Non-repudiation - CompTIA Security+ SY0-701 - 1.2

Summary

This script delves into the concept of nonrepudiation in cryptography, emphasizing the importance of verifying the integrity and origin of data sent to third parties. It explains how hashes ensure data consistency and how digital signatures, using a private-public key pair, provide proof of origin, thus offering a high assurance of authenticity. The script illustrates these concepts with practical examples, such as creating a hash for a large document and the process of adding and verifying digital signatures in electronic communications.

Takeaways

• 🔒 Cryptography ensures nonrepudiation by verifying the sender's identity and the integrity of the data sent to a third party.
• 🖋 In contracts, signatures serve as proof of agreement, similar to how cryptography uses digital signatures to confirm the origin of data.
• 🔑 Proof of integrity in cryptography is achieved through hashing, which creates a unique fingerprint of the original data to detect any alterations.
• 🔄 A hash is a message digest that changes even with the slightest modification in the data, ensuring data consistency and accuracy.
• 👤 Hashing alone does not associate data with an individual; it only verifies data integrity, not the sender's identity.
• 🌐 Practical example: Project Gutenberg's encyclopedia volume one was hashed to demonstrate how even a minor change affects the hash value.
• 🔄 If a file's hash is recalculated and compared to the original, any changes in the data can be detected, providing proof of integrity.
• 🔒 Proof of origin is an additional layer of integrity that verifies the identity of the data sender, akin to authentication in message source verification.
• 🖊 Digital signatures provide nonrepudiation by using a private key known only to the sender, ensuring the data's origin and integrity.
• 🔓 The public key associated with the sender's private key is used to verify the digital signature, confirming the data's authenticity and origin.
• 💼 In practice, adding a digital signature to a document is often a simple action, but it involves complex cryptographic processes behind the scenes.

Q & A

What is the fundamental purpose of nonrepudiation in cryptography?

The fundamental purpose of nonrepudiation in cryptography is to ensure that a third party can verify the authenticity and origin of the data sent by a sender, similar to signing a contract.

How does proof of integrity ensure the accuracy and consistency of data?

Proof of integrity ensures the accuracy and consistency of data by using a hash function to create a unique fingerprint of the data. Any change in the data will result in a different hash, indicating the data's integrity has been compromised.

What is a hash in the context of cryptography?

A hash in cryptography is a short string of text created from the data in the plaintext, often referred to as a message digest or fingerprint. It is used to verify the integrity of the data by detecting any changes.

Why is a hash alone not sufficient to verify the origin of the data?

A hash alone is not sufficient to verify the origin of the data because it only confirms the data's integrity but does not associate the data with a specific individual or sender.

How does the concept of digital signatures provide nonrepudiation?

Digital signatures provide nonrepudiation by using a private key known only to the sender to encrypt a hash of the data. The public key associated with the private key is then used to decrypt and verify the signature, ensuring the data's authenticity and origin.

What is the practical example given in the script to illustrate the concept of hashing?

The practical example given is the downloading and hashing of volume one of the Gutenberg Encyclopedia, which is 8.1 megabytes of data. Any change in the file, no matter how small, results in a different hash value.

How can one verify if a downloaded file has been tampered with?

One can verify if a downloaded file has been tampered with by performing a hash of the downloaded file and comparing it to the original hash. A mismatch indicates that the file has been altered.

What is the process involved when a user clicks the 'add a digital signature' option?

When a user clicks 'add a digital signature,' a hashing algorithm first creates a hash of the plaintext. This hash is then encrypted with the sender's private key and sent along with the plaintext. The recipient uses the sender's public key to decrypt the hash and verify it against a hash they create from the received plaintext.

How does the use of a private key in digital signatures ensure the data's origin?

The use of a private key in digital signatures ensures the data's origin because the private key is unique to the sender. The recipient uses the corresponding public key to decrypt the signature, confirming that the data could only have come from the holder of the private key.

What is the significance of using both a hash and a digital signature in verifying the integrity and origin of data?

Using both a hash and a digital signature in verifying the integrity and origin of data provides a two-fold security measure. The hash ensures the data has not been altered, while the digital signature confirms the identity of the sender, providing proof of origin.

How does the process of digital signature verification work?

The process of digital signature verification involves the recipient using the sender's public key to decrypt the digital signature, revealing the original hash. This hash is then compared to a newly created hash from the received plaintext. A match confirms both the integrity and the origin of the data.

Outlines

00:00

🔒 Ensuring Data Integrity and Nonrepudiation

This paragraph introduces the concept of nonrepudiation in cryptography, which is the assurance that data sent by a sender cannot be denied by the sender later. It compares this to signing a contract and explains the use of a hash function to create a unique 'fingerprint' of the data, ensuring its integrity. The hash function is used to verify that the data received is unchanged from the original. The paragraph also touches on the limitations of a hash in proving the origin of the data and hints at digital signatures as a solution for this issue.

05:02

📜 The Process of Digital Signatures in Cryptography

This paragraph delves into the process of creating and verifying digital signatures to ensure both the integrity and the origin of a message. It uses an example of Alice sending a message to Bob and explains the steps involved: hashing the plaintext message, encrypting the hash with Alice's private key, and sending it along with the message. Bob then uses Alice's public key to decrypt the signature and verify it against a hash of the received message. This process confirms that the message is unaltered and originated from Alice, providing nonrepudiation and authentication.

Keywords

💡Cryptography

Cryptography is the practice and study of techniques for secure communication in the presence of third parties. It is central to the video's theme as it discusses how cryptography ensures the integrity and origin of data. The script mentions cryptography in the context of verifying the sender of data and protecting the data's integrity, like signing a contract.

💡Nonrepudiation

Nonrepudiation refers to the assurance that a party cannot deny the validity of a transaction or the authenticity of a message. In the video, nonrepudiation is achieved through cryptographic methods, ensuring that the sender of data cannot later deny having sent it, which is illustrated by comparing it to signing a contract.

💡Proof of Integrity

Proof of integrity is the assurance that the data received is exactly the same as the data that was sent. The script explains this concept by discussing the use of hashes to verify that the data has not been altered, which is crucial for maintaining the authenticity of the information.

💡Hash

A hash is a short string of characters generated from a larger set of data, often used to verify the integrity of the data. The video script describes how a hash functions as a fingerprint for data, changing if any part of the data is altered, which is demonstrated by hashing the Gutenberg Encyclopedia volume.

💡Message Digest

A message digest is the output of a hash function, often used to ensure data integrity. The script uses the term interchangeably with 'hash' to describe the process of creating a unique representation of data that can be used to detect changes.

💡Fingerprint

In the context of the video, a fingerprint metaphorically represents the hash value of data. It is used to illustrate how even a minor change in data results in a completely different hash value, similar to how a person's fingerprint is unique and changes with the person.

💡Proof of Origin

Proof of origin is the ability to verify the source of data. The script explains that while a hash can verify data integrity, it does not prove who sent the data. The concept is expanded upon by introducing digital signatures as a method to provide proof of origin.

💡Digital Signature

A digital signature is a cryptographic mechanism used to verify the authenticity of digital messages or documents. The script describes how a digital signature is created using a private key and verified with a public key, ensuring that the data came from the stated sender and has not been altered.

💡Private Key

A private key is a secret piece of information used in cryptographic processes, known only to its owner. In the script, the private key is used to encrypt the hash of a message, creating a digital signature that can be verified with the corresponding public key.

💡Public Key

A public key is the counterpart to a private key and can be shared openly. The script explains that the public key is used to decrypt a digital signature, allowing anyone with the public key to verify the authenticity of the data and its origin.

💡Alice and Bob

Alice and Bob are characters often used in cryptography to represent the sender and receiver in a communication scenario. The script uses Alice and Bob to illustrate the process of creating and verifying a digital signature, showing how nonrepudiation is achieved in a practical example.

Highlights

Ensuring the third party can verify the sender's identity is a fundamental aspect of cryptography.

Cryptography features are compared to signing a contract with a personal signature.

Nonrepudiation in cryptography is achieved through proof of integrity and proof of origin.

Proof of integrity confirms the received data is unchanged from its original state.

Hashing is used to create a unique fingerprint of data, ensuring its integrity.

A hash value changes with even the slightest alteration in the data.

Hashing alone cannot verify the identity of the data sender.

Digital signatures provide an additional layer of integrity by associating data with a specific individual.

A digital signature uses a private key known only to the sender.

Public and private keys are used in tandem for verifying digital signatures.

Digital signatures offer nonrepudiation, confirming the sender's identity.

The process of adding a digital signature to a document is often automated and user-friendly.

Alice and Bob's conversation illustrates the digital signature process.

A hashing algorithm creates a hash of the plaintext before a digital signature is applied.

The hash is encrypted with the sender's private key to form a digital signature.

The recipient uses the sender's public key to decrypt and verify the digital signature.

Verification of a digital signature involves comparing the original and received hashes.

Understanding the digital signature process aids in recognizing the importance of integrity and proof of origin in transactions.

Transcripts

One of the important foundations of cryptography

is ensuring that when someone sends data to a third party,

that that third party is able to verify that information really

came from the sender.

This is something that we think of all the time as part

of a contract.

We sign a contract at the bottom.

It's our name.

It's our signature.

And if somebody was to look at this contract later,

they could see our signature and could reasonably

say that the contract was signed by us.

Just like our signed contract, we

have a similar set of features in cryptography.

And today, we'll look at how this nonrepudiation works,

using proof of integrity and using proof of origin

with high assurance of authenticity.

Proof of integrity means that any data that we've received we

can verify that it's exactly the same data that

was originally sent.

This means that our data is accurate, consistent.

And we know that nothing inside of the data we've received

has been changed.

In cryptography, we can accomplish this

by using a hash.

A hash is a short string of text that we

can create based on data that is contained within the plaintext.

This is sometimes referred to as a message digest or something

like a fingerprint.

This means that if anything changes with that data,

we'll have a different fingerprint

or a different hash.

This is the same as an actual fingerprint.

If the person changes, you'll see that the fingerprint

is very different.

Although a hash is very good at verifying

the integrity of the data, it doesn't associate that data

with a particular individual.

We can verify that the data that we've received

is exactly the same as the data that was sent.

But we can't verify who sent the data.

However, there are ways to provide

that additional integrity.

We'll talk about those in just a moment.

Let's see how this hashing works by using a practical example.

There is an organization called Project Gutenberg

on the internet, and they have published the Gutenberg

Encyclopedia.

I downloaded volume one of that encyclopedia,

and it's 8.1 megabytes of data.

And then I ran an application that took all of that data

and created a hash or a fingerprint

of that particular encyclopedia volume one.

And here is the exact hash that I've created from that volume.

Now, if I was to change one character inside of that file,

anywhere, although the size of the file

is exactly the same after making the change,

somewhere in that haystack of data,

there is some type of difference.

But it would be very difficult for a human

to read through all of that data, 8.1 megabytes of text,

and somehow determine where that individual change might be.

But if you perform a hash of the changed data,

you'll see that the hash value that I create

is very different than the hash value that was original.

So if I have downloaded this file, perform my own hash,

and compare it to the original, I

can see that something has indeed

changed with this particular volume one of the Gutenberg

Encyclopedia.

At this point, we might want to download again

to see if we happen to get a corrupted or modified version.

Or perhaps we perform a diff or a comparison between those two

files to see exactly where the change might

be between the original version of the volume

one and the version that we received.

By using these hashes, we're able to provide

proof of integrity.

We know if anything was changed when the information was

sent from the original sender.

But we can also add to this an additional level

of integrity called proof of origin,

where we can verify the person that sent the data to us.

Sometimes you'll see this referred

to as an authentication when we are looking

at the source of the message.

By using a digital signature, we provide nonrepudiation.

So not only do we know the person

that has sent that data to us, but anyone else

could examine this transaction and verify

that the information we received really

did come from the sending party.

Just as someone can use a pen and paper to sign a contract

and send it to you, in cryptography

we use a digital signature.

This digital signature uses a private key

that is only known to the person who's sending the data.

No one else has a copy of this private key.

To verify that private key was used,

we use the public key associated with that private key.

And that way, we can assure that the information we received

is not only the same as what was sent,

but we know that it had to be sent by the person who

provided the digital signature.

In practical terms, adding a digital signature to a document

is usually created by clicking a box that

says add a digital signature.

And a lot of cryptography happens behind the scenes.

Let's lift the hood a little bit and see what that process might

be that's taking place when you check that box to add

a digital signature.

We'll start with a conversation that's occurring

between Alice and Bob.

Alice is sending a message to Bob that says, "You're hired,

Bob."

And the first thing that Alice will do

is provide a digital signature before she sends it.

The first thing that happens when

she clicks that checkbox for the digital signature

is that a hashing algorithm creates

a hash of that plaintext.

In this case, the plaintext is "You're hired, Bob."

Once that hash is created, we now

need to have some way to verify that it really came from Alice.

And since Alice is the only one with her private key,

we're going to encrypt that hash with Alice's private key,

take that encrypted hash, send it along with the plaintext,

so that Bob is going to receive a message that says,

"You're hired, Bob."

And then, attached to that message,

is a digital signature.

In most cases, Alice is going to send that message over

to Bob using email or some other type of electronic delivery.

Bob is going to receive exactly that message

that was sent that says "You're hired,

Bob" with the digital signature included with it.

Bob is going to use Alice's public key, which

is a key available to anybody, and he's

going to examine the digital signature

and decrypt it using that public key.

Once that decryption takes place,

we have the original hash that was created

of that plaintext message.

And at this point, Bob wants to see

if the hash that was in that digital signature

matches the hash of what he received.

So he's going to perform the same hashing function

that Alice originally performed.

Bob is going to take the original plaintext, run it

through the same hashing algorithm

to come up with a hash of what he received.

Bob can now do a comparison to see

if the hash that was included with the digital signature

is the same as the hash that he manually

created from the plaintext.

And if that matches, we not only know

that the information we received is

exactly the same as the information that was sent,

but we know that it had to be sent from Alice.

As I mentioned earlier, this entire process

of creating a digital signature and verifying

the digital signature is something

that's usually created through clicking

a button on your screen, or it's something

that happens automatically.

You normally never see this process occur.

But having an understanding of the digital signature process

and the verification of the digital signature process

can help you better understand the integrity that we're

looking for and the proof of origin that's

so important when working with transactions like this.