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Lecture 53

Digital signatures are mechanisms that provide authenticity and integrity for electronic documents, similar to physical signatures on checks. They must be unforgeable, authentic, tamperproof, non-reusable, and allow no repudiation, which can be achieved using public key cryptosystems. The lecture emphasizes that digital signatures are crucial for secure communication and will be followed by a discussion on certificates.

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7 views6 pages

Lecture 53

Digital signatures are mechanisms that provide authenticity and integrity for electronic documents, similar to physical signatures on checks. They must be unforgeable, authentic, tamperproof, non-reusable, and allow no repudiation, which can be achieved using public key cryptosystems. The lecture emphasizes that digital signatures are crucial for secure communication and will be followed by a discussion on certificates.

Uploaded by

rehab.s.m.alyousuf
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Foundations of Computer Security

Lecture 53: Digital Signatures

Dr. Bill Young


Department of Computer Sciences
University of Texas at Austin

Lecture 53: 1 Digital Signatures


Digital Signatures

Suppose you write a (physical) check. What would you like to be


true?
A check is a tangible object authorizing the transaction.
The signature on the check confirms authenticity.
In the case of an alleged forgery, a third party may be called
to judge authenticity.
The check is not alterable or alterations can be easily
detected.
The signature is part of the check, so cannot be easily
removed and re-used.
Can we define a mechanism for signing a document digitally that
has analogous characteristics?

Lecture 53: 2 Digital Signatures


Digital Signatures Properties

Suppose S sends a message M to R with signature f (S, M): We’d


like the signature to have certain properties:
unforgeable: it should be difficult for anyone but S to produce
f (S, M);
authentic: R can verify that S signed the document M;
no repudiation: S cannot deny producing the signature;
tamperproof: after being transmitted, M cannot be modified;
not reusable: the signature cannot be detached and reused for
another message.

Lecture 53: 3 Digital Signatures


Digital Signatures (Cont.)

Public key systems are well-suited for digital signatures. Recall


that some algorithms, RSA in particular, have the following
characteristic:

{{M}K }K −1 = M = {{M}K −1 }K .

So, if S wishes to send message M to R in a way that has some of


the characteristics of a digitally signed message, S could send

{{M}K −1 }KR .
S

Most often, it’s not the M but a hash of M that is signed. Why?

What assurance does R gain from this interchange?

Lecture 53: 4 Digital Signatures


Digital Signatures Properties

S sends to R the following message:

{{M}K −1 }KR .
S

This scheme has the desired properties:


unforgeable: only S can use KS−1 ;
authentic: a third party can verify the signature with KS ;
no repudiation: only S can use KS−1 ;
tamperproof: only R can remove the outer layer of encryption;
not reusable: the signature is tightly bound to the message M.

Lecture 53: 5 Digital Signatures


Lessons

Digital signatures function much as physical signatures.


Ideally a signature should be: unforgeable, authentic,
tamperproof, non-reusable, and allow no repudiation.
Public key cryptosystems facilitate creating digital signatures.

Next lecture: Certificates

Lecture 53: 6 Digital Signatures

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