Cryptography and Network Security Chapter 13 pot

Cryptography and
Cryptography and
Network Security
Network Security
Chapter 13
Chapter 13
Fourth Edition
Fourth Edition
by William Stallings
by William Stallings
Lecture slides by Lawrie Brown
Lecture slides by Lawrie Brown

Chapter 13 –
Chapter 13 –
Digital Signatures &
Digital Signatures &
Authentication Protocols
Authentication Protocols
To guard against the baneful influence exerted by strangers
To guard against the baneful influence exerted by strangers
is therefore an elementary dictate of savage prudence.
is therefore an elementary dictate of savage prudence.
Hence before strangers are allowed to enter a district, or
Hence before strangers are allowed to enter a district, or
at least before they are permitted to mingle freely with
at least before they are permitted to mingle freely with

the inhabitants, certain ceremonies are often performed
the inhabitants, certain ceremonies are often performed
by the natives of the country for the purpose of disarming
by the natives of the country for the purpose of disarming
the strangers of their magical powers, or of disinfecting,
the strangers of their magical powers, or of disinfecting,
so to speak, the tainted atmosphere by which they are
so to speak, the tainted atmosphere by which they are
supposed to be surrounded.
supposed to be surrounded.
—
—
The Golden Bough
The Golden Bough
, Sir James George Frazer
, Sir James George Frazer

Digital Signatures
Digital Signatures

have looked at
have looked at
message authentication
message authentication

but does not address issues of lack of trust
but does not address issues of lack of trust

digital signatures provide the ability to:
digital signatures provide the ability to:


verify author, date & time of signature
verify author, date & time of signature

authenticate message contents
authenticate message contents

be verified by third parties to resolve disputes
be verified by third parties to resolve disputes

hence include authentication function with
hence include authentication function with
additional capabilities
additional capabilities

Digital Signature Properties
Digital Signature Properties

must depend on the message signed
must depend on the message signed

must use information unique to sender
must use information unique to sender

to prevent both forgery and denial
to prevent both forgery and denial

must be relatively easy to produce
must be relatively easy to produce


must be relatively easy to recognize & verify
must be relatively easy to recognize & verify

be computationally infeasible to forge
be computationally infeasible to forge

with new message for existing digital signature
with new message for existing digital signature

with fraudulent digital signature for given message
with fraudulent digital signature for given message

be practical save digital signature in storage
be practical save digital signature in storage

Direct Digital Signatures
Direct Digital Signatures

involve only sender & receiver
involve only sender & receiver

assumed receiver has sender’s public-key
assumed receiver has sender’s public-key

digital signature made by sender signing
digital signature made by sender signing
entire message or hash with private-key
entire message or hash with private-key

can encrypt using receivers public-key

can encrypt using receivers public-key

important that sign first then encrypt
important that sign first then encrypt
message & signature
message & signature

security depends on sender’s private-key
security depends on sender’s private-key

Arbitrated Digital Signatures
Arbitrated Digital Signatures

involves use of arbiter A
involves use of arbiter A

validates any signed message
validates any signed message

then dated and sent to recipient
then dated and sent to recipient

requires suitable level of trust in arbiter
requires suitable level of trust in arbiter

can be implemented with either private or
can be implemented with either private or
public-key algorithms
public-key algorithms


arbiter may or may not see message
arbiter may or may not see message

Authentication Protocols
Authentication Protocols

used to convince parties of each others
used to convince parties of each others
identity and to exchange session keys
identity and to exchange session keys

may be one-way or mutual
may be one-way or mutual

key issues are
key issues are

confidentiality – to protect session keys
confidentiality – to protect session keys

timeliness – to prevent replay attacks
timeliness – to prevent replay attacks

published protocols are often found to
published protocols are often found to
have flaws and need to be modified
have flaws and need to be modified

Replay Attacks
Replay Attacks


where a valid signed message is copied and
where a valid signed message is copied and
later resent
later resent

simple replay
simple replay

repetition that can be logged
repetition that can be logged

repetition that cannot be detected
repetition that cannot be detected

backward replay without modification
backward replay without modification

countermeasures include
countermeasures include

use of sequence numbers (generally impractical)
use of sequence numbers (generally impractical)

timestamps (needs synchronized clocks)
timestamps (needs synchronized clocks)

challenge/response (using unique nonce)
challenge/response (using unique nonce)

Using Symmetric Encryption
Using Symmetric Encryption

as discussed previously can use a two-
as discussed previously can use a two-
level hierarchy of keys
level hierarchy of keys

usually with a trusted Key Distribution
usually with a trusted Key Distribution
Center (KDC)
Center (KDC)

each party shares own master key with KDC
each party shares own master key with KDC

KDC generates session keys used for
KDC generates session keys used for
connections between parties
connections between parties

master keys used to distribute these to them
master keys used to distribute these to them

Needham-Schroeder Protocol
Needham-Schroeder Protocol

original third-party key distribution protocol
original third-party key distribution protocol


for session between A B mediated by KDC
for session between A B mediated by KDC

protocol overview is:
protocol overview is:
1.
1.
A->KDC:
A->KDC:
ID
ID
A
A


||
||
ID
ID
B
B


||
||
N
N
1
1
2

2
. KDC ->
. KDC ->


A: E
A: E
Ka
Ka
[Ks
[Ks


||
||
ID
ID
B
B


||
||
N
N
1
1
|| E
|| E
Kb

Kb
[
[
Ks
Ks
||
||
ID
ID
A
A
] ]
] ]
3.
3.
A ->
A ->


B:
B:
E
E
Kb
Kb
[
[
Ks
Ks
||

||
ID
ID
A
A
]
]
4.
4.
B ->
B ->


A:
A:
E
E
Ks
Ks
[
[
N
N
2
2
]
]
5.
5.
A ->

A ->


B:
B:
E
E
Ks
Ks
[f(
[f(
N
N
2
2
)]
)]

Needham-Schroeder Protocol
Needham-Schroeder Protocol

used to securely distribute a new session
used to securely distribute a new session
key for communications between A & B
key for communications between A & B

but is vulnerable to a replay attack if an old
but is vulnerable to a replay attack if an old
session key has been compromised
session key has been compromised


then message 3 can be resent convincing B
then message 3 can be resent convincing B
that is communicating with A
that is communicating with A

modifications to address this require:
modifications to address this require:

timestamps (Denning 81)
timestamps (Denning 81)

using an extra nonce (Neuman 93)
using an extra nonce (Neuman 93)

Using Public-Key Encryption
Using Public-Key Encryption

have a range of approaches based on the
have a range of approaches based on the
use of public-key encryption
use of public-key encryption

need to ensure have correct public keys
need to ensure have correct public keys
for other parties
for other parties

using a central Authentication Server (AS)
using a central Authentication Server (AS)


various protocols exist using timestamps
various protocols exist using timestamps
or nonces
or nonces

Denning AS Protocol
Denning AS Protocol

Denning 81 presented the following:
Denning 81 presented the following:
1.
1.
A ->
A ->


AS:
AS:
ID
ID
A
A


||
||
ID
ID
B

B
2.
2.
AS ->
AS ->


A: E
A: E
PRas
PRas
[
[
ID
ID
A
A
||PU
||PU
a
a
||T] || E
||T] || E
PRas
PRas
[
[
ID
ID
B

B
||PU
||PU
b
b
||T]
||T]
3.
3.
A ->
A ->


B: E
B: E
PRas
PRas
[
[
ID
ID
A
A
||PU
||PU
a
a
||T] || E
||T] || E
PRas

PRas
[
[
ID
ID
B
B
||PU
||PU
b
b
||T] || E
||T] || E
PUb
PUb
[E
[E
PRas
PRas
[K
[K
s
s
||
||
T]]
T]]

note session key is chosen by A, hence AS need
note session key is chosen by A, hence AS need

not be trusted to protect it
not be trusted to protect it

timestamps prevent replay but require
timestamps prevent replay but require
synchronized clocks
synchronized clocks

One-Way Authentication
One-Way Authentication

required when sender & receiver are not in
required when sender & receiver are not in
communications at same time (eg. email)
communications at same time (eg. email)

have header in clear so can be delivered
have header in clear so can be delivered
by email system
by email system

may want contents of body protected &
may want contents of body protected &
sender authenticated
sender authenticated

Using Symmetric Encryption
Using Symmetric Encryption

can refine use of KDC but can’t have final

can refine use of KDC but can’t have final
exchange of nonces, vis:
exchange of nonces, vis:
1.
1.
A
A
->
->
KDC:
KDC:
ID
ID
A
A


||
||
ID
ID
B
B


||
||
N
N
1

1
2
2
. KDC
. KDC
->
->
A: E
A: E
Ka
Ka
[Ks
[Ks


||
||
ID
ID
B
B


||
||
N
N
1
1
|| E

|| E
Kb
Kb
[
[
Ks
Ks
||
||
ID
ID
A
A
] ]
] ]
3.
3.
A
A
->
->
B:
B:
E
E
Kb
Kb
[
[
Ks

Ks
||
||
ID
ID
A
A
] || E
] || E
Ks
Ks
[M]
[M]



does not protect against replays
does not protect against replays

could rely on timestamp in message, though
could rely on timestamp in message, though
email delays make this problematic
email delays make this problematic

Public-Key Approaches
Public-Key Approaches

have seen some public-key approaches
have seen some public-key approaches


if confidentiality is major concern, can use:
if confidentiality is major concern, can use:
A
A
->
->
B: E
B: E
PUb
PUb
[Ks] || E
[Ks] || E
Ks
Ks
[M]
[M]

has encrypted session key, encrypted message
has encrypted session key, encrypted message

if authentication needed use a digital
if authentication needed use a digital
signature with a digital certificate:
signature with a digital certificate:
A
A
->
->
B: M || E
B: M || E

PRa
PRa
[H(M)] || E
[H(M)] || E
PRas
PRas
[T||ID
[T||ID
A
A
||PU
||PU
a
a
]
]

with message, signature, certificate
with message, signature, certificate

Digital Signature
Digital Signature
Standard
Standard
(DSS)
(DSS)

US Govt approved signature scheme
US Govt approved signature scheme


designed by NIST & NSA in early 90's
designed by NIST & NSA in early 90's

published as FIPS-186 in 1991
published as FIPS-186 in 1991

revised in 1993, 1996 & then 2000
revised in 1993, 1996 & then 2000

uses the SHA hash algorithm
uses the SHA hash algorithm

DSS is the standard, DSA is the algorithm
DSS is the standard, DSA is the algorithm

FIPS 186-2 (2000) includes alternative RSA &
FIPS 186-2 (2000) includes alternative RSA &
elliptic curve signature variants
elliptic curve signature variants

Digital Signature
Digital Signature
Algorithm
Algorithm
(DSA)
(DSA)

creates a 320 bit signature
creates a 320 bit signature


with 512-1024 bit security
with 512-1024 bit security

smaller and faster than RSA
smaller and faster than RSA

a digital signature scheme only
a digital signature scheme only

security depends on difficulty of computing
security depends on difficulty of computing
discrete logarithms
discrete logarithms

variant of ElGamal & Schnorr schemes
variant of ElGamal & Schnorr schemes

Digital Signature
Digital Signature
Algorithm
Algorithm
(DSA)
(DSA)

DSA Key Generation
DSA Key Generation

have shared global public key values (p,q,g):
have shared global public key values (p,q,g):


choose q, a 160 bit
choose q, a 160 bit

choose a large prime
choose a large prime
p = 2
p = 2
L
L


•
where L= 512 to 1024 bits and is a multiple of 64
where L= 512 to 1024 bits and is a multiple of 64
•
and q is a prime factor of
and q is a prime factor of
(p-1)
(p-1)

choose
choose
g = h
g = h
(p-1)/q
(p-1)/q


•
where

where
h<p-1, h
h<p-1, h
(p-1)/q
(p-1)/q
(mod p) > 1
(mod p) > 1



users choose private & compute public key:
users choose private & compute public key:

choose
choose
x<q
x<q



compute
compute
y = g
y = g
x
x
(mod p)
(mod p)

DSA Signature Creation
DSA Signature Creation

to
to
sign
sign
a message
a message
M
M
the sender:
the sender:

generates a random signature key
generates a random signature key
k, k<q
k, k<q



nb.
nb.
k
k
must be random, be destroyed after
must be random, be destroyed after
use, and never be reused
use, and never be reused


then computes signature pair:
then computes signature pair:
r = (g
r = (g
k
k
(mod p))(mod q)
(mod p))(mod q)
s = (k
s = (k
-1
-1
.H(M)+ x.r)(mod q)
.H(M)+ x.r)(mod q)



sends signature
sends signature
(r,s)
(r,s)
with message
with message
M
M

DSA Signature Verification
DSA Signature Verification


having received M &
having received M &
signature
signature
(r,s)
(r,s)



to
to
verify
verify
a signature, recipient computes:
a signature, recipient computes:
w = s
w = s
-1
-1
(mod q)
(mod q)
u1= (H(M).w)(mod q)
u1= (H(M).w)(mod q)
u2= (r.w)(mod q)
u2= (r.w)(mod q)
v = (g
v = (g
u1
u1
.y

.y
u2
u2
(mod p)) (mod q)
(mod p)) (mod q)

if
if
v=r
v=r
then signature is verified
then signature is verified

see book web site for details of proof why
see book web site for details of proof why

Summary
Summary

have discussed:
have discussed:

digital signatures
digital signatures

authentication protocols (mutual & one-way)
authentication protocols (mutual & one-way)

digital signature algorithm and standard
digital signature algorithm and standard