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E_BLUETOOTH_ATT_INSUFFICIENT_ENCRYPTION_KEY_SIZE
Encrypted does not imply an adequate key size
E_BLUETOOTH_ATT_INSUFFICIENT_ENCRYPTION_KEY_SIZE maps ATT error 0x0C. The connection has encryption, yet the key size associated with the bond or current pairing is below the minimum configured for the attribute. This can occur with legacy bonds, constrained devices, or security parameters that permitted a shorter key during pairing.
A reconnect normally reuses the same long-term key and therefore preserves the failure. The remedy is to establish a bond whose negotiated key size meets both peers’ capabilities and the server policy; reducing the server requirement merely to make the request pass can weaken the intended protection.
Security evidence worth recording
- Capture the negotiated encryption key size rather than only an encrypted/not-encrypted flag.
- Check the minimum and maximum key-size parameters used during pairing.
- Delete the old bond on both devices before testing a new negotiation.
- Confirm that the peripheral does not silently fall back to an older security mode.
References
- Bluetooth Core: encryption key size negotiation
- Bluetooth Core: Encryption Key Size Too Short
- Apple Core Bluetooth: insufficient encryption key size
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