MIAMI/WASHINGTON // CIA agents have written books about it. The former US president, George W Bush, has explained why he thought it was necessary and legal.
Yet the Al Qaeda suspects who were subjected to harsh interrogation techniques, and the lawyers charged with defending them at the Guantanamo Bay military tribunals, are not allowed to talk about the treatment they consider torture.
Defence lawyers say this, along with other Kafkaesque legal restrictions on what they can discuss with their clients and raise in the courtroom, undermine their ability to mount a proper defence on charges that could lead to the death penalty.
Those restrictions will be the focus of a pretrial hearing this week.
Prosecutors say every utterance by the alleged Al Qaeda murderers - and what their lawyers in turn pass on to the court - must be strictly monitored precisely because of the men's intimate personal knowledge of the highly classified CIA interrogation methods they endured in the agency's clandestine overseas prisons. Defence lawyers called that view extreme.
"Everything is presumptively top secret," said Cheryl Bormann, who represents Walid bin Attash, after the May court appearance of the men charged with plotting the September 11 attacks on New York's World Trade Centre. "So if my client had a tuna-fish sandwich for lunch, I couldn't tell you that."
At one point during the hearing, another of Mr bin Attash's lawyers, US air force captain Michael Schwartz, was explaining why his client refused to cooperate. Just as things got interesting, a security officer cut the audio feed to the media and others observing proceedings from behind a soundproof glass wall with a 40-second audio delay.
"The reason for that is the torture my client was subjected to by the men and women wearing the big-boy pants down at the CIA, it makes it impossible," Capt Schwartz said during the blocked portion of the arraignment, according to a declassified partial transcript.
Prosecutors have said in court filings that any revelations about the interrogations of the accused men could cause "exceptionally grave damage".
Civil libertarians argue that if those interrogation methods really are top secret, then the CIA had no business revealing them to Al Qaeda suspects.
Defence lawyers will challenge the secrecy rules during the pretrial hearing that begins on Wednesday at Guantanamo Bay US naval base.
Prosecutors have about 75,000 pages of evidence to hand over to defence lawyers in the September 11 case - but they won't do so until the judge, army colonel James Pohl, issues protective orders to safeguard the material.
Hundreds of men suspected of supporting Al Qaeda or the Taliban were rounded up in Afghanistan, Pakistan and elsewhere and shipped to Guantanamo in response to the September 11 attacks.
Of the 779 who have been held at Guantanamo since the prison operation began in 2002, 168 remain.
The CIA took custody of the "high-value" captives believed to have top-level information that could help the US and its allies to prevent further attacks. It held them incommunicado for three or four years and transferred them between secret overseas prisons, questioning them with interrogation methods that defence lawyers say amounted to torture and which the US administration has since banned.
Some details of the programme - including waterboarding (simulated drowning), mock executions and sleep deprivation - have already been disclosed by Mr Bush and the CIA. Jose Rodriguez, a former CIA official, recently defended them in interviews while promoting his book, Hard Measures: How Aggressive CIA Action After September 11 Saved American Lives.
Yet in both the September 11 case and that of Abd Al Rahim Al Nashiri, who is accused of sending suicide bombers to ram a boat full of explosives into the side of the USS Cole off the coast of Yemen in 2000, the government presumes that every word spoken by the accused, in the past and in the future, is classified at the highest level - "Top Secret," with "Sensitive Compartmented Information," which is routinely shortened to TS/SCI.
The men's words are also "born classified," a status their lawyers said has previously been used only to safeguard details about nuclear weapons. So are all the documents and legal motions related to their cases, which cannot be made public unless they are cleared by a department of defence security classification review team.
How that team works is a secret.
"I've never seen them. I've never communicated [with them]. No one has ever been able to tell me that," said James Connell, a lawyer for the September 11 accused, Ali Abdul Aziz Ali.
The Pentagon would say only that the review team includes both civilians and uniformed military personnel and that it can take up to 15 business days to make its decisions.
According to the court documents, proscribed topics include details of the men's capture, where they were held and under what conditions, names and descriptions of anyone who transferred, detained or interrogated them and the methods used to get information.
Defence lawyers say the classification system used at Guantanamo violates a 2009 order by Barack Obama, the US president, that prohibits the use of secrecy labels to conceal lawbreaking or prevent political embarrassment. They say it also "eviscerates" the legal defence protections that Congress set down in the law that authorises the Guantanamo tribunals.
The secrecy rules mean that every lawyer, paralegal and expert on the prosecution and defence teams must undergo an extensive background check and obtain a TS/SCI clearance. Once they get that clearance, they are briefed on what has to stay secret. The document that forms the basis of the presumptive classification is itself secret.
"It is ridiculous," said army captain Jason Wright, one of the lawyers for the accused September 11 mastermind, Khalid Sheikh Mohammed. "The briefing is classified, so I can't discuss what I can and cannot discuss."
Mr Mohammed's lawyers have asked the UN special rapporteur for torture, Juan E Mendez, to investigate claims that their client was tortured. But they could share with Mr Mendez only the information that has been publicly declassified.
The American Civil Liberties Union (ACLU) has filed a challenge arguing that the government has no legal authority to classify information it not only disclosed to the accused but also forced them to learn.
"The question here is: can the government subject people to torture and abuse and then prevent them from talking about it?" said Hina Shamsi, the director of the ACLU's National Security Project.
The ACLU said the claim of broad authority to gag defendants infringes on the US public's right to open trials and goes far beyond what the courts have allowed, namely that censorship must be narrowly tailored and aimed at protecting a compelling government interest.
"We don't think the government has any interest in classifying personal observations about conduct banned by the president of the United States," said Ms Shamsi. "The commission certainly will not be seen as legitimate if the proceedings revolve around judicially approved censorship of the defendants' accounts of government misconduct."
Prosecutors acknowledged the public has a right to witness the proceedings, but urged the judge not to substitute the ACLU's judgment on what should be classified for that of intelligence professionals.
Security rules restrict not only what can be made public but also what the lawyers can talk about with their clients.
In December 2011, the officer then in charge of the Guantanamo detention operation, Rear Admiral David Woods, issued orders forbidding defence lawyers from discussing certain topics during client visits.
Those included "historical perspectives on jihadist activities". That apparently means the lawyers cannot discuss part of the actual charges with their clients because the charges specifically mention Osama bin Laden's 1996 Declaration of Jihad Against the Americans.
The men are alleged to be Al Qaeda terrorists but the order prohibits their lawyers from talking to them about "groups engaged in terrorist activities".
"It's sort of an illusory idea, which is that we are going to give you a lawyer but you're not going to be able to talk about the central thing that is important for you to talk about with a lawyer," said David Nevin, another of Mr Mohammed's lawyers.
The chief prosecutor, Brig Gen Mark Martins, disputed this interpretation of Mr Woods's order and said defence lawyers' conversations are not restricted.
"They can talk to their clients about anything. What they can't do is take a document that may have classified information related to sources and methods and - unless it is cleared as disclosable to the client - they can't show them that document."
Under US law, communications between defendants and their lawyers are confidential and cannot be used as evidence. Again, the rules are different at Guantanamo.
The military and civilian defence lawyers, mostly from the Washington area, are not allowed to telephone their clients at Guantanamo and can communicate with them only during visits or in writing.
The September 11 defence lawyers have refused to send legal mail to their clients until prison camp inspectors agree to stop reading it.
The review teams are made up of Pentagon lawyers, translators and former intelligence officers - people from the same agencies that detained and interrogated the defendants and are now prosecuting them.
The lawyers say that submitting case-related documents for screening would force them to illegally disclose trial strategy, violating the right to a fair trial. They said it was also an ethical violation that could put their law licences at risk.
Not every word from the September 11 accused is treated as confidential for long. In 2009 they sent a note to the judge proclaiming themselves "terrorists to the bone" and calling the charges against them "badges of honour".
The note was rapidly cleared for public release and posted on a Pentagon website, leaving defence lawyers fuming that potential evidence from their clients had been released to the public before they even knew of its existence.
