Cryptography Reference
In-Depth Information
the computer functioned correctly. This 'human-computer gap' may allow
a signer to claim that he did not create, or was not aware of creating, a digital
signature.
PRACTICAL DIFFERENCES:
Cost
. Handwritten signatures are cheap to deploy. Digital signature schemes
require a supporting infrastructure that provides technology, keymanagement,
etc. This makes digital signature schemes more expensive to implement.
Longevity
. Handwritten signatures can be created for the lifetime of the signer,
and are often valid beyond the signer's lifetime. Restrictions on the longevity
of an agreement signed using a handwritten signature can be put in place by
stating such limitations in the underlying data. Digital signatures can be created
and verified until the time that the relevant key pair expires (see Section 10.2.1)
or the verification key is revoked (see Section 11.2.3). Digital signatures may
also be regarded as being valid beyond this time, depending on the application.
To extend the lifetime of a digital signature, digital data can be resigned to
create a new digital signature on the data, or be archived by a trusted third
party (see Section 12.6.5).
Acceptability
. Handwritten signatures are well accepted and their limitations
understood. Despite their failure to provide strong security services, they
are deployed in applications throughout the world for reasons of cost and
availability. Digital signatures have not earned this level of use and their
inherent complexity provides several possibilities for challenging the security
services that they provide.
Legal recognition
. Handwritten signatures are well respected and recognised in
law throughout the world. Digital signatures are increasingly being recognised
and appropriate laws being developed, but many jurisdictions do not yet
recognise them, while others only recognise them for specific purposes.
FLEXIBILITY DIFFERENCES:
Binding to the underlying data
. Handwritten signatures are, to an extent,
physically bound to the object that they relate to, typically a piece of paper.
Note however that this binding can be 'loose', for example, when a handwritten
signature is only applied to the cover or last page of a document. In contrast,
digital signatures can be sent and stored separately from the underlying
data.
Support for multiple signatures
. Some people have different handwritten
signatures that they reserve for different purposes. However, any one individual
is only likely to be able to reliably generate a small number of different
handwritten signatures. On the other hand, there is no limit to the number
of signature keys that any one individual can hold (they could have different
signature keys for different applications) and hence no limit to the number
