It was an ordinary number, just a dozen digits: 873682505331.
But it gave U.S. intelligence and law enforcement the key to the prosecution of four of Osama bin Laden's followers this year for their roles in the 1998 terrorist bombings of two American embassies in East Africa.
The number rang bin Laden's satellite telephone, a laptop-sized device that linked his hideout in the mountains of Afghanistan to a global network of followers - and to the National Security Agency at Fort Meade, which intercepted the plotters' calls.
While not explicit enough to allow U.S. authorities to move in time to prevent the bombings, the intercepts helped track the terrorists and send them to prison.
Since the East Africa bombings, however, the NSA has had far less success in picking up bin Laden's communications, according to people knowledgeable about U.S. intelligence.
Possibly put on guard by the embassy bombing investigation, the Saudi dissident and his terrorist lieutenants evidently have become far more cautious in their communications.
"They've been very careful and compartmented," said one former senior intelligence official.
James Bamford, author of two books on the NSA, was told by agency sources that in the late 1990s, NSA officers would sometimes play a tape of bin Laden talking with his mother to impress visitors holding high-level security clearances. But early this year, Bamford said, a source told him that the agency "had totally lost bin Laden's calls."
"They lost all track of him," Bamford said. "It could be that he uses couriers for really important communications. Or it could be he's using encryption."
If it is true that the NSA can no longer monitor bin Laden's communications - and the agency isn't saying - the loss might have played a role in the failure of U.S. intelligence to pick up warning of Tuesday's terrorist attacks. Officials have called bin Laden their prime suspect.
Now, the loss will hinder investigators as they try to hunt down those who conceived, planned and paid for the bloodiest terrorist assault in U.S. history.
It has already triggered a new debate over government control of encryption, posing the authorities' desire to intercept terrorists' communications against Americans' freedom to communicate without fear of government snooping.
Over the past decade, encryption - software or hardware used to scramble communications so that only the intended recipients can understand them - has become widely available and virtually unbreakable. And, as officials have long feared, encryption is becoming a deadly weapon in the terrorists' arsenal.
In the aftermath of Tuesday's attacks, Sen. Judd Gregg, a New Hampshire Republican, called for a global prohibition on encryption products that do not have a back door built in for government surveillance.
"This is something we need international cooperation on ... to get the information that allows us to anticipate and prevent what occurred in New York and Washington," Gregg told the Senate.
Unworkable and unwise
Cryptographer Phil Zimmermann - the creator of PGP, for Pretty Good Privacy, a powerful and widely used e-mail encryption program - said such a ban would be unworkable and unwise. He said PGP is used by human rights activists worldwide collecting information on government repression and even genocide. "If we make momentous political decisions under such incredible emotional pressure," he said, "we're bound to make terrible decisions."
Bin Laden and his allies may be using several methods to hide from the NSA. He may simply have dropped all electronic communications, insisting that sensitive messages be hand-carried. His associates in urban areas may make calls from random public telephones using untraceable, prepaid phone cards, and using vague language.
Or possibly, experts say, he may be using new, powerful encryption that even the NSA can't break. Cryptographers say the long historical race between code makers and code breakers is over, and the code makers won.
"There was a time when NSA held a monopoly in this field," said Zimmermann, of Burlingame, Calif. "That's no longer the case."
William P. Crowell, the NSA deputy director, told a Senate committee in 1996 that if all the personal computers that then existed were set to work to break a single message encrypted with PGP, it would require 12 million times the estimated age of the universe to break it.
"It's just going to be real tough for the NSA, and it's only going to get tougher," said Stephen T. Walker, a former NSA and Defense Department employee who built a Maryland software company that he sold in 1998 for $350 million.
Several terrorists have been caught using encryption in recent years. Ramzi Yousef, the convicted mastermind of the 1993 World Trade Center bombing, used encryption to protect computer files giving details of a plan to crash 11 U.S. airliners.
After months of work, NSA experts broke the encryption. The plot was foiled.
This year's trial of the embassy bombing plotters revealed that bin Laden associates began to use encryption before 1998. Wadih El-Hage, one of the four convicted, sent encrypted e-mails under various names, including "Norman" and "Abdus Sabbur," to associates in the bin Laden organization.
But the trial transcript shows the terrorist ring talked constantly, if guardedly, on unencrypted phone lines as they coordinated the embassy bombings in Kenya and Tanzania, which killed 224 people and injured several thousand.
Bin Laden's phone used the Inmarsat satellite network, originally created for maritime use but now linking 210,000 phones worldwide. An associate bought the phone Nov. 1, 1996, from a New York supplier, and a total of 2,200 minutes of prepaid time was used over the next two years, according to records presented at trial.
He made calls all over the world, to England, Yemen, Sudan, Iran, Saudi Arabia, Pakistan and Azerbaijan, as well as 50 calls to Kenya, where the bombings were being plotted.
Federal prosecutors spent hours laying out hundreds of intercepted calls for a Manhattan jury, using transcripts and call records to sketch the web of conspiracy.
Assistant U.S. Attorney Kenneth Karas dubbed one London cellular telephone that often called the satellite phone in Afghanistan "the Jihad phone," used to coordinate bin Laden's jihad, or holy war, against the United States.
"That's the phone that bin Laden and the other co-conspirators [used] to carry out their conspiracy to murder U.S. nationals. ... It is the phone that gives you a window into how it is that Al Qaeda [the Base, bin Laden's network] operates," he told the jury.
That window evidently has narrowed considerably since 1998, though it might not have closed completely. Sen. Orrin G. Hatch, a Utah Republican, said after Tuesday's attacks that the United States had intercepted a call between two bin Laden associates suggesting their involvement.
Such information is highly classified, and intelligence officials were furious that Hatch had disclosed it, fearing the targets would be warned not to use telephones, the Chicago Tribune reported.
Despite the NSA's reputation as a "vacuum cleaner" that gathers up all communications worldwide, officials have said in recent years that the agency risks being overwhelmed by the ballooning volume of global communications.
"Forty years ago, there were 5,000 stand-alone computers, no fax machines and not one cellular phone," NSA Director Michael V. Hayden, an Air Force lieutenant general, said in a speech last year. "Today, there are over 180 million computers - most of them networked. There are roughly 14 million fax machines and 40 million cell phones, and those numbers continue to grow."
Contrary to popular myth, the job of plucking crucial calls of a suspected terrorist from the flood of communications is not a simple matter of plugging keywords such as bomb into the NSA's computers, electronics experts say. While text messages such as e-mails can be searched rapidly by a computer, voice communications are a different matter.
"When you type an A on your keyboard, it sends seven bits of data, and those seven bits mean A everywhere on the planet," said Steve Uhrig, president of SWS Security in Harford County, which manufactures and sells electronic surveillance systems.
"But there are thousands of ways to say bomb, and the technology just doesn't exist yet to interpret it very accurately," he said.
Even in hunting a terrorist's text message, the NSA's job is akin to searching for a ring that accidentally fell into the trash - after the trash has been dumped at a landfill. "Sifting bin Laden out of the enormous volume is a huge challenge," Uhrig said.
And that's if the message is not encrypted. If it is encrypted, and even if the NSA has the theoretical ability to break the code, the agency might have no way of telling that a particular message is worth trying to break.
If the terrorist assault sparks a new debate over government control of encryption, it will echo a battle fought in the early 1990s. Then, cryptographers demanded that the U.S. government drop restrictions on the export of encryption products. They said strong encryption, crucial for such contemporary business purposes as using credit cards online, was already available on the Internet and from foreign companies.
The NSA and FBI fought to keep the export controls, fearing strong encryption would protect terrorists and drug traffickers. They also proposed the Clipper chip, a plan for the government to keep the key to an electronic back door to all encryption. After an outcry from civil libertarians and industry officials, the Clipper plan was dropped and export restrictions on encryption were greatly relaxed.
'Both sides proven right'
Walker, the former Defense employee, said Tuesday's attacks are a reminder that both sides in the old debate had a point.
"Now we have an incredibly ugly situation where both sides have been proven right," Walker said. "You can't stop encryption. But it can be used for terrible purposes."
PGP inventor Zimmermann - who once was threatened with federal prosecution for allegedly making PGP available over the Internet - said that to build a government back door into the encryption used today by countless individuals, businesses and organizations for all kinds of purposes would be a serious mistake:
"If we install blanket surveillance systems, it will mean the terrorists have won. The terrorists will have cost us our freedom."Copyright © 2015, The Baltimore Sun