algorithm Brute-force attack • Strength of hash function depends solely on the length of the hash code produced by the algorithm SHA most widely used hash algorithm Developed by NIST in 1993 and revised in 1995 as SHA-1 that produces hash value of 160 bits In 2002 NIST defined 3 new version of SHA with 256, 384 and 512 bits of hash values. SHA-3 in 2015 Additional secure hash function applications: Passwords • Hash of a password is stored by an operating system Intrusion detection • Store H(F) for each file on a system and secure the hash values
Public-Key Encryption Structure Publicly proposed by Diffie and Hellman in 1976 Based on mathematical functions rather than operations on bits Asymmetric • Uses two separate keys • Public key and private key • Public key is made public for others to use
Misconceptions 1. Is public-key encryption is more secure from cryptanalysis than symmetric encryption? The security of any encryption scheme depends on (1) the length of the key and (2) the computational work involved in breaking a cipher. Nothing in principle about either symmetric or public-key encryption that makes one superior to another on cryptanalysis. 2. Is public-key encryption general- purpose technique made symmetric encryption obsolete? The computational overhead of current public-key encryption schemes, there seems no foreseeable likelihood that symmetric encryption will be abandoned. 3. Finally, there is a feeling that key distribution is trivial when using public-key encryption, compared to the rather cumbersome handshaking involved with key distribution centers for symmetric encryption. For public-key key distribution, some form of protocol is needed, often involving a central agent, and the procedures involved are no simpler or any more efficient than those required for symmetric encryption.
Public Key Encryption The public key of the pair is made public for others to use, while the private key is known only to its owner. So one key for encryption and a different but related key for decryption. 1. Each user generates a pair of keys to be used for the encryption and decryption of messages. 2. Each user places one of the two keys in a public register or other accessible file. This is the public key. The companion key is kept private. As Figure 2.6a suggests, each user maintains a collection of public keys obtained from others. 3. If Bob wishes to send a private message to Alice, Bob encrypts the message using Alice’s public key. 4. When Alice receives the message, she decrypts it using her private key. No other recipient can decrypt the message because only Alice knows Alice’s private key. All participants have access to public keys, and private keys are generated locally by each participant and never need to be distributed. As long as a user protects his or her private key, incoming communication is secure. At any time, a user can change the private key and publish the companion public key to replace the old public key.
Plaintext Readable message or data that is fed into the algorithm as input Encryption algorithm Performs transformations on the plaintext
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- Summer '19