Clause 7.4: Skinny tweakey schedule

What the clause requires

What the public text does not state

• The body of clause 7.4
• The Skinny schedule constants and permutation

What it means for you

The Skinny schedule is the second schedule in the document and the one that has to handle three tweakey lengths. The public text gives the title and the page only. The record does not describe the permutation, the round constants or the update rule, because those details are in the purchased text.

The schedule matters to the buyer for the same reason as the Deoxys schedule: it is where the key and the tweak are combined, and it is where a change of tweak length becomes a different code path. Clause 5 s separation rule is about instances with different tweak sizes, so the schedule is the place to see whether the implementation really keeps them apart.

The evidence line is a design review request. Ask for a trace from the schedule as written to the code, and for a test that exercises more than one tweakey length. If the implementation supports only one length, say so and record the limitation. That is a complete answer even without the clause body.

Evidence to keep

What an auditor or a customer is likely to ask for

Artefact	Owner	Why it matters
Design note tracing the Skinny sub-tweakey derivation	Engineering	The schedule is where the key and tweak are combined.
Test that exercises each supported tweakey length	Engineering	Clause 5 separates instances by tweak length.
Purchase record for the base standard, so clause 7.4 can be read	Standards owner	The clause is not in the public preview without it.

How to verify it

Ask how many tweakey lengths the implementation supports and how the code keeps them apart. A single-length implementation is a valid answer if it is recorded.

Dates

The kit plan for clause 7.4 sets the evidence review at 17 June 2027, 90 days before the estimated publication date of 15 September 2027.

Cross references

• 5 Requirements on the usage of tweakable block ciphers
• 7 Skinny
• 7.1 Skinny versions
• A.3 Skinny-64/192 example
• A.4 Skinny-128/256 example
• A.5 Skinny-128/384 example

Where this comes from