This document summarizes symmetric and asymmetric cryptography. Symmetric cryptography involves both parties agreeing on an encryption algorithm and key beforehand to encrypt and decrypt messages. Asymmetric cryptography uses public and private key pairs, where the public key encrypts messages and the private key decrypts them, allowing encryption without pre-sharing keys. It also discusses digital signatures, where messages are signed with a private key and verified with the corresponding public key. Common attacks on public-key cryptography like man-in-the-middle attacks are addressed. The document is intended for a computer science course on cryptography fundamentals and security mechanisms.

1. Symmetric versus Asymmetric
Cryptography
Prof. Neeraj Bhargava
Kapil Chauhan
Department of Computer Science
School of Engineering & Systems Sciences
MDS University, Ajmer

2. Presenting cryptography
Top concern in security mechanism
Basic knowledge in computer security
 Integrity
 Availability

3. Two kinds of Cryptography
Symmetric
1) Ram and Manoj agree on a
cryptosystem
2) Ram and Manoj agree on a
key
3) Ram takes her plaintext
message and encrypts it using
the encryption algorithm and
the key. This creates a cipher
text message
4) Ram sends the cipher text
message to Manoj.
5) Manoj decrypts the cipher
text message with the same
algorithm and key and reads
it
Asymmetric
1) Ram and Manoj agree on a
public-key cryptosystem
2) Manoj sends Alice his public
key
3) Ram encrypts her message
using Manoj’s public key and
sends it to Bob
4) Ram decrypts Ram’s message
using his private key
5) Ram decrypts the cipher text
message with the different
algorithm and key.

4. Public-key algorithms
Public key are not a substitute for symmetric algorithms.
Public key are not used to encrypt messages, they are used to encrypt
keys .

5. Digital Signatures with Encryption
1) Ram signs the message with her private key
SR(M)
2) Ram encrypts the signed message with Manoj’s public
key and sends it to Manoj
EM(SR(M))
3) Manoj decrypts the message with his private key
DM(EM(SR(M))) = SR(M)
4) Manoj verifies with Ram’s public key and recovers the
message
VR(SR(M)) = M

6. Attacks against
Public-key Cryptography
How Ram gets Manoj’s public key?
 from secure database
How to protect the public key?
 database is read-only to everyone, only writable to Trent
 Trent can sign each public key by his own private key (Key
Certification Authority or Key Distribution Center)

7. Security Mechanisms
Specific use of certain algorithms, protocols, and
procedures to provide one or more security services
Examples:
Authentication – use password, fingerprint.
Access Control – specify access rights based on the user id,
role/group to specific transactions .
Data Confidentiality – encrypt information using a specific
algorithm
Data Integrity – detect and prevent unauthorized change to content
Non-Repudiation – use electronic signature to ensure authenticity
Availability – increase filter malicious traffic.

8. Assignment Question
Q 1. Explain following terms:
A) Private key
B) Public key
Q 2. Explain cryptography in information security and
discuss the types of attacks an encrypted message.
Q3. Explain the digital signature with hash function and
describe the model of digital signature.