User manual

CY4672 Reference Design Guide, Document # 001-16968 Revision ** 31
WirelessUSB™ Protocol 2.2
2.6.2.1 AES Key Management
The encrypt key is stored on the keyboard and the decrypt key is stored on the bridge during compil-
ing time. There is no dynamic encrypt key exchange in the running time.
2.6.3 Encryption and Power Consumption Trade Off
If the keyboard encryption is enabled, each key code is encrypted into an 8 byte key code (TEA) or
16 byte key code (AES). When a single key is pressed, a non-encrypted key down packet consists of
a 16-bit Preamble + 2 bytes SOP + 1 byte packet header + 1 byte key code + 2 bytes CRC; an
encrypted key down packet consists of the same overhead packets plus 8 or 16 byte key code
instead of one byte key code. This results in an increase in the average power consumption when
encryption is enabled.
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