They caution that once you add unproven servers, a variety of software and the quirks of individual users into the equation, any assumptions about the bedrock security of the system are open to question.
"Once you sign with a digital signature, (that signature is) going to be equated with you," said Adam Shostack, director of technology for privacy-enhanced software maker Zero-Knowledge Systems Inc. "Unfortunately, the computing base is not that secure. Someone could get access to the keys on your machine and sign documents in your name."
On Oct. 1, the Electronic Signatures in Global and National Commerce Act will become law, clearing the way for people to use digital signatures to sign online contracts, agree to software licenses and secure business-to-business transactions.
Digital signatures are generally based on two technologies: Public-key encryption and a digital-fingerprinting technology known as hashing -- each of which solves a different aspect of the digital signature problem and can be proven mathematically secure.
Most documents are long, and scrambling the text using reasonably secure encryption would take even the latest computers a long time. Using a hash function, however, the document can be turned into a much shorter digital fingerprint, usually 1KB or so. A single change in the document would significantly change the resulting hash, so anyone can check if the document has been forged just by rehashing it.
Yet, hashing only guarantees the integrity of the document. It doesn't associate the document with any particular person. That's where another technology, called public-key cryptography, comes in. Public-key encryption uses one set of numbers (called the private key) to scramble a message and a second (called the public key) to unscramble it.
Someone who wants to sign a document will encrypt the hash of the text with a private key. The user would then publish the public key and the encrypted hash or give it to a third party for safekeeping.
The result: Anyone who wants to verify whether this is a document that the user signed can hash the text and compare that with the hash on file, decrypted with the user's public key. If the two are the same, that would confirm the user signed the document.
In theory, each step is secure. But in the real world, a lot can go wrong.
Companies called certification authorities store digital signatures and act as a cross between a notary public and a phone book. As digital signatures become a more accepted method to sign online contracts, such companies could become critical parts of the electronic signature system. Yet, these third parties -- the two others being those who sign a contract -- can be a key weakness in the digital signature chain.
In a paper outlining their concerns with public-key encryption, cryptographer Bruce Schneier and Intel Corp. (intc) security specialist Carl Ellison likened security to a chain.
"It's only strong as the weakest link," they wrote. "The security of any (certification authority)-based system is based on many links and they are not all cryptographic. People are involved."
Those people -- consumers and sellers alike -- typically don't worry about security, and they have no safety net under the new law. Where the maximum anyone can be charged for a forged credit card transaction is $50, people will be responsible for any use of their e-signatures.
Digital signatures can also add more privacy problems to e-commerce systems. Typically, in order to confirm that a signature belongs to a person, a signature will not just include a digital fingerprint of the document to be signed, but also information about the user as well.
"The amount of information that people load into digital signatures is bad for privacy," said Zero-Knowledge's Shostack. Moreover, using a third party to store keys and signed documents can create problems as well; everyone with whom the user does business will be known to the third party.
Yet, others argue such releases of information actually add to the security of the system.
"Privacy is all well and good until I am trying to sue you because you sold me a lemon of a house," said Gary McGraw, vice president of corporate technology for security service provider Cigital Inc.
"People have to make decisions about how much information to give away."
A bigger problem, said McGraw, is that the technology is still not easy to use.
"There has been a lot of money pumped into cryptography, but the problem is that there has not been enough effort to hide the technology away from the user, he said."
With such usability and security problems unresolved, digital signatures may have a long road ahead to acceptance, even with the go-ahead from the U.S. government.