1. Protecting An Organization’s Network
Network Security
Protecting An Organization’s Network

2. A Note About Security
Humans are usually the most susceptible point in any security scheme
A worker who is malicious, careless, or unaware of an organization’s information policy can compromise the best security
William Stallings

3. A Note About Security
If your site has multiple access points, the overall security of the site is only as strong as the security of the least secure access point
Thus, you must ensure that all points of access are secure
Again, frequently the weakest link is people
This includes such access points as , their personal web sites, etc.

4. A Note About Security
A recent demonstration found that 24% of passwords could be determined, by:
Searching dictionaries of names, places, and other words
Using the username
Using information about the user, such as their first name
A much higher percentage would result if variations of the above were used
e.g. marge123

5. Security Terminology
Authentication: The process of determining the identify of a client or other entity
Encryption: The process of obfuscating data so that it cannot be examined in its current form
Public key: An object used in the encryption process that is publicly available
Private key: An object used in the encryption process that is kept private within an organization

6. Authentication: IPSec
Uses IP datagrams to provide security features such as authentication
Authentication is provided for each datagram
An IP datagram, such as those used for TCP connections will be assigned an authentication header (AH)

7. Authentication: IPSec
A normal TCP datagram:
IPH
TCPH
TCP Data
A TCP datagram with IPSec authentication information:
IPH AH
TCPH
TCP Data

8. Authentication Headers
Each authentication header contains authentication information that relates the sender of the message to the message data
If this data were static, it would be fairly simple to ‘forge’ datagrams with another entity’s authentication information
Clearly this data must be dynamic, relating to the data in the message, and difficult to reproduce by another entity

9. Authentication Headers1
Next Header
Points to the TCP header 1
Payload Length
Length of the entire header 2
Unused
Reserved for future use 4
Security Parameters Index
The security scheme used 4
Sequence Number
A unique number for each packet ?
Data
Data for the security scheme

10. Authentication Data
The data portion of the authentication header is where we place authentication information
This authentication must be difficult to forge
IPSec (and many other schemes) use message digests for this purpose

11. Message Digests
A message digest is a small piece of information created by examining a larger piece of information
In this case, the larger bit of information is the data of the datagram
Sound familiar?
It should, a checksum is very similar to a digest
A digest normally is a larger entity so there is a higher degree of detecting changes
Various algorithms are used to create digests
E.g. SHA-1, MD5

12. Authentication Information
A well-known algorithm is used to generate the digest
So how is that valid authentication information?
Its not, the digest ensures the data received is the same as the data sent
The digest is encrypted using the sender’s private key

13. Public Key Encryption
Public key encryption (encrypting with pairs of public and private keys) will be discussed at a later time
Suffice it to say, that either:
The sender encrypts the data using the receiver’s public key, and the receiver decrypts the data using its own private key
The sender encrypts the data using its own private key, and the receiver decrypts the data using the sender’s public key

14. Public Key Encryption A B A’s Private Key B’s Public Key B’s Private

15. Public Key Encryption A B A’s Private Key B’s Public Key B’s Private

16. Public Key Encryption A B A’s Private Key B’s Public Key B’s Private

17. Public Key Encryption A B A’s Private Key B’s Public Key B’s Private

18. Public Key Encryption
Clearly, each key can be used to encrypt, and each key can be used to decrypt
Thus public key encryption is bi-directional
The private key is used to encrypt when the receive wants to ensure the data comes from the correct recipient
No-one else would have the private key
The public key is used to encrypt when the data must not be seen by external entities
Only the private key can be used to decrypt

19. Authentication Information
The private key of the sender is used to encrypt the message digest
Since only the sender would have the private key, the resulting encrypted digest is unique
Since the digest would change with even a minor change to the data, the encrypted ensures data has not been tampered with
Thus this combination of digests and public key encryption ensures data integrity and provides authentication

20. Encryption in IPSec IPSec also supports encryption
In IPSec, it is called Encapsulating Security Payload (ESP)
The operation is similar to how IPSec handles authentication, except that the datagram’s data portion is encrypted
The ESP header describes the technique used for encryption

21. Encryption in IPSec
Since the sender normally would use its private key to encrypt datagrams, additional authentication is not normally required
Encryption schemes also normally include methods for ensuring data integrity

22. VPNs
Recall that virtual private networks uses encryption to keep their data secure between sites
VPNs frequently use IPSec’s ESP feature to accomplish this
The external router for each site would employ IPSec ESP on incoming and outgoing datagrams

23. VPNs
The advantage of using IPSec to implement VPNs is that it is a well-known technology
Thus it may be used, assuming the ubiquitous nature of IPSec on the Internet

24. Securing the World Wide Web
Web Security
Securing the World Wide Web

25. The Need The World Wide Web requires certain access points
If your site is to have a world wide web server, it requires at least one port that external entities may connect to
e.g. 80 (HTTP), 443 (HTTPS), etc.
If users on your site will access the web through a client, it requires ports to do so
This may be through a single machine, called an HTTP proxy server

26. The Threats There are four kinds of web-related threats:
Integrity threats: Data and/or system files are modified or destroyed
Confidentiality threats: Private data is examined by the intruder
Denial of Service: Web service is disrupted, preventing other clients from using it
Authentication: The identify of an entity is forged, making it seem like a request or command is coming from someone else
Unchecked code attacks: User-supplied data is assumed to be benign, but when used it could be code which is executed (e.g. Javascript or SQL)

27. Integrity Threats Data integrity can be compromised in several ways:
Files stored on the server are modified
Memory, containing data, is modified
Network messages are modified
The result is the loss of data, or some compromised data
This compromised data may include such things as password files, malicious shell scripts, etc.

28. Integrity Threats
The common theme to all of these threats is that data is changed
Checksums/digests make it possible to ensure data is not changed
If data is changed, the checksum/digest will indicate the change
That may not prevent loss of data alone, but it will (at least) alert administrators (or automatic sentry programs) of the change
Example: Nimbda virus

29. Nimbda Virus
Nimbda virus exploited a vulnerability in Internet Information Server (a web server) to modify files
The outgoing web page was modified
Actually, Nimbda virus is also an example of a denial of service attack
It is used as an example here, because of its widespread effect and notoriety

30. Confidentiality Threats
The types of confidentiality threats are:
Reading files from the server
Reading contents of a server’s memory
Reading network messages
The results are:
Invasion of privacy
Lost data (packets read are typically not placed back on the network)

31. Confidentiality Threats
Encryption prevents any data from being interpreted
Encryption allows only owners of the right ‘key’ to unlock the contained data
Datagrams might be examined and removed from the network, but timeouts would allow that information to arrive, eventually
Example: Packet sniffers, Spyware

32. Packet Sniffers
Packet sniffers allow anyone on a network to intercept any datagram passing through a machine
Instead of passing the datagram along, first the packet sniffers read the packet’s data
Examples: SpyNet, CommView, Ethereal

33. Denial of Service Attacks
Sometimes called overload attacks, there are a few types:
Overloading a server with connections or requests
Overloading the threads of a server
Overloading the network connection to a server (buffer overload)
Overloading a disk
Overloading memory

34. Denial of Service Attacks
Overloading a server can cause the service to become unavailable (thus ‘denial of service’)
Example: Nimbda virus
Once Nimbda virus infiltrates a site (using an integrity attack), each incoming request executes a program (part of the virus)
This program attacks a number of other sites, trying to install the virus there
Eventually, the attacks themselves can cause denial of service (DoS), even on machines where the Nimbda virus could not install itself

35. Denial of Service Attacks
DoS attacks can be reduced/avoided in a variety of ways:
Routers can filter out duplicate packets
Reduce the effect of a request
Attempt to recognize ‘bogus’ attacks and eliminate any unnecessary processing as a result
Require authenticated access wherever possible
Although, authentication itself may be a target of DoS
However, there is no way to make DoS attacks impossible at this time

36. Authentication Attacks
Sometimes used in combination with integrity attacks
Where data is modified and the identity of the sender is forged, so that the data is accepted
Authentication attacks typically involve users modifying source IP addresses (and things like IPSec authentication headers) to forge their identities
Although the authentication schemes used by IPSec now make this difficult

37. Authentication Attacks
Authentication can be accomplished using techniques such as those used by IPSec
Including a message digest, encrypted with the sender’s private key allows the receiver to validate the identity of the sender
Examples: Using a packet sniffer, modifying the packets, placing them back on the network

38. Unchecked Code Attacks
User-supplied data should always be validated
Positive validation: Checking to make sure the data meets your expectations
e.g. the expected format of an address
Negative validation: Checking for code, special characters and other enablers which might indicate that something malicious is in the user data
e.g. a <script> tag

39. Unchecked Code Attacks
Imagine a form that asks for an address
Please enter your address and password to log into our site
Imagine that when the form submission fails, the printed the address in the browser for verification
Are you sure the correct address?
Now imagine that the user types in some nasty JavaScript code (<script>…</script>)
The code will be put into the response page, and possibly executed

40. Guarding Against Attacks
For each type of attack, I have suggested a few examples of technologies that can be used to prevent the attacks
Of course, none of these solutions is fool proof
Recall the comment about the weakest link in the chain
e.g. A private key falls into the wrong person’s hands
e.g. A certificate is sent unencrypted via

41. Guarding Against Attacks
The combination of the following techniques may represent a security scheme for a given site:
Message digests
Guards against integrity attacks
Encrypting the digest with a private key
Guards against authentication attacks
Encrypting the message with a public key
Guards against confidentiality attacks
Employing duplicate-removing routers, and requiring authenticated access wherever possible
Guards against DoS attacks
User input validation
Guards against unchecked code attacks

42. Guarding Against Attacks
One fact is always true about site security:
An ounce of prevention is worth a pound of cure
Frankly if attacks can be avoided or halted before they occur, we will not need to deal with the side-effects of intrusions
The Code Red worm gives an example:
The security hole was discovered by a popular security site, and a patch was created by them many months in advance
The security hole was made public by this site
Many clones of the Code Red worm were created given this new information, which also flourished on unpatched sites
Microsoft subsequently released an official patch for IIS more than 2 months before the code red worm became prevalent

43. Another Attack Categorization
The previous scheme categorized attacks by their purpose
Another possibility is to categorize attacks by how messages propagate:
Interruption: Messages are removed from the network, so they are never received
Interception: Messages are read from the network without interrupting the message reaching its destination
Modification: Messages are removed from the network and replaced by another, modified version
Fabrication: Messages are created, usually in order to appear to come from a different source, for a different purpose

44. Encryption and Authentication
In Depth

45. Authentication and Encryption
These two technologies are frequently inter-related
Both can be closely related to private keys
There are really two categories of encryption techniques:
Conventional (single-key) encryption: The same key (shared by both sides) is used to encrypt and decrypt the data
Public key encryption: One key is used for encryption (usually the private key), and another for decryption (usually the public key)

46. Single Key Encryption
Somehow, a shared key must be distributed between both programs
Single key encryption algorithms must be symmetric:
They must provide a means to use the same key to decrypt and encrypt the data
Usually this means there are two (reverse) algorithms

47. Single Key Encryption
To ensure security, the key exchange must occur in some secure way
If someone intercepts the key, the encrypted message can be decrypted
Assuming they have the decryption algorithm
This separation of encryption algorithm from the key is an advance in cryptography
Encryption algorithms in the 30’s and 40’s involved only an encryption algorithm

48. Terminology Plaintext: Data prior to the encryption process
Or data after the decryption process
Ciphertext: Data after the encryption process
Cipher: An encryption algorithm
Secret key: A piece of data used by the encryption algorithm to generate ciphertext
The secret key is also used by the decryption algorithm to generate plaintext

49. Encryption Algorithms: Ancient
Encryption methods (not applied to data per se, but to spoken or telegraph/written messages) usually involved the replacement of common parts of the messages with other characters/words
One common example is the replacement of letters with some other letter in the alphabet
This scheme was used by Julius Caesar, and as such it is called the Caesar Cipher
Later, these schemes employed some changing factor
Such as a table containing mappings between letters
This table represents a changeable key that both parties must know

50. Caesar Cipher Problems
If the relationship between ciphertext letters and plaintext letters follows some pattern (as they did in Caesar’s method), the key can be easily determined
One way to combat these problems is to convert groups of letters at a time
Thus AB might translate to XY, but AC might translate to WV (not XV or something like that)

51. The Playfair Cipher
The Playfair cipher was created to convert groups of two letters at a time
Thus each translation occurs on two letters, so there are 262 (or 676) combinations
This method is more difficult to crack since all 676 combinations must be found to be able to crack all messages
However, if a partial list of mappings is known, it may reveal the message (or part of it), which will reveal more mappings
mee? me a? ??e barn => meet me at the barn

52. The Playfair Cipher
The details of the Playfair cipher are irrelevant to this discussion, since the key can easily be determined
This scheme does, however, remove some of the structure of the message (e.g. word groupings), which is a good idea
For example, we cannot examine the ciphertext, looking for commonly used letters, to determine which represent common letters in the plaintext (such as R,S,T,E)
We need to maximize this effect in order to make it more difficult to determine the key

53. Encryption Algorithms: Single Key
Keys have to be complicated enough that the encryption algorithm could be well-known and the ciphertext is still relatively secure
While overly simplistic, the Caesar Cipher (as well as other ancient algorithms) represents a symmetric algorithm, as required by single key encryption
One shortcoming is that the key can easily be determined through iteration
Ciphertext should never be enough information to determine the secret key
Algorithms which do not suffer from this include:
DES (Data Encryption Standard), IDEA (International Data Encryption Algorithm), BlowFish, RC5, RC2, CAST-128

54. DES Algorithm
DES is a complicated algorithm, the details of which are not important here
Suffice it to say that DES is more secure than the other (ancient) algorithms discussed here
However, DES is vulnerable to brute-force attacks
Attacks where various key values are tried until the message appears to be decrypted

55. DES Algorithm
One solution for this, is to apply the DES algorithm multiple times
If the code breaker finds one of the keys, it will be hard for him/her to realize it, since the output will be ciphertext again
Applying DES twice makes it difficult to break using brute force, however techniques exist for cracking double DES
Thus, an alternative to DES is triple DES, where DES is applied three times
Thus the two parties must share 3 different keys
This scheme is called Triple-DES or 3DES

56. Key Distribution
Single key encryption relies on the secrecy of the key in order to work
There are a few ways to transfer secret keys:
The key is generated by A, and delivered (physically) to B
For example, a floppy disk may be used for this purpose
Once one key is securely transferred to B, A or B could send the other a new key, by encrypting the new key using the old key
The keys are generated simultaneously using the same (private) algorithm and some shared data

57. Key Generation
Generating a secret key is fairly simple, random sequences of characters are usually sufficient
However, care must be taken to avoid pseudo-randomly generated keys
These can be identified and keys may be found using prediction algorithms
Computers are incapable of generating truly random numbers, but algorithms which generate very difficult to predict sequences can be used above easy to predict ones

58. Public Key Cryptography
Cryptography Using Keys Which Are Publicly Distributed

59. Public Key Cryptography
Public key cryptography involves ciphers that use different keys for encryption than decryption
PKC uses a key pair:
A public key, which is made publicly available
A private key, which is kept secret
Since there are two keys, the private key need never be transferred, so there is less likelihood of it being discovered

60. Key Pairs In some systems, keys have a sort of symmetry
Either key can decrypt data encrypted with the other key
e.g. RSA encryption
Thus if the private key is used to encrypt, anyone with the public key can decrypt the data
This does not keep data private
However, only the owner of the true private key could have generated the encrypted data
If the public key is used to encrypt, only someone who has the private key can decrypt the data
This keeps the data private
However, it is possible for anyone to replace the data with other encrypted data

61. Key Pairs Thus there are two uses for PKC:
Encryption using the private key for authentication
Encryption using the public key for data encryption
Key pairs must be carefully generated to support this symmetry
The details of generating key pairs is usually specific to each PKC technique
e.g. RSA

62. RSA
The RSA PKC scheme is very popular, and is used by many technologies:
PGP (Pretty Good Privacy): An implementation of an encryption scheme (typically used for )
One useful feature of the RSA algorithm is that it uses variable sized keys
Larger keys can be used where additional security is required
Smaller keys can be used for short periods (too short for the private key to be ‘broken’) and where computation and/or network bandwidth is a concern

63. RSA
428 bit RSA keys can be cracked by old desktop PC in less than 1 year
Info: Athlon XP CPU (~4000 MIPS)
So why the heck do banks (and other sites that use SSL) use only 128 bit encryption?
Firstly, the US government will not allow larger keys
They mandate that because they must be able to crack private keys when necessary
Secondly, SSL-enabled browsers generates these keys for each SSL session
Sessions typically last minutes or hours, not months

64. A Problem
RSA (and other PKC schemes) suffer from one problem, similar to the distribution of the secret key in SKC:
How do we export public keys?
At first glance, this may seem like an easy problem
Public keys need not be hidden, since little damage can be done with them
The danger is not with reading the public key, but replacing it (i.e. an integrity attack on the public key)

65. A Problem
If a 3rd party replaced the public key with their own, they could use their own private key to decrypt data and for authentication
Thus the site would grant full access to the 3rd party
Obviously we want to avoid this
We need to distribute public keys in such a way as to eliminate this possibility

66. Public Key Distribution
Secure public key distribution really boils down to one problem:
How do we know that the public key we receive is really from who we think?
The answer is simple: certificates
Certificates provide a way of distributing public keys, while also providing source authentication

67. Certificates Certificates are quite simple:
They are an encrypted version of your public key
Certificates are encrypted using the private key of a certification authority (CA)
Public keys of CAs are well known
In fact, the public keys of several CAs are built into browsers for SSL
Since the public key of a CA is well known, we do not need to transfer it (and thus run the risk of it being tampered with)
The CA public key can be used to decrypt the certificate, and thus extract the public key of the owner of the certificate

68. Certification Authorities
CAs job is to create certificates
The certificate (i.e. public key) of at least one well-known CA is installed into browsers
For other (non-browser) uses of PKC, the certificate/public key of a CA must be known
If the incoming certificate is issued by (encrypted using the private key of) a different CA, the certificate can still be verified
The public key of the certificate’s CA can be downloaded (they are also publicly available), and used to extract the public key in the certificate
The CA’s public key is contained in a certificate authorized by another CA
Eventually, through a chain of certificates, the certificate can be validated, and the public key extracted

69. SSL
A Common Scheme

70. Secure Socket Layer (SSL)
SSL was designed to create a communication model similar to that available through TCP
However, SSL employs additional security techniques, such as encryption
Thus SSL gives the appearance of normal socket communication, while providing security features

71. SSL
SSL can use several different encryption algorithms, including IDEA, DES, 3DES
However, due to the streaming nature of TCP, normally uses one of the schemes that encrypts a stream byte by byte: RC4
This includes key sizes of 40 (RC4-40) and 128 bits (RC4-128)

72. SSL Operation SSL begins when clients connect to servers
This stage is marked by each side exchanging hello messages
Next, certificates are exchanged
The server sends its certificate to the client, followed by the client sending its certificate to the server
Then, the two agree upon an encryption algorithm and parameters
Now, it is possible for the client and server to exchange information using the agreed encryption algorithm and the certificates (which contain public keys)

73. Security

74. E-Mail Security There are two aspects to E-Mail security:
privacy
Using encryption (or some other means) to keep data contained in private
Virus protection
Since is a simple portal through which any entity may send messages, those messages must not affect the user’s system or the network

75. Privacy
The most common way to ensure data privacy through is to use PGP
PGP: Pretty Good Privacy
A freeware tool that uses single key cryptography (and public key cryptography) to encrypt text
It uses public key cryptography for digital signatures (obviously)
The tool allows users to choose the encryption method: 3DES, IDEA (SKC), RSA (PKC)

76. E-Mail Virus Protection
Microsoft recently release a “patch” for Outlook, which Outlook 2002 (shipped with Office XP) had already preinstalled
This patch essentially prevents receiving any files which are suspect (.EXE, .COM, etc.)
Other (better) solutions are to have virus checkers virus scan all incoming files
Norton Antivirus, McAfee, PC-cillin, and others already perform these scans, if configured to do so

77. Malicious Programs
A program with 2 goals; to propagate itself to other machines, and to do some harm to this machine, possibly:
Trap door: A modification that allows a secret entry point to a system (accidental or purposeful; by an employee?)
Logic bomb: Code hidden within a seemingly harmless program that will activate when certain conditions are met (e.g. a time elapses)
Trojan horse: A program that appears to be useful, but actually has some unwanted behaviour
Virus: A program that propagates itself by embedding its own code into other executable programs
Worm: A program that uses network connections to propagate
Bacteria: A program that does no harm, except propagate