Well, it eliminates most of the possible keys.  Out of 2^256 possible keys, only 2^64 are now possible.<p>This would be serious if someone found a way to easily identify <i>which</i> keys were still possible.  From there, you could find the real key simply by a process of elimination.  It would be somewhat lengthy, but not unreasonably so.  But so far as I know, nobody yet knows how to easily identify which keys are still possible.<p>Note:  I&#x27;m ignoring IVs here, partly because I don&#x27;t know if HTTP2 uses them.  They may make the process of elimination impossible (at least in realistic amounts of time).