Digital Signature Technology

Exchanging documents over the Internet is common in e-commerce. Such documents often contain sensitive information—for example, legal contracts, information concerning technological innovation, financial transactions. To prevent hackers from intercepting and reading e-commerce documents traveling through e-space, you must encrypt those documents. If you want your documents to be truly secure, however, you must sign them digitally. A digital signature on an e-commerce document serves as a guarantor of data origin, integrity, and nonrepudiation. When a customer digitally signs an online purchase order, for example, the merchandiser—through the document's digital signature—can identify the customer who originated the order, can verify that no one tampered with the contents of the order in transit, and has proof that a particular customer made a specific order.

Digital signatures have been with us since 1976, when Diffie and Hellman introduced the digital signature as an application of public key cryptography. Only recently, however, have businesses and governments started to use digital signature technology to protect sensitive documents on the World Wide Web. In September 1998, President Bill Clinton and Irish Prime Minister Bertie Ahern digitally signed an intergovernmental e-commerce document that is the world's first such document to use digital signature technology. Microsoft used digital signature technology to develop Authenticode technology, which secures Web-downloadable codes.

As the need for digital signature technology grows, several software companies, including Entrust Technologies and Network Associates, have delivered commercial security software that lets users employ digital signatures to secure e-commerce documents. In this article, I'll explain digital signature technology. I'll also discuss some currently available digital signature software products and offer guidelines to help you plan your company's digital signature solution.

What Is a Digital Signature?
Digital signature technology grew out of public key cryptography. In public key cryptography, you have two keys: a private key and a public key. When you send a document to someone, you use your private key to sign the document. When recipients receive the signed document, they use the sender's public key to authenticate the document.

Figure 1 illustrates the digital signature process. Suppose you want to send a digitally signed document to John. After you create the document, you pass it through a message hash algorithm. The algorithm generates a hash of the document that is a checksum of the contents of the document. You then encrypt the message hash with your private key. The result is a digital signature. You append this digital signature to the document to form a digitally signed document, then send it to John.

When John receives the document, he passes the document contents through the same message hash algorithm that you used, and creates a new hash. At the same time, John uses your public key to decrypt your digital signature, thereby converting the signature to the original hash. John then compares the newly generated hash and the original hash. If the hashes match, John can be sure that the document he received is really from you and that no one altered it during transmission. If the hashes don't match, John knows that tampering or a transmission error changed the document contents.

The most commonly used message hash algorithms are Message Digest 5 (MD5) and Secure Hash Algorithm 1 (SHA-1). MD5 can produce a 128-bit hash, and SHA-1 can produce a 160-bit hash. The hash algorithm is a one-way function that generates a one-way hash. Therefore, no one can derive original document contents from a message hash. The chance that two documents will have the same hash is almost zero. For example, the possibility that MD5 will output the same hash for two different documents is 1/2¹²⁸. (2¹²⁸ translates into about 1,500 documents for every square meter of the earth's surface.)

A digital signature is superior to a traditional handwritten signature. A skilled forger can alter the contents of a document with a handwritten signature or move a signature from one document to another without being detected. With digital signature technology, however, any change in a signed document—such as content modification or signature replacement—causes the digital signature verification process to fail.

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