---
title: 'Lecture 07 Digital Signatures'
disqus: hackmd
---

:::info
ST2504 Applied Cryptography
:::

Lecture 07 Digital Signatures
===

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## Table of Contents

[TOC]

Digital Signature
---
- msg auth
- through hash, MAC or encryption
- doesn't address issues of lack of trust
- handwritten signature
- useless on internet
- bind signatory to msg
- signature provides
- auth of msg content
- 3rd parties not just receiver can verify
- author, date & time of msg
- non-repudiation

### Requirements
- must be bit pattern that depend on msg signed
- must use info unique to sender
- prevent forgery & denial
- easy to produce
- easy to recognise & verify
- shld be computationally infeasible to forge
- create new msg with existing digital signature
- create fraudulent digital sign for given msg
- shld be practical to save digital sign in storage

### Application
- sign docs, files, emails
- Eg. PDF, Word PGP email
- sign server's cert
- provide as online signing service
- Eg. DocuSign

### How it Works
- digital sign - auth mechanism that enable creator of msg to attach code that acts as signature
- formed by taking hash of msg & encrypting msg with creator's private key
- guarantees src & integrity of msg


Types of Digital Signatures
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### Direct Digital Signatures
- involve only sender & receiver
- assumed
- sender & receiver have own key pairs - private & public
- receiver has sender's pub key
- sender has receiver's pub key
- sign 1st then encrypt msg & signature
- digital sign made by sender's private key either
- sign/encrypt entire msg
- sign/encrypt hash only
- for efficiency of scheme
- encrypt using receiver's pub key
- validity of signature depends on sender's private key
- sender may deny sending msg
- private key stolen
- timestamp on msg not useful
- msg might be back dated


### Arbitrated Digital Signatures
- implementing arbitrated digital signature invites 3rd pt into process called __trusted arbiter__
- use of arbiter (trusted 3rd pt)
- arbiter receive signed msgs from sender
- validate content & origin from subj, msg & signature
- arbiter dated msg & indicated that msg verified
- sent to recipients
- arbiter may/may not see msg
- suitable lvl of trust in arbiter needed
- implemented with priv & pub key encryption
- need
- complete trust from both sender & receiver that arbiter wont only time-stamp & forward doc as instructed but also not alter data
- prevent sender from disowning msg

Digital Signature Schemes
---
### Mechanism of Signature Schemes
- 3 algos
- key gen algo
- randomly select key pair
- signature algo
- msg + priv key = signature
- signature verification algo
- pub key + signature = true/false (validated/not validated)

### Digital Signature Standard (DSS)
- US gov pproved signature scheme
- designed by NIST & NSA in early 90s
- published as FIPS-186 in 1991
- FIPS 186-2 (2000) inclludes alt RSA & elliptic curve signature variants
- revised in 1993, 1996 & 2000
- uses SHA hash algo
- DSS is standard, DSA is algo

### Digital Signature Algorithm (DSA)
- based on asymmetric key algo
- digital signature scheme
- 320bit signature (length of sign)
- 512-1024 bit security key
- smaller & faster than RSA
- security > discrete logarithm (computationally hard prob)


#### Key Generation
- by signing person



#### Signing

#### Verification


Summary
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###### tags: `ACG` `DISM` `School` `Notes`