Annex A.5: Skinny-128/384 example

What the clause requires

What the public text does not state

• The example values
• Any amendment change to the example

What it means for you

The Skinny-128/384 example is the widest Skinny version: a 128-bit block and a 384-bit tweakey. It is the version where the key and tweak split has the most room to move, and therefore the version where the inventory fields matter most. The block is the same as the 256-bit version; the input is 128 bits longer.

The clause 5 separation rule is the reason this page exists separately. An implementation that supports both this version and the 256-bit version must keep keying material apart across the two, because the tweak lengths differ. A configuration that reuses a key across the pair is outside the clause even if both individual vectors pass.

The evidence list adds a cross-version test to the familiar three. Run the same key in both Skinny versions and show that the system either refuses it or treats it as a different instance. That is a negative test, and it is the one most likely to find a real defect.

Evidence to keep

What an auditor or a customer is likely to ask for

Artefact	Owner	Why it matters
Known-answer test record for Skinny-128/384 with the five fields and the build	Engineering	The annex supplies the expected output.
Configuration record with the key and tweak lengths that sum to 384	Engineering	The key floor and the split both need to be visible.
Cross-version key separation test against Skinny-128/256	Security architecture	Clause 5 forbids reuse across differing tweak lengths.

How to verify it

Ask for the vector and the negative test. If the same key is accepted in both Skinny versions, the separation rule is not implemented.

Dates

The kit plan for annex A.5 sets the evidence review at 17 June 2027, 90 days before the estimated publication date of 15 September 2027.

Cross references

• A Annex A, Numerical examples
• 5 Requirements on the usage of tweakable block ciphers
• 7 Skinny
• 7.4 Skinny tweakey schedule

Where this comes from