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Security
Terminology
Traditional Unix Security
TCP Wrapper
Cryptography
Kerberos
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Terminology
 Authentication:
identifying someone (or
something) reliably. Proving you are
who you say you are.
 Authorization:
permission to access a
resource.
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Terminology
 Encryption:
Scramble data so that only
someone with a secret can make sense
of the data.
 Decryption: Descrambling encrypted
data.
 DES: Data Encryption Standard: secret
key cryptographic function standardized
by NBS (NIST).
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Terminology (cont.)
 Secret
Key Cryptography: a
cryptographic scheme where the same
key is used to encrypt and decrypt.
 Public Key Cryptography: a
cryptographic scheme where different
keys are used for encryption and
decryption.
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Terminology (more!)
 Firewall:
a network component that
separates two networks and (typically)
operates in the upper layers of the OSI
reference model (Application layer).
 Screening
Router: a discriminating
router that filters packets based on
network layer (and sometimes transport
layer) protocols and addresses.
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Unix Network Security
Some basic approaches:
1. Do nothing and assume requesting
system is secure.
2. Require host to identify itself and
trust users on known hosts.
3. Require a password (authentication)
every time a service is requested.
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Traditional Unix Security (BSD)
on option 2 – trust users on
trusted hosts.
 Based
– if the user has been authenticated by a
trusted host, we will trust the user.
 Authentication
of hosts based on IP
address! (doesn’t deal with IP spoofing)
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Reserved Ports
 Trust
only clients coming from trusted
hosts with source port less than 1024.
– Only root can bind to these ports.
 We
trust the host. The request is
coming via a trusted service (a reserved
port) on the host.
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Potential Problem
 Anyone
who knows the root password
can replace trusted services.
 Not all Operating Systems have a
notion of root or reserved ports!
 It’s easy to impersonate a host that is
down.
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Services that use the BSD
security model
 lpd
– line printing daemon.
 rshd
– remote execution.
 rexec
– another remote execution.
 rlogin
– remote login.
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BSD Config Files
 /etc/hosts.equiv
 /etc/hosts.lpd
– list of trusted hosts.
– trusted printing clients.
– user defined trusted hosts
and users.
 ~/.rusers
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lpd security
check client's address for reserved port
and
check /etc/hosts.equiv for client IP
or
check /etc/hosts.lpd for client IP
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rshd, rexecd, rlogind security
 As
part of a request for service a
username is sent by the client.
 The
username must be valid on the
server!
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rshd security
1. check client’s address for reserved
port
if not a reserved port – reject request.
2. check for password entry on server for
specified user.
if not a valid username – reject
request.
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rshd security (cont.)
3. check /etc/hosts.equiv for client’s IP
address.
if found – process request.
4. check users ~/.rhosts for client's IP
address.
if found – process request, otherwise
reject.
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rexecd security
client sends username and password to
server as part of the request (plaintext).
1. check for password entry on server for
user name.
2. encrypt password and check for match.
rexecd is rarely used!
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rlogind security
 Just
like rshd.
trusted host (user) not found –
prompts for a password.
 If
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Special Cases
 If
username is root requests are treated
as a special case:
– look at /.rhosts
– often disabled completely.
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TCP Wrapper
 TCP
wrapper is a simple system that
provides some firewall-like functionality.
 A single host (really just a few services)
is isolated from the rest of the world.
 Functionality includes logging of
requests for service and access control.
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TCP Wrapper Picture
Single Host
TCP based
Servers
TCP
wrapper
(tcpd)
TCP
Ports
The World
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tcpd
tcpd daemon checks out incoming
TCP connections before the real server
gets the connection.
 The
 tcpd
can find out source IP address
and port number (authentication).
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tcpd (cont.)
 A log
message can be generated
indicating the service name, client
address and time of connection.
 tcpd
can use client addresses to
authorize each service request.
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Typical tcpd setup
 inetd
(the
) is told to start
tcpd instead of the real server.
 tcpd
checks out the client by calling
getpeername on descriptor 0.
 tcpd
decides whether or not to start the
real server (by calling exec).
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tcpd configuration
 The
configuration files for tcpd specify
which hosts are allowed/denied which
services.
 Entire domains or IP networks can be
permitted or denied easily.
 tcpd can be told to perform RFC931
lookup to get a username.
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Cryptography
Reference:
Network Security
PRIVATE Communication in a PUBLIC World.
by Kaufman, Perlman & Speciner.
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Secret Key Cryptography
• Single key used to encrypt and decrypt.
• Key must be known by both parties.
• Assuming we live in a hostile
environment (otherwise - why the need
for cryptography?), it may be hard to
share a secret key.
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Public Key Cryptography
(a.k.a. asymmetric cryptography)
• Relatively new field - 1975 (as far as we
know, the NSA is not talking).
• Each entity has 2 keys:
– private key (a secret)
– public key (well known).
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Using Keys
• Private keys are used for decrypting.
• Public keys are used for encrypting.
plaintext
encryption
ciphertext
public key
ciphertext
decryption
plaintext
private key
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Digital Signature
• Public key cryptography is also used to
provide digital signatures.
plaintext
signing
signed message
private key
signed message
verification
plaintext
public key
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Transmitting over an insecure
channel.
Alice wants to send Bob a private message.
Apublic is Alice’s public key.
Aprivate is Alice’s private key.
Bpublic is Bob’s public key.
Bprivate is Bob’s private key.
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Hello Bob,
Wanna get together?
Alice
Bob
encrypt using Bpublic
decrypt using Bprivate
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OK Alice,
Your place or mine?
Alice
Bob
decrypt using Aprivate
encrypt using Apublic
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Bob’s Dilemma
• Nobody can read the message from
•
Alice, but anyone could produce it.
How does Bob know that the message
was really sent from Alice?
• Bob may be comforted to know that only
Alice can read his reply.
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Alice can sign her message!
• Alice can create a digital signature and
•
prove she sent the message (or
someone with knowledge of her private
key).
The signature can be a message digest
encrypted with Aprivate.
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Message Digest
• Also known as “hash function” or “one•
•
way transformation”.
Transforms a message of any length
and computes a fixed length string.
We want it to be hard to guess what the
message was given only the digest.
– Guessing is always possible.
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Alice’s Signature
•
•
•
•
Alice feeds her original message through a
hash function and encrypts the message
digest with Aprivate.
Bob can decrypt the message digest using
Apublic.
Bob can compute the message digest
himself.
If the 2 message digests are identical, Bob
knows Alice sent the message.
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Revised Scheme
Alice
Sign with Aprivate
Bob
check signature using Apublic
decrypt using Bprivate
encrypt using Bpublic
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Why the digest?
• Alice could just encrypt her name, and
then Bob could decrypt it with Apublic.
• Why wouldn’t this be sufficient?
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Implications
• Suppose Alice denies she sent the
message?
• Bob can prove that only someone with
Alice’s key could have produced the
message.
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Another possible problem
•
Suppose Bill receives a message from Alice
including a digital signature.
“meet me at the library tonight”
•
Bill sends the same message to Joe so that it
looks like the message came from Alice.
Bill includes the digital signature from the
message Alice sent to him.
Joe is convinced Alice sent the message!
•
•
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Solution?
• Always start your messages with:
– Dear Bill,
• Create a digest from the encrypted
message and sign that digest.
• There are many other schemes as well.
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Speed
• Secret key encryption/decryption
•
algorithms are much faster than public
key algorithms.
Many times a combination is used:
– use public key cryptography to share a
secret key.
– use the secret key to encrypt the bulk of
the communication.
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Secure Protocols
• There are a growing number of
applications for secure protocols:
– email
– electronic commerce
– electronic voting
– homework submission
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Secure Protocols
• Many application protocols include the
use of cryptography as part of the
application level protocol.
– The cryptographic scheme employed is
part of the protocol.
– If stronger cryptographic tools become
available we need to change the protocol.
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SSL and TLS
• Secure Sockets Layer (SSL) is a
different approach - a new layer is
added that provides a secure channel
over a TCP only link.
• TLS is Transport Layer Security (IETF
standard based on SSL).
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SSL layer
Application
SSL
TCP
IP
Application
SSL
TCP
IP
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Advantages of SSL/TLS
• Independent of application layer
• Includes support for negotiated
encryption techniques.
– easy to add new techniques.
• Possible to switch encryption algorithms
in the middle of a session.
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HTTPS Usage
• HTTPS is HTTP running over SSL.
– used for most secure web transactions.
– HTTPS server usually runs on port 443.
– Include notion of verification of server via a
certificate.
– Central trusted source of certificates.
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