CC2420-RTR1 datasheet(46/92 Pages) TI

46 / 92 page

CC2420

SWRS041B

Page 46 of 89

flag setting is stored in the most significant

byte of the nonce. The flag byte used for

encryption and authentication is then

generated as shown in Figure 26.

The frame counter part of the nonce must

be incremented for each new packet by

software.

CTR Flag

bits 7:6

CBC Flag

bits 7:6

Res

Adata

SECCTRL0.SEC_M

MSB in CC2420 nonce RAM

CTR mode flag byte

CBC-MAC flag byte

Res

Figure 26.

CC2420 Security Flag Byte

21.3

Stand-alone encryption

Plain AES encryption, with 128 bit

plaintext and 128 bit keys [2], is available

using

stand-alone

encryption.

The

plaintext is stored in stand-alone buffer

located at RAM location 0x120, as can be

seen from Table 6 on page 31.

A stand-alone encryption operation is

initiated by using the SAES command

strobe.

The

selected

key

(SECCTRL0.SEC_SAKEYSEL) is then used

to encrypt the plaintext written to the

stand-alone buffer. Upon completion of the

encryption operation, the ciphertext is

written back to the stand-alone buffer,

thereby overwriting the plaintext.

Note that RAM write operations also

output data currently in RAM, so that a

new plaintext may be written at the same

time

reading

out

the

ciphertext.

21.4

In-line security operations

CC2420 can do MAC security operations

(encryption, decryption and authentication)

frames

within

the TXFIFO and

RXFIFO. These operations are called in-

line security operations.

As with other MAC hardware support

within

CC2420, in-line security operation

relies on the length field in the PHY

header. A correct length field must

therefore

used

for

all

security

operations.

The key, nonce (does not apply to CBC-

MAC), and SECCTRL0 and SECCTRL1

control registers must be correctly set

before

starting

any

in-line

security

operation.

The in-line security mode is set in

SECCTRL0.SEC_MODE

to one of the

following modes:

•

Disabled

•

CBC-MAC (authentication)

•

CTR (encryption / decryption)

•

CCM (authentication and encryption /

decryption)

When enabled, TX in-line security is

started in one of two ways:

•

Issue a STXENC command strobe. In-

line security will be performed within

the TXFIFO, but a RF transmission

will not be started. Ciphertext may be

read back using RAM read operations.

•

Issue

STXON

STXONCCA

command strobe. In-line security will

be performed within the TXFIFO and a

RF transmission of the ciphertext is

started.

When enabled, RX in-line security is

started as follows:

•

Issue a SRXDEC command strobe. The

first frame in the RXFIFO is then

decrypted / authenticated as set by

the current security mode.

CC2420-RTR1 数据表(PDF) 46 Page - Texas Instruments

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