Understanding Digital Signatures and Non-Repudiation | Security+ SY0-701 Course

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Non-repudiation helps us prove without doubt that a message or file came from the sender it claims to, and that it hasn’t been altered along the way.

What is Non-repudiation?
Non-repudiation ensures that a sender cannot deny sending a message or performing an action, and that the message or data has not been changed in transit. It provides verifiable proof of both authenticity and integrity.

A good real-world example is a cheque. When someone writes a cheque, they sign it and write the amount in both numbers and words. The bank can then compare the signature to the one on file and verify the cheque hasn’t been altered. This is a form of non-repudiation.

In the digital world, we use different tools, but the goal is the same. We want to prove who sent a message or file, and make sure it hasn’t been tampered with.

Why Do We Need It?
Non-repudiation is essential in emails, financial transactions, software distribution, and even official HR or legal communications. Without non-repudiation:

We cannot trust that a message truly came from the sender it claims to. We cannot confirm that the message or file hasn’t been altered. We risk accepting fraudulent communications or modified documents

To achieve non-repudiation, we must verify two things:

- Integrity
- Authenticity

Let’s look at each one in more detail.

Verifying Integrity with Hashing
Integrity ensures that the message or file hasn’t been changed. One of the most effective ways to confirm integrity is through hashing.

Hashing is the process of converting data into a fixed-length string of characters. The result is called a hash.

If even a single character in the original data changes, the hash will change completely. This makes it easy to detect any alterations to files, emails, or other data.

For example, you hash a document and send the document and hash to someone. They hash the document again when they receive it. If their hash matches the original, the file has not been altered

Verifying Authenticity with Digital Signatures
Authenticity proves that the sender is who they say they are. In digital systems, we use digital signatures to achieve this. A digital signature is an encrypted hash that is created using the sender’s private key. Only the matching public key can decrypt it.

Here’s how it works:

A hash is generated from the document or message

The sender encrypts that hash using their private key. This creates the digital signature

The recipient can use the sender’s public key to decrypt the hash. If successful, this proves the data came from the sender

Note: Digital signatures use the private key to sign (encrypt the hash), and the public key is used to verify. In other contexts, like confidentiality, the roles of the keys are reversed.

The recipient also hashes the document themselves and compares it to the decrypted hash to ensure integrity

Non-repudiation is critical in cybersecurity. It provides the confidence that digital messages, files, or transactions are both genuine and unchanged. Without it, we would be unable to verify identities or detect tampering, making secure digital communication impossible.