Security Basics

9 March, 2006 mark.baker@computer.org

Security Basics

Prof Mark Baker

ACET, University of Reading Tel: +44 118 378 8615 E-mail: Mark.Baker@computer.org Web: http://acet.rdg.ac.uk/~mab

Basic Security - Outline• Concerns.• Objectives.• Basic Definitions• Security Components:

– Symmetric/asymmetric systems,– Public Key Encryption.

• Public Key Infrastructure:– Certificates,– Signatures.

• Summary.

Security Concerns• Unauthorized access to resources.• Masquerade as authorized user or end

system.• E-mail forgery.• Malicious attacks.• Monitoring and capture of network traffic.• Exploitation of software bugs.

Contributing Factors• Increased Internet use:

– Home broadband,– Greater coverage (wired and wireless):– More ubiquitous on-line use:

• Education,• Business,• Games,• Shopping…

• Lack of awareness of threats and risks.• Wide-open network policies.• Unencrypted network traffic.• Complexity of security measurements and

administration.• Software bugs.• Availability of cracking tools .

The Actors

Attack Sophistication vs. Intruder Technical Knowledge

1980 1985 1990 1995 2000 2005 2010

Automated Probes/Scans

Password GuessingSelf-Replicating Code

Password Cracking

Exploiting Known Vulnerabilities

Disabling Audits

Hijacking Sessions

Sweepers

Sniffers

Distributed Attack Tools

Denial of Service

Network Management Diagnostics

Web Attacks

“Stealth”/AdvancedScanning TechniquesHigh

Intruders

Back Doors

Zombies

Morphing

Malicious Code

Attack Sophistication

Intruder Knowledge

Packet Spoofing

Sources: Carnegie Mellon University, 2002 and Idaho National Laboratory, 2005

Basic Security Terms• Classic security concerns deal more with data:

– Confidentiality – data only available to those authorised,– Availability – you can get it when you want it,– Integrity – data has not been changed.

• Additional concerns deal more with people and transactions:– Trust – who you are and what you are authorized to do,– Non-repudiation – you can’t deny doing something you

did,– Auditability – I can check what you did to the data,– Reliability – the system does what I want, when I want it

to,– Privacy – within certain limits no one should know who I

am or what I do.

Basic Security Objectives• Confidentiality: prevent/detect/deter

improper disclosure of information.• Integrity: prevent/detect/deter improper

modification of information.• Availability: prevent/detect/deter improper

denial of access to services.

Security TermsAuthentication:• The process by which a person or other entity

proves that it is who (or what) it says it is. • Want to authenticate the person or entity that you

are dealing before transferring something valuable, such as information or money, to or from, it.

• Authentication is achieved by presenting some unique identifying entity to the endpoint that is undertaking the process:– An example of this process is the way you authenticate

yourself with an ATM: here you insert your bank card (something you have) and enter your personal identification number (PIN, something you know).

Identification• Being able to identify yourself to a

computer is absolutely essential:– ATM, e-banking,– Access to e-mail, computer accounts,– Access to personal information (e.g., staff or

student portal).

Non-computer identification• Bank teller knows you by sight (good).• Bank teller checks your picture against a photo ID

(dodgy).• Bank back office compares cheque signature to

one on record (dodgy).• All examples of biometric identification.

Computer Identification• How we identify a human to a computer?

– Username/Passwords (common),– Token, e.g. ATM card,– Cryptographic protocols,– Combinations, e.g. token and password,– Biometrics, e.g. face recognition, finger prints,

and retina/iris scans.

Passwords• Most common identification technique:

– Variants: such as “PIN” (number), memorable date, mothers maiden name.

• Problem: we are not well-suited to remembering passwords:– Especially rarely used ones,– We can also confuse passwords used in similar

contexts.

Vulnerabilities• Users reveal passwords to outsiders.• Users reuse passwords.• Users choose “easy to guess” passwords.• Password observed on entry.• Password obtained from system files.

Biometric identification• Passwords are pretty useless at identifying

people.• Can we identify them by their properties?

– Face, handwriting, retina, DNA, voice, signature, fingerprint…

• “How humans identify other humans”.

Other issues• Cost:

– Voice recognition is cheap,– Eye (iris) scanning is expensive.

• User comfort:– Face recognition is nice (look into camera),– DNA matching is not (blood/skin sample).

• Theoretical accuracy:– Iris is unique (determined while an embryo),– DNA is shared by identical twins,– Voice can be imitated.

• Excluded population:– Voice does not work on mute people,– Fingerprints do not work on amputees,– DNA works on everyone!

• Variability:– Dirty fingers, or sick (cold) for voice.

Security TermsAuthorisation:• Is the act of providing the rights to perform some

action:– Typically based on based on what are known as Access

Control Lists (ACLs), which for some set of resources, a list of user names and their rights are provided.

• For example, the mere possession of a security badge does not grant you the right to enter a restricted area, such as the administration room:– An examples could be a guest list for an event or a door

lock that reads your badge.

Setting Up Access Rights• Classify users into groups:

– Patients, doctors, chemist, lab, NHS admin, …

• Classify resources into groups:– Prescriptions, blood test results, diagnoses, patient

contact details, …

• Classify access rights:– Read, write, delete, modify, append, …– Domain specific: number AIDS cases per region.

Access Control Lists (ACL)• Specify the access permissions of each group for

each resource (or resource type):– (doctors, blood-test.db) – read access.– (lab, blood-test.db) - read, write access.

• Program-specific permissions:– Allows application-specific restrictions:

• (NHS, blood-test.db, SPSS) – AIDS/region

Security TermsTrust:• Trust is the “assured reliance on the character,

ability, strength, or truth of someone or something”.

• A distributed environment requires explicit statements of trust, such as:– “who is trusted to do what”,– Also obligations of all the parties involved in the trust

relationship.

• Trust percolates through almost every stage of today’s security infrastructure and can be seen as a key issue with the world of information assurance.

Security TermsIntegrity:• This is the assurance that the data has not

changed since it was written:– e.g., prevent a potential intruder-in-the-middle from

changing messages.

• Data integrity can be checked using:– A check-sum, which is a simple error-detection scheme

where each transmitted message is accompanied by a numerical value based on the number of set bits in the message:

• Checked by the receiving station - if different the receiver can assume that the message has been garbled.

– Hash functions, any one-way function that reduces variable sized data to a fixed length “hash code”:

• If the hashes of two documents differ, then the documents differ.

Security TermsConfidentiality:• This is the act of ensuring no one but authorised

parties (who know some secret) can understand the data.

• There are two mechanisms used to ensure data confidentiality, the more common encryption, and steganography: – With encryption an algorithm or function (encrypt) that

transforms plain text to cypher text where the meaning is hidden, but which can be restored to the original plain text by another algorithm (decrypt).

– Steganography, on the other hand is where a message is hidden in another message or image:

• It is used when it is necessary to conceal the fact that a secret message is being transmitted.

Security ComponentsEncryption and Decryption:

– Encryption is the conversion of data into a form, called a ciphertext, which cannot be easily understood by unauthorised entities.

– Decryption is the process of converting encrypted data back into its original form, so it can be understood.

• Most security technologies rely, to some degree, on encryption of text or data:– For example, encryption is used in the creation of

certificates and digital signatures, for the secure storage of secrets or transport of information.

• Encryption can be anything from a simple process of substituting one character for another, in which case the key is the substitution rule, to some complex mathematical algorithm.

Security ComponentsEncryption and Decryption:• We assume that the more difficult it is to decrypt

the ciphertext, the better. • Trade-off - if the algorithm is too complex and it

takes too long to use, or requires keys that are too large to store easily, it becomes impractical to use: – Need a balance between the strength of the encryption;

that is, how difficult it is for someone to discover the algorithm and the key, and ease of use.

• There are two main types of encryption in use for computer security, referred to as symmetric and asymmetric key encryption.

Symmetric Key• Symmetric key cryptography, also called private or

secret key cryptography, is the classic cryptographic use of keys: – Here the same key is used to encrypt and decrypt the data.

Encrypt withsecret key

Decrypt withsecret key

Plaintext Plaintext

InternetCiphertext

Symmetric Key• Key management is an issue.• Each pair of communicating entities needs a shared

key:– For an n-party system, there are n(n-1)/2 distinct keys in

the system and each party needs to maintain n-1 distinct keys.

• How to reduce the number of shared keys in the system:– Centralised key management:

• Session keys.– Public keys.

K2 K3K5

Asymmetric Keys• In asymmetric key cryptography, different keys are

used for encrypting and decrypting a message. • In that case, one key can be made public while the

other is kept private. • There are advantages to this public-key–private-key

arrangement, often referred to as public key cryptography: – The necessity of distributing secret keys to large numbers

of users is eliminated,– The algorithm can be used for authentication as well as for

creating cipertext.

Public Key Encryption• Jill has two keys: public and private:

• Jill publishes her public key:• Such that the key is publicly known!

• Jill keeps her private key secret.• Other people use Jill’s public key to encrypt messages for Jill.• Jill uses her private key to decrypt messages.• Only Jill can decrypt since only she has the private key.

Message rfwekfsEncrypt

Public key

Message rfwekfsDecrypt

Private key

• Security: To compute the private key from the public key is assumed difficult.

Secure Message Exchange Using Asymmetric Keys

J ill Mark

Send J ill'spublic Key

Use J ill's private keyto encrypt

Decrypt withJ ill's public Key

PrivateKey

PublicKey

Public key vs. Symmetric key

Symmetric key Public key

Typically both share same key

Typically faster x100! Typically slower

Two parties MUST trust each other

Two parties DO NOT need to trust each other

Two separate keys: a public and a private key

Examples:DES, IDEA, RC5, AES, …

Examples:RSA, ElGamal Encryption, ECC…

Public Key Infrastructure• Many applications need key distribution.• Anyone can derive keys, so there is a need to

have a mechanism to assure that keys belong to entities they claim to come from.

• In PKI a Certification Authority (CA) validates keys.• Distribution in PKI is done via a hierarchy of CAs.• A CA:

– Checks real-world credentials,– Gets key from user in person,– Signs Certificate (“cert”) validating key.

• Then a certificate is attached to assure an end point that an entity is who it claims to be:– If the end point trusts the CA, then it will trust that entity

and who it claim to be.

Certification Authority• CAs issue digital certificates after verifying

that a public key belongs to a certain owner: – Driving licenses, identification cards and

fingerprints are examples of documentation required.

• Some examples of CAs are:

The e-Science CA

Public Key Certificate• A public key certificate is a file that contains a

public key, together with identity information, such as a person's name, all of which is signed by a certification authority (CA):– Similar in concept to a passport signed by the national

government.

• The CA is a guarantor who verifies that the public key belongs to the named entity.

• Certificates are required for the large-scale use of public-key cryptography, since anybody can create a public-private key pair:– So in principle, if the originator is sending private

information encrypted with the recipient’s public key, a malicious user can fool the originator into using their public key, and so get access to the information, since it knows its corresponding private key.

Public Key Certificate• But if the originator only trusts public keys that

have been signed ("certified") by an authority, then this type of attack can be prevented.

• In large-scale deployments one user may not be familiar with another’s certificate authority (perhaps they each have a different company CA), so a certificate may also include a CA's public key signed by a higher level CA, which is more widely recognised.

• This process can lead to a hierarchy of certificates, and complex graphs representing trust relations.

E-Science Certificate

Digital Certificate – Info.

Version

Serial number

Certificate issuer

Certificate holder

Validity period (note that the certificate is not valid before or after this period),Attributes, known as certificate extensions that contain additional information such as allowable uses for this certificate,

Digital signature from the certification authority to ensure that the certificate has not been altered and to indicate the identity of the issuer,Public key of the owner of the certificate,

Message digest algorithm used to create the signature.

Table 1: The Contents of Digital Certificate

Sender

CA CA Public Key

Sender Public Key

Signed Document

Decrypt Message

Recipient

The Role of the Certification Authority

Digital Signatures• Integrity is guaranteed in public-key

systems by using digital signatures:– This is a method of authenticating digital

information, in the same manner that an individual would sign a paper document to authenticate it.

• A digital signature is itself a sequence of bits conforming to one of a number of standards.

• Most digital signatures rely on public key cryptography to work.

Digital Signatures• Often, a cryptographically strong hash

function is applied to the message.– A hash function is an algorithm which creates a

digital representation in the form of a "hash value" of a standard length, which is typically much smaller than the message but nevertheless unique to it.

• The resulting message digest is encrypted instead of the entire message:– This makes the signature significantly shorter

than the message and saves considerable time since hashing is generally much faster, byte for byte, than public-key encryption.

Basic Features of a Digital Signature

• Private key: sender uses the private key to sign the document.

• Public key: recipient uses the public key to authenticate the document.

• Message hash algorithm: perform a mathematical calculation on the document and generate a hash value unique to the message.

• Encryption algorithm: accept the private key and a hash value to generate a digital signature or accept a public key and a digital signature to generate a hash value.

How does Digital Signature Work?

Digital signatures

• Only the signer (who has a private key) can generate a valid signature.

• Everyone (since the corresponding public key is published) can verify if a signature with respect to a message is valid.

Message rfwekfsSign

Private key

Message

rfwekfsVerify

Public key

Valid/Invalid

(fixed-length signature)

Adding A Digital Signature

A Digital Signed Email

Security – Summary• Security Concerns:

– Confidentiality – data only available to those authorised,– Availability – you can get it when you want it,– Integrity – data has not been changed.– Trust – who you are and what you are authorized to do,– Non-repudiation – you can’t deny doing something you

did,– Auditability – I can check what you did to the data,– Reliability – the system does what I want, when I want it

• Public Key Infrastructure:– Secret key,– Public key,– Certificates,– Digital Signatures.

Questions?

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