Clause 4: Symbols

What the clause requires

The clause fixes the notation used by the algorithm clauses: k, Nr, n and t for lengths and rounds; assignment, concatenation and exclusive-or; M for the diffusion matrix; X for the internal state; GF and the base field; lambda for a sub-tweakey; rho for the rotation table; h and PT for the two tweakey schedules; and the integer range notation.

What the public text does not state

• Whether the amendment changes the notation

What it means for you

Symbols look like housekeeping and are not. The document uses the same letters in the algorithm clauses and in the annexes, and an implementation review that reads k as the tweakey length instead of the key length will misstate the security claim. The record keeps the distinction: k is the key length, n is the block length and t is the tweak length.

The notation also tells you what the standard expects an implementation to expose. The internal state is an n-bit value, the diffusion matrix is named, and the sub-tweakey is a value per round. A conformance review can follow the same symbols from the text to the code, which is why this clause is worth a page even though it imposes no duty on an organisation.

For the ISMS file, this clause is supporting material. It does not create evidence an auditor will ask for by name. It does make the technical review cheaper, and it is the clause to cite when an engineering team says the standard is ambiguous about which length is which.

Evidence to keep

What an auditor or a customer is likely to ask for

Artefact	Owner	Why it matters
Implementation design document that uses k, n and t with the same meaning as the standard	Engineering	Prevents a length confusion in the security claim.
Review note that maps each symbol to the code variable	Security architecture	Makes a technical review repeatable.

How to verify it

Ask for the design document and check the three lengths. If the same letter is used for the key and the tweakey, the document has drifted from the standard.

Dates

The kit plan for clause 4 sets the evidence review at 17 June 2027, 90 days before the estimated publication date of 15 September 2027.

Cross references

• 3 Terms and definitions
• 5 Requirements on the usage of tweakable block ciphers
• 6 Deoxys-TBC

Where this comes from