"There was a time when NSA held a monopoly in this field," said Zimmermann, of Burlingame, Calif. "That's no longer the case."
"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.
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."