ATAES132-SH-ER Atmel, ATAES132-SH-ER Datasheet - Page 108

• Manufacturer Part Number: ATAES132-SH-ER
• Description: EEPROM AES 32Kbit EE I2C
• Manufacturer: Atmel

Specifications of ATAES132-SH-ER

• Rohs: yes
• Maximum Clock Frequency: 1 MHz
• Operating Supply Voltage: 6 V
• Maximum Operating Temperature: - 40 C to + 85 C
• Mounting Style: SMD/SMT
• Package / Case: SOIC-8
• Interface Type: I2C
• Factory Pack Quantity: 2300

I.5.
Data Decryption
The following example shows how the encrypted data and integrity MAC are calculated for a 128 bit data block write to the
device with up to 14 bytes of authenticate-only data. This operation involves five passes through the AES crypto engine, all
five using the same key. If there are more than 14 bytes of authenticate-only data and/or more than 128 bits of data being
written, then 1, 2 or 3 more passes through the AES crypto engine are required.
There are two passes through the AES crypto engine in CTR mode to create the key block that is used to decrypt the data and
the MAC. The inputs to the crypto engine for those blocks are labeled A0 and A1, and the outputs are A’0 and A’1
respectively. A’0 & A’1 are the blocks sent to the system as the output parameters of the EncRead and decrypt commands.
There are three passes through the AES crypto engine in CBC mode to create the expected MAC value. The inputs to the
crypto engine for those blocks are labeled B0, B1 and B2, and the outputs are B’0, B’1 and B’2 respectively.
This sequence is also used for the Decrypt and KeyLoad commands, in addition to EncWrite.
A0 is composed of the following 128 bits:
A’0 is XOR’d with the encrypted input MAC and stored in the internal SRAM as the MAC T
A1 is composed of the following 128 bits:
A’1 is XOR’d with the encrypted input data and stored in the internal SRAM as the message M
B0 is composed of the following 128 bits:
B1 is the XOR of B’0 with the following 128 bits:
B2 is the XOR of B’1 with the following 128 bits:
B’2 is the cleartext MAC. If this matches the stored T value, then the write to memory proceeds. If there is no match, the
device returns an error flag and does not modify memory.
1 byte flag, fixed value of b0000 0001
12 byte nonce, as generated by the nonce command
1 byte MacCount, 1 for first MAC generation
2 byte counter field – always 0x00 00 for A0
1 byte flag, fixed value of b0000 0001
12 byte nonce, as generated by ATAES132 during nonce command
1 byte MacCount, 1 for first MAC generation
2 byte counter field – always 0x00 01 for A1
1 byte flag, fixed value of b0111 1001
12 byte nonce, as generated by the nonce command
1 byte MacCount, 1 for first MAC generation
2 byte length field – max 0x00 20 if 256 bits of encrypted data, min 0x00 01 for one byte
2 byte length field, size of authenticate-only data
14 byte data to be authenticated only
16 bytes of cleartext message M
Atmel ATAES132 Preliminary Datasheet
8760A−CRYPTO−5/11
108