CC1010-RTR1 datasheet(75/152 Pages) TI

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CC1010

SWRS047

Page 75 of 152

16.9 DES Encryption / Decryption

DES encryption / decryption is supported

by hardware in

CC1010. Blocks of data

ranging from 1 to 256 bytes can be

encrypted / decrypted in one operation by

the DES module. Multiple encryption /

decryption operations can also be used on

larger data blocks.

Encryption is the process of encoding an

information bit stream to secure the data

content. The DES algorithm is a common,

simple and well-established encryption

routine. An encryption key of 56 bits is

used to encrypt the message. The receiver

must use the exact same key to decrypt

the message, otherwise the message will

scrambled.

The

encryption

and

decryption

operations

the

DES

algorithm are symmetrical operations with

the same computational requirements.

The operations produce the same number

of output bytes as input bytes. The

strength of an encryption algorithm is

determined by the number of bits in the

key, the more the better. The DES

algorithm offers a low to medium level of

security. If higher levels of security are

required, a triple DES algorithm can be

used. Triple DES can be achieved by

running the DES algorithm three times

sequentially using three different 56-bit

encryption keys. The keys must be used in

reverse order when decrypting.

The DES algorithm works internally on

entities of 8 bytes. The Output Feedback

Mode (OFB) and Cipher Feedback Mode

(CFB) are DES modes of operation that

permit data lengths that are not a multiple

of eight bytes. The operation mode is

selected through the CRPCON.CRPMD

control bit. The same DES mode of

operation

must

used

both

for

encryption and decryption to yield correct

results. CFB is recommended, as it is

more secure than OFB.

CRPCON.ENCDEC should be cleared when

encrypting data and set when decrypting

data.

56 bit DES keys are stored in external

RAM, as shown in Table 24. The location

given

the

CRPKEY,

containing the 8 most significant address

bits. New keys are loaded only at the

beginning of an encryption / decryption if

CRPCON.LOADKEYS is set. If not, the

same keys as used in the previous run will

be used again.

The DES keys do not contain parity bits. If

DES keys with parity bits are given, the

parity bits must be removed before

performing encryption / decryption. The

keys are therefore stored as 7 successive

bytes in RAM.

After running the DES, a output block O of

length CRPCNT bytes is generated by

encrypting / decrypting the input block I of

same length as O using key K1 as follows:

O=EK1(I)

(encryption)

O=DK1(I)

(decryption)

The following is an example on how to use

the single DES algorithm hardware in

CC1010. First the 56-bit encryption key

must be stored in the external RAM. Then

the CRPKEY register must be written to

point to the start of the encryption key.

Note that the encryption key must start on

a RAM address location divisible by 8.

Then the data bit stream to encrypt must

be stored in the external RAM. The data

bit stream must consist of at least 1 byte

up to a maximum of 256 bytes, and it must

also start on a RAM address location

divisible by 8. The CRPDAT register must

be written to point to the start of the data

bit stream, and CRPCNT must be written to

give the number of bytes to be encrypted.

Then

the

CRPINI0,

CRPINI1,

CRPINI2,

CRPINI3,

CRPINI4,

CRPINI5,

CRPINI6,

CRPINI7

registers must be written to contain the

DES initialisation vector used in the OFB

and CFB modes of operation. For

simplicity it can be set to all zeros. Note

that the initialisation vector must be the

same for both encryption and decryption to

yield correct results. To initiate the

encryption the CRPCON register must be

written. The bits in this register select

encryption/decryption,

feedback

mode,

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