Key Expiry Number (KEN)

The Key Expiry Number is an 8 bit binary number which is used to check whether the decoder key in a payment meter has expired. The Key expiry Number represents the last 8 bits of the TID field. If the most significant 8 bits of the TID value in the token are greater that the KEN stored in the payment meter, the token is rejected with a key expiry error.

The KEN bit positions in the token field are shown below in bold - all numbers in binary format unless otherwise stated:

Class	Sub-class	Random number	TID showing KEN value bits	Amount	CRC
00 (credit token)	0000 (electricity)	0101 (5 decimal)	110000000010101001010101 (2016/12/11 15:49)	0010011100010000 (1000 decimal as per amount formula in IEC62055-41)	0001100101110000 (1970 Hex)

Usage

The table below shows various values of TID and the token validity given various KEN values stored in the payment meter.

Key Expiry Number (stored in the payment meter)	KEN Value of token (most significant 8 bits of the 24 bit TID field)	Token validity
0xff (Hexadecimal) = 255 (decimal) = 11111111 (binary)	0x00 - 0xff (01/01/1993 - 24/11/2024 20:15 for 1993 base date) 0x00 - 0xff (01/01/2014 - 24/11/2045 20:15 for 2014 base date) 0x00 - 0xff (01/01/2035 - 24/11/2066 20:15 for 2035 base date)	token accepted since TID <= KEN (all tokens accepted. Effectively KEN is disabled if set to 0xff)
0xaa (Hexadecimal) = 170 (decimal) = 10101010 (binary)	0x00 - 0xaa (01/01/1993 - 08/03/2014 21:20 for 1993 base date) 0x00 - 0xaa (01/01/2014 - 08/03/2035 21:20 for 2014 base date) 0x00 - 0xaa (01/01/2035 - 08/03/2066 21:20 for 2035 base date) 0xab - 0xff (23/04/2014 21:36 - 24/11/2024 20:15 for 1993 base date) 0xab - 0xff (23/04/2035 21:36 - 24/11/2045 20:15 for 2014 base date) 0xab - 0xff (23/04/2066 21:36 - 24/11/2066 20:15 for 2035 base date)	token accepted since TID <= KEN token accepted since TID <= KEN token accepted since TID <= KEN token rejected since TID > KEN token rejected since TID > KEN token rejected since TID > KEN

References: IEC62055-41