23-06-2012, 11:55 AM
Seminar on E-commerce Systems
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Cryptography: Algorithms and Keys
A method of encryption and decryption is called a cipher.
Generally there are two related functions: one for encryption and other for decryption.
Some cryptographic methods rely on the secrecy of the algorithms.
Such methods are mostly of historical interest these days.
All modern algorithms use a key to control encryption and decryption.
Encryption key may be different from decryption key.
Secrete Key Cryptography
Also called symmetric or single-key algorithms.
The encryption and the decryption key are the same.
Techniques based on a combination of substitution and permutation.
Stream ciphers: operate on single bit or byte.
Block ciphers: operate on blocks (typically 64 bits)
Advantage: simple, fast.
Disadvantage: key exchange, key management.
Examples: DES,RC4, IDEA, Blowfish, AES, etc.
Secrete Key Assurances
Confidentiality
is assurance that only owners of a shared secrete key can decrypt a message that has been encrypted with the shared secrete key
Authentication
is assurance of the identify of the person at the other end of the line (use challenge and response protocols)
Integrity
is assurance that a message has not been changed during transit and is also called message authentication (use message fingerprint)
Non-repudiation
is assurance that the sender cannot deny a file was sent. This cannot be done with secrete key alone (need trusted third party or public key technology)
Asymmetric Algorithms
Also called public-key algorithms.
Encryption key is different from decryption key.
Furthermore, one cannot be calculated from other.
Encryption key is often called the public key and decryption key is often called the private key.
Advantages: better key management.
Disadvantages: slower, more complex.
Both techniques are complementary.
Examples: RSA, Diffie-Hellman, El Gamal, etc.