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Hashes, Message Digests

How it works:. A makes a message digest from the plaintext message.A signs the message digest and sends it with the message to B.B independently recreates the message digest from the received plaintext message.B decrypts the message digest received with the message from A.A checks if the message

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Hashes, Message Digests

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    1. Hashes, Message Digests A condensed representation of a message. Encrypting whole message is very expensive. //for authenticity, integrity, and non-repudiation.// A signed message digest (or hash) of the message is much more efficient. <> Usually much shorter than the message. <> Much faster to encrypt and decrypt. <> Acts as a proxy for a lengthy message. <> Make public-key cryptography more popular. <> Like digital fingerprint or checksum of message.

    2. How it works: A makes a message digest from the plaintext message. A signs the message digest and sends it with the message to B. B independently recreates the message digest from the received plaintext message. B decrypts the message digest received with the message from A. A checks if the message digest in the recd. message is identical to the one he created.

    3. Security Properties and Assurances: Message integrity: if the message is changed, its message digest will not match. Authentication: the message digest is signed with the private key of the sender (A). Non-Repudiation: the message digest is signed with the private key of the sender. //The message can be encrypted for secrecy.//

    4. Hash Properties: One-way: It is extremely difficult to invert the hash function or message digest. //insures original message not recoverable.// Collision Freeness: A collision occurs when two or more distinct messages hash to the same value. <> Collisions are inevitable. <> But bad hash functions can produce lot more collisions. Collisions should be avoided. Why?

    5. Examples of Hash Functions Non-keyed hash functions: MD5 (128-bit digest) SHA-1 (Secure Hash Algorithm) (160-bit digest)

    6. Keyed Message Digests Depend upon a secret key for one-wayness and collision free. Such a digest is called Message Authentication Code (MAC). Two items: a file and a secret key. Using secret key, an encryption procedure, a node computes a keyed hash value. > It is much smaller than the message. > It can not be inverted (decrypted).

    7. Generation of MAC The file is divided into chunks of 64 bytes. First two chunks are combined to make a chunk of 64 bytes. DES using secret is applied to the new chunk. Output of DES operation is combined with the third 64 byte chunk. (to give a 64 byte chunk). DES is applied to this new 64 byte chunk. The output of DES is combined with the fourth 64 byte chunk. So on,...

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