Lecture on Cryptographic Hash and Digital Signatures

Introduction to Cryptographic Hash

Definition: Represents data as a short string of text, also known as a message digest or fingerprint.
Characteristics:
- Not encryption; you cannot recreate data from a hash.
- Used for data integrity; verify downloaded documents match originals.
- Utilized in creating digital signatures for authentication, non-repudiation, and integrity.

SHA-256:
- Common hashing algorithm.
- Produces 256 bits represented as 64 hexadecimal characters.
- Characteristics: a minor change in input results in a significantly different hash.
Collision:
- Occurs when different inputs produce the same hash.
- Example: MD5 algorithm (collision problem identified in 1996).
- Recommendation: Avoid using MD5 due to collision vulnerability.

File Verification:
- Ensures downloaded files match originals (e.g., ISO files from websites).
Storing Passwords:
- Use hashed values (plus salt) instead of storing plain text passwords.
- Salt adds randomness, preventing rainbow table attacks.
Rainbow Tables:
- Pre-compiled lists of hashes for various inputs.
- Salted hashes render rainbow tables ineffective.

Purpose: Ensures message integrity and authenticity (similar to physical signatures).
Functionality: Authentication, Integrity, and Non-repudiation.
Process of Creating a Digital Signature:
- Sender (e.g., Alice) hashes the plain text and encrypts the hash with her private key.
- Recipient (e.g., Bob) uses sender's public key to decrypt the digital signature.
- Verifies integrity by comparing decrypted hash with a newly performed hash on the plain text.

Example Process:
- Alice sends a message with a digital signature to Bob.
- Bob verifies the message through his email client using the digital signature.
- Ensures both the content integrity and validates sender authenticity.
- If hashes match, the message is verified; if not, it indicates tampering or authenticity issues.