Comey admitted it. It is by virtue of the Laws as they are written.  Section 18, and others.  He had conversations with the President, and now he is liable for it under US Code as the FBI Director.  Jay Sekulow who has been in front of the Supreme Court 12 times and won 9 of them if I remember correctly just said that on FOX and it needs to be reviewed.  This is there to protect everybody including the country.  What Comey did is no different than what J. Edgar Hoover did.  That is why we have classifications for these items. You don't get to leak this stuff w/o consequences.   

Questions raised by Comey leaking contents of his Trump memo
abcnews.go.com/US/questions-raised-comey-admitting-leaked-contents...trump/story?...
3 hours ago - WATCH Trump attorney says Comey admitted he leaked 'privileged' information ... one of which he testified was classified," Kasowitz said this afternoon. ... I need to document it and preserve it in a way that this committee gets ..."
Google it for more.