2. CRYPTOGRAPHY
• A word with Greek origins, means “ secret writing ”.
• The term to refer to the science and art of transforming
messages to make them secure and immune to attacks .
• Applications of cryptography includes ATM cards, computer
passwords, and electronic commerce .
3. CRYPTOGRAPHY ISSUES
•
•
•
•
•
Confidentiality: Only sender, intended receiver should “understand”
message contents.
End-Point Authentication : Sender and receiver want to confirm
identity of each other.
Message Integrity : Sender and receiver want to ensure message not
altered (in transit, or afterwards) without detection.
Message Nonrepudiation : Message nonrepudiation means that a
sender must not be able to deny sending a message that he or she,
in fact, did send.
Entity Authentication : In entity authentication (or user identification)
the entity or user is verified prior to access to the system resources
4. PLAINTEXT AND CIPHERTEXT
• The original message , before being transformed, is called
plaintext .
• After the message is transformed , it is called ciphertext .
• An encryption algorithm transforms the plaintext into
ciphertext; a decryption algorithm transforms the ciphertext
back into plaintext.
• Example:
• Plaintext: HELLO
• Ciphertext: KHOOR
5. CIPHER
• Encryption and Decryption algorithms are referred as ciphers.
• Also used to refer to different categories of algorithms in
cryptography.
• Example ( Traditional Substitution Ciphers ):
• Monoalphabetic cipher
• Polyalphabetic cipher
• Plaintext: HELLO
• Ciphertext: ABNZF
6. KEY
• A key is a number (or a set of numbers) that the cipher
operates on , as an algorithm.
• To encrypt a message, we need an encryption algorithm, an
encryption key, and the plaintext.
• To decrypt a message, we need a decryption algorithm, a
decryption key, and the ciphertext. These reveal the original
plaintext.
• Types:
• Shared key, Public key and Private key .
7. CRYPTOGRAPHY CATEGORIES
• We can divide all the cryptography algorithms (ciphers) into two
groups :
• Symmetric key (also called secret-key) cryptography
algorithms and
• Asymmetric key (also called public-key) cryptography
algorithms .
8. SYMMETRIC KEY
CRYPTOGRAPHY
• The same key encrypts and
decrypts the plaintext.
• The shared key is kept secret
between Alice and Bob.
• Examples of Symmetric
algorithms:
• DES, 3DES, AES, IDEA,
BLOWFISH, TWOFISH, RC4, RC5,
SAFER etc.
ASYMMETRIC KEY
CRYPTOGRAPHY
• Only 1 shared key is involved. • Here 2 keys : a private and a
public key are involved.
• The Public key encrypts the
plaintext while the private
key decrypts it.
• The private key is just kept
secret by the Bob while the
public key is made public .
• Examples of Asymmetric
algorithms:
• Diffie-Hellman, RSA, El Gamal,
9. MODERN ROUND CIPHERS
•
•
•
•
The ciphers of today are called round ciphers because they involve
multiple rounds , where each round is a complex cipher made up of
the simple ciphers.
The key used in each round is a subset or variation of the general key
called the round key .
If the cipher has N rounds, a key generator produces N keys, K1,
K2,...., KN, where K1 is used in round 1, K2 in round 2, and so on.
Modem symmetric-key ciphers: DES and AES are referred to as block
ciphers because they divide the plaintext into blocks and use the
same key to encrypt and decrypt the blocks.
10. DES - DATA ENCRYPTION STANDARD
• The algorithm encrypts a 64-bit plaintex t block using a 64-bit
key .
• DES has two transposition blocks (P-boxes) and 16 complex
round ciphers (they are repeated).
• Although the 16 iteration round ciphers are conceptually the
same, each uses a different key derived from the original key.
• The initial and final permutations are keyless straight
permutations that are the inverse of each other. The
permutation takes a 64-bit input and permutes them according
11. RSA ALGORITHM
• It uses two numbers, e and d, having a special relationship to
each other, as the public and private keys.
• Selecting Keys
Bob use the following steps to select the private and public keys:
1. Bob chooses two very large prime numbers p and q .
2. Bob multiplies the above two primes to find n, the modulus for
encryption and decryption. In other words, n = p X q .
3. Bob calculates another number φ = (p -1) X (q - 1) .
4. Bob chooses a random integer e . He then calculates d so that d x e = 1
mod φ .
5. Bob announces e and n to the public ; he keeps φ and d secret .
12. RSA ALGORITHM
• Encryption
•
•
•
•
Anyone who needs to send a message to Bob can use n and e .
For example, if Alice needs to send a message to Bob, she can change
the message , usually a short one, to an integer . This is the plaintext.
She then calculates the ciphertext , using e and n, as c = p e (mod n) .
Alice sends C, the ciphertext, to Bob.
• Decryption
• When Bob receives the ciphertext, he uses his private key d to decrypt
the message:
p = Data c d Communications (mod n)
13. CONCLUSION
• By using of encryption techniques a fair unit of confidentiality,
authentication, integrity, access control and availability of data
is maintained.
• Using cryptography Electronic Mail Security, Mail Security, IP
Security, Web security can be achieved.
