[ne0n]

It's right there in the article.

> “When Emma contacted me saying the source sent her the same docs too, I took a hash of my original HTML file and it checked out, so she has a copy of the same file as me,” Lee told Motherboard in a Twitter direct message. A hash is a cryptographic fingerprint of a file; if someone has tampered the file at all, those hashes won’t match.

> Lee said his source provided an HTML file of the DMs, and then Lee logged into the Twitter account himself and downloaded the direct messages with an automated tool.

> “I confirmed that they were authentic (Twitter itself would have had to doctor them) and that the source didn't modify the content in the copy he gave me,” Lee told Motherboard.