Digital Signatures and Digital Certificates

Summary

This script explains digital signatures using the concept of asymmetric key cryptography. It illustrates the process of creating a hash value using SHA-256, encrypting it with the sender's private key, and embedding it in the document. The recipient uses the sender's public key to decrypt and verify the hash, ensuring document integrity. The script also introduces digital certificates, issued by certification authorities, to authenticate the sender's identity, making digital signatures legally binding and crucial for secure online transactions.

Takeaways

• 🔐 Digital signatures use asymmetric key cryptography, where a pair of mathematically related keys is used for encryption and decryption.
• 🔑 Asymmetric keys consist of a public key for encryption and a private key for decryption, ensuring that only the intended recipient can decrypt the message.
• 💼 The public key can be shared openly without compromising security, while the private key must remain confidential.
• 📜 Digital signatures are essential for verifying the authenticity and integrity of documents, ensuring they have not been tampered with.
• 🖋️ A digital signature involves creating a hash of the document using an algorithm like SHA-256, which is then encrypted with the sender's private key.
• 🗂️ The encrypted hash (digital signature) is embedded in the document, which can then be sent to the recipient.
• 🔄 Upon receipt, the recipient uses the sender's public key to decrypt the hash and recalculates the hash from the document's content to verify its integrity.
• 🛡️ Digital certificates, issued by trusted certification authorities, vouch for the sender's identity and contain a public key, adding a layer of trust to digital signatures.
• 🏦 The Electronic Communications Act of 2000 in the UK made digital signatures legally binding, facilitating secure online transactions and the rise of cryptocurrencies.
• 🌐 Cryptocurrencies like Bitcoin rely on digital signatures for secure transactions, where the list of transactions is updated and verified through cryptographic means.

Q & A

What is the fundamental concept behind symmetric key cryptography?

Symmetric key cryptography uses a single key for both encryption and decryption. The same key must be kept secret and shared between the communicating parties.

How does asymmetric key cryptography differ from symmetric key cryptography?

Asymmetric key cryptography uses a pair of mathematically related keys: a public key for encryption and a private key for decryption. The public key can be shared openly, while the private key must be kept secret.

Why is it crucial that the private key in asymmetric cryptography is kept secret?

The private key must be kept secret because if it falls into the wrong hands, anyone could decrypt messages intended for the key owner, compromising the security of the communication.

What role does the public key play in the process of digital signatures?

In digital signatures, the public key is used to verify the authenticity of the signature. It can decrypt the hash value that was encrypted with the sender's private key, ensuring that the document was indeed sent by the claimed sender.

Why are digital signatures necessary in digital communication?

Digital signatures are necessary to ensure the authenticity and integrity of a document. They provide proof that the document was sent by the claimed sender and has not been altered since it was signed.

How does the hashing process using SHA-256 contribute to the security of digital signatures?

SHA-256 contributes to the security of digital signatures by creating a unique hash value from the document's content. Any change in the document would result in a different hash value, thus detecting tampering.

What is a digital certificate and what is its purpose in the context of digital signatures?

A digital certificate is a file issued by a certification authority that contains the public key of the certificate holder along with other identifying information. Its purpose is to guarantee the identity of the public key owner, thus enhancing the trust in digital signatures.

How does a certification authority verify the identity of an individual applying for a digital certificate?

A certification authority verifies the identity of an individual by requiring them to provide various details about themselves during the application process. The authority then checks these details to confirm the applicant's identity.

What is the Electronic Communications Act, and how does it relate to digital signatures?

The Electronic Communications Act is a UK law passed in 2000 that made digital signatures legally binding. This legislation has allowed businesses to conduct transactions and sign agreements digitally with the same legal force as handwritten signatures.

How do digital signatures contribute to the security of cryptocurrencies?

Digital signatures are fundamental to the security of cryptocurrencies as they ensure the authenticity of transactions. They verify that transactions come from the rightful owner of the cryptocurrency and have not been tampered with.

Outlines

00:00

🔐 Asymmetric Key Cryptography and Digital Signatures

This paragraph introduces the concept of digital signatures and their reliance on asymmetric key cryptography. It explains the process of encryption and decryption using a pair of mathematically related keys, where one key encrypts the message and the other decrypts it. The example of Albert and Sheila illustrates how one person can send a secret message to another using public and private keys. The paragraph also discusses the importance of asymmetric keys in digital signatures, where the choice of which key is public or private is flexible, enabling the creation of a digital signature. It sets the stage for understanding why digital signatures are necessary by presenting a scenario where Carol and Bob have a dispute over a contract, highlighting the need for a secure and verifiable way to prove the authenticity of documents.

05:04

📜 The Process of Digital Signatures

This paragraph delves into the technical process of creating a digital signature. It explains how a document's content is hashed using an algorithm like SHA-256 to create a unique digest or hash value. This hash value is then encrypted with the sender's private key to form the digital signature, which is embedded in the document. The recipient can decrypt this signature using the sender's public key to verify its origin. The paragraph also emphasizes the importance of digital certificates, which are issued by trusted certification authorities to guarantee the sender's identity and the authenticity of the public key. It discusses how the recipient can use the digital certificate to ensure they are communicating with the correct person, and how the Electronic Communications Act in the UK made digital signatures legally binding, facilitating business transactions online.

10:07

🔏 Summary of Digital Signatures and Certificates

The final paragraph summarizes the key points about digital signatures and certificates. It reiterates that digital signatures use asymmetric key cryptography to ensure the integrity and authenticity of documents. The process involves hashing the document's content to create a digest, encrypting this digest with the sender's private key, and embedding it into the document. The recipient then decrypts the digest using the sender's public key and recalculates the hash to verify the document's integrity. The paragraph also explains the role of digital certificates, which are issued by certification authorities to guarantee the sender's identity and the validity of the public key. It concludes by emphasizing the importance of digital signatures in secure communication and the legal recognition of their binding nature.

Keywords

💡Digital Signatures

Digital signatures are a form of cryptographic technology that ensures the authenticity and integrity of electronic documents. In the video, digital signatures are discussed as a method to verify that a document was sent by a specific person and has not been altered. They are crucial for secure communication and transactions over the internet, as exemplified by the scenario where Jack sends a digitally signed document to Jill, ensuring she can trust its origin and content.

💡Symmetric Key Cryptography

Symmetric key cryptography is a method of secure communication that uses a single key for both encryption and decryption of information. The video script briefly contrasts this with asymmetric key cryptography, highlighting that digital signatures actually rely on the latter. Symmetric key cryptography is not directly used for digital signatures but serves as a foundational concept in understanding the broader field of cryptography.

💡Asymmetric Key Cryptography

Asymmetric key cryptography, also known as public key cryptography, uses a pair of keys: one public and one private. In the video, it is explained that if a message is encrypted with one key, only the other can decrypt it. This property is fundamental to digital signatures, as it allows for the creation of a signature that can be verified by anyone with the public key but can only be created by the holder of the private key.

💡Public Key

The public key in asymmetric cryptography is a key that can be freely distributed and is used to encrypt data or verify a digital signature. In the context of the video, it is mentioned that Sheila could send Albert her public key, which he would use to encrypt a message that only she, with her private key, could decrypt. Similarly, in digital signatures, the public key is used to verify the authenticity of a signed document.

💡Private Key

The private key in asymmetric cryptography is a key that must be kept secret by its owner and is used to decrypt data or create a digital signature. The video script emphasizes the importance of the private key by illustrating a scenario where Bob's actions could be verified by his signed documents. In digital signatures, the private key is used to sign a document, ensuring that only the owner of the corresponding public key can verify it.

💡Hash Function

A hash function, such as SHA-256 mentioned in the video, is an algorithm that takes an input (or 'message') and returns a fixed-size string of bytes, typically a digest that uniquely represents the original input. The video explains that even a minor change in the input results in a completely different hash value, which is crucial for ensuring the integrity of a document in digital signatures.

💡Digital Certificate

A digital certificate is an electronic document used to prove the ownership of a public key. Issued by a trusted third party, it binds the identity of an individual or organization to their public key. In the video, it is explained that Jack can apply for a digital certificate, which includes his public key and is signed by a certification authority, providing a trusted means for Jill to verify that the public key indeed belongs to Jack.

💡Certification Authority (CA)

A certification authority is a trusted entity that issues digital certificates, thereby attesting to the ownership of a public key. The video mentions companies like Verisign, GlobalSign, and Symantec as examples of CAs. They play a critical role in the digital signature process by vouching for the identity of the certificate holder, adding a layer of trust to digital transactions.

💡Cryptographic Hash

A cryptographic hash, created using a hash function, is a unique fixed-size representation of data. The video script uses the term in the context of creating a 'digest' of a document that is then encrypted with the sender's private key to form a digital signature. The hash ensures that any change in the document would result in a different hash, thus detecting tampering.

💡SHA-256

SHA-256, or Secure Hash Algorithm 256, is a cryptographic hash function from the SHA-2 family. The video script describes it as the algorithm most commonly used to create a hash of a document for digital signatures. It is designed to be a one-way process, making it computationally infeasible to reverse-engineer the original document from its hash, ensuring the security of the digital signature.

Highlights

Digital signatures utilize asymmetric key cryptography, which involves a pair of mathematically related keys.

In asymmetric cryptography, one key encrypts data that can only be decrypted by the other key.

A public key can be shared openly, while the private key must be kept secret by its owner.

The choice of which key is public and which is private is flexible in asymmetric cryptography.

Digital signatures are essential for verifying the authenticity and integrity of digital documents.

A hash function like SHA-256 is used to create a unique digest of the document's content.

The hash value changes significantly with even the slightest alteration in the document.

The sender encrypts the hash using their private key to create a digital signature.

The digital signature is embedded in the document, which can then be sent to the recipient.

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

The recipient also calculates the hash of the received document to verify its integrity.

If the calculated hash matches the decrypted hash, the document is confirmed to be unaltered.

Digital certificates, issued by a trusted certification authority, guarantee the sender's identity.

A digital certificate contains the sender's public key, ensuring the recipient can trust the signature.

The Electronic Communications Act of 2000 in the UK made digital signatures legally binding.

Digital signatures are foundational for secure communication and transactions on the internet.

Cryptocurrencies like Bitcoin rely on digital signatures for secure transactions.

Transcripts

digital signatures rely on a symmetric

key cryptography so before we talk about

digital signatures let's quickly remind

ourselves how a symmetric key

cryptography

works with asymmetric key cryptography

there's a pair of mathematically related

keys if you encrypt a message with one

of the keys than the other key and only

the other key can be used to decrypt it

this is Albert and this is Sheila if

Sheila wants Albert to send her a secret

message

she needs an asymmetric a pair of keys a

computer program can generate these for

her she sends Albert a copy of one of

the keys and she keeps the other key to

herself instead of sending Albert a key

she could put a key in a public place

for him to go and fetch himself it

doesn't matter if someone else gets a

copy of the public key Albert uses the

public key to encrypt his message when

Albert has encrypted the message he

sends a Sheila the ciphertext only

Sheila can decrypt the ciphertext

because only Sheila has the matching key

the private key truth be told most

people don't need to worry about what's

really going on because the whole

process is taken care of

behind the scenes my programs built into

web browsers or email applications but

what do asymmetric keys have to do with

digital signatures well it so happens

that it doesn't matter which one of the

pair of keys is made public and which

one is kept private if you encrypt a

message with either one than the other

and only the other can be used to

decrypt it as long as she doesn't change

her mind later

Sheila can decide which one of the keys

will be private and which one will be

public this is the crucial feature of

asymmetric key cryptography that makes

digital signatures possible now let's

think about why we even need digital

signatures

this is Carol and this is Bob Bob's

builder carol has a leaky roof and she

asks Bob to fix it

Carol agrees to pay Bob a thousand

pounds to fix her roof Bob agrees to

start work on Monday and have it

finished by Wednesday

Carol writes details of the agreement on

a piece of paper and she signs it and

she sends it to Bob in the post no

computers here Bob signs the piece of

paper and sends a copy back to Carol in

the post Carol sends Bob the money bob

writes a note to carol to say he's got

the money he signs this receipt and he

sends it to Carol in the post on Monday

Bob doesn't turn up he's gone on holiday

for a week to spend the money on

Thursday it rains all day and all of

Carol's furniture is destroyed poor

Carol Carol says it's Bob's fault but

Bob denies ever having agreed to do the

work Carol takes Bob to court the judge

looks at the contract and the receipt

which Bob signed Bob is ordered to pay

the money back to Carol and to give her

some more money for new furniture and

Bob is sent to jail where he belongs

once upon a time not so long ago if you

wanted proof that a document was sent to

you by a particular person

it needed a handwritten signature on it

only written signatures were legally

binding but these days it's possible to

put a digital signature on a document

this is Jack and this is Jill Jack wants

to send a document to Jill by email no

paper this time now let's be clear

there's nothing secret about the

document neither of them care if

somebody else reads it Jill just wants

to be sure that it definitely came from

Jack and that nobody else has made any

changes to it on the way before his

document is sent some software on Jack's

computer prepares the digital signature

the purpose of this software is to

create something called a hash of the

document these days most computers do

this using an algorithm called sha-256

which was invented by the USA's National

Security Agency sha-256 takes a copy of

the document text and subjects it to a

sequence of complex mathematical

calculations and other transformations

remember as far as the computer is

concerned the document consists of

binary ones and zeros the result is

called a hash value it's also referred

to as a digest of the document the

hashing process has been designed so

that even the tiniest difference in the

original document would result in a

completely different hash value this

part of the signing process is not

encryption because the transformations

done by sha-256

are practically impossible to reverse

you can't take a hash value and use it

to work out what was in the original

document like baking a cake hashing is a

one-way process but if you were to apply

the same process to the same document

you would get exactly the same hash some

software on Jack's computer now encrypts

the hash using Jack's private key and

the encrypted hash is embedded in the

original document the document now has a

digital signature Jack sends Jill a copy

of the signed document he also sends her

a copy of the public key alternatively

he can put the public key on a website

for Jill to go and fetch Jill's computer

decrypt the digital signature using

Jack's public key if she can decrypt it

she knows it came from jack Jill's

computer then uses sha-256 to calculate

the hash value again using the text of

the document if the hash value that

Jill's computer calculates

the same as the hash value that was sent

by Jack she can be pretty sure that it

hasn't been tampered with since it was

created remember Jack and Jill really

don't care if someone else has seen the

signed document it's not a secret and it

doesn't matter if someone else gets a

hold of Jack's public key Jill simply

wants to be sure that the document was

sent by Jack of course anyone else could

have been pretending to be jack from the

start a criminal could create a fake

document hash it with sha-256 and

generate an asymmetric pair of keys

using their computer so how can Jill be

really sure that she's communicating

with jack well that's where digital

certificates come in for a fee Jack can

apply for a digital certificate to a

well known and well trusted organization

called a certification Authority

certification authorities include

companies like Verisign global sign and

Symantec to name but a few as part of

the application process

Jack's computer generates an asymmetric

pair of keys and he sends the public key

to the certification Authority along

with various details about himself

the certification Authority carefully

checks that Jack is who he says he is

then they send them a special type of

file called a digital certificate this

contains details about Jack along with

information about the certification

Authority and an expiry date bound to

this digital certificate is jack's

public key jack still has the

corresponding private key which never

left his computer Jack must of course

keep his private key safe so now when

Jack sends a signed document to Jill he

can also send her a copy of the whole

certificate or put it in a public place

for her to go and get

this means that when Jill wants to

decrypt something that Jack is encrypted

she can inspect this certificate first

and if she's happy to trust it she can

use the public key within the public key

that has been guaranteed by the

certification authority to belong to

Jack

essentially the certification authority

is vouching for Jack

needless to say applying to a

certification Authority for a digital

certificate is itself a very secure

process anything the certification

authority sent to Jack was digitally

signed by them using their own digital

certificate and this was provided by an

even higher certification authority in

the year 2000 a law was passed in the UK

called the Electronic Communications Act

this law made digital signatures legally

binding and this has allowed businesses

to thrive on the web since then we've

seen the rise of crypto currencies like

a Bitcoin a cryptocurrency is

fundamentally a secure list of who paid

who how much updating this list depends

on digital signatures to summarize

digital signatures rely on a symmetric

key cryptography a documents contents

are hashed to create a digest for

example using sha-256 the digest is

encrypted by the sender using their

private key the digest is then embedded

in the document which can be sent the

recipient decrypt the digest using the

sender's public key the recipient also

calculates a hash from the documents

contents using the same hash algorithm

if the recalculated digest matches the

decrypted digest it can be assumed that

the document hasn't been tampered with

since it was sent a digital certificate

is issued by a certification Authority

which guarantees the sender's identity

the digital certificate contains a

public key along with other information

about the sender and an expiry date