Transcript Security Services in Information Systems

Security Services in
Information Systems
Basic Concepts
Key exchange: Diffie-Hellman protocol
1. Picks a  GF(p) at random
2. Computes TA = ga mod p
3. Sends TA
4. Receives TB
5. Computes KA = TBa mod p
Machine A
1. Picks b  GF(p) at random
2. Computes TB = gb mod p
3. Receives TA
4. Sends TB
5. Computes KB = TAb mod p
Machine B
Where K = KA = KB, Because:
TBa = (gb)a = gba = gab = (ga)b = TAb mod p
Man-in-the-Middle Attack
• Consider the following scenario:
Anita
Middleperson
Betito
ga = 8389
gx = 5876
gb = 9267
8389
5876
5876
9267
Shared key KAX:
Shared key KBX
5876a = 8389x
9267x = 5876b
• After this exchange, the middle-person attacker simply decrypts any
messages sent out by A or B, and then reads any possibly modifies
them before re-encrypting with the appropriate key and transmitting
them to the correct party.
• Middle-person attack is possible due to the fact that DHC does not
authenticate the participants. Possible solutions are digital signatures
and other protocol variants.
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CONTRASEÑA
Key Distribution Problem
[proposed solutions]
Key Distribution/Management
and Authentication
• two closely related subjects
• why?
Key Distribution
• symmetric schemes require both parties to
share a common secret key
– issue is how to securely distribute this key without
revealing that key to an adversary
• many attacks are based on poor key
management and distribution
– rather than breaking the codes
• This is, actually, the most difficult problem in
developing secure systems
Key Distribution
various key distribution alternatives for parties A
and B :
1. A can select key and physically deliver to B
–
–
does not scale for a large and distributed group
how many keys do we need for N users?
2. third party can select & deliver key to A & B
–
–
similar comment as 1
sometimes finding a “trusted” third party is another
problem
3. if A & B have communicated previously can use
previous key to encrypt a new key
–
good option but initially several keys to be distributed
4. if A & B have secure communications with a third
party C, C can relay key between A & B
–
only N master keys are enough
Session Key / Master Key
• The idea is having a master encryption key
(master key) to generate random and
temporary session keys
• can be implemented in several ways
– Basic D-H is such an example
• public/private keys are master keys, exchanged key is a
session key
– Kerberos is another example
– SSL uses three layers
• D-H for master key, master key for the session key
Session Key / Master Key
• Session key lifetime is a trade-off
– if small,
• more secure since attacker can obtain less
ciphertext to analyze
• But this creates more delay
– If large
• less secure, but less delay
Key Distribution Facts
• Conservation of trust principle
– a secure communication cannot be based on
nothing; either there should be an initial direct
contact or an indirect one
• Either physical delivery or a trusted third party
– physical delivery is the only option to avoid a third
party
• most basic system is PIN entry
• the case in Bluetooth
– otherwise regardless of symmetric or asymmetric
encryption, you need a trusted third party
• even D-H does not work without a third party, why?
A Key Distribution Example
• Symmetric crypto based
• Each user shares a symmetric master key
with the KDC (Key Distribution Center)
– e.g. Ka, Kb, Kc, …
– possibly physically distributed
• Basic idea:
– whenever a user A wants to communicate with B,
it requests a session key (Ks) from KDC
• Protocol is in the next slide
Nonce Definition
• Nonce: The present or particular
occasion.
• Nonce word: A word occurring,
invented, or used just for a particular
occasion.
A Key Distribution Example
An Alternative
• In the previous figure, can KDC send
message 3 directly to B?
– If not, why?
– If so, what are pros and cons?
Hierarchies of KDCs
• we may have several KDCs connected to
each other in a tree topology
– each leaf KDC serves to a local community
• intra-domain communication passes thru only
the local KDC, however inter-domain
communication requires several KDC-KDC
interaction
• master key delivery is only in local domains
Decentralized Key Control
• We may avoid using KDC in a small
group by having a master key for each
pair
Key Management in PKC
• In other words
– distribution of public keys
– use of PKC to distribute secret keys
• public/private key as a master key
Distribution of the Public Keys
• the most important barrier against the
deployment of PKC in applications
• Basic question?
– how can I make sure about the legitimacy of a
public key?
– how can I make sure that Bob’s public key really
belongs to Bob, not to Charlie?
• Why this is so important?
– Name spoofing attacks become possible
• remember the man-in-the-middle attacks in anonymous
Diffie-Hellman
Distribution of the Public Keys
• Some methods
– Public Announcement
– Publicly available databases/directories
– Centralized distribution
– Certificates
Public Announcement
• Broadcast your public key to the public
– via newsgroups, mailing lists, from
personal website, etc.
– major weakness is anyone can easily
pretend as yourself
• so attacks are possible
Publicly available
directories/databases
• There exists a directory/database for
{name, public key} pairs
• write controlled
– a trusted administrator
• if administered thoroughly, good
– but a proper administration is difficult
• need secure mechanisms for registration,
update, delete.
Centralized Distribution - PublicKey Authority
• Similar to directory/database approach, but
access to the directory is automated via a
secure protocol
– users interact with directory to obtain any desired
public key securely
– requires real-time access to directory when keys
are needed
– users should know public key for the directory
• the directory/database contains {name, public
key} pairs
– write permit is restricted
Centralized Distribution - PublicKey Authority PROTOCOL
Centralized Distribution - PublicKey Authority
• Disadvantages
– authority is an active entity and may create
a performance bottleneck
– database should be kept secure to prevent
unauthorized modification
Public-Key Certificates
• certificates allow key exchange without realtime access to public-key authority
• a certificate binds identity to public key
– usually with other info such as period of validity,
rights of use, issuer’s info, etc
• all contents signed by a trusted Certification
Authority (CA)
• can be verified by anyone who knows the CA
public-key
• CA must make sure about the identity of the
cert owner
Public-Key Certificates
Public-Key Certificates
• Certificates are widely used in practice
– previous slides only show the idea
• need lots of polishing for practical use
– is a single CA sufficient?
– what happens if the CA’s public key is not known?
• how to distribute CA public keys?
– what happens if a certificate is revoked?
• We will discuss the use of certificates later
What can you do with securely
distributed public keys?
• Digital signatures
– talked about them
• confidentiality
– theoretically possible but slow
– instead session keys can be distributed
• those session keys are used for symmetric
encryption
Distribution of Secret Keys using
PKC
• Several methods exist
• Diffie-Hellman is one way
• we will see some alternatives
Simple Secret Key Distribution
• proposed by Merkle in 1979
– A generates a new temporary public key pair
– A sends B its public key and identity
– B generates a session key and sends it to A
encrypted using the supplied public key
– A decrypts the session key and both use it
Simple Secret Key Distribution
• problem is that an opponent can intercept
and impersonate both halves of protocol
– man-in-the-middle attack
– result: A, B think that they exchanged Ks securely
but C also knows Ks and use it to eavesdrop the
communication passively
KUc
C
EKUa[Ks]
EKUc[Ks]
Public-Key Distribution of Secret
Keys
• assumption: public-keys are securely exchanged a
priori
• First three steps are for authentication purposes
• Last step provides both the secrecy and authenticity
of the session key
In practice
• Most systems offer a three-level
approach
– use of PKC to exchange master-key
– use of master-key to exchange session
keys
• most important advantage is at
performance
A closer look to authentication
• making sure of peer entity’s identity
– mutual or one-way
– non-repudiation is not an aim here
• generally implemented as a protocol
– basic idea: making sure that other party knows a
common secret
– also used for session key distribution
• two types
– message authentication
• mostly one-way
– peer entity authentication
• connection oriented approach
Two key issues
• Protection of any secret information
• Timeliness
– to prevent replay attacks
• a valid message is copied and later resent
– Why replays are bad?
• at minimum, disrupt proper operation by
presenting messages that appear genuine but
actually are not
• may also cause impersonation and key
compromise
Countermeasures - 1
• Sequence numbers
– not a practical method
– parties should keep track of the sequence
numbers
– and should take care of message losses,
duplications in a secure manner
• complicates stuff
Countermeasures - 2
• Timestamps
– message contains a timestamp
– accepts only fresh messages based on this
timestamp
– sometimes used but some practical
problems
• clocks must be synchronized in a secure
manner (some attacks are merely based on
failure on clock synchronization)
• tolerance to network delays
Countermeasures - 3
• Challenge/response
– Initiator sends a nonce (one-time challenge
phrase or number) and expects that nonce
in the response
– easier to implement
– but may require extra message transfer
Authentication using Symmetric
Encryption
• We start with well-known NeedhamSchroeder protocol
– actually have seen it as a key distribution protocol
• There exists a Key Distribution Center (KDC)
– each party shares own master key with KDC
– KDC generates session keys used for connections
between parties
– master keys used to distribute these keys to them
Needham-Schroeder Protocol
• original three-party key distribution protocol
– for session between A and B mediated by a trusted
KDC
– KDC should be trusted since it knows the session
key
• protocol overview
1. A→KDC: IDA || IDB || N1
2. KDC→A: EKa[Ks || IDB || N1 || EKb[Ks||IDA] ]
3. A→B: EKb[Ks||IDA]
4. B→A: EKs[N2]
5. A→B: EKs[f(N2)]
• 4 and 5 are to prevent a kind of a replay attack
– against replay of message 3 by an attacker
•
Needham-Schroeder (NS)
Protocol
but still vulnerable to a replay attack
– if an old session key has been compromised, then
message 3 can be resent to B by an attacker X
impersonating A
– after that X intercepts message 4 and send a
message 5 to B as if it is A
– now X can impersonate A for the future
communications with the session key
– unlikely but a vulnerability
• modifications to address this problem
– timestamps (Denning 81), B contacted at the
beginning (Needham Schroeder 87)
• see http://www.lsv.ens-cachan.fr/spore/index.html for a repository
of security protocols
NS Protocol with timestamps
• proposed by Dorothy Denning
• A and B can understand replays by checking
the timestamp in the message
– even if attacker knows Ks, he cannot generate
message 3 with a fresh timestamp since he does
not know Kb
– open question: why do we need messages 4 and
5?
Needham – Schroeder Protocol
Revisited
• Amended by the creators themselves in 1987
by putting B in the loop early in the protocol
1.
2.
3.
4.
5.
6.
7.
A -> B
:
B -> A
:
A -> KDC :
KDC -> A :
A -> B :
B -> A :
A -> B :
A
EKb[A, Nb]
A, B, Na, EKb[A, Nb]
EKa [Na, B, Ks, EKb [Ks, Nb, A]]
EKb [Ks, Nb, A]
Ks[Nb]
Ks[f(Nb)]
Synchronization problem
• Proper use of timestamps requires synchronized
clocks
• e.g. when sender’s clock is ahead of recipients
clock
– attacker can intercept the message and replay later
• Neuman proposed use of nonces with timely
session keys to constitute tickets in 93
– nonces are used to avoid replays
– timestamps are expiration dates of the session key
established
– attacker only has a limited time to break the session
key
• in original NS, attacker has unlimited time to break the
session key
Neuman Protocol
• EKb[IDA || Ks || Tb] is a ticket that can be
used later within the limit of Tb
Authentication using Public-Key
Encryption
• We have given an example method that
assumes public keys are known
• There exists protocols that also
distributes public keys
– using a central Authentication Server (AS)
or KDC
– using timestamps or nonces
One-Way Authentication
• required when sender & receiver are not
online at same time
– e-mail is a typical application
– protocol should not rely on the processing of B
• A symmetric encryption approach
1. A→KDC: IDA || IDB || N1
2. KDC→A: EKa[Ks || IDB || N1 || EKb[Ks||IDA] ]
3. A→B: EKb[Ks||IDA] || EKs[M]
• Provides authentication that the sender is A
• does not protect against replays (of msg. 3)
– could rely on timestamp in message, but delays in
email make this problematic
Public-Key Approaches
• have seen some public-key approaches
• if confidentiality is major concern, can use:
A→B: EKUb[Ks] || EKs[M]
– digital envelopes
• if authentication needed use a digital
signature with a digital certificate:
A→B: M || EKRa[H(M)] || EKRas[T||IDA||KUa]
– message, signature, certificate
Overview on IP Security
Internetwork Protocol (IP)
• Aim
– provide interconnection across different
networks
• implemented in every network and in
routers
• IP is an unreliable protocol
– IP datagrams may be lost
– IP datagrams may arrive out of order
– TCP takes care of those problems
Internetwork Protocol (IP)
Where to provide security?
• Application-layer?
– S/MIME, PGP – email security
– Kerberos – client server
– SSH – secure telnet
• Transport level?
– SSL / TLS
– between TCP and Application
• IP level
– IPSec
IPv4
• The IP version that most LANs are
currently using
Data (Payload) follows the header
IPv6
• Next generation IP
– driving force was the inadequateness of IPv4
address space
• IPv6 header
– modular approach
– base header + extension headers
– base header is longer than v4, but number of
fields is smaller
IPv6 header
Is IP Secure?
• Content (Payload) is not encrypted
– confidentiality is not provided
– IP sniffers are available on the net
• IP addresses may be spoofed
– authentication based on IP addresses can be
broken
• So IP is not secure
IPSec
• general IP Security mechanisms
• provides authentication and confidentiality at
IP level
– also has key management features
• Applications
– VPNs (Virtual Private Networks)
• Interconnected LANs over the insecure common carrier
network (mostly the Internet)
• router-to-router
– Secure remote access, e.g. to ISPs
• individual-to-router
• IPSec is mandatory for IPv6, optional for v4
– many manufacturers support IPSec in their v4
products
IPSec Application Scenarios
AH – Anti-replay Service
• Detection of duplicate packets
• Sequence numbers
– associated with SAs
– 32-bit value
– when an SA is created, initialized to 0
• when it reaches 232-1, SA must be terminated
• not to allow overflows
– sender increments the replay counter and puts
into each AH (sequence number field)
• Problem: IP is unreliable, so the receiver may
receive IP packets out of order
– Solution is using windows
AH – Antireplay
Service
•Fixed window
size W (default is
64)
–employed by
the receiver
• If a received packet falls in the window
– if authenticated and unmarked, mark it
– if marked, then replay!
• If a received packet is > N
– if authenticated, advance the window so that this packet is at
the rightmost edge and mark it
• If a received packet is <= N-W
– packet is discarded; this is an auditable event
Key Management in IPSec
• Ultimate aim
– generate and manage SAs for AH and ESP
– asymmetric
• receiver and initiator have different SAs
• can be manual or automated
– manual key management
• sysadmin manually configures every system
– automated key management
• on demand creation of keys for SA’s in large systems
Key Management in IPSec
• Complex system
– not a single protocol (theoretically)
– different protocols with different roles
• intersection is IPSec
• but may be used for other purposes as well
• Several protocols are offered by IPSec WG of IETF
– Oakley, SKEME, SKIP, Photuris
– ISAKMP, IKE
• IKE seems to be the IPSec key management protocol but it
is actually a combination of Oakley, SKEME and uses
ISAKMP structure
• See IPSec WG effort at
http://www.ietf.org/html.charters/ipsec-charter.html
Internet Key Exchange.
X.509 Authentication
X.509 Authentication Protocols
• X.509 is a ITU-T standard part of the
“directory services”
– mostly for certificates, but also propose
three generic authentication protocols
– one-way authentication
– two-way authentication
– three-way authentication
– use both nonces and timestamps
• nonces are unique only within the lifetime of
timestamp
X.509 one-way authentication
• Proves that the message generated by A and
intended for B
– also proves that message is not a replay
– proves the identity of the sender, but not the
recipient
– optionally includes a session key
tA: timestamp
rA: nonce
sgnData: Data signed
X.509 two-way Authentication
• both parties verify identity of each other
• reply message
– generated by B
– not a replay (guaranteed by tB and rB)
•
X.509 three-way
Authentication
Nonces are signed and echoed back
– each side can check the replay
– timestamps are not needed
• synchronized clocks are not needed either
• what is a potential risk in such a case?
X.509 Certificate Format