A signature, by law, consists of three parts:
A formal affirmative act by a person,
Understanding of the intent or significance of the act (as specified in the document), and
Creation of evidence of the act (traditionally the mark, seal, or signature on the document).
To summarize: person, intent, and evidence.
Not all business transactions require a signature. Telephone orders have been honored for years. Banks, based only on brief "PINs," transfer funds. Knowledge of an identifier and password (Login) are often sufficient identification to allow computer keystrokes to create a contract, even though we may not have a signature. Even breaking a shrink-wrap is considered a sufficient personal act, intent, and evidence. However, many laws and regulations have explicit reference to "full handwritten", "in ink", "initials", and so forth.
The recent "Electronic Signature in Global and National Commerce Act" redefined a signature as an electronic sound, symbol, or process (evidence), associated with a record (intent), executed by a person with the intent to sign the record (person). Regulatory agencies, which have rules requiring signatures, are making similar changes (although not all regulators are bound by that new law, nor have they made changes to allow electronic signatures yet).
One of the speakers made an important distinction between electronic signatures and digital signatures. By his definitions (the distinction is important, but the terminology is not universal),
The electronic signature could be the writing of the name (like a traditional signature) or a thumbprint, or other measure. Sending that information (such as the image of the signature) doesn't protect it from being intercepted, and being reused for other purposes. That electronic signature is generally not sufficient.
If a cryptographic process is associated with the electronic signature (see the document on PKI encryption), and the signature is tied to the data being signed, then we have a digital signature.
Although we don't have a lot of experience with digital signatures, it appears that they are secure and will be widely accepted.
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