dual-use digital signature vulnerability

[Amir Herzberg]

Anne & Lynn Wheeler wrote:

> 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):
... skipped (it was long :-)
> 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)
I don't see here any problem or attack. Indeed, there is difference 
between signature in the crypto sense and legally-binding signatures. 
The later are defined in one of two ways. One is by the `digital 
signature` laws in different  countries/states; that approach if often 
problematic, since it is quite tricky to define in a general law a 
binding between a person or organization and a digital signature. The 
other way however is fine, imho: define the digital signature in a 
(`regular`) contract between the parties. The contract defines what the 
parties agree to be considered as equivalent to their (physical) 
signature, with well defined interpretation and restrictions.

-- 
Best regards,

Amir Herzberg
Associate Professor, Computer Science Dept., Bar Ilan University
http://amirherzberg.com (information and lectures in cryptography & 
security)
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