Transcript hello world - boun.edu.tr

Security
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SECURITY
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Fundamental Requirements:
1. Privacy
2. Integrity
3. Authentication
4. Non-repudiation
5. Availability
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SECURITY
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Privacy issue:
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Integrity Issue
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How do you ensure that that the information you transmit over the
Internet has not been captured or passed on to a third party without
your knowledge.
How do you ensure the information you send or receive has not been
compromised or altered
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SECURITY
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Authentication issue:
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Non-Repudiation Issue:
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How do you legally prove that a message was sent or received
Availability Issue:
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How do sender and receiver of a message prove their identities to
each other
How do we ensure that the network and the computer system it
connects will stay in operation continuously
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Cryptography
• Cryptography: transforms data using a key (a string of digits that acts as a
password) to make the data incomprehensible to all but the sender and the
intended receiver
•Plaintext: unencrypted data
•Ciphertext: encrypted data
•Cipher/Cryptosystem: technique/algorithm for encrypting messages
•Simple examples of cryptosystem:
-Substitution
-Transposition
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Secret key (Symmetric) Cryptography
• uses same (symmetric) keys to encrypt/decrypt a message
• fundamental problem: before two people can communicate, they must first find
a way to exchange the symmetric key securely
• Point-to-point key exchange
• Centralized: Key distribution center generates a session key
• DES algorithms developed by NSA and |IBM in the 1950s
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Public Key (Asymmetric) Cryptography
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developed by Diffie & Hellman (Stanford Univ) 1976
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Two inversely related keys are used:
1.
Public key : freely distributed
2.
Private key: kept secret by its owner
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Either the public key or the private key can be used to encrypt or
decrypt a message
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If the public key is used to encrypt a message, only the corresponding
private key can decrypt it
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Vice versa: if the private key is used to encrypt a message, only the
corresponding public key can decrypt it (this can be used to
authenticate the sender of the message)
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Public Key (Asymmetric) Cryptography
• The defining property of a secure public key is that it is computationally
infeasible to deduce the private key from the public key
•Public key algorithms require large amounts of computer power
•Symmetric systems are faster
•RSA : most commonly used public key algorithm (developed by Rivest,
Shamir, Adleman, MIT Professors, in 1977)
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Key Agreement Protocols
• Public key algorithms can be used to allow two parties to agree upon a key to
be used as secret key to be used for symmetric key encryption over insecure
medium
•Digital Envelope:
-message is encrypted using a symmetric key
-Symmetric key is encrypted using public key
-Attach encrypted symmetric key to encrypted message and send the entire
package
-To decrypt: receiver first decrypts the symmetric key using the receiver’s
private key. Then the symmetric key is used to decrypt actual message
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SECURITY-Hash Function
• Also known as “message digest”
• Mathematical function that gives message a hash value
• The chance that two different messages will have the same message digest is
statistically insignificant
• Collision occurs when multiple messages have the same hash value
• It is computationally infeasible to compute a message from its hash value or
to find two messages with the same hash value
• Example: MD5
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SECURITY- Digital signatures
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Solve problems of integrity and authentication
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Like a written signature, authenticates sender’s identity
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To create a digital signature
1.
Run original plaintext message through hash (message digest)
2.
Encrypt message digest using sender’s private key (creates a digital
signature and authenticates the sender)
3.
Encrypt original message with receiver’s public key
4.
Send (encrypted message+digital signature+hash function) to the
receiver
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SECURITY – Digital Signatures
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Receiver:
1.
Receives the package
2.
Uses sender’s public key to decipher the digital signature and reveal
the message digest
3.
Uses receiver’s own private key to decipher the original message
4.
Applies the hash function to the original message
5.
Compare the deciphered message digest to the result of hash
function
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SECURITY – Digital Signatures
• Digital signatures do not provide the proof that a message has been sent
• A time-stamping agency (third party) can help to solve the non-repudiation
problem by digitally signing the time-stamp
• US government recently passed digital-signature legislation that makes
digital signatures as legally binding as hand-written signatures
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Public Key Infrastructure (PKI)
• How does a customer know that the web site it is accessing belongs to a
trustworthy merchant and not to a third party site that is acting as
merchant to steal credit-card information
• PKI integrates public-key cryptography with “digital certificates” and
“certification authorities (CA)” to authenticate parties in a transaction
• Digital Certificate: is a digital document issued by a CA and includes:
-name of the subject (being certified)
-Subject’s public key
-Expiration date
- … plus other relevant information
• CA is a financial institution or other trusted third party such as VeriSign or
Thawte
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CA
• CA takes the responsibility for authentication, it checks the validity of
information before issuing a digital certificate
• Digital certificates are publicly available in CA certificate repositories
• CA signes the certificate by encrypting either the public key or a hash value of
the public key using the CA’s own private key
• CA has to verify every individual’s public key. Thus users must trust the
public key of a CA.
• A certificate authority is a chain of certificates starting with the root
certification authority IPRA (Internet Policy Registration Authority)
• Root only signs certificates for policy creation authorities (organizations that
set policies for obtaining digital certificates)
• Policy creation authorities sign digital certificates for Cas
• CA s sign digital certificates for individuals, organizations
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SECURE SOCKETS LAYER (SSL)
• SSL protocol developed by Netscape
• Built into web browsers and numerous other products
• When you use the Internet, the connection between you and any other point
can be routed through dozens of independent systems (unauthorized people
can steal confidential information, credit card numbers etc by eavesdropping)
• SSL protocol was developed to transfer information privately and securely
across the Internet
• SSL is the de facto standard for encrypted and authenticated communications
between clients and servers on the Internet
• Virtually all online purchases and monetary tansactions on the Internet are
secured by SSL
• URL starts with https:
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SSL
• SSL usage is not just limited to securing e-commerce transactions; other SSL
usage examples:
-financial institutions, insurance companies, B2B, private organizations
• SSL ensures that connection is private and secure by providing authentication
and encryption
• Authentication confirms the server and optionally the client are who they say
they are
• Encryption creates a secure ‘tunnel’ between the client and the server which
prevents any unauthorized system from reading the data
• SSL-enabled clients: Netscape, MS Internet Explorer, Firefox, Chrome etc.
• SSL-enabled servers: Apache, MS IIS, etc.
• Clients and Servers confirm each other’s identities using digital certificates
which are issued by CA.
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SSL
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SSL is comprised of two protocols:
1.
Handshake Protocol: (key exchange)
2.
Record Protocol (bulk data transfer)
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SSL- Handshake Protocol
1.
Authenticates the server to the client (optionally the client to the
server) using public-key encryption (asymmetric) techniques
2.
Allows client and server to negotiate the cipher suite to be used
3.
Allows the client and the server to generate symmetric session keys
4.
Establishes the encrypted session
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Once key exchange is complete, client and server use symmetric
session keys to encrypt all communication between them (SSL Record
Protocol)
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Symmetric encryption algorithm such as DES or RC4 is used.
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SSL Negotiation Steps
1.
Initially request for SSL session comes from the browser to the web server
2.
Web server sends the browser its digital certificate (contains info about the
server and server’s public key)
3.
Browser verifies that certificate is valid and that a CA listed in the client’s
list of trusted CA’s issued it. Browser also checks expiration date and web
server domain name
4.
Once browser has determined that the server certificate is valid, browser
generates a 48-byte master secret. This master secret is encrypted using
server’s public key and is then sent to the Web server
5.
Web server receives the encrypted master secret from the browser and
decrypts it using the server’s private key
6.
Both web server and the browser have the same secret key
7.
Communicate securely by encrypting data using symmetric technique
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SSL
Negotiation
Steps
SSL Negotiation Steps
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• SSL sits on top of TCP at the transport layer
• SSL operates independently and transparently of other protocols so it will
work with any application layer and transport layer protocol
• This allows clients & servers to establish secure SSL connections w/o
requiring knowledge of other party’s code
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SECURITY ATTACKS
• Denial-of-service: attack occurs when a network’s resources are taken up
by unauthorized individual, leaving the network unavailable for legitimate
users
• Another type of attack: modifies routing tables of a network, thus
disabling network ability or funneling all data to one address in the
network
• Distributed denial service attacks ( attack does not come from one single
source, but rather from multiple sources
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Denial of Service Attack
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A "denial-of-service" attack is characterized by an explicit
attempt by attackers to prevent legitimate users of a service
from using that service. Examples include
- attempts to "flood" a network, thereby preventing
legitimate network traffic
- attempts to disrupt connections between two machines,
thereby preventing access to a service
- attempts to prevent a particular individual from
accessing a service
- attempts to disrupt service to a specific system or
person
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Illegitimate use of resources may also result in denial of
service. For example, an intruder may use your anonymous
ftp area as a place to store illegal copies of commercial
software, consuming disk space and generating network
traffic
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Modes of Attack
• Consumption of scarce, limited, or non-renewable resources
- Network connectivity,
- Using Your Own Resources Against You
- Bandwidth Consumption
- Consumption of Other Resources
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Destruction or alteration of configuration information
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Physical destruction or alteration of network components
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Further Reading
http://www.cert.org/tech_tips/denial_of_service.html
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