blob: store the first share of the Blob in the blob's proof

[vgonkivs]

To protect against the length attack we should store the first share of the blob in the blob's proof.
A length attack is an attack when a malicious node can give us invalid proof and commitment and it could be proved against the header's DataRoot correctly. It can happen when the proof will contain only part of the subtreeRoots.

[nashqueue]

From the first share of the Blob you can get the Sequence Length. From the sequence Length, you can check the length of the blob. If i don't, i can create a correct inclusion proof for 3/4 of a blob that is actually 4 shares long. The malicious commitment would be PFB2

[vgonkivs]

#3610 (comment)