Summary
Highlights
Non-repudiation in cryptography ensures that the recipient of data can verify the sender's identity and that the data has not been altered. Similar to signing a physical contract, digital methods like proof of integrity and proof of origin provide high assurance of authenticity.
Proof of integrity guarantees that received data is identical to the original. This is achieved using a hash, a short string of text derived from the data, often called a message digest or fingerprint. Any change to the data, even a single character, will result in a completely different hash, making it easy to detect alterations. However, a hash alone doesn't identify the sender.
Using Project Gutenberg's encyclopedia as an example, the video demonstrates how a hash created from a large file drastically changes if even one character is altered. This shows how hashes can quickly reveal any modification to data, prompting recalculation or comparison to identify the change.
To add proof of origin and authenticate the sender, digital signatures are used. This provides non-repudiation, meaning the sender cannot deny sending the data. A digital signature uses a sender's private key, known only to them, and is verifiable by anyone using a corresponding public key.
When Alice sends a message to Bob, she first creates a hash of the message. This hash is then encrypted with Alice's private key, forming the digital signature. Both the plaintext message and the encrypted hash (digital signature) are sent to Bob. Bob uses Alice's public key to decrypt the digital signature, revealing the original hash. He then independently hashes the received plaintext message and compares his calculated hash with the one from the digital signature. If they match, it confirms the data's integrity and Alice's authorship.
While creating and verifying digital signatures often happens automatically with a simple click, understanding the underlying process is crucial. This knowledge helps in appreciating the integrity and proof of origin provided by digital signatures, which are vital for securing transactions and communications.