How much damage have Edward Snowden's revelations caused to British national security?
Much of the focus of MI5 director-general Andrew Parker's speech last night was on the damage caused by the deliberate leaking of intelligence secrets, such as the classified material release by American security contractor Edward Snowden.
Parker didn't mention Snowden himself, but it was clear who he was referring to.
The thrust of his argument was that the leaks from Snowden had gifted an advantage to al-Qaeda and its affiliates by revealing the capabilities of GCHQ's electronic eavesdropping, thus allowing them to avoid it.
He said: "What we know about the terrorists, and the detail of the capabilities we use against them together represent our margin of advantage.
"That margin gives us the prospect of being able to detect their plots and stop them. But that margin is under attack."
Put simply, if the bad guys know how much GCHQ and its sister agencies know about them, they will behave differently, and be more difficult to monitor and to stop.
But the information stolen by Edward Snowden has further implications beyond simply alerting terrorists to electronic surveillance, damaging though that is.
Giving evidence at the High Court in August, deputy national security adviser Oliver Robbins spelled out just how much damage had been caused to the security and intelligence agencies.
Robbins was explaining why the police and intelligence services should be permitted to continue to examine material seized from David Miranda (the partner of Guardian journalist Glenn Greenwald) at Heathrow.
Miranda was carrying a number of USB drives. One of the drives contained 58,000 highly classified UK intelligence documents.
It was material which had come originally from Snowden, but it does not represent the totality of material in his possession, files which he has taken to China, then Russia.
According to Robbins, GCHQ's assessment of the amount of classified UK intelligence documents which Snowden would have access to during his work for the National Security Services (NSA) is consistent with the volume of documents on the hard drive.
In other words, he had bulk-downloaded everything he could find.
Exactly why he would have done this isn't clear. He wouldn't have required that volume of documents in order to make his initial point about the extent of NSA or GCHQ mass-surveillance.
That point seems to have been made quite simply by publishing a couple of PowerPoint presentations.
Such a volume of documents would of course be highly interesting to foreign intelligence agencies, particularly those who have historically been hostile to British and Western Interests - Russia, say, or China.
Continuing his evidence Mr Robbins asserted that "I can say with confidence…that the material seized is highly likely to describe techniques which have been crucial in life-saving counter-terrorist operations…the compromise of these methods would do serious damage to UK national security, and ultimately risk lives."
One area of particular concern, Robbins went on, was "the protection of staff identities …Anything which reveals or indicates the identities of UK security and intelligence agencies would be of value to elements hostile to the national interest of the United Kingdom, including foreign intelligence agencies and terrorists who actively seek such information."
He continued: "A particular concern for Her Majesty's Government is the possibility that the identity of a UK intelligence officer might be revealed.
"It is known that contained in the seized material are personal information that would allow staff to be identified, including those deployed overseas."
In addition, a large proportion of the material discovered on the hard drive is classified as either Secret or Top Secret.
According to Robbins' statement, "The compromise of Top Secret information would (according to the Security Policy Framework) be likely to have one or more of the following consequences:
To threaten the internal stability of the UK or friendly countries
To lead directly to widespread loss of life
To cause exceptionally grave damage to the effectiveness or security of UK or allied forces or to the continuing effectiveness of security or intelligence operations
To cause exceptionally grave damage to relations with friendly governments
And to cause long-term damage to the UK economy
The idea of Edward Snowden, in Russia with reams of such information represents a nightmare for Britain's spies.
Whatever his motives for travelling there, it's hard to imagine that Russia's security and intelligence services wouldn't be delighted to get their hands on exactly this kind of information, and could be highly persuasive in their pursuit of it.