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CC1010 Datasheet(PDF) 75 Page - Texas Instruments |
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CC1010 Datasheet(HTML) 75 Page - Texas Instruments |
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75 / 152 page  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 be scrambled. The encryption and decryption operations in 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 be 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 is given by the register 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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