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AES hardware accelerator (AES) RM0440

1508/2126 RM0440 Rev 4

encryption) or ciphertext (CTR decryption) input. In CTR mode, the MODE[1:0] bitfield

setting 01 (key derivation) is forbidden and all the other settings default to encryption mode.

The sequence of events to perform an encryption or a decryption in CTR chaining mode:

1. Ensure that AES is disabled (the EN bit of the AES_CR must be 0).

2. Select CTR chaining mode by setting to 010 the CHMOD[2:0] bitfield of the AES_CR

3. Initialize the AES_KEYRx registers, and load the AES_IVRx registers as described in

Table 319.

4. Set the EN bit of the AES_CR register, to start encrypting the current counter (EN is

automatically reset when the calculation finishes).

5. If it is the last block, pad the data with zeros to have a complete block, if needed.

6. Append data in AES, and read the result. The three possible scenarios are described in

Section 34.4.4: AES procedure to perform a cipher operation.

7. Repeat the previous step till the second-last block is processed. For the last block,

apply the two previous steps and discard the bits that are not part of the payload (if the

size of the significant data in the last input block is less than 16 bytes).

Suspend/resume operations in CTR mode

Like for the CBC mode, it is possible to interrupt a message to send a higher priority

message, and resume the message that was interrupted. Detailed CBC suspend/resume

sequence is described in Section 34.4.8: AES basic chaining modes (ECB, CBC).

Note: Like for CBC mode, the AES_IVRx registers must be reloaded during the resume operation.

34.4.10 AES Galois/counter mode (GCM)

Overview

The AES Galois/counter mode (GCM) allows encrypting and authenticating a plaintext

message into the corresponding ciphertext and tag (also known as message authentication

code). To ensure confidentiality, GCM algorithm is based on AES counter mode. It uses a

multiplier over a fixed finite field to generate the tag.

GCM chaining is defined in NIST Special Publication 800-38D, Recommendation for Block

Cipher Modes of Operation - Galois/Counter Mode (GCM) and GMAC. A typical message

construction in GCM mode is given in Figure 519.

ST STM32G431 User Manual

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