ok, this is a long posting about what i might be able to reasonable assume
if a digital signature verifies (posting to c.p.k newsgroup):

basically the relying-party has certified the environment that houses the private key and the environment that the digital signature was done in ... then the verification of the digital signature might be assumed to imply one-factor or possibly two-factor authentication (i.e. if the relying-party has certified that a private key is housed in a secure hardware token and can never leave that hardware token, then the verification of the digital signature might imply one-factor, "something you have" authentication).

that establishes the basis for using digital signature for authentication purposes ... being able to assume that verification of the digital signature possibly implies "something you have" authentication (or something similar).

just the verification of the digital signature, however doesn't do anything to establish any implication about a legal signature where the "signer" is assumed to have read and agrees to the contents of the thing being signed (intention to sign the content of the document as agreement, approval, and/or authorization).

lets assume for argument sake that some sort of environment can be certified that provides a relying party some reasonable assurance that the signer has, in fact, read and is indicating agreement, approval, and/or authorization ... then there might possible be the issue of the dual-use vulnerability.

the dual-use comes up when the person is 'signing" random challenges as purely a means of authentication w/o any requirement to read the contents. Given such an environment, an attack might be sending some valid text in lieu of random data for signature. Then the signer may have a repudiation defense that he hadn't signed the document (as in the legal sense of signing), but it must have been a dual-use attack on his signature (he had signed it believing it to be random data as part of an authentication protocol).

Anne & Lynn Wheeler http://www.garlic.com/~lynn/

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