P2P Security
1.0
30/September/2003
Niels Olof Bouvin
Aarhus Universitet
P2P Security - overall topics for this talk
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General security concerns
relevant to all distributed systems
Secure P2P systems and techniques
Security
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Basic security concerns
(Lack of) Internet security
Cryptography
Closing remarks on security
Basic security concerns - all the things that can go wrong
Dangers of distributed systems
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Trust
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who can you trust?
Identity theft
pretending to be you (or someone you trust)
Privacy
preventing others listening in on the conversation
Censorship & attacks
denying you the right to know
(Lack of) Internet security
The Internet
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The Internet is vast and not safe at all
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data packets going from machine to machine before they reach you
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Many standards and protocols established back in safer days
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SMTP, NNTP, telnet, ...
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Today there are plenty of sociopaths, who would delight in destroying your data or
machine
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see iloveyou, Code Red, SQL Slammer, SoBig.F, Swen, etc. etc.
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not to mention industrial espionage etc.
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Spammers, anyone?
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it has been claimed that recent worms are behind DDoS attacks against antispam sites...
Who can you trust?
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Surely you can trust well-established Web sites?
Several important open source ftp servers have been 'owned' over the years
thus leaving black hats free to insert code of their own in the cvs trees...
This also happened for Microsoft last year (apparently without any ensuing nastiness)...
And of course numerous sites have been hacked for credit card numbers etc.
Email
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Email remains (despite spammers) the most successful CSCW and file sharing
application on the planet
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The vast majority of email is not encrypted
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neither in transit nor at source/destination
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Standards for email encryption exist
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PGP (Pretty Good Privacy
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S/MIME
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These are generally somewhat cumbersome and are (therefore) not widely used
Email
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Email headers can be easily spoofed
faking the sender of an email is trivial
One of the reasons behind the "success" of email worms
users naturally trust email received from friends, colleagues, and family...
Similar points can be raised for instant messaging
File systems
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File systems are generally, by default, not encrypted
Thus not funny at all to loose the company laptop PC...
Without encryption your data is only as safe as the hard disk it resides on
Cryptography
Cryptography
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Fact: Messages can be intercepted. But intercepted data is worthless, if the interceptor
cannot read it
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(the people involved are traditionally known as Alice, Bob, and Carol (the
latter usually trying to intercept and decrypt messages)
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Cryptography is very old (at least as old as the Roman Empire), and has been based on
a long number of techniques
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letter substitution or various permutations (e.g. ROT-13)
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one time pads
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complex machines (e.g. Enigma)
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etc.
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Today cryptography is based on advanced, hard-to-solve mathematical problems
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Regardless of the method used, a key is used to signify how the plain text is
transformed into cipher text
Symmetric cryptography
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The same key is used to encrypt and decrypt the message
Advantages
symmetric cryptography is fast
Disadvantages
the key must be securely exchanged between Alice and Bob
if the key is compromised, the entire communication is instantly readable
Asymmetric cryptography
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Keys come in pairs:
a public key known to all
a private (or secret) key known only by the individual user
A message encrypted with the public key can be decrypted only by the private key
So if Alice encrypts a message with Bob's public key, only Bob can decrypt it
with his private key
A message encrypted with the private key can be decrypted only by the public key
So if Alice encrypts a message with her private key, all can verify (using
Alice's public key) that Alice is the author
Asymmetric cryptography
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Advantages
as the private key is never shared, the system is secure
the system can also be used to authenticate (or "digitally sign") messages
Disadvantages
only as secure as the private key...
much slower than symmetric cryptography
Establishing trust
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How does Alice know Bob is really Bob, and not Carol claiming to be Bob?
Asymmetric cryptography often relies on CAs - Certification Authorities
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these, using out-of-band methods, establish the correct identity of Bob, and
assigns a (signed) certificate to Bob
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Alice can then verify that some CA has vouchsafed Bob, and if she trusts the
CA, she can trust Bob
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A problem with these certificates is the cost...
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A less centralised approach is taken by PGP (Pretty Good Privacy), where Bob relies
on associates to confirm his identity
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if Alice knows (and trusts) any of these associates, she can trust Bob's identity
Symmetric/asymmetric cryptography
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Not an either/or situation - often used in combination
Asymmetric cryptography is used for the initial communication to establish identity
and (securely) exchange a randomly generated symmetric key used for the rest of the
session
This is the method used by SSL (Secure Socket Layer), which handles e.g. https
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the Web server provides the Web browser with its CA signed certificate (this
makes man-in-the-middle attacks harder)
the Web browser generates a random key, encrypts it with the Web server's
public key, and returns it to the Web server
as only the Web server can decrypt the key, the Web server and Web browser
can now initiate a symmetric encrypted session
Secure hashes
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Secure (or cryptographic) hashes are used to verify the integrity of a message
Most common are MD5 (128 bit) and SHA-1 (160 bit)
It is computationally infeasible to create two different messages with identical secure
hash codes (it requires brute force and 2^128 or 2^160 are big)
Thus, if the (MD5/SHA-1) hash code of a message is known, we can check whether
the message has been modified by computing the hash code of the message
Given the quality of the secure hash, it is just as good to encrypt the (compact) hash
code with your private key for authentication as encrypting the entire message
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Closing remarks on security
Security - a purely technical problem?
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Security can be addressed through a number of technical means
However, these valiant efforts are all for naught
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in the face of inexperience and general cluelessness
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The most successful hackers have operated, not through absurd Hollywood computer
guru excellence, but through social engineering
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hacking is considerably easier if you can get people to tell you their password
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Script-kiddies usually operate through kits exploiting well-published security holes
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leaving their victims with a valuable lesson if nothing else
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Most worms spread because people apparently just can't help clicking on attachments...
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where is the problem? With Outlook or the users?
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"Machine destroying email" used to be an Internet urban legend... but no more
(this is progress?)
Security in P2P
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P2P systems previously seen...
Groove
Various techniques for anonymity
P2P systems previously seen...
Gnutella/Napster/KaZaA/... - the vast majority of file sharing tools
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No security to speak of
No user authentication
in some cases for quite obvious reasons...
but users are not anonymous...
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No on-the-wire encryption
eavesdropping is trivial on most file sharing networks, as a number of people
have discovered to their cost
No content integrity checking
the retrieved file is not what it seems...
Freenet
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No authentication (to real world identities) as such, but can authenticate pseudonyms,
allowing e.g. only the original author to update a document
Each resource in a Freenet node space is encrypted and integrity checked with SHA-1
hash
Network traffic not encrypted, but as the resources are encrypted, this is less of a
problem
Routing is performed in a way to foil surveillance
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JXTA
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Per default:
authentication at the client
not on-the-wire encryption
not strong checking of group membership
However
secure pipes can be used
Arbitrary (as in "write your own") group membership checking can done
Groove
Security concerns for the corporate world - or this place for that matter
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Most are placed behind a firewall
excellent protection for most attacks
unless black hats somehow get inside
can make it quite difficult to collaborate with people on the other side of the
firewall
As email pass through the firewall, it is a popular choice for collaboration and sharing
but as noted before email is usually not safe
mail filters check for spam, worms and trojans, but cannot be completely
updated at all times
Groove - P2P for the corporation
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Aims to provide a secure shared space between coworkers
Encryption is the default and quite transparent to the user
thus avoiding the hassle of PGP & S/MIME in email
Provides
chat with threaded persistent discussions
shared writing space
file sharing
calendar
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(according to their Web site, they now also integrate with Microsoft Office)
Identity in Groove - a system with many, many keys
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A user in Groove has an account (possibly on a number of devices) with
A pass phrase to login to the client and to generate the
master symmetric key
The user can have a number of identities, each with
an asymmetric key pair for signing and verification
an asymmetric key pair for encryption
a digital fingerprint
a Diffie-Hellman key pair per member per shared space
Shared spaces - mutual trusting
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A mutual trusting shared space has
a symmetric key used by all members to verify/authenticate messages
a symmetric key used by all members to encrypt messages
All space data is stored on all clients using a symmetric storage key per member per
shared space (encrypted with the user's master key)
There is not guarantee for identity - users could in principle spoof identities when
posting to the group
Shared spaces - mutual trusting messages
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A delta (a chunk of data) is sent from Bob to the group containing:
a header
a body encrypted using the group's symmetric encryption key
a digest (SHA-1) of header and body authenticated using the group's
symmetric authentication key
Upon receiving this delta, Alice can
verify the integrity of the delta (and thus ensuring that the delta was sent by a
group member)
decrypt the delta
store the delta locally using her own encryption
Shared spaces - mutual suspicious
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Mutual suspicious shared spaces are used in situation where the identity of the
individual user must be ensured
Each member has a Diffie-Hellman key pair per other member
D-H allows Bob to compute a shared symmetric key with Alice using Bob's private DH key and Alice's public D-H key (and vice versa)
By using this symmetric encryption communication between Alice and Bob is secured
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Shared spaces - mutual suspicious messages
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A delta is sent from Bob to the group containing:
a header
a body encrypted using the group's symmetric encryption key
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digests (SHA-1) of header and body authenticated using each D-H derived key
pair, including Bob/Bob
Upon retrieval, Alice can then verify that the delta was indeed sent by Bob
Initiating membership of a shared space
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Bob is chair and sends Alice an invitation to a shared space
cryptographic context (parameters for D-H)
Bob's public keys
Alice then replies with
a one-time key encrypted in Bob's public key
Alice's D-H public key
Bob can then
decrypt the one-time key, use it to send an encrypted message only Alice can
respond to and thus verifying that she intends to join the space
send the group keys to Alice
send Alice's D-H public key to the other members of the space
Leaving a shared space
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If Carol leaves the shared space, the group key is recomputed, so that she no longer
can access the shared space's material
Group keys are not recomputed, when people join the space, as they should have
access to the history of the group
Dependency on keys are stored, so that other members (who perhaps were off-line)
can still access data
Communication in Groove
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Usually peers communicate using point-to-point connections
Peers are not always online or immediately available (e.g. they could be behind a
firewall)
Relays are used for tunnelling through firewalls, handle fan-out of messages, and do
storing and forwarding
Various techniques for anonymity
Mix networks - defeating traffic analysis
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Mix networks are used to ensure that a sender and receiver cannot both be known
A mix network consists of a number of known mixes - routers with asymmetric key
pairs
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A sender chooses a path through the mix network (m_1, ..., m_n), and encrypts the
message (with final destination) with m_n's public key, encrypts this message (with m_(n1)->m_n) with m_(n-1)'s public key and so on
The message is then sent to m_1, who decrypts the message using its private key, and
sends it to the next mix, who repeats the process
Only m_1 knows the sender and only m_n knows the receiver and neither knows the
route of the message (not even their own position on the path)
Crowds - defeating Web browsing tracking
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A number of members participate in a crowd, and they are known to each other
If a member, Bob, wishes to retrieve a Web page, Bob sends a request for the URL to
a random member, Carol (using symmetric encryption). Carol can then choose to retrieve
the Web page or forward the request to another crowd member, Alice, and so on.
Eventually a member chooses to retrieve the Web page, and the Web page is returned along
the request's path
Summary and pointers
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A number of proven technologies exists, which can enable safe computing
The success of these technologies hinges on ease-of-use, as they are (in themselves)
quite complicated
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There are two directions in the area
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ensuring anonymity and privacy on the Internet
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ensuring identity and integrity in a working environment
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Next time:
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Accountability
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Reputation
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can get complicated if users are anonymous...
Project description time!
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You must formulate a P2P project for your group
You will present the result of this project to the rest of us during the first half of
December
(and deliver a report about it to Klaus and I by the middle of January)
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Project topics
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Anything goes! As long as
it is P2P in nature
it is sufficiently ambitious (neither too much nor too little)
You can
develop new routing and searching algorithms
create a neato P2P application
extend JXTA in some direction
create a P2P framework The Way It Ought Be Done And Not The Way JXTA
Did It
etc., etc.
Only one requirement prior to starting:
send an URL to a short project description (no more than a page) to Klaus and
I and get it approved
Created by JackSVG