EM2420 ETC [List of Unclassifed Manufacturers], EM2420 Datasheet - Page 44

• Manufacturer Part Number: EM2420
• Description: 2.4 GHz IEEE 802.15.4 / ZigBee RF Transceiver
• Manufacturer: ETC [List of Unclassifed Manufacturers]

MAC Security Operations (Encryption and Authentication)
The EM2420 features hardware IEEE
802.15.4 MAC security operations, which
are exposed in the EmberNet API. This
includes counter mode (CTR) encryption /
decryption, CBC-MAC authentication and
CCM encryption + authentication. All
security operations are based on AES
encryption [2] using 128 bit keys. Security
operations
transmit and receive FIFOs on a frame
basis.
The EM2420 also includes stand-alone
AES encryption, in which one 128 bit
plaintext is encrypted to a 128 bit
ciphertext.
The
command strobes are used to start
security operations in the EM2420 as will
be described in the following sections. The
ENC_BUSY status bit (see Table 5) may be
used to monitor when a security operation
has been completed. Security command
strobes issued while the security engine is
busy will be ignored, and the ongoing
operation will be completed.
Table 6 on page 29 shows the EM2420
RAM memory map, including the security
related data located from addresses
0x100 through 0x15F. RAM access (see
the RAM access section on page 27) is
used to write or read the keys, nonces,
and stand-alone buffer. All security related
data is stored little-endian, i.e. the least
significant byte is transferred first over the
SPI interface during RAM read or write
operations.
For a complete description of IEEE
802.15.4 MAC security operations, please
refer to [1].
Keys
All security operations are based on 128
bit keys. The EM2420 RAM space has
storage space for two individual keys
(KEY0 and KEY1). Transmit, receive and
stand-alone encryption may select one of
these
SEC_TXKEYSEL,
SEC_SAKEYSEL control bits (SECCTRL0).
Ember Part Number: 120-0049-000C
SAES,
two
are
keys
STXENC
performed
SEC_RXKEYSEL
individually
and
within
SRXDEC
in
and
EM2420 Datasheet
the
the
As can be seen from Table 6 on page 29,
KEY0 is located from address 0x100 and
KEY1 from address 0x130.
A way of establishing the keys used for
encryption and authentication must be
decided for each particular application.
IEEE 802.15.4 does not define how this is
done; it is left to the higher layer of the
protocol.
ZigBee
Cryptography (ECC) based approach to
establish keys. For PC based solutions,
more processor intensive solutions such
as Diffie-Hellman may be chosen. Some
applications
programmed keys, e.g. for remote keyless
entry where the key and lock are delivered
in pairs. A push-button approach for
loading keys may also be selected.
Nonce / counter
The receive and transmit nonces used for
encryption / decryption are located in RAM
from
respectively. They are both 16 bytes.
The nonce must be correctly initialized
before receive or transmit CTR or CCM
operations are started. The format of the
nonce is shown in Table 7. The block
counter must be set to 1 for compliance
with [1]. The key sequence counter is
controlled by a layer above the MAC layer.
The frame counter must be increased for
each new frame by the MAC layer. The
source address is the 64 bit IEEE address.
1 byte
Flags
Table 7. IEEE 802.15.4 Nonce [1]
The block counter bytes are not updated
in RAM, only in a local copy which is
reloaded for each new in-line security
operation. I.e. the block counter part of the
nonce does not need to be rewritten. The
EM2420 block counter should be set to
0x0001 for compliance with [1].
addresses
8 bytes
Source
Address
uses
may
4 bytes
Frame
Counter
an
0x110
also
EM2420
Eliptic
1 byte
Key
Sequence
Counter
and
use
Page 44 of 89
0x140
Curve
2 bytes
Block
Counter
pre-