1. Section 4.6: Digital Signatures
A problem that occurs with a public key cryptosystem deals with message authenticity.
The receiver wants to be certain that the message came from the intended sender.
An enemy can send a message using the recipient’s public key and pose as someone friendly.
This problem can be overcome by having the sender use his or her decryption exponent to “sign” the message before encrypting it with the recipient’s public key.
The recipient can then decrypt the sent message using his or her’s decryption exponent and then “unlock” the signature using the sender’s public encryption exponent modulus…

2. RSA Digital Signature Scheme
In the discussion that follows:
Y is the numerical representation of the plaintext.
ZS is the signed version of the plaintext.
Z is the numerical representation of the ciphertext.
ZS is the encryption of the message signature
Trixie – The sender of messages
Sophie – The receiver of messages
Allie – the opponent or enemy.
ms and es is the public key of the sender.
ds is the sender’s secret decryption exponent.
mr, and er is the public key of the recipient.
dr is the receiver’s secret decryption exponent…

3. RSA Digital Signature Scheme
Case 1 – Signing a Message
The plaintext message Y is sent, signed, and verified as follows:
Initial setup:
The Sender
Define the modulus ms and encryption exponent es and decryption exponent ds.
Make the parameters ms and es public and keep ds secret.
Message Signing
Create a digital Signature (Message Signing)
Signature Verification
The recipient receives the message.
Example 1: Trixie sends a message.
Example 2: Allie poses as Trixie…

4. RSA Digital Signature Scheme
Case 2 – Signing and Encryption
Given a plaintext message Y, the message can be sent, signed, and the signature verified using the following steps:
Initial Setup
Message Signing
Signature Verification
Example 3: Trixie sends a message to Sophie.
Example 4: Allie poses as Trixie…

5. Case 3 – Hashing, Signing and Encryption
In the previous two examples the encrypted message and the signature are of the same length. To save storage and computational time, a common method involves generating a signature using the hash value of a message.
Initial Setup
Message Signing
Signature Verification
Example 5: Trixie transmits another message…!