Figure 104. Ctr Encryption And Decryption Principle - ST STM32WL5 Series Reference Manual

RM0453
Counter (CTR) mode
Legend
The CTR mode uses the AES core to generate a key stream. The keys are then XORed
with the plaintext to obtain the ciphertext as specified in NIST Special Publication 800-38A,
Recommendation for Block Cipher Modes of Operation.
Note: Unlike with ECB and CBC modes, no key scheduling is required for the CTR decryption,
since in this chaining scheme the AES core is always used in encryption mode for producing
the key stream, or counter blocks.

Figure 104. CTR encryption and decryption principle

Counter
key
Encrypt
Plaintext block 1
Ciphertext block 1
Counter
key
Decrypt
input
output
Plaintext block 1
XOR
Ciphertext block 1
Encryption
+1 Counter
value value + 1
key
Encrypt
Plaintext block 2
Ciphertext block 2
Decryption
+1 Counter
value value + 1
key
Decrypt
Plaintext block 2
Ciphertext block 2
RM0453 Rev 1
AES hardware accelerator (AES)
+1 Counter
value + 2
key
Encrypt
Plaintext block 3
Ciphertext block 3
+1 Counter
value + 2
key
Decrypt
Plaintext block 3
Ciphertext block 3
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