Among the tricks of the spy trade the book reportedly reveals is MI6's accidentally discovered method of transferring secret messages.

BBC: Spy tactics revealed by book

Research groups in the U.S., Europe and Canada devised ways to use DNA to encode trademarks and secret messages in cells.

WSJ: Harvard Researchers Turn Book Into DNA Code

There is also some debate over a technique called steganography, which involves embedding secret messages within publicly available files and images on the internet, but there is little evidence so that al-Qaeda has used this.

BBC: NEWS | In Depth | A web wise terror network

By the eighties, the goal of objectivity had been replaced, at least in the universities, by the postmodern view that there is no such thing as objectivity: every statement is subjective, partial, full of biases and secret messages.

NEWYORKER: The English Wars

The evidence included about 80 government witnesses, bank records, contract agreements, text messages and secret video and audio recordings.

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Based on their extensive surveillance of the secret agents and their messages to handlers back in Moscow, U.S. counterintelligence officials believe the grooming of Mr. Foley was part of a long-term goal for some of the group's children to become spies when they got older.

WSJ: Russian Spy Ring Aimed to Make Children Agents

It is true that during WWII the BBC had broadcast coded messages to British secret agents behind enemy lines, and that some journalists had worked with MI6 in producing propaganda.

BBC: The 'secret agents' of the UK press

Aside from being ineffective - friends cannot read private messages - it encourages an erosion of trust and could lead children to be tempted into using a "secret" account, away from prying eyes.

BBC: Child safety measures to protect against internet threats