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A new book on Mossad's efforts to intercept terrorist communications states that Isis and Al-Qaeda are using Reddit and eBay to hide encrypt messagesGetty

Islamic State (Isis) and Al-Qaeda terrorists have been turning to Reddit, eBay and pornography to send coded messages to their followers, according to an author with sources inside Mossad.

Gordon Thomas claims Al-Qaeda operatives were found by the Israeli intelligence agency's cybercrime unit to be using steganography – a method of concealing secret information within digital files – to send messages to other operatives, encrypted in goods being offered to sale on eBay.

The author wrote Gideon's Spies: The Secret History Of The Mossad in 1999 – a book that gave readers a never-before-seen insight into the agency that was formed in 1949.

His 2015 update to the book includes two chapters on Mossad's cybercrime practices in recent years, including the fact that its operatives now routinely monitor social bookmarking website Reddit, where messages can be hidden easily due to the plethora of subreddit sections and millions of posts of comments in each section of the website.

"More than once, [Reddit] had led an operator to a terrorist using hexadecimal characters and prime numbers. Decoded, they sometimes indicated an attack was being planned or even about to happen," Thomas writes in an excerpt of the book published by the New York Post.

Concealed message increase since Paris shootings

In fact, since the Charlie Hebdo shootings in Paris in January, both Mossad and UK intelligence agency MI5 have seen an increase in concealed messages being sent by IS and Al-Qaeda to their followers, and MI5 has warned Al-Qaeda is plotting "large scale massacre" in Britain.

Other than eBay and Reddit, Thomas was told by sources that the terrorists are also hiding messages in pornographic pictures that "conceal documents and orders for the next target".

Terrorist organisations are also turning to the Dark Web, a section of the internet with millions of webpages that are not being indexed by search engines such as Google and cannot easily be navigated to using a standard web browser.

The Dark Web is estimated to be 5,000 larger than any other part of the web and Mossad's highly skilled cryptologists and codebreakers have to wade in to retrieve intelligence.

If they discover crucial information about an imminent attack on a particular country, the information is passed to the head of cybersecurity in that nation's security agency.

MI5 and Mossad have also employed linguists fluent in Arabic, Pashto and Urdu in order to translate online magazines, where both IS and Al-Qaeda are publishing instructions to their members.