SSK and TSK lengths

[railabouni]

Hi,
in https://github.com/OP-TEE/optee_docs/blame/master/architecture/secure_storage.rst#L344 SSK and TSK are 128-bit while each of them is a result of HMAC-SHA256 (should be 256-bit long each)
What am I missing?

Thank you!

[jockebech]

The key comes from the HMAC-SHA256 as you're saying, but it is truncated to a 128-bit key here (called from here and the define is here).

[jockebech]

But I agree, the docs are a bit unclear here. Should be updated to reflect what actually is happening without making assumptions.

[railabouni]

yea, was a bit confused cause in: https://github.com/OP-TEE/optee_docs/blame/master/architecture/secure_storage.rst#L362
did say "128 bits of k" after "k = SHA256(FEK);" :)

Thank you! Joakim

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