1. Richard Henson University of Worcester October 2016
COMP3371 Cyber Security
Richard Henson
University of Worcester
October 2016

2. Week 4: Public Key Encryption & PKI
Objectives
Explain public-private key encryption (PKE)
Explain need for the sender of data to identify themselves; why digital signatures are necessary in the real world; how they can be implemented
Explain PGP and PKI as two reliable techniques for sending data securely from one place to another… including verification of the sender
Apply PKE to the sending of secure

3. Symmetric v Asymmetric Key
one encryption/decryption key only
Asymmetric (public key encryption, PKE)
encryption: shared public key
decryption: unshared private key
each algorithm a one way function

4. Authentication of Transmitted Data
Two potential issues with data sending:
is it intact & unmodified? (integrity)
date/timestamp etc…
can original authorship (authenticity) be established
i.e. is the sender really is who he/she claims to be
Requirements for Authentication:
inputs (sender): secret key, message
output: message authentication code

5. When is Encryption alone not enough?
On local network
covered through username/password
network system should verify authenticity
BUT… when data is on the move to a computer or device from OUTSIDE the network…
It could come from ANYONE…

6. Authentication Methods
Paper correspondence?
by physical signature/wax, stamped seal
Many available digital methods of providing a sender signature to data
e.g. Windows SIGVER (file signing)
method of checking incoming files to ensure that they are from a Microsoft approved source
Linux uses a similar technique

7. Security & Wireless Data
Wireless media more prone to interception
WAP (wireless access protocol)
encryption only not enough
open access, decryption too easy…
Requires authentication as well for safe transmission (best WPA-2)
use a known SSID to provide authentication of remote device
other devices won’t get access…

8. Asymmetric (two key) encryption
Attributed to Diffie and Hellman (US, ‘76)
However, British scientists were secretly working on it much earlier…
Ellis, at GCHQ, made the first breakthrough in 1970
Based on two keys:
public key - known to everyone
private or secret key - known only to the recipient of the message

9. Mechanism of PKE Jane receives encrypted message
John wants to send a secure message to Jane…
uses Jane's public key to encrypt the message
Jane receives encrypted message
then uses her private key to decrypt it
Original public key method did not support either encryption or digital signatures…
therefore vulnerable to third party in the middle eavesdroppers

10. Public Key Encryption (PKE)
Can work in two ways:
private key encryption, public key decryption
public key encryption, private key decryption
Private key on sender’s computer
Unencrypted data
Encrypted data
Data sent through the Internet
Public key on recipient computer
Encrypted data
Decrypted data
Received by
recipient’s computer

11. Public Key Encryption (PKE)
The public and private keys must be related in such a way that
only the public key can be used to encrypt messages
only the corresponding private key can be used to decrypt them
In theory it is virtually impossible to deduce the private key if you know the public key

12. Practical Public Key Encryption systems
Include public-private key and authentication of sender
Variety of techniques developed:
Pretty Good Privacy (PGP)
Digital Certificates & Public Key Infrastructure (PKI)

13. PGP (Pretty Good Privacy)
Developed by Philip Zimmerman (early 1990s)
official repository held at the Massachusetts Institute of Technology
spec for v2.0 at RFC #1991
Based on public-key method…
plus authentication using a “web of trust”. Quote from RFC…
“As time goes on, you will accumulate keys from other people that you may want to designate as trusted introducers. Everyone else will each choose their own trusted introducers. And everyone will gradually accumulate and distribute with their key a collection of certifying signatures from other people, with the expectation that anyone receiving it will trust at least one or two of the signatures. This will cause the emergence of a decentralized fault-tolerant web of confidence for all public keys.”
Convenient way to protect messages on the Internet:
effective
easy to use
free

14. Using PGP (or not..)
To encrypt a message using PGP, the receiver needs the PGP encryption package
Zimmerman made it available for free download from a number of Internet sources
Such an effective encryption tool that the U.S. government actually brought a lawsuit against Zimmerman!
Problem:
PGP made public…
therefore available to enemies of the U.S.

15. US gov v Zimmerman (PGP)
Actual Lawsuit:
selling munitions overseas without a license (used >40 bit encryption)
unpopular…
after a public outcry, quietly dropped, law changed in 2000
Still illegal to download PGP from US to many other countries

16. Trust Ever seen the film “Meet the Parents?”
Web of trust (personal) not practicable for trust “in the business sense”
Business Trust
“you may not trust me, but you do trust my business enough to accept that you’ll get paid!”
PGP web of trust wouldn’t be practicable!
New model developed by business (embedded into PKI)

17. Verisign Trust System Web of Trust (PGP)
OK for academics (“good” people?)
but bad” people can do business
Need for a more practical alternative
developed so that people could trust strangers in business transactions
financial institutions provide the “trust”

18. LDAP and Public Key “lookup”
Public Key “lookup” developed - system that could be used with PKE
Protocol: LDAP (Lightweight Directory Application Protocol)
Netscape spec:
“historic” involvement of Microsoft in Internet Infrastructure
implemented in VB (!)
Microsoft/Netscape/Internet Engineers put all together… Public Key Infrastructure (PKI)

19. The Public Key Repository
Store of public keys so they can only be used securely
readily accessible via the Internet and LDAP
enabled public key lookup to occur transparently i.e. without intervention from the user
Infrastructure complete by 1999
Implemented through Windows 2000 architecture
Active Directory
many still never heard of it or how to implement it even in 2016!

20. Digital Signatures/Digital-IDs
Unique 'security code' appended to an electronic document
the digital equivalent of a signature on a paper document
authenticates the sender
permits the authenticity of the document to be proven
also used the ensure the integrity of the message sent
Signature and public key supplied packaged within a digital certificate
usually 30-day trial, then ~£100 for 2-year lease

21. Digital Certificate
Randomly generated number that creates, via algorithm:
the public-private key pair
the attachment to an electronic message known as a digital signature
Service for those wishing to send encrypted data (inc )
acquire digital certificate from Certificate Authority (CA)

22. Certificate Authorities
Trusted third-party organizations that issues the digital certificates used to create public-private key pairs
Started with Verisign
Many more followed
Role of CA:
guarantee that the individual granted the unique certificate is, in fact, who he or she claims to be.

23. Certificate Authorities
Authentication.. CA has an arrangement with a financial institution, such as a credit card company
finance company provides it with information to confirm an individual's claimed identity
Soon became a critical component in security and e-commerce
guarantee that the two parties exchanging information really are who they claim to be

24. Supplying Digital Certificates
Online via CA…
Digital certificates contain:
the applicant's private key
a digital signature
CA makes its own public key readily available via LDAP
digital certificate attached to the message
recipient of the encrypted message uses CA's public key to decode the digital certificate

25. Digital Certificate (continued)
The recipient:
verifies the digital signature as issued by the CA
obtains the sender's public key and digital signature held within the certificate
With this information, the recipient can send an encrypted reply

26. Digital Signatures: an increasing role in society…
Increased online delivery of traditionally paper based correspondence & services…
contracts
government forms such as tax returns
anything else that would require a hand-written signature for authentication…
Information sent WITHOUT a digital signature…
has NOT been authenticated!
proof of identity of sender? Should still be FAXed

27. The trouble with HTTP
General Internet principle of “anyone can go anywhere”
On a Windows system with www access:
TCP can link directly to HTTP
session layer authentication not invoked
HTML data transferred directly to the presentation and application layers for display
Problem:
the data is visible to anyone else on the Internet who may have access to that machine and the data path to it!

28. Secure HTTP and the user authentication problem
Makes use of the potential for requiring authentication at the session layer
SSL protocol can require a username/password combination before data passes through the socket from transport layer to application layer
application
authentication
required
transport

29. Computer Authentication
SSL is able to use the PKI
When a user first attempts to communicate with a web server over a secure connection:
that server will present the web browser with authentication data
presented as a server certificate (remember those?)
verifies that the server is who and what it claims to be
Works both ways…
server may in return request client authentication

30. SSL and Encryption
Authenticating the user & server only helps when the data is at its at its source or destination
data also needs to be protected in transit…
SSL working at level 5/6 also ensures that it is:
encrypted before being sent
decrypted upon receipt and prior to processing for display

31. Is an SSL Digital Certificate Really Necessary?
Yes:
for sites involved in e-commerce and therefore involving digital payment
any other business transaction in which authentication of identity is important
No:
if an administrator simply wants to ensure that data being transmitted and received by the server is private and cannot be snooped by anyone eavesdropping on the connection
In such cases, a self-signed certificate is sufficient

32. Https & “Web of Trust”
Based on individual trust networks built up between individuals
Possible to “self sign” a digital certificate
if someone trusts you, a self-signature may be all they need
OpenPGP identiity certificates are designed to be self-signed

33. General Tips on Running SSL
Designed to be as efficient as securely possible but encryption/decryption is computationally expensive from a performance standpoint
not strictly necessary to run an entire Web application over SSL
customary for a developer to decide which pages require a secure connection and which do not

34. When to use SSL Whenever web pages require a secure connection e.g.:
login pages
personal information pages
shopping cart checkouts
any pages where credit card information could possibly be transmitted

35. Running HTTPS Client-server service like http and ftp
runs on the Web server
uniquely designed so it will not run on a server without a server certificate
Once set up, https requires users to establish an encrypted channel with the server
i.e. rather than
Unless the user uses https…
get an error, rather than the pop up that proceeds the secure web page

36. HTTPs and encryption
Even if https channel set up with server certificate, still potential problems
use of an encrypted channel running https between user's Web browser and Web server
BOTH must support the encryption scheme used to secure the channel
e.g. 128-bit RSA

37. Accessing a Web Page using HTTPS
Prefix the address with instead of
and the system will do the rest
Any pages which absolutely require a secure connection should have a facility to:
check the protocol type associated with the page request
take the appropriate action if https: is not specified

38. Proof that Web Page has been delivered securely using SSL
At one time.. a pop up would appear…
informed the client that they are entering a secure client-server connection
must be acknowledged to continue
Default browser settings now bring up https page automatically if all is well

39. A Practical Limitation on the Use of SSL
SSL “handshake”, where the client browser accepts the server certificate, must occur before the HTTP request is accessed
As a result:
the request information containing the virtual host name cannot be determined prior to authentication
it is therefore not possible to assign multiple certificates to a single IP address
name-based virtual hosts on a secured connection can therefore be problematic

40. Next session will explore… Authentication and access control to websites, remote organisational servers It will also introduce Active Directory and Firewalls