it.
- Check for stale bonds or firmware policy changes before removing user pairing data, because the observed key size can be a compatibility symptom rather than a local API fault.
References
- Bluetooth SIG: Attribute Protocol (ATT)
- Bluetooth SIG: Generic Access Profile security requirements
- Microsoft: Bluetooth GATT client
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