Security Basic Notions

Information Security

Preservation of confidentiality, integrity, and availability of information

Other properties can also be involved, such as authenticity, accountability, nonrepudiation, and availability

Network Security

Protection of networks and their service from unauthorized modification, destruction, or disclosure

Provision of assurance that the network performs its critical functions correctly and there are no harmful side effects

Communications Security

Protection of communications through network devices

Device/System Security

Protection of device/system against intruders that could gain access to the system in various ways to perform unauthorized actions

Key objectives to achieve information and network security

Confidentiality

Integrity

Availability

Authenticity

Accountability

CIA Triad

Confidentiality

Integrity

Availability

Confidentiality

Data confidentiality: Assurance that private or confidential information is not made available or

disclosed to unauthorized individuals

Privacy: Assurance that individuals control or influence what information related to them may be

collected and stored and by whom and to whom that information may be disclosed

Integrity

Data integrity: Assurance that data and programs are changed only in a specified and authorized manner (incl. authenticity)

System integrity: Assures that a system performs its intended function in an unimpaired manner, free from deliberate or inadvertent unauthorized manipulation (incl. accountability)

Availability

Assurance that a service is not denied to authorized users

Types of Attacks

Passive Attacks

Active Attacks

Passive Attacks

Attempt to learn or make use of information without affecting system resources

• Eavesdropping

• Traffic Analysis

Difficult to detect, but (relatively) easy to prevent

Active Attacks

Involve active actions on information flow

• Masquerade/Impersonation

• Replay Attacks

• Data deletion

ITU-T X.800 Recommendation

Standard that establishes Security Service Framework

Security Service

a capability that supports one or more security objectives

Six (6) fundamental security services

• Authentication

• Access Control

• Confidentiality

• Integrity

• Non-Repudiation

• Availability

Authentication

Assurance that an entity is really the one who it claims to be (assurance can be provided in multiple ways)

Can be one-way or mutual

Peer Authentication

In a connection-oriented transfer, it ensures confidence that the entity you are communicating with is the one who claims to be

Usually performed at:

Connection Initiation

Connection Refresh

Re-synchronization

Connection Initiation

Establishment phase where two communication systems first greet each other and build a secure relationship

Connection Refresh

Maintenance phase that keeps an ongoing session secure over a long period of time

Re-synchronization

Data Authentication

In a connection-less transfer, it ensures that the source/destination of the data are the intended ones

Access Control / Authorization

Offers the assurance that the entity performing a given action has the rights to do so

Data Confidentiality

Guarantee that data are available only to intended parties

Protection against unauthorized disclosure

Specific portions of data may be protected

Prevention of Eavesdropping Attacks (e.g., reading a file)

Data Integrity

Assurance that data have not been modified maliciously

• No insertion, modification,reordering

Prevention of Active Attacks

Non-Repudiation

Ensures that an entity cannot deny having done an action (e..g, either the sender or the receiver cannot deny to have sent/received a particular message)

Related to Data Authenticity, but extends the concept (time, data)

Services Availability

Property of a system to be always available and ready to serve requesting entities

Security Mechanism/Algorithm

A mathematical procedure applied to secure data

Security Protocols

Sequence of operations providing one or more security services to the data or the communication, through one or more security algorithms

Security Algorithms

AES

SHA-3

HMAC

Classes: Hashing, Key Agreement, Encryption, Symmetric and Asymmetric Crypto

Security Protocols

ECDH, TLS, WPA

Classes: Key Agreement

Generic Network Security Model Details

The channel is shared and publicly-available (also to attackers)

Sender and Receiver cooperate to securely exchange messages

• Algorithms

• Secrets

Trusted Third-Party (TTP) may be involved to distribute secrets and synchronize communications

Secure Transformation via Cryptography

Cryptography allows to hide the message content through a secure transformation

Such transformation is also reversable: anyone in possession of the correct information can re-obtain the original message, i.e., the plaintext, starting from the secure message, i.e., the ciphertext

Elements of a Cryptosystem

• Plaintext: m

• Ciphertext: c

• Encryption Key: k

• Encryption Algorithm: E(k,m) = E k (m) = c

• Decryption Key: k’

• Decryption Algorithm: D(k’,c) = D k (m) = m

3 Dimensions of Cryptographic Systems

The type of operations used for transforming plaintext to ciphertext

The number of keys used

The way in which the plaintext is processed

The type of operations used for transforming plaintext to ciphertext

Substitution

Transposition

The number of keys used

Symmetric, or secret-key, encryption

Asymmetric, or public-key encryption

The way in which the plaintext is processed

Block cipher

Stream cipher

Symmetric Cryptography features

• The encryption and decryption keys are the same (k = k’)

• Limited computational overhead

• Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure

• Algorithm should be strong enough

Caesar’s Algorithm

• Substitution cipher

• Involves replacing each letter of the alphabet with the letter standing three places further down the alphabet (substitution cipher)

• Alphabet is wrapped around so that the letter following Z is A

Attacking an Encryption System

Objective: Recover the key in use rather than simply to recover the plaintext of a single ciphertext

Two approaches to Encryption System attack

Cryptanalysis

Brute-force

Cryptanalysis

Rely on the nature of the algorithm plus some knowledge of the general characteristics of the plaintext

Brute-force

• Try every possible key on a piece of ciphertext until an intelligible translation into plaintext is obtained

• On average, attackers should try half of the possible keys to have success

• Longer keys, higher security

Vernam Cipher

• Stream Cipher

• Original version re-used keys after long periods of time, thus enabling cryptanalysis with sufficient ciphertext

• Random key for each plaintext, no correlation with the plaintext Security depends on the randomness of the key

• Theoretically unbreakable, but hardly feasible

  • Key Generation and Distribution

  • Perfect Security

Stream Ciphers

• Encryption of digital stream bit per bit or byte per byte

• Keystream is of the same size of the plaintext

• If the keystream is random, the cipher is unbreakable

• Practical issues with actual deployment

  • Very long keystream for high-volume data

  • Distribution of such keystream to intended receivers

Block Cipher

• Encryption of a block of plaintext producing a block of ciphertext of the same size

• Typical block size: 64 or 128 bits

• Widely used in modern encryption systems

Feistel Cipher

Alternating two basic operations

• Substitutions: fixed replacement of groups of plaintext with groups of ciphertext

• Permutations: the order in which the elements appear in the sequence is randomly changed

Steps of Feistel Cipher

• Spit plaintext into half

• Right side enters function with key k, XORd with left side

• Left side switches positions with right side

• Process repeated however many times

• Same steps can be repeated to decrypt the ciphertext

Data Encryption Standard (DES)

• Data are encrypted in 64-bit blocks using a 56-bit key

• The algorithm transforms 64-bit input in a series of steps into a 64-bit output

• The same steps, with the same key, are used to reverse the encryption

DES Steps

• 64 bit key is generated (56 bits of actual key is used, 8 bits are parity bits)

• 16 subkeys are generated from this initial key

• Feistel cipher applied, 48 bit subkey is used in the function

• Final 32 bit swap of both halves applied after 16 rounds

Limitations of DES

Keys are too short (56 or 64 bits)

• Vulnerable to brute-forcing attacks in very short time

Differential and Linear Cryptanalysis can also break DES with less complexity than brute-forcing

Advanced Encryption Standard (AES)

Encryption and Decryption Operations are different circuits

10 rounds

Advanced Encryption Standard Round Steps

1. Substitute Bytes (changing values with fixed lookup table)

2. Shift Rows

3. Shift Columns

4. Add Round Key (XOR with Round Key derived from Main Key)

5. Round Keys are generated using Key Expansion that generates words from preceding words

Mode of Operation

Specific way the cryptographic algorithm is used, in combination with the key and the plaintext, to generate the ciphertext

Mode of Operation Types

Electronic Code Book

Cipher Block Chaining

Cipher Feedback

Output Feedback

Counter Mode

Electronic Code Book

Each block is encrypted separately

Simple to implement

Prone to cryptanalysis

Block order can be altered (no integrity)

Cipher Block Chaining (CBC)

Blocks ordering dependence

Error Propagation (errors propagate forever…)

Cipher Feedback (CFB)

Dependence on the past (same input, different output)

Works on group of s bits

• S s → first s bits of S

Errors propagate to 1 + b/s parts of the plaintext

Not very fast (you need the plaintext..)

Steps of Cipher Feedback

b bit segment is the shift register that updates per iteration

b bit block split into b-s and s bit long segments

entire b bit segment is encrypted with function E and key k

s and b-s bit segments are swapped over

s is XORd with plain text segment, propagated to shift register, old s bits are discarded

due to this mechanism, errors can propagate for another b/s times

Output Feedback

Similar mechanism to Cipher Feedback but instead the s bit is directly propagated to the shift register without XOR on plaintext

Counter Mode

Start with a base value, increment it by 1 for every block of cipher text and encrypt it

Very fast

AES - Galois Counter Mode (GCM)

Extends Counter Mode with Authentication through creation of Auth Tag

mult H is a hashing function

Assymetric Cryptography

Encryption and Decryption Keys are different

Usage of complex problems over modular groups

Many different schemes

Diffie Hellman (DH)

Both parties agree on a g and p value (even attacker knows this)

One party picks unique value “a,“ the other picks unique value ”b”

Exponent-Modulo calculation is done on a and b separately to create A and B

A and B are exchanged

A and B are raised to the parties other chosen number, to create the final key

Note: Does not have authentication mechanism built in, MITM can occur

Rivest Shamir Adleman (RSA)

Private key generated through two huge primes (p and q)

Signature is created using private key and exponential-modulo calc

Signature sent alongside original plaintext, receiving party unscrambles signature to verify authenticity

Usages of Asymmetric Cryptography

Confidentiality – Symmetric Key Distribution

Message Authentication

Data Confidentiality and Authentication (opposite also possible, both public and private key can be used to encrypt/decrypt information)

Elliptic Curve Cryptography (ECC)

Elliptic Curve arithmetic, in place of modular arithmetic

Advantage: reduced computational complexity, and many existing problems are harder to solve

ECC Basic Concepts

Any elliptic curve can be identified through six (6) parameters:

• a,b parameters of the curve

• G generator

• n order

• g cyclic group

• γ, co-factor

Any scalar number a can be mapped on a curve through the generator G: A = a ⋅ G (it is possible to obtain A on the curve by jumping by G a times)

The addition of any two points on the curve gives another point: C = A + B (drawing line between A and B creates a new intersection at C)

Elliptic Curve Diffie Hellman

Publicly agreed: curve equation, base point on curve

Parties A and B pick their scalars, public key calculated using point scalar multiplication

These new calculation results are exchanged

Cryptographic key management

The process of administering or managing cryptographic keys for a cryptographic system

What does Cryptographic Key Management Entail?

It involves the generation, creation, protection, storage, exchange, replacement, and use of keys and enables selective restriction for certain keys

In addition to access restriction, key management also involves the monitoring and recording of each key’s access, use, and context

A key management system also includes key servers, user procedures, and protocols

The security of the cryptosystem is dependent upon successful key management

Secure Key Generation

Secure = random and unpredictable

Performed using Random Number Generation

2 requirements for Security Viable random numbers

Randomness

• Uniformly-distributed 0s and 1s

• All bits independent from each other

Unpredictability

• You cannot predict the next bits looking at previous ones

Pseudorandom Number Generation and Functions

True random number generator: True random source converted to binary

Psuedorandom number generator: deterministic algorithm
Psuedorandom function: number generated depends on context info (ex. a string)

Randomness Tests

The generated bit stream needs to appear random even though it is deterministic

NIST SP 800-22

• Uniformity, Scalability, Consistency

• Frequency Tests (freq. of 0 and 1 in the generated stream are uniform)

• Runs Tests (sequences of continuous 0 and 1 is as expected)

• Maurer’s Tests (compression should lead to consistent information loss)

Algorithms for PRNG

Purpose-built algorithms (Algorithms designed specifically and solely for the purpose of generating pseudorandom bit streams)

Algorithms based on existing cryptographic algorithms (when processed, the cipher text stream looks indistinguishable from a random sequence)

Symmetric Key Distribution Options

Others should not see the key

Keys should also be changed frequently, to prevent compromise

• A can select a key and physically deliver it to B

• A third party can select the key and physically deliver it to A and B

• If A and B have previously and recently used a key, one party can transmit the new key to the other, encrypted using the old key

• If A and B each has an encrypted connection to a third-party C, C can deliver a key on the encrypted links to A and B

Asymmetric Key Distribution and Publication

Private key should be distributed (similiarly to symmetric)

Public key should be published

One-way Hashing Functions

One-Way: for any given h, it is computationally impossible to obtain the message m used to generate h

Weak Collision Resistance (a.k.a. Pre-image Resistance): given x, it is computationally hard to find y ≠ x s.t. H(x) = H(y)

Strong Collision Resistance: it is computationally hard to find any pair of messages (x,y) s.t. H(x) = H(y)

Secure Hash Algorithm Parameters

Message Digest Size: size of the memory used within the algorithm to process the data

Message Size

Block Size: input message is processed in chunks of this size, a batch of the message

Word Size: Chunk of the message derived from block

Number of Steps: number of times the core compression function is applied

Applications of One-Way Hashing Functions

Message Integrity Verification

• Digest of the message at the sender matches reconstruction on the receiver (receiver uses same hash function to check integrity)

Signature Generation and Verification (hashes are also encrypted)

Hashing and Confidentiality

Applying one-way hashes does not allow to recover plaintext information, but only to verify it

Thus, confidentiality cannot be provided

One-way Hashes usage (Services)

Integrity

Authenticity