Key block size

Question

In the TLS-Details lecture (lecture 04) on slide 19 it says that the required keyblock size for the TLS_RSA_WITH_AES_128_CBC_SHA256 cipher suite is 96 bytes,
(32*2) for MAC keys.
(16*2) for Encryption keys.
But is that correct? wouldn't we also generate the IVs as AES in CBC mode requires them? so the resulting keyblock would be of size 128 bytes.

Answer 1

Hi,

The IV in the key block is only used in TLS 1.0 or with AEAD.

Comments

But then, how is CBC mode used without an IV? Isn't it necessary? or is TLS using random IVs then?

---

In TLS 1.0, the IV is derived from the key block. In newer versions, the IV is generated for each record and sent with it.

---

Oh alright, thank you, that was confusing.