Message
Message
Message
K
E
K
(a) Using symmetric encryption
Compare
D
H
H
H
H
H
Message
Message
Message
PR
a
E
PU
a
(b) Using public-key encryption
Compare
D
Message
Message
Message
(c) Using secret value
Compare
K
K
K
K
Source A
Destination B
Figure 2.5
Message Authentication Using a One-Way Hash Function.
H

Hash Function
Requirements

Security of Hash
Functions

Public-Key Encryption
Structure

Plaintext
Readable message or data that is fed into the algorithm as input
Encryption algorithm
Performs transformations on the plaintext
Public and private key
Pair of keys, one for encryption, one for decryption
Ciphertext
Scrambled message produced as output
Decryption key
Produces the original plaintext

User encrypts data using his or
her own private key
Anyone who knows the
corresponding public key will be
able to decrypt the message
Mike
Bob
Plaintext
input
Transmitted
ciphertext
Plaintext
output
Encryption algorithm
(e.g., RSA)
Decryption algorithm
Bob's private
key
Bob's public
key
Alice's
public key
ring
Joy
Ted
(b) Encryption with private key
X
PU
b
PR
b
Y
= E[
PR
b
,
X
]
X
=
D[
PU
b
,
Y
]
Figure2.6
Public-Key Cryptography
Bob
Alice

Algorithm
Digital Signature
Symmetric Key
Distribution
Encryption of
Secret Keys
RSA
Yes
Yes
Yes
Diffie-Hellman
No
Yes
No
DSS
Yes
No
No
Elliptic Curve
Yes
Yes
Yes
Table 2.3
Applications for Public-Key
Cryptosystems

Requirements for Public-Key
Cryptosystems

Asymmetric Encryption
Algorithms

Digital Signatures
Used for authenticating both source and data
integrity
Created by encrypting hash code with private key
Does not provide confidentiality
Even in the case of complete encryption
Message is safe from alteration but not
eavesdropping

Unsigned certificate:
contains user ID,
user's public key,
as well as information
concerning the CA
Signed certificate
Recipient can verify
signature by comparing
hash code values
Figure 2.7
Public-Key Certificate Use
Generate hash
code of unsigned
certificate
Encrypt hash code
with CA's private key
to form signature
H
H
Bob's ID
information
CA
information
Bob's public key
E
D
Decrypt signature
with CA's public key
to recover hash code
Use certificate to
verify Bob's public key
Create signed
digital certificate

Digital
Envelop
es
Protects a message
without needing to
first arrange for
sender and receiver
to have the same

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- Cryptography