2. 2
Encryption
• Encryption is said to occur when data is passed through a series of mathematical
operations that generate an alternate form of that data; the sequence of these
operations is called an algorithm.
• To hide the meaning
Encryption Algorithms
Cipher
Text
Key
Plain
Text
3. 3
Encryption
• The two forms of data:
plaintext- unencrypted data and the as
Ciphertext- encrypted data
• The security of encryption lies in the ability of an algorithm to generate
ciphertext that is not easily reverted to the original plaintext.
• Two fundamental approaches are in use:
conventional encryption(symmetric encryption) and
public-key encryption (asymmetric encryption).
4. Conventional encryption
• Symmetric encryption :
single secret key is used for encryption and decryption
the only type of encryption used prior to the development of a public key encryption.
The most commonly used symmetric encryption algorithms are block ciphers.
Block encryption algorithms like DES, triple DES and AES are examples for
symmetric key encryption algorithms.
A block cipher process the plain text input in fixed-sized blocks and produces a
block of cipher text of equal size for each plaintext block.
5. Symmetric encryption principles:
A symmetric key encryption scheme has five ingredients
• Plaintext: This is the original message or data that is fed into the
algorithm as input.
• Encryption algorithm: The encryption algorithm performs various
substitutions and transformations on the plain text.
• Secret Key: The secret key is also input to the algorithm. The exact
number of substitutions and transformations performed by the
algorithm depend on the key.
• Cipher text: This is the scrambled message produced as output. It
depends on the plain text and secret key. For any given message two
different keys will produce two different cipher texts.
6. Symmetric encryption principles:
• Decryption algorithm: This is essentially the encryption algorithm run
in reverse. It takes cipher text and secret key as input and produces
original plain text.
Simplified model of symmetric encryption.
7. Continued..
There are two requirements for secure use of symmetric encryption:
1. The encryption algorithm must be strong enough.
2. Sender and receiver must have obtained copies of the secret key in secure fashion and must
keep the key secure.
Note: Security of symmetric encryption depends on the secrecy of the key, not the secrecy of
the algorithm.
8. Cryptography
o Cryptographic systems are generally classified based on the three independent dimensions:
1. The type of operation used for transforming plaintext to cipher text: All the
encryption algorithms are based on two general principles,
o Substitution: Here each element in the plain text is mapped into another element.
o Transposition: Here the elements in the plain text are rearranged.
o Most product systems involve multiple stages of substitutions and transpositions.
2. The number of keys used: Here user may use symmetric or asymmetric keys.
o Symmetric (single key): Both sender and receiver use the same key. E.g. DES,
AES
o Asymmetric (two-keys, or public-key encryption): Sender and receiver use a
different key. E.g. RSA(Rivest, Shamir, Adelson) and Diffie and Hellmann
3. The way in which plain text is processed: It can be in terms of block or stream.
o Block cipher: Encrypts/decrypts a block at a time.
o Stream cipher: Encrypts/decrypts one element a time or process the input elements
continuously.
9. Cryptanalysis
o The process of attempting to discover the plain text or key is known as cryptanalysis. The
strategy used by the cryptanalyst depends on the
o Nature of the encryption scheme and
o The information available to the cryptanalyst.
10. Data Encryption Standard (DES):
o The most widely used encryption scheme
o The algorithm is referred to the Data Encryption Algorithm (DEA)
o DES is a block cipher
o The plaintext is processed in 64-bit blocks
o The key is 56-bits in length
o The DES structure is a minor variation of the Feistal structure.
o There are sixteen rounds of processing. From original 56-bit key sixteen sub keys are
generated, one of which is used for each round.
