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16.2 Digital Signatures

16.2 Digital Signatures. By: John Barton. What is a Digital Signature?. The idea comes from the idea of signing a document by its author (authenticating it). When you send a document electronically you can also sign it and that is called a Digital Signature.

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16.2 Digital Signatures

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  1. 16.2 Digital Signatures By: John Barton

  2. What is a Digital Signature? • The idea comes from the idea of signing a document by its author (authenticating it). • When you send a document electronically you can also sign it and that is called a Digital Signature. • A Digital Signature provides security to the author of the document because the signature represents his/her integrity, authentication, and the document becomes nonrepudiation (the author cannot denying his/her signature later).

  3. The Two Forms of Digital Signatures Are… • Signing the Whole Document • Signing the Digest

  4. Bob (Bob's public key) (Bob's private key) Signing the Whole Document • When Bob wishes to send a document he first encrypts the message with his private key • Then Bob sends the ciphered text to the recipient who uses the public key to decrypt it. • After decryption the document can be read as Bob first sent it.

  5. "Hey Bob, how about lunch at Taco Bell. I hear they have free refills!" HNFmsEm6Un BejhhyCGKOK JUxhiygSBCEiC 0QYIh/Hn3xgiK BcyLK1UcYiY lxx2lCFHDC/A HNFmsEm6Un BejhhyCGKOK JUxhiygSBCEiC 0QYIh/Hn3xgiK BcyLK1UcYiY lxx2lCFHDC/A "Hey Bob, how about lunch at Taco Bell. I hear they have free refills!" Keys • Bob has two parts to signing the whole document, a public key and a private key. • He keeps the private key for himself • He can then give out the public key… well as you might have guessed the public… so smart these computer guys. • In this example Susan is using her public key to send Bob a message.

  6. Bob's Co-workers: Anyone can get Bob's Public Key, but Bob keeps his Private Key to himself Pat Doug Susan Problems with Signing the Whole Document • The process is very inefficient. • You cannot use any secret keys. • This method does not provide any secrecy, because anyone can use the public key to read a message.

  7. Signing the Digest • The sender of a digest makes a miniature form of the document and signs it. • To create the digest hash functions are employed. • Hash Functions take any sized documents and fix their lengths at either a 128-bit digest or a 160-bit digest.

  8. Properties of Hash Functions to Guarantee Success • Hashing should be one way, the digest can only be created form the message not vice versa. • Hashing should only be one-to-one.

  9. Mission Control… Message Sent • Check out the Diagrams on pg 313 and 314 to follow along better • Bob sends a message. • The message gets hashed, it becomes a digest. • The digest becomes encrypted using Bob’s private key…the encrypted message is now considered a signed digest. • The total package is known as a the Message Plus Signed Digest.

  10. Roger That…Message Received • The Message Plus Signed Digest is received by the recipient's computer and split into two parts…to ensure the message’s privacy. • There is the decrypted part and the hashed part. • Both become digests and then compared. • If they are the same the privacy is ensured and Bob is happy.

  11. In Conclusion • Signing Two Ways – Whole or Digest • Two Types of Key - Public or Private • Hashing – (slows down Bob’s reaction time) but it speeds up the process of signing a digest. • When Everything Goes Right • BOB IS HAPPY Bob

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