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Write short note on Digital Signatures
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Digital Signatures

  • Expected question: Suppose Bobby has got message M. private key sk1 and public key pk1 and Bob has got private key sk2 and publick key pk2. Bobby computes $X = E_{sk1(M)}$, $Y = E_{pk1(H(M))}$ where E is encryption and H is hash. Now she sends this (x, Y) to Bob. State the security goals achieved and not achieved.

  • It involves the standard process of encryption and decryption.

  • Working: Let the sender be A and the receiver be B.

  1. A encryption the plaintext to ciphertext using the public key of B.

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  2. Now A does not directly send the ciphertext to B. It first creates a message digest is encrypted with A's private key. this is called the digital signature.

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  1. Now A concatenates the ciphertext (CT1) and the digital signture (DS!) and sends it to B. B receives the message as CT2 and DS2.

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  1. B decrypts ciphertext CT2 into plaintext PT2 using its private key.

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  1. Now to check if the message actually came from A, B does the following: B decrypts A's digital signature using A's public key. This gives B the message digest. Let us call the message digest as MD2.

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  2. B applied the Hash Function to PT2 to get Message Digest MD3. Now B checks if MD3 and MD2 are the same. If yes then the message was actually sent by A.

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