Digital Signature in Information Security

1. Information Security
Week 9

2. Digital signature
• A digital signature is basically a way to ensure that an electronic document (e-mail,
spreadsheet, text file, etc.) is authentic. Authentic means that you know who
created the document and you know that it has not been altered in any way since
that person created it.
• Digital signatures rely on certain types of encryption to ensure authentication.
Encryption is the process of taking all the data that one computer is sending to
another and encoding it into a form that only the other computer will be able to
decode. Authentication is the process of verifying that information is coming from a
trusted source. These two processes work hand in hand for digital signatures.

3. Digital signature
• A digital signature is a mathematical technique which validates the
authenticity and integrity of a message, software or digital
documents. It allows us to verify the author name, date and time of
signatures, and authenticate the message contents. The digital
signature offers far more inherent security and intended to solve the
problem of tampering and impersonation (Intentionally copy another
person's characteristics) in digital communications.

4. Application of Digital Signature
• The important reason to implement digital signature to
communication is:
• Authentication
• Non-repudiation
• Integrity

5. Application of Digital Signature
• Authentication
• Authentication is a process which verifies the identity of a user who wants
to access the system. In the digital signature, authentication helps to
authenticate the sources of messages.
• Non-repudiation
• Non-repudiation means assurance of something that cannot be denied. It
ensures that someone to a contract or communication cannot later deny
the authenticity of their signature on a document or in a file or the sending
of a message that they originated.
• Integrity
• Integrity ensures that the message is real, accurate and safeguards from
unauthorized user modification during the transmission.

6. Algorithms in Digital Signature
• Key Generation Algorithms : Digital signature are electronic signatures, which
assures that the message was sent by a particular sender. While performing
digital transactions authenticity and integrity should be assured, otherwise the
data can be altered or someone can also act as if he was the sender and expect a
reply.
• Signing Algorithms: To create a digital signature, signing algorithms like email
programs create a one-way hash of the electronic data which is to be signed. The
signing algorithm then encrypts the hash value using the private key (signature
key). This encrypted hash along with other information like the hashing algorithm
is the digital signature. This digital signature is appended with the data and sent
to the verifier. The reason for encrypting the hash instead of the entire message
or document is that a hash function converts any arbitrary input into a much
shorter fixed length value. This saves time as now instead of signing a long
message a shorter hash value has to be signed and moreover hashing is much
faster than signing.

7. Algorithms in Digital Signature
• Signature Verification Algorithms : Verifier receives Digital Signature
along with the data. It then uses Verification algorithm to process on
the digital signature and the public key (verification key) and
generates some value. It also applies the same hash function on the
received data and generates a hash value. Then the hash value and
the output of the verification algorithm are compared. If they both
are equal, then the digital signature is valid else it is invalid.

8. How Digital Signature Works…
• The steps followed in creating digital signature are :
• Message digest is computed by applying hash function on the message and then message digest
is encrypted using private key of sender to form the digital signature. (digital signature =
encryption (private key of sender, message digest) and message digest = message digest
algorithm(message)).
• Digital signature is then transmitted with the message.(message + digital signature is transmitted)
• Receiver decrypts the digital signature using the public key of sender.(This assures authenticity,as
only sender has his private key so only sender can encrypt using his private key which can thus be
decrypted by sender’s public key).
• The receiver now has the message digest.
• The receiver can compute the message digest from the message (actual message is sent with the
digital signature).
• The message digest computed by receiver and the message digest (got by decryption on digital
signature) need to be same for ensuring integrity.

10. Types of
Digital
Signature

11. Types of Digital Signature
• Certified Signatures
• The certified digital signature documents display a unique blue ribbon
across the top of the document. The certified signature contains the name
of the document signer and the certificate issuer which indicate the
authorship and authenticity of the document.
• Approval Signatures
• The approval digital signatures on a document can be used in the
organization's business workflow. They help to optimize the organization's
approval procedure. The procedure involves capturing approvals made by
us and other individuals and embedding them within the PDF document.
The approval signatures to include details such as an image of our physical
signature, location, date, and official seal.

12. Types of Digital Signature
• Visible Digital Signature
• The visible digital signature allows a user to sign a single document
digitally. This signature appears on a document in the same way as
signatures are signed on a physical document.
• Invisible Digital Signature
• The invisible digital signatures carry a visual indication of a blue
ribbon within a document in the taskbar. We can use invisible digital
signatures when we do not have or do not want to display our
signature but need to provide the authenticity of the document, its
integrity, and its origin.

13. Digital Certificate
• Digital certificate is issued by a trusted third party which proves
sender's identity to the receiver and receiver’s identity to the sender.
A digital certificate is a certificate issued by a Certificate Authority
(CA) to verify the identity of the certificate holder. The CA issues an
encrypted digital certificate containing the applicant’s public key and
a variety of other identification information. Digital certificate is used
to attach public key with a particular individual or an entity.

14. Digital certificate contains:-
• Name of certificate holder.
• Serial number which is used to uniquely identify a certificate, the
individual or the entity identified by the certificate
• Expiration dates.
• Copy of certificate holder's public key.(used for decrypting messages
and digital signatures)
• Digital Signature of the certificate issuing authority.
• Digital ceritifcate is also sent with the digital signature and the
message.

16. Digital certificate vs digital signature :
Digital signature is used to verify authenticity, integrity, non-
repudiation ,i.e. it is assuring that the message is sent by the known
user and not modified, while digital certificate is used to verify the
identity of the user, maybe sender or receiver. Thus, digital signature
and certificate are different kind of things but both are used for
security. Most websites use digital certificate to enhance trust of their
users.

17. Digital certificate vs digital signature :
FEATURE DIGITAL SIGNATURE DIGITAL CERTIFICATE
Basics / Definition
Digital signature is like a fingerprint
or an attachment to a digital
document that ensures its
authenticity and integrity.
Digital certificate is a file that
ensures holder’s identity and
provides security.
Process / Steps
Hashed value of original message is
encrypted with sender’s secret key
to generate the digital signature.
It is generated by CA (Certifying
Authority) that involves four steps:
Key Generation, Registration,
Verification, Creation.
Security Services
Authenticity of Sender, integrity of
the document and non-
repudiation.
It provides security
and authenticity of certificate
holder.
Standard
It follows Digital Signature
Standard (DSS).
It follows X.509 Standard Format

18. one-way hash function
• A one-way hash function, also known as a message digest, fingerprint or
compression function, is a mathematical function which takes a variable-
length input string and converts it into a fixed-length binary sequence.
• Hash functions are extremely useful and appear in almost all information
security applications.
• A hash function is a mathematical function that converts a numerical input
value into another compressed numerical value. The input to the hash
function is of arbitrary length but output is always of fixed length.
• Values returned by a hash function are called message digest or
simply hash values. The following picture illustrated hash function −

19. one-way hash function

20. Features of Hash Functions
• The typical features of hash functions are −
• Fixed Length Output (Hash Value)
• Hash function coverts data of arbitrary length to a fixed length. This process is often
referred to as hashing the data.
• In general, the hash is much smaller than the input data, hence hash functions are
sometimes called compression functions.
• Since a hash is a smaller representation of a larger data, it is also referred to as
a digest.
• Hash function with n bit output is referred to as an n-bit hash function. Popular hash
functions generate values between 160 and 512 bits.
• Efficiency of Operation
• Generally for any hash function h with input x, computation of h(x) is a fast
operation.
• Computationally hash functions are much faster than a symmetric encryption.

21. Properties of Hash Functions
• In order to be an effective cryptographic tool, the hash function is desired to
possess following properties −
• Pre-Image Resistance
• This property means that it should be computationally hard to reverse a hash function.
• In other words, if a hash function h produced a hash value z, then it should be a difficult
process to find any input value x that hashes to z.
• This property protects against an attacker who only has a hash value and is trying to find
the input.
• Second Pre-Image Resistance
• This property means given an input and its hash, it should be hard to find a different input
with the same hash.
• In other words, if a hash function h for an input x produces hash value h(x), then it should
be difficult to find any other input value y such that h(y) = h(x).
• This property of hash function protects against an attacker who has an input value and its
hash, and wants to substitute different value as legitimate value in place of original input
value.

22. Properties of Hash Functions
• Collision Resistance
• This property means it should be hard to find two different inputs of any
length that result in the same hash. This property is also referred to as
collision free hash function.
• In other words, for a hash function h, it is hard to find any two different inputs
x and y such that h(x) = h(y).
• Since, hash function is compressing function with fixed hash length, it is
impossible for a hash function not to have collisions. This property of collision
free only confirms that these collisions should be hard to find.
• This property makes it very difficult for an attacker to find two input values
with the same hash.
• Also, if a hash function is collision-resistant then it is second pre-image
resistant.

23. Design of Hashing Algorithms
• At the heart of a hashing is a mathematical function that operates on
two fixed-size blocks of data to create a hash code. This hash function
forms the part of the hashing algorithm.
• The size of each data block varies depending on the algorithm.
Typically the block sizes are from 128 bits to 512 bits. The following
illustration demonstrates hash function −

24. • Hashing algorithm involves rounds of above hash function like a block
cipher. Each round takes an input of a fixed size, typically a
combination of the most recent message block and the output of the
last round.
• This process is repeated for as many rounds as are required to hash
the entire message. Schematic of hashing algorithm is depicted in the
following illustration −

25. Message Authentication Code (MAC)
• threat that exist for data is the lack of message authentication. In
this threat, the user is not sure about the originator of the message.
Message authentication can be provided using the cryptographic
techniques that use secret keys as done in case of encryption.
• MAC algorithm is a symmetric key cryptographic technique to provide
message authentication. For establishing MAC process, the sender
and receiver share a symmetric key K.
• Essentially, a MAC is an encrypted checksum generated on the
underlying message that is sent along with a message to ensure
message authentication.

26. Message
Authentication Code
(MAC)
• The process of using MAC
for authentication is
depicted in the following
illustration −

27. Message Authentication Code (MAC)
• Let us now try to understand the entire process in detail −
• The sender uses some publicly known MAC algorithm, inputs the
message and the secret key K and produces a MAC value.
• Similar to hash, MAC function also compresses an arbitrary long input
into a fixed length output. The major difference between hash and
MAC is that MAC uses secret key during the compression.
• The sender forwards the message along with the MAC. Here, we
assume that the message is sent in the clear, as we are concerned of
providing message origin authentication, not confidentiality. If
confidentiality is required then the message needs encryption.

28. Message Authentication Code (MAC)
• On receipt of the message and the MAC, the receiver feeds the
received message and the shared secret key K into the MAC algorithm
and re-computes the MAC value.
• The receiver now checks equality of freshly computed MAC with the
MAC received from the sender. If they match, then the receiver
accepts the message and assures himself that the message has been
sent by the intended sender.
• If the computed MAC does not match the MAC sent by the sender,
the receiver cannot determine whether it is the message that has
been altered or it is the origin that has been falsified. As a bottom-
line, a receiver safely assumes that the message is not the genuine.

29. Limitations of MAC
• There are two major limitations of MAC, both due to its symmetric nature of
operation −
• Establishment of Shared Secret.
• It can provide message authentication among pre-decided legitimate users who have shared
key.
• This requires establishment of shared secret prior to use of MAC.
• Inability to Provide Non-Repudiation
• Non-repudiation is the assurance that a message originator cannot deny any previously sent
messages and commitments or actions.
• MAC technique does not provide a non-repudiation service. If the sender and receiver get
involved in a dispute over message origination, MACs cannot provide a proof that a message
was indeed sent by the sender.
• Though no third party can compute the MAC, still sender could deny having sent the message
and claim that the receiver forged it, as it is impossible to determine which of the two parties
computed the MAC.

30. Birthday attack in Cryptography
Birthday attack is a type of cryptographic attack that belongs to a class
of brute force attacks. It exploits the mathematics behind the birthday
problem in probability theory. The success of this attack largely
depends upon the higher likelihood of collisions found between
random attack attempts and a fixed degree of permutations, as
described in the birthday paradox problem.

31. Birthday paradox problem –
Let us consider the example of a classroom of 30 students and a teacher. The
teacher wishes to find pairs of students that have the same birthday. Hence
the teacher asks for everyone’s birthday to find such pairs. Intuitively this
value may seem small. For example, if the teacher fixes a particular date
say October 10, then the probability that at least one student is born on that
day is 1 – (364/365)30 which is about 7.9%. However, the probability that at
least one student have same birthday as any other student is
around 70% using the following formula:
• 1 - 365!/((365 - n!) * (365n)) (substituting n = 70 here)

32. Birthday paradox problem –
• Assumptions –
1. Assuming a non leap year(hence 365 days).
2. Assuming that a person has equally likely chance of being born on any
day of the year.
Let us consider n = 2.
P(Two people have the same birthday) = 1 – P(Two people having different
birthday)
= 1 – (365*365)*(364*365)
= 1 – 1*(364/365)
= 1 – 364/365
= 1/365.
So for n people the probability that all of them have different birthday is:
P(N people having different birthdays) = (365/365)*(365-1/365)*(365-
2/365)*….(365-n+1)/365.
= 365!/((365-n)! * 365n)

33. public key certificate
• A public key certificate provides a safe way for an entity to pass on its
public key to be used in asymmetric cryptography. The public key certificate
avoids the following situation: if Charlie creates his own public key and
private key, he can claim that he is Alice and send his public key to Bob. Bob
will be able to communicate with Charlie, but Bob will think that he is
sending his data to Alice.
• A public key certificate can be thought of as the digital equivalent of a
passport. It is issued by a trusted organization and provides identification
for the bearer. A trusted organization that issues public key certificates is
known as a certificate authority (CA). The CA can be likened to a notary
public. To obtain a certificate from a CA, one must provide proof of identity.
When the CA is confident that the applicant represents the organization it
says it represents, the CA signs the certificate attesting to the validity of the
information contained within the certificate.

34. public key certificate
• A public key certificate contains several fields, including:Issuer - The issuer is the
CA that issued the certificate. If a user trusts the CA that issues a certificate, and
if the certificate is valid, the user can trust the certificate.
• Period of validity - A certificate has an expiration date, and this date is one piece
of information that should be checked when verifying the validity of a certificate.
• Subject - The subject field includes information about the entity that the
certificate represents.
• Subject's public key - The primary piece of information that the certificate
provides is the subject's public key. All the other fields are provided to ensure the
validity of this key.
• Signature - The certificate is digitally signed by the CA that issued the certificate.
The signature is created using the CA's private key and ensures the validity of the
certificate. Because only the certificate is signed, not the data sent in the SSL
transaction, SSL does not provide for non-repudiation.

35. public key certificate
• If Bob only accepts Alice's public key as valid when she sends it in a public
key certificate, Bob will not be fooled into sending secret information to
Charlie when Charlie masquerades as Alice.
• Multiple certificates may be linked in a certificate chain. When a certificate
chain is used, the first certificate is always that of the sender. The next is
the certificate of the entity that issued the sender's certificate. If there are
more certificates in the chain, each is that of the authority that issued the
previous certificate. The final certificate in the chain is the certificate for a
root CA. A root CA is a public certificate authority that is widely trusted.
Information for several root CAs is typically stored in the client's Internet
browser. This information includes the CA's public key. Well-known CAs
include VeriSign, Entrust, and GTE CyberTrust.
•

36. Man-in-the-Middle Attacks
• A Man-in-the-Middle Attack (MITM) is a form of cyber eavesdropping in
which malicious actors insert themselves into a conversation between two
parties and intercept data through a compromised but trusted system. The
targets are often intellectual property or fiduciary information. MITM
aggressors will also use malware to open the communications channel with
the hopes of creating zombie machines or building vast networks of
comprised systems. Man-in-the-Middle Attacks can be used as way into
systems in order to execute an advanced persistent threat (APT).
• In many cases organizations are unaware their session and/or data has
been tampered with until it is much too late. If a MITM attack is successful,
organizations experience negative brand perception, reduced customers’
confidence and ultimately a damaged bottom line.

37. How Man-in-the-
Middle Attacks work
• MITM attacks can be executed in
a number of different ways that
exploit communications between
other parties. Whether by
passive or active means, an
MITM attack finds a way
between a user and an entity and
attempts to conceal the breach
and information theft. Below are
common ways Man-in-the-
Middle Attacks manipulate
communication systems.

38. Types of Man-in-the-Middle Attacks
• Email Hijacking – attackers gain access to a user’s email account and watch transactions
to and from the account. When the time is right, for instance the user is exchanging
funds with another party, the attacker takes advantage of the situation by attempting to
intercept the funds by spoofing one or all members of the conversation.
• Wi-Fi Eavesdropping – a passive way to deploy MITM attacks, Wi-Fi eavesdropping
involves cyber hackers setting up public Wi-Fi connections, typically with an unsuspecting
name, and gain access to their victims as soon as they connect to the malicious Wi-Fi.
• Session Hijacking – session hijacking is when an attacker gains access to an online
session via a stolen session key or stolen browser cookies.
• DNS Spoofing – an attacker engages in DNS spoofing by altering a website’s address
record within a DNS (domain name server) server. A victim unknowingly visits the fake
site and the attacker will attempt to steal their information.
• IP Spoofing – similar to DNS spoofing, IP Spoofing sees an attacker attempt to divert
traffic to a fraudulent website with malicious intent. Instead of spoofing the website’s
address record, the attacker disguises an IP (internet protocol) address.

39. How to Prevent Man-in-the-Middle Attacks
• Implement a comprehensive Email Security Solution – an email security solution is a vital tool in
an organizations security architecture that will help minimize the risks associated with MITM. It
secures email activity pro-actively while staff focuses efforts elsewhere.
• Implement a Web Security Solution – a strong web security tool provides visibility to web traffic
generated by both the system and end user at protocol and port layers. Similar to an email
security tool, this implementation protects your organization’s web traffic so the security team
can cover more ground.
• Educate Employees – Making training relevant to the employee is key to training effectiveness.
Prepare your workforce for these advanced attacks by educating them on the dynamics, patterns,
samples and frequency of attack methods attempted on other organizations. Case studies are a
valuable resource when putting together educational materials and awareness programs.
• Keep credentials secure - Check your user credentials often. Make sure your passwords are
secure, complex and updated every three months at a minimum. This will aid in your company’s
protection by keeping credentials fresh and more difficult to crack.
• Forcepoint can help you prevent and guard against Man-in-the-Middle attacks. Both Forcepoint’s
Email Security and Web Security tools offer best-in-class protection. For more, visit Forcepoint’s
product overview.