Clause 6.3: Deoxys-TBC decryption

What the clause requires

Decryption applies the inverse transformations in the reverse order: the sub-tweakey exclusive-or, the inverse column mixing, the inverse row rotation and the inverse byte substitution, with the sub-tweakey step repeated after the last round.

What the public text does not state

• The inverse test values, which are not in the ISO public preview
• Whether the amendment changes the inverse round

What it means for you

The decryption clause mirrors the encryption clause and is worth a separate page because the failure modes are different. A decryption routine can be correct in isolation and still not invert the encryption routine if the round keys are applied in the wrong order or the final step is misplaced. The standard gives the inverse matrix and the inverse S-box so that the two directions can be checked against each other.

The practical test is a round trip plus a known-answer test. A round trip alone proves only that the same bug appears twice. The annex vector is what ties the pair to the standard. That is why the record treats the vector as the evidence and the round trip as a smoke check.

For an ISMS file this page is supporting material. It becomes relevant when a product claim includes decryption, which most do, and when a test lab asks for the conformance evidence. The evidence list is the same shape as the encryption page, with the inverse direction named.

Evidence to keep

What an auditor or a customer is likely to ask for

Artefact	Owner	Why it matters
Known-answer test for the inverse direction, or a round trip against the published vector	Engineering	Proves the inverse matches the standard, not only itself.
Code review note confirming the inverse order and the final sub-tweakey step	Security architecture	The two common defects are order and the final step.
Test lab report if the module is validated	Compliance	An external report is stronger evidence than an internal test.

How to verify it

Encrypt the published plaintext, decrypt the published ciphertext, and compare both with the annex. A round trip on your own data is not enough.

Dates

The kit plan for clause 6.3 sets the evidence review at 17 June 2027, 90 days before the estimated publication date of 15 September 2027.

Cross references

• 6 Deoxys-TBC
• 6.2 Deoxys-TBC encryption
• A.1 Deoxys-TBC-256 example
• A.2 Deoxys-TBC-384 example

Where this comes from