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Referring to the AES specification:

https://nvlpubs.nist.gov/nistpubs/fips/nist.fips.197.pdf

Printed pages 35-37...

The first detailed walkthrough is encryption, the second is decryption.... I don't understand what is the third, "equivalent inverse cipher (decrypt)"?

How can there be two decryption techniques?

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  • $\begingroup$ Read section 5.3.5 of the linked specification (pg. 23 ff.). $\endgroup$
    – yoyo
    Feb 13, 2023 at 23:32

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You are correct that the third example in the AES specification (printed pages 35-37) is not a decryption technique, but rather an "equivalent inverse cipher" that demonstrates the inverse of the cipher's encryption algorithm.

there is only one decryption technique in AES, which is the inverse of the encryption algorithm. The third example in the specification is simply another way to express the inverse of the encryption algorithm, and it is not a separate decryption technique.

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