CIS 3500 1 - Metropolitan State University of Denverrowdysites.msudenver.edu/~fustos/cis3500/pdf/chapter26.pdf · CIS 3500 2 General Cryptographic Concepts nPlaintext needs to be

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CIS 3500 1

Cryptographic Concepts

Chapter #26:

Cryptography and Public Key Infrastructure

Chapter Objectives

n Identify the different types of cryptography

n Learn about current cryptographic methods

n Understand how cryptography is applied for security

n Given a scenario, utilize general cryptography concepts

n Compare and contrast basic concepts of cryptography

Cryptographic Concepts2


n Cryptography is the science of encrypting, or hiding,

information

n These complicated methods are cryptographic algorithms,

also known as ciphers

n The word “cipher” comes from the Arabic word sifr,

meaning empty or zero


General Cryptographic Concepts

n Historical ciphers were simple to use, simple to break

n More advanced transposition and substitution ciphers

n Ciphers were automated (electro)mechanical device

n German Enigma machine from World War II

n When setting up a cryptographic scheme, it is important


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CIS 3500 2

General Cryptographic Concepts

n Plaintext needs to be protected – it is encrypted into ciphertext

n We use an algorithm and a key

n Cryptanalysis – attempt to return the encrypted message to its

original form

n Differential cryptanalysis – comparing the input plaintext to the

output ciphertext to try to determine the key

n Linear cryptanalysis – uses both plaintext and ciphertext, puts the

plaintext through a simplified cipher to try to deduce what the key

is likely to be in the full version


Fundamental Methods

n The choice of algorithm depends on the type of cryptographic operation

that is desired

n Choice of key is then tied to the specific algorithm

n Cryptographic operations include

n encryption for the protection of confidentiality

n hashing for the protection of integrity

n digital signatures to manage non-repudiation, and

n specialty operations such as key exchanges

n Mathematical specifics can be very complex

n Data is characterized by its usage: data-in-transit, data-at-rest, or data-

in-use; block form or stream form



Algorithms

n Encryption schemes are based upon an algorithm

n step-by-step problem-solving procedure

n made up of mathematical steps for encrypting and decrypting

information

n They are designed to use a key – a special piece of data used in

both the encryption and decryption processes

n The algorithms are the same, but every implementation uses a

different key

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Symmetric Algorithms

n Usually use same key for encryption and decryption

n Require sender and receiver to agree on a key before they

communicate securely

n Security lies with the key

n Also called secret key algorithms, single-key algorithms, or

one-key algorithms


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Modes of Operation

n In symmetric or block algorithms, there is a need to deal

with blocks of identical data to prevent multiple blocks of

ciphertext that would identify the blocks of identical input

n There are five common algorithmic modes that are detailed

in NIST SP 800-38A, Recommendation for Block Cipher

Modes of Operation: Methods and Techniques

n Electronic Code Book (ECB), Cipher Block Chaining (CBC),

Cipher Feedback Mode (CFB), Output Feedback Mode

(OFB), and Counter Mode (CTR)Cryptographic Concepts9 Cryptographic Concepts

Asymmetric Algorithms

n Use different keys for encryption and decryption

n Decryption key cannot be calculated from the encryption

key

n Anyone can use the key to encrypt data and send it to the

host; only the host can decrypt the data

n Also known as public key algorithms

n RSA, Diffie-Hellman, ECC, and ElGamal are asymmetric

protocols

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ECC (Elliptic Curve Cryptography)

n ECC works on the basis of elliptic curves

n y2 = x3 + ax2 + b

n They have a special property — you can add two points on

the curve together and get a third point on the curve

n Users agree on a curve and a fix point

n With a secret random number calculate a public key

n That can be used to encrypt messages

11 Cryptographic Concepts

Symmetric vs. Asymmetric

Type of Algorithm Advantages Disadvantages

Symmetric Single key

Faster

Less computation

Requires sender and receiver to agree on a

key before transmission of data

Security lies only with the key

High costs

Asymmetric Encryption and decryption keys

are different

Decryption key cannot be

calculated from encryption key

Security of keys can be compromised when

malicious users post phony keys

Slower

Lots of computing power needed

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CIS 3500 4

Hashing

n Hashing algorithm is a special mathematical function that performs one-

way encryption – no feasible way to retrieve the plaintext

n There is no feasible way to generate two different plaintexts that compute

to the same hash value

n Store computer passwords - the hash value is reproducible

n Can be compromised with a collision attack, in which an attacker finds

two different messages that hash to the same value

n Hash algorithms: Secure Hash Algorithm (SHA) series, the RIPEMD

algorithms, and the Message Digest (MD) hash of varying versions (MD2,

MD4, MD5)


Salt, IV, Nonce

n The term salt refers to addition of a high-entropy piece of

data concatenated with the material being hashed

n Salts are useful when the text is short and low in entropy

n An initialization vector, or IV, is used in several ciphers,

particularly in the wireless space

n IVs can add randomness and are used in block ciphers

n A nonce is similar to a salt or an IV, but it is only used

once


Weak/Deprecated Algorithms

n The challenge is understanding which algorithms have fallen to

attacks and avoiding their use

n Hash algorithms, such as MD5, should be considered inappropriate -

manufactured collisions have been achieved

n Even newer hash functions have issues, such as SHA-1 and soon SHA-

256

n The Data Encryption Standard, DES and 3DES, have fallen from favor

n There new forms of these functions are widely available, e.g. AES

that are computationally efficient


Key Exchange

n Cryptographic mechanisms use an algorithm and a key

n In symmetric encryption, the secrecy depends upon the secrecy of the key

n Key exchange is the central foundational elemen

n In asymmetric systems, the key exchange problem is one of key

publication

n Early key exchanges were performed by trusted couriers

n The Diffie-Hellman key exchange is one example of secure key exchange

n It depends upon two random numbers, each chosen by one of the parties

and kept secret


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CIS 3500 5

Cryptographic Objectives

n Cryptographic methods exist for a purpose: to protect the

integrity and confidentiality of data

n There are many associated elements to enable a system-

wide solution

n Perfect forward secrecy, non-repudiation, key escrow, and

others enable successful cryptographic implementations


Digital Signatures

n A digital signature is a cryptographic implementation

designed to demonstrate authenticity and identity

associated with a message

n Digital signatures are based upon both hashing functions

and asymmetric cryptography

n User can decrypt the hash with the public key of the

originator, that user knows that the hash was encrypted by

the corresponding private key – nonrepudiation


Diffusion

n Diffusion is a principle that the statistical analysis of

plaintext and ciphertext results in a form of dispersion

n A change in one character of plaintext should result in

multiple changes in the ciphertext in a manner that

changes in ciphertext do not reveal information as to the

structure of the plaintext


Confusion

n Confusion is a principle to affect the randomness of an

output

n The concept is operationalized by ensuring that each

character of ciphertext depends on several parts of the key

n Confusion places a constraint on the relationship between

the ciphertext and the key employed, forcing an effect that

increases entropy


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Collision

n A collision is when two different inputs have the same output on a

cryptographic function such as a hash

n Inputs to a hash function are infinite (unlimited in length) and the

number of unique outputs is limited by hash size, collisions have to

occur

n The issue is whether one can manufacture a collision

n If two inputs can be generated that produce the same hash value, this

enables the movement of a digital signature from an original to a near

duplicate, resulting in the failure of the digital signature to protect an

original


Steganography

n Steganography gets its meaning from the Greek word

“steganos,” meaning covered

n Hide data inside other data

n The most common application is the concealing of a text

message in a picture file – difficult to detect

n They can also be hidden in video and audio files

n Editing the LSB (least significant bit)


Obfuscation

n Obfuscation is the masking of an item to render it

unreadable yet still usable

n Code obfuscation is the process of making the code

unreadable by adding complexity at the time of creation

n Makes it impossible to easily understand, copy, fix, maintain

n Using cryptographic functions to obfuscate materials is more

secure in that it is not reversible without the secret element


Stream vs. Block

n Block operations are performed on blocks of data, enabling both

transposition and substitution operations

n Stream data has become more common with audio and video

across the Web – bit by bit or byte by byte (substitution


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CIS 3500 7

Key Strength

n The strength of a cryptographic function typically depends

upon the strength of a key

n Direct comparison of key strength is not easily done

n Some cryptographic systems have fixed key lengths, such

as 3DES, while others, such as AES have multiple lengths,

AES-128, AES-192, and AES-256


Session Keys

n A session key is a symmetric key used for encrypting messages

during a communication session

n It is generated from random seeds and is used for the duration of

a communication session

n Session keys provide significant levels of protection during the

communication session – perfect forward secrecy

n Offer the advantages of symmetric encryption, speed, strength

and simplicity, and, with key exchanges possible via digital

methods, significant levels of automated security


Ephemeral Key

n Ephemeral keys are cryptographic keys that are used only

once after generation

n When an ephemeral key is used as part of the Diffie-

Hellman scheme, it forms an Ephemeral Diffie-Hellman

(EDH) key exchange

n An EDH key exchange generates a temporary key for each

connection, never using the same key twice

n This provides for perfect forward secrecy


Secret Algorithm

n Algorithms: published details and those whose steps are

kept secret

n Secrecy has its uses in security: passwords, or how to

apply security elements, or use of multiple rounds of

multiple hash functions

n It makes reverse engineering difficult, if not impossible

n The drawback of keeping a cryptographic algorithm secret

is that it reduces the testing of the algorithm


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Data-in-Transit

n Transport encryption is used to protect data-in-transit, or

data that is in motion

n It is at risk of interception

n When utilizing the TCP/IP protocol, Transport Layer

Security (TLS) is one specific method of managing security

at the transport level

n Secure Sockets Layer (SSL) is another example


Data-at-Rest

n Protecting data-at-rest is the most prominent use of

encryption – referred to as data encryption

n Whole disk encryption of laptop data is an example of data-

at-rest protection

n The same concept applies to data being stored in the cloud,

where encryption can protect against unauthorized reading


Data-in-Use

n Data-in-use is the term used to describe data that is stored

in a non-persistent state of either RAM, CPU caches, or CPU

registers - concern

n RAM scraping malware are occurring

n This data can be encrypted

n New techniques, such as Intel’s Software Guard Extensions

(SGX), promise a future where sensitive data can be

protected from all other processes


Random/Pseudo-Random Number Generation

n Many cryptographic functions require a random number

n A true random number has no correlation to previous or

future random numbers, and has a uniform distribution

n The next number cannot be predicted

n True random numbers are virtually impossible to generate


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CIS 3500 9

Random/Pseudo-Random Number Generation

n Pseudo-random numbers statistically appear to be random

with respect to frequency distribution, because they are

algorithmically generated, if one knows the algorithm and the

seeds, one can predict future values

n For cryptographic purposes unpredictability important

n Specialized random/pseudo-random number generation

algorithms that minimize the predictability element

n Use a cryptographically secure random number generator to

prevent introducing flaws into the encryption protections


Key Stretching

n Key stretching takes what would otherwise be weak keys

and “stretches” them to make the system more secure

n With computational power hash functions can be computed

very quickly

n To make the problem more difficult, either the keyspace

must be increased or the computation must be slowed down

n Key stretching: increasing the computational complexity by

adding iterative rounds of computations


Implementation vs. Algorithm Selection

n When using cryptography for protection of data, several

factors need to be considered

n One of the first decisions is which algorithm to select

n The algorithm must be matched to the intended use, and

deprecated algorithms must be avoided


Cryptographic Modules

n A cryptographic module is a hardware or software device

that performs cryptographic operations

n Crypto modules use a hardware, software, or hybrid

cryptographic engine, and cryptographic keys that do not

leave the boundary

n Maintaining all secrets within a specified protected

boundary is a foundational element of a secure

cryptographic solution


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CIS 3500 10

Cryptographic Service Provider

n A cryptographic service provider (CSP) is a software library

that implements cryptographic functions – encoding and

decoding

n In Windows, the Microsoft CryptoAPI (CAPI) is a CSP for all

processes that need specific cryptographic function

n This provides a standard implementation of a complex set

of processes


Perfect Forward Secrecy

n Perfect forward secrecy is a property of a public key system

n A key derived from another key is not compromised even if

the originating key is compromised in the future

n This is especially important in session key generation:

if perfect forward secrecy were not in place, then past

messages that had been recorded could be decrypted


Security Through Obscurity

n Security through obscurity is the concept that security can

be achieved by hiding what is being secured

n This alone has never been a valid method of protecting

secrets

n But this does not mean obscurity has no role in security

n It can make hard for an attacker to easily guess critical

pieces of information


Common Use Cases

n Cryptographic services are being employed in more and

more systems

n There are many common use cases associated with them

n Examples include implementations to support low power,

low latency, high resiliency, confidentiality, integrity, and

non-repudiation


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CIS 3500 11

Stay Alert!

There is no 100 percent secure system, and

there is nothing that is foolproof!

Documents

CIS 3500 1 - Metropolitan State University of Denverrowdysites.msudenver.edu/~fustos/cis3500/pdf/chapter26.pdf · CIS 3500 2 General Cryptographic Concepts nPlaintext needs to be