ENC624J600-I Datasheet, PDF(132/168 Page) Microchip Technology – Stand-Alone 10/100 Ethernet Controller with SPI or Parallel Interface

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ENC624J600-I Datasheet, PDF (132/168 Pages) Microchip Technology – Stand-Alone 10/100 Ethernet Controller with SPI or Parallel Interface

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ENC424J600/624J600

To initialize decryption using a known encryption key:

1. Verify that AESST is clear, indicating that the

engine is Idle.

2. Configure AESLEN<1:0> to select the correct

key size.

3. Use the DMA to transfer the encryption key to

address 7C00h. Keys shorter than 256 bits

should be left-aligned.

4. Configure AESOP<1:0> (ECON1<10:9>) to

â00â.

5. Set AESST to initiate the key expansion.

6. Wait until the hardware clears the AESST flag.

15.3.2 CONTEXT SWITCHING

After each block is complete, the internal state may be

saved in order to switch encryption keys or operations.

Context switching may only be performed when

AESST (ECON2<11>) is clear, indicating that the

engine is Idle. The values comprising the context varies

depending on the mode selected. The sections

describing each mode details which values must be

saved.

15.3.3 BLOCK MODES

Block ciphers are commonly used in one of five modes

as described by the NIST Special Publication 800-38A,

âRecommendations for Block Cipher Modes of

Operation: Methods and Techniquesâ. The use of the

AES engine in each of these modes is described in the

following sections.

Four modes are natively supported in hardware:

â¢ Electronic Code Book (ECB)

â¢ Cipher Block Chaining (CBC)

â¢ Cipher Feedback (CFB)

â¢ Output Feedback (OFB)

The fifth mode, Counter (CTR), can be used with the

addition of software support for the counter.

15.3.3.1 Electronic Code Book Mode (ECB)

The Electronic Code Book mode applies the AES

encryption function directly to each plaintext block. No

feedback is included, so the encryption of each block is

completely independent of any previous block.

Assuming a given session key, any plaintext block will

always yield to the same ciphertext block (and vice

versa). If this is an undesirable property, a different

block mode should be selected. Figure 15-1 shows the

use of ECB mode for encryption and decryption.

FIGURE 15-1:

ECB ENCRYPTION AND DECRYPTION

ENCRYPTION

DECRYPTION

Plaintext0

Key

Plaintext1

Key

Ciphertext0

Key

Ciphertext1

Key

DATA KEY

ENCRYPTER

DATA KEY

ENCRYPTER

DATA

KEY

DECRYPTER