PV222
Security Architectures
Lecture 2
Web Security
22nd April 2011 PV222: Security Architectures: Lec 2 2
Lecture Overview
What is the web?
The web components: HTTP and HTML
HTTP, state and cookies
OWASP Top 10
SSL/TLS
HTML, HTTP and the Web
22nd April 2011 PV222: Security Architectures: Lec 2 4
The World Wide Web: In the beginning…
The Internet began as a US Government experiment in the late
1960s.
However, it was the early 1990s before the Internet was widely
available outside of the government and academic sectors.
At the same time some scientists in CERN (the European
Particle Physics Laboratory) released an authoring language and
distribution system.
This was the birth of the Hypertext Markup Language (HTML).
At its core it’s a multimedia-enabled, integrated electronic
document language.
Key to its success was the hypertext linking of documents,
whereby documents automatically reference other documents.
22nd April 2011 PV222: Security Architectures: Lec 2 5
Clients, Servers and Browsers
To access HTML documents, we run browsers on
client machines.
The browser links to web severs over the Internet
to access and retrieve electronic documents.
All web activity begins on the client side, when a
users starts his or her browser.
The browser begins by loading a home page HTML
document from either local storage, or from a server
over some network.
This request (and the server’s reply) is formatted
according to the HyperText Transfer Protocol
(HTTP) standard.
22nd April 2011 PV222: Security Architectures: Lec 2 6
HTML
HTML is a document-layout and hyperlinkspecification
language.
It defines the syntax and placement of special,
embedded directions that aren’t displayed by the
browser, but tell it how to display the contents of the
document.
This includes the text, images, and other supported media.
It also tells the browser how to make the document
interactive through special hypertext links.
These connect one document with other documents – on
any other computer – as well as with other Internet
resources, such as FTP.
22nd April 2011 PV222: Security Architectures: Lec 2 7
The Web Standards
The World Wide Web Consortium (W3C) was formed with the
charter to define the standard versions of HTML.
Beyond HTML, the W3C has the broader responsibility of
standardising any technology related to the World Wide Web.
http://www.w3c.org
Even broader in each than W3C, the Internet Engineering Task
Force (IETF) is responsible for defining and managing every
aspect of Internet technology.
The IETF defines all of the technology of the Internet via official
documents known as Requests For Comments or RFCs.
http://www.ietf.org
22nd April 2011 PV222: Security Architectures: Lec 2 8
A First HTML Document
My first HTML document
My first HTML document
Hello, World Wide Web!
Greetings from
O’Reilly & Associates
Composed with care by:
Geraint Price
©2000 and beyond
22nd April 2011 PV222: Security Architectures: Lec 2 9
A First HTML Document
22nd April 2011 PV222: Security Architectures: Lec 2 10
HTML Embedded Tags
HTML is an embedded language: you insert the language’s
directions or tags into the same document that you and your
readers load into a browser to view.
The browser uses the information inside the HTML tags to decide
how to display or otherwise tread the subsequent contents.
For instance, the tag that follows the word “Hello” in the
simple example tells the browser to display the following text in
italic.
Most tags define and affect a discrete region of your HTML
document.
The region begins with the start tag, and finishes with the end
tag. An end tag is the start tag’s name preceded by a forward
slash (/).
For example, the end tag that matches the “start italicizing”
tag is .
22nd April 2011 PV222: Security Architectures: Lec 2 11
Hyperlinks
What makes HTML so useful on the Internet
is hypertext.
Hypertext gives you the ability to retrieve and
display a different document simply by
clicking on an associated word or phrase
(hyperlink) in the HTML document.
To include a hyperlink to some other
document, you need to know the document’s
unique address and how to put an anchor in
the HTML document.
22nd April 2011 PV222: Security Architectures: Lec 2 12
URLs – I
Every document and resource on the Internet has a
unique address known as its uniform resource
locator (URL).
A URL consists of:
the document’s name preceded by the hierarchy of
directory names in which the file is stored (pathname);
the Internet domain name of the server that hosts the file;
the software and manner by which the browser and the
document’s host server communicate to exchange the
document (protocol).
This information is arranged:
protocol://server_domain_name/pathname
22nd April 2011 PV222: Security Architectures: Lec 2 13
URLs – II
Here are some example URLs:
http://www.kumquat.com/docs/catalog/price_list.html
price_list.html
http://www.kumquat.com/
ftp://ftp.netcom.com/pub/
The first example is what’s known as an absolute or complete URL, as
it includes every part of the URL format: protocol, server, and
pathname.
Browsers also let you use relative URLs and automatically fill in any
missing portions.
The second example is the simplest relative URL.
Relative URLs are also useful if you don’t know a directory or document
name.
The third example points to a kumquat.com’s server, and leaves is up to
the server to decide which file to send back to the client.
22nd April 2011 PV222: Security Architectures: Lec 2 14
The HTTP Protocol
The web protocol, i.e. the set of rules by
which data is transferred between web
browsers and web servers is called HTTP, for
HyperText Transfer Protocol.
This is a very simple “request/reply” protocol
running over TCP (the Transmission
Control Protocol).
Requests are directed from a web browser to
a resource at a specific address.
HTTP is an application-level protocol.
22nd April 2011 PV222: Security Architectures: Lec 2 15
HTTP: Overview
There are two main versions of HTTP:
Version 1.0 (HTTP/1.0 defined in RFC 1945) and Version
1.1 (HTTP/1.1 defined in RFC 2616).
The fundamental unit of HTTP communication is a
message (a structured sequence of bytes).
HTTP is a request-response protocol.
The client (e.g. web browser) submits a request message
to the server.
The server, which stores content, or provides resources,
returns response message to the client.
22nd April 2011 PV222: Security Architectures: Lec 2 16
HTTP: Requests
A client’s HTTP request message:
Request line, such as GET /images/logo.png
HTTP/1.1
Headers, such as Host and Date.
An empty line.
An optional message body.
In the HTTP/1.1 protocol, all headers except
Host are optional.
22nd April 2011 PV222: Security Architectures: Lec 2 17
HTTP: Responses
A server’s response consists of:
a status line, including the protocol version
number, and a success/error code, and
a MIME-like message, containing server
information, content meta-information (headers),
and content.
The content will typically be written in HTML.
Web Security
22nd April 2011 PV222: Security Architectures: Lec 2 19
What is Web Security?
Garfinkel and Spafford (in Web Security,
Privacy & Commerce) define web security
as:
1. “Securing the web server and the data that is on
it.”
2. “Securing information that travels between the
web server and the user.”
3. “Securing the end user’s computer and other
devices that people use to access the Internet.”
22nd April 2011 PV222: Security Architectures: Lec 2 20
Securing the Web Server
Securing the web server is a three part
process:
First, the computer itself must be secured using
traditional computer security techniques.
Second, special programs that provide web
services must be secured.
Finally, you need to examine the operating system
and the web service to see if there are any
unexpected interactions between the two.
22nd April 2011 PV222: Security Architectures: Lec 2 21
Simplification of Services
One of the best strategies for improving a web
server’s security is to minimise the number of
services provided by the host on which the web
server is running.
If you have to provide both a web server and a mail server,
the safest strategy is to put them on different computers.
Chose an operating system that and web server that don’t
come with lots of extra defaults and unnecessary options.
The more complex the system, the more interactions and
the more that can go wrong.
You should limit the number of users who have the ability
to log into the computer.
22nd April 2011 PV222: Security Architectures: Lec 2 22
Securing Information in Transit
Much of the initial emphasis in the field of web
security surrounded securing the information as it
travels over the Internet.
There are many ways in which you can secure
information that travels through a network:
Physically securing the network, so that eavesdropping is
impossible.
Hide the information that you wish to secure within
information that appears innocuous.
Encrypt the information so that it cannot be decoded by any
party who is not in possession of the proper key.
Of these option, encryption is the only technique
that is feasible on a large-scale public network.
22nd April 2011 PV222: Security Architectures: Lec 2 23
Securing the User’s Computer
In the early days of the Web, browsers were regularly being
exposed as insecure.
However, during that period, the main cause of problems for end
users were viruses and worms.
Many computer security professionals had long maintained that
education was the way to secure end user’s computers.
In recent years, however, some people have revised their
opinion, and are now putting their hopes on strong end user
computer security technology.
The reason is that computer systems are getting too complicated
for most end users to make rational security decisions.
22nd April 2011 PV222: Security Architectures: Lec 2 24
HTTP is stateless
The HTTP protocol does not require the
server to maintain any protocol state.
That is, the server does not keep any
information to enable consecutive requests
from a single user agent to be linked.
Hence HTTP does not support “sessions”,
e.g. as might be required to support
e-commerce.
22nd April 2011 PV222: Security Architectures: Lec 2 25
Cookies
HTTP Cookies are simple means of enabling
browser sessions with a server.
The idea is that the server sends back state
information in its response header, in the
form of a Cookie.
The Cookie is then resubmitted with the next
request to the same server.
A Cookie might, for example, specify the
current contents of your shopping basket.
22nd April 2011 PV222: Security Architectures: Lec 2 26
Cookie contents
A cookie header (in a response header)
contains:
attribute, the data payload;
domain scope, enables sharing of cookies by web
hosts with specified domain name;
path scope, limits the URI path to which the
cookie should be sent back;
expiration, the expiry date of the Cookie;
SSL flag, if set the Cookie should only be sent
back via an HTTPS (HTTP over SSL) connection.
22nd April 2011 PV222: Security Architectures: Lec 2 27
Cookies and privacy
Whilst Cookies are an invaluable tool for ecommerce
and other uses of the web, they
also constitute a privacy threat.
Clearly, a server can use Cookies to track
individual user PCs (even if the server cannot
automatically discover the owner of a
particular PC).
We look at one way this tracking can pose a
threat.
22nd April 2011 PV222: Security Architectures: Lec 2 28
Tracking cookies
Web-based advertising agencies, e.g. DoubleClick,
Focalink, Globaltrack, and ADSmart put
advertisements on web sites.
These web pages contain an ![]()
tag, pointing to
a URL on the advertising agency’s server.
When a web browser sees this ![]()
tag, it
contacts the agency server to retrieve the graphic.
The first time the graphic is downloaded, the user
browser will receive an agency cookie containing a
random ID.
22nd April 2011 PV222: Security Architectures: Lec 2 29
Tracking cookies
Every time the browser visits a site containing
the agency’s advertisements, it sends the
cookie (the random ID) along with the URL of
the page that is being read (using the referer
field) to the agency.
This enables the agency to track a single
user’s behaviour across multiple web sites.
22nd April 2011 PV222: Security Architectures: Lec 2 30
Countermeasures
Software can be used to detect tracking
cookies and eliminate them (and, in some
cases, even prevent them being loaded).
Sources of software include:
www.spybot.info (for Spybot Search and Destroy),
and
www.lavasoftusa.com (for Ad-Aware 6.0)
22nd April 2011 PV222: Security Architectures: Lec 2 31
Referer field
One of the fields in the header of an HTTP request
message is the Referer field.
This allows the client to specify, for the server’s
benefit, the address (URI) of the resource from
which the URI of this request was obtained.
In most browsers, when you look at a new page, the
browser will send the URL of the current page in the
referer field.
Under the HTTP definitions, this is means to be an
option for the user, but according to Garfinkel and
Spafford, they have never seen a browser where it
is optional.
OWASP Top Ten
22nd April 2011 PV222: Security Architectures: Lec 2 33
OWASP Top Ten – I
The Open Web Application Security Project
(OWASP) is an open community dedicated to
improving the security of web applications.
The OWASP Top Ten is a project to collate
information on what the most critical web application
security flaws are.
Designed to educate designers, developers and
architects about the consequences of the most
common web application security vulnerabilities.
Goal is education… it is not a standard or policy.
22nd April 2011 PV222: Security Architectures: Lec 2 34
OWASP Top Ten – II
1. Cross Site Scripting (XSS)
2. Injection Flaws
3. Malicious File Execution
4. Insecure Direct Object Reference
5. Cross Site Request Forgery (CSRF)
6. Information Leakage and Improper Error Handling
7. Broken Authentication and Session Management
8. Insecure Cryptographic Storage
9. Insecure Communications
10. Failure to Restrict URL Access
22nd April 2011 PV222: Security Architectures: Lec 2 35
Cross-Site Scripting (XSS) – I
XSS flaws occur whenever an application takes data that
originated from a user and sends it to a browser without first
validating that content.
XSS allows attackers to execute scripts in the victim’s browser.
Using this technique, the attacker can:
Hijack user sessions;
Deface websites;
Inset hostile content;
Conduct phishing attacks;
etc…
Usually JavaScript, but any scripting language which is
supported by the victim’s browser can be misused.
22nd April 2011 PV222: Security Architectures: Lec 2 36
Cross-Site Scripting (XSS) – II
Three types of XSS.
Reflected:
A page will reflect user supplied data directly back to the
user.
Stored:
Takes hostile data, stores it in a file, database or other
backend system, then at a later stage displays the data to
the user.
Very dangerous in Blogs, forums, etc. where a large
number of users will see input from other users.
DOM injected:
The site’s JavaScript code and variables are manipulated
rather than the HTML.
22nd April 2011 PV222: Security Architectures: Lec 2 37
Cross-Site Scripting (XSS) – III
The best protection for XSS is a combination of:
“whitelist” validation of all incoming data;
appropriate encoding of all outgoing data.
Here are some of the validation principles:
Input validation: validate against length, type, syntax, etc.
Use “known good” acceptance strategy.
Strong output encoding: ensure that all user-supplied
data is correctly encoded (e.g. HTML or XML).
Specify the output encoding: for example, ISO 8859-1
character encoding.
Do not use “blacklist” validation: it’s stronger to use a
known “good list” than a known “bad list”.
22nd April 2011 PV222: Security Architectures: Lec 2 38
Injection Flaws – I
Injection occurs when user-supplied data is sent to a
command interpreter as part of a comment or query.
Attackers trick the interpreter into executing
unexpected commands via supplying specially
crafted data.
Injection flaws allow attackers to create, read,
update, delete any data available to the application.
Most famous type of injection query is SQL Injection
attack.
22nd April 2011 PV222: Security Architectures: Lec 2 39
Injection Flaws – II
SQL Injection occurs when:
data is entered to a program from an untrusted source;
the data is used to dynamically generate an SQL query.
This can lead to the loss of the following services:
Confidentiality: any data held in the database can be
read.
Authentication: if SQL commands are used to verify
username and passwords, then an attacker can possibly
log-in without prior knowledge of the password.
Authorisation: if the authorisation data is held by a
database, then this can be modified.
Integrity: it may be possible to modify or delete sensitive
information.
22nd April 2011 PV222: Security Architectures: Lec 2 40
Injection Flaws – III
Avoid the use of interpreters where possible!
If you must use an interpreter, then use a safe API
(e.g. one with strong typing).
Even if you use a strong API, validation is still
recommended.
Examples of recommended precautions:
Input validation: use a “known good” acceptance strategy.
Use strongly types APIs: this helps reduce the types of
input that will automatically be accepted.
Enforce least privilege: when connecting to databases
and other back-ends.
Avoid detailed error messages: these might be useful to
an attacker.
22nd April 2011 PV222: Security Architectures: Lec 2 41
Malicious File Execution – I
Any web server is vulnerable where the application
allows filenames or files from the user.
This allows attackers to perform:
Remote code execution;
Remote root kit execution.
As well as allowing remote execution, it can be used
to access local file systems.
Other methods of attack:
Hostile data being uploaded to session files, log data, and
via image uploads (e.g. in a forum environment).
22nd April 2011 PV222: Security Architectures: Lec 2 42
Malicious File Execution – II
Some ways in which this attack can be prevented:
Strongly validate user input: again, accept only “known
good” input.
Add firewall rules: these should prevent web servers from
making connections to external websites.
Use a taint checking mechanism: some languages allow
any variables that have user input to be flagged as
potentially dangerous.
Implement a chroot jail: this is a type of sandboxing
technique to isolate applications from each other.
22nd April 2011 PV222: Security Architectures: Lec 2 43
Insecure Direct Object Reference – I
A direct object reference when some internal object
(e.g. file, directory, database record or key) is used
as a URL or form parameter.
This can allow an attacker to manipulate other
objects without authorisation.
For example:
In internet banking applications, the account number is
often used as the database primary key.
It is then tempting to use this account number directly on a
web interface.
If no extra check is done to verify the user, an attacker can
manipulate this parameter to see or change all accounts.
22nd April 2011 PV222: Security Architectures: Lec 2 44
Insecure Direct Object Reference – II
The best way to protect against this is to not
to use a direct object reference.
Instead use an indirect mapping which is easier to
validate.
Avoid exposing your private object references
to users wherever possible.
Validate any private object reference with the
“known good” approach.
Verify authorisation to all referenced objects.
22nd April 2011 PV222: Security Architectures: Lec 2 45
Cross Site Request Forgery – I
A CSRF attack forces a logged-on victim’s browser
to send a request to a vulnerable web application.
The vulnerable application then performs the
chosen action on behalf of the victim.
The malicious code is usually not on the attacked
side – hence “Cross Site”.
A typical CSRF attack against a forum might take
the form of directing a user to invoke some function,
such as the application’s logout page:
The following tag in any webpage viewed by the
user will automatically log them out:
22nd April 2011 PV222: Security Architectures: Lec 2 46
Cross Site Request Forgery – II
Applications must ensure that they are not relying
on credentials or tokens that are automatically
submitted by browsers.
The only solution is to use a custom token that the
browser will not “remember” and then automatically
include with a CSRF attack.
Insert custom random tokens into every form and URL.
Also check that there are no XSS vulnerabilities in
your application.
For sensitive data or value transactions, reauthenticate
or use transaction signing.
22nd April 2011 PV222: Security Architectures: Lec 2 47
Information Leakage and Improper Error
Handling – I
Applications can unintentionally leak
information about their configuration, internal
workings or violate privacy through a variety
of application problems.
Web applications will often leak information
about their internal state through detailed or
debug error messages.
Often, this information can be leveraged to
launch or even automate more powerful
attacks.
22nd April 2011 PV222: Security Architectures: Lec 2 48
Information Leakage and Improper Error
Handling – II
Applications frequently generate error messages
and display them to users.
Sometimes this can reveal useful information to an
attacker.
Some common ways in which this might happen:
Where an error displays too much detailed information: e.g.
a stack trace; failed SQL statements.
Functions that respond with different results based on
different inputs: for example, responding to an incorrect
username/password combination with different error codes
depending on which parts were wrong.
22nd April 2011 PV222: Security Architectures: Lec 2 49
Information Leakage and Improper Error
Handling – III
Applications should use a standard exception
handling architecture to prevent additional
information leaking to an attacker.
Effective practice might include:
Disable or limit detailed error handling.
Ensure that secure paths that have multiple outcomes
return similar or identical error messages.
Different layers (e.g. database layer, web server layer) will
return exceptional results. Ensuring that these are sanitised
to prevent information leakage is important.
Always return a “standard” error screen can prevent an
automated tool from finding out if a serious error occurred.
22nd April 2011 PV222: Security Architectures: Lec 2 50
Broken Authentication and Session
Management – I
Many web applications allow authentication or
session management through tokens or session
cookies.
Failure to protect these tokens or cookies can allow
an attacker to hijack a user’s account or login
session.
Authentication relies on secure credential
communication and storage.
Ensure that SSL is the only option for authenticated parts
of the application.
All credentials are stored in a hashed or encrypted form.
22nd April 2011 PV222: Security Architectures: Lec 2 51
Broken Authentication and Session
Management – II
Other things that might be done:
Only use the inbuilt session management mechanism.
Do not allow the login process to start from an unencrypted
page.
Ensure that every page has a logout link, and that upon
logout all server side state and client side cookies are
destroyed.
Use a timeout period that automatically logs out an inactive
session.
Do not rely on spoofable credentials as the sole form of
authentication: e.g. IP addresses; referrer headers.
22nd April 2011 PV222: Security Architectures: Lec 2 52
Insecure Cryptographic Storage – I
Protecting key sensitive data using
cryptography has become an important part
of application security.
The most common application flaws are:
Not encrypting sensitive data.
Using home grown algorithms.
Insecure use of strong algorithms.
Continued use of proven weak algorithms (MD5,
SHA-1, RC3, RC4, etc…)
Hard coding keys, and not protecting key storage.
22nd April 2011 PV222: Security Architectures: Lec 2 53
Insecure Cryptographic Storage – II
Ensure that everything that should be
encrypted is actually encrypted.
Ensure that the cryptography is properly
implemented:
Do not create cryptographic algorithms.
Do not use weak algorithms.
Generate keys offline and store private keys with
care.
Ensure that encrypted data stored on disk is not
easy to decrypt.
22nd April 2011 PV222: Security Architectures: Lec 2 54
Insecure Communications
Applications frequently fail to encrypt sensitive network traffic.
Encryption (usually SSL) should be used for all authenticated
connections, especially Internet-accessible web pages.
In addition, encryption should be used whenever sensitive data
(e.g. credit card, health information) is transmitted.
Applications that can fallback or forced out of encrypting can be
abused by attackers.
Encrypting communications with back-end servers is also
important, as the information they carry is more sensitive and
more extensive.
Under PCI (Payment Card Industry) Data Security Standard
requirement 4, you must protect cardholder data in transit.
22nd April 2011 PV222: Security Architectures: Lec 2 55
Failure to Restrict URL Access
Frequently, the only protection for a URL is that links
to that page are not presented to an unauthorised
user.
However, a motivated, skilled or lucky attacker may
be able to find and access these pages.
Web applications must enforce access control on all
URLs and business functions.
It is also important that the authorisation is checked
regularly during the process.
Otherwise an attacker might be able to skip the
authentication phase and forge the parameters for the
subsequent steps.
22nd April 2011 PV222: Security Architectures: Lec 2 56
Other Attacks on Servers
Web servers themselves may be the victims of
attacks via HTTP requests.
For example, to cause buffer overflow in a web
server, an attacker might induce errors at Web traffic
ports by entering large character strings to find a
susceptible overflow field.
Once a field spills over into a code-executing field,
an attacker will enter another string that will spill a
command into the executable field.
Buffer overflows can give an attacker access to a
range of sensitive server functions.
22nd April 2011 PV222: Security Architectures: Lec 2 57
URL Obfuscation Attacks
URL Obfuscation attacks are mechanisms used to
trick users to visit an attacker’s website.
Examples of such attacks are: using strings; using @ sign;
URL redirection.
Using strings:
http://254.231.52.42/ebay/account_update/now.php
Using @ sign:
http://www.citybank.com/update.pl@254.231.52.42/usb
/upd.pl
URL redirection:
http://usa.visa.com/track/dyredir.jsp?rDirl=http://
200.251.251.10/.verified/
SSL/TLS
22nd April 2011 PV222: Security Architectures: Lec 2 59
SSL/TLS overview
SSL = Secure Sockets Layer. Current version
is v3.
TLS = Transport Layer Security. TLS 1.0 is
similar to SSL 3.0 with minor tweaks.
TLS is defined in RFC 2246.
SSL/TLS provides security “at TCP layer”. In
fact, it usually provides a thin layer between
TCP and HTTP.
22nd April 2011 PV222: Security Architectures: Lec 2 60
SSL/TLS basic features
SSL/TLS widely used in Web browsers and
servers to support “secure e-commerce” over
HTTP.
Built into Microsoft IE, Netscape, Mozilla, Apache,
IIS, …
Presence of SSL protected link indicated by the
browser padlock symbol.
22nd April 2011 PV222: Security Architectures: Lec 2 61
SSL architecture
SSL architecture involves two layers:
SSL Record Protocol
Lower layer providing secure, reliable channel to upper
layer.
Upper layer carrying:
SSL Handshake Protocol,
Change Cipher Spec. Protocol,
Alert Protocol,
HTTP,
Any other application protocols.
22nd April 2011 PV222: Security Architectures: Lec 2 62
SSL architecture
SSL
Handshake
Protocol
SSL Record
Protocol
TCP
SSL
Change
Cipher
Spec
Protocol
SSL
Alert
Protocol
HTTP,
other apps
22nd April 2011 PV222: Security Architectures: Lec 2 63
SSL Record Protocol
Carries application data and “management” data.
Sessions:
Sessions created by handshake protocol.
Defines set of cryptographic parameters (encryption and
hash algorithm, master secret, certificates).
Carries multiple connections to avoid repeated use of
expensive handshake protocol.
Connections:
State defined by nonces, secret keys for MAC and
encryption, IVs, sequence numbers.
Keys for many connections derived from single master
secret created during handshake protocol.
22nd April 2011 PV222: Security Architectures: Lec 2 64
SSL Record Protocol
SSL Record Protocol provides:
Data origin authentication and integrity.
MAC using algorithm similar to HMAC, based on MD5 or
SHA-1 hash algorithms.
MAC protects 64 bit sequence numb for anti-replay.
Confidentiality.
Bulk encryption using symmetric algorithm (IDEA, RC2-
40, DES-40 (exportable), DES, 3DES, RC4-40 and RC4-
128.
22nd April 2011 PV222: Security Architectures: Lec 2 65
SSL Record Protocol
Data from application/upper layer SSL
protocol partitioned into fragments (max size
214 bytes).
MAC first, then pad (if needed), and finally
encrypt.
Prepend header containing: Content type,
version, length of fragment.
Submit to TCP.
22nd April 2011 PV222: Security Architectures: Lec 2 66
SSL Handshake Protocol
SSL needs secret keys:
Used for MAC & encryption at Record Layer.
Different keys in each direction.
These keys are established as part of the
SSL Handshake Protocol.
The SSL Handshake Protocol is a complex
protocol with many options.
22nd April 2011 PV222: Security Architectures: Lec 2 67
SSL Handshake Protocol security goals
Entity authentication of participating parties
(“client” and “server”).
Server nearly always authenticated, client more
rarely.
Appropriate for most e-commerce applications.
Establishment of a fresh, shared secret.
Shared secret used to derive further keys for SSL
Record Protocol.
Secure ciphersuite negotiation (including
encryption and hash algorithms).
22nd April 2011 PV222: Security Architectures: Lec 2 68
SSL Handshake Protocol – key exchange
SSL supports several key establishment
mechanisms.
Most common is RSA encryption.
Client chooses pre_master_secret, encrypts it
using public RSA key of server, and sends to
server.
Can also create pre_master_secret using
one of several variants of Diffie-Hellman key
establishment protocol.
22nd April 2011 PV222: Security Architectures: Lec 2 69
SSL Handshake Protocol – entity
authentication
SSL supports several different entity
authentication mechanisms.
Most common based on RSA:
The ability to decrypt pre_master_secret and
generate correct MAC using keys derived from
pre_master_secret authenticates the server to
the client.
DSS or RSA signatures on nonces (and other
fields, e.g. Diffie-Hellman values).
22nd April 2011 PV222: Security Architectures: Lec 2 70
SSL key deriviation
Keys used for MAC and encryption derived
from pre_master_secret:
Derive master_secret from
pre_master_secret and client/server nonces
using MD5 and SHA-1.
Derive key material from master_secret and
client/server nonces, by repeated use of hash
functions.
Split key material into MAC and encryption keys
as needed.
22nd April 2011 PV222: Security Architectures: Lec 2 71
SSL Handshake Protocol run
We choose the most common use of SSL for
illustration:
No client authentication.
Client sends pre_master_secret using
Server’s public encryption key from Server
certificate.
Server authenticated by ability to decrypt to obtain
pre_master_secret, and construct correct
finished message.
22nd April 2011 PV222: Security Architectures: Lec 2 72
SSL Handshake Protocol run
M1: C → S: ClientHello
Client initiates connection.
Sends client version number.
3.1 for TLS.
Sends ClientNonce.
28 random bytes plus 4 bytes of time.
Offers list of ciphersuites.
key exchange and authentication options, encryption
algorithms, hash functions, e.g.
TLS_RSA_WITH_3DES_EDE_CBC_SHA.
22nd April 2011 PV222: Security Architectures: Lec 2 73
SSL Handshake Protocol run
M2: S → C: ServerHello, ServerCertChain,
ServerHelloDone
Sends server version number.
Sends ServerNonce and SessionID.
Selects single ciphersuite from list offered by client, e.g.
TLS_RSA_WITH_3DES_EDE_CBC_SHA.
Sends ServerCertChain message.
Allows client to validate server’s public key.
(optional) CertRequest message.
Omitted in this protocol run – no client authentication.
Finally, ServerHelloDone.
22nd April 2011 PV222: Security Architectures: Lec 2 74
SSL Handshake Protocol run
M3: C → S: ClientKeyExchange,
ChangeCipherSpec, ClientFinished
ClientKeyExchange contains encryption of
pre_master_secret under server’s public key.
ChangeCipherSpec indicates that client is updating
cipher suite to be used in this session.
Sent using SSL Change Cipher Spec. Protocol.
(optional) ClientCertificate,
ClientCertificateVerify messages.
Only when client is authenticated.
Finally, ClientFinished message.
A MAC on all messages sent so far (both sides).
MAC computed using master_secret.
22nd April 2011 PV222: Security Architectures: Lec 2 75
SSL Handshake Protocol run
M4: S → C: ChangeCipherSpec,
ServerFinished
ChangeCipherSpec indicates that server is
updating cipher suite to be used on this session.
Sent using SSL Change Cipher Spec. Protocol.
Finally, ServerFinished message.
A MAC on all messages sent so far (both sides).
MAC computed using master_secret.
Server can only compute MAC if it can decrypt
pre_master_secret in M3.
22nd April 2011 PV222: Security Architectures: Lec 2 76
SSL Handshake Protocol run
Summary:
M1: C → S: ClientHello
M2: S → C: ServerHello,
ServerCertChain, ServerHelloDone
M3: C → S: ClientKeyExchange,
ChangeCipherSpec, ClientFinished
M4: S → C: ChangeCipherSpec,
ServerFinished
22nd April 2011 PV222: Security Architectures: Lec 2 77
SSL Handshake Protocol run
1. Is the client authenticated to the server in this
protocol run?
2. Can an adversary learn the value of
pre_master_secret?
3. Is the server authenticated to the client?
1. No.
2. No. Client has validated server’s public key; only the
holder of the private key can decrypt
ClientKeyExchange to learn pre_master_secret.
3. Yes. ServerFinished includes MAC on nonces
computed using key derived from pre_master_secret.
22nd April 2011 PV222: Security Architectures: Lec 2 78
Other SSL Handshake options
Many optional/situation-dependent protocol
messages:
M2 (S → C) can include:
ServerKeyExchange (e.g. for DH key exchange).
CertRequest (for client authentication).
M3 (C → S) can include:
ClientCert (for client authentication).
ClientCertVerify (for client authentication).
22nd April 2011 PV222: Security Architectures: Lec 2 79
Other SSL protocols
Alert protocol.
Management of SSL session, error messages.
Fatal errors and warnings.
Change cipher spec protocol.
Not part of SSL Handshake Protocol.
Used to indicate that entity is changing to recently
agreed ciphersuite.
Both protocols run over Record Protocol (so
peers of Handshake Protocol).
22nd April 2011 PV222: Security Architectures: Lec 2 80
SSL and TLS
TLS 1.0 = SSL 3.0 with minor differences:
TLS signalled by version number 3.1
Use of HMAC for MAC algorithm.
Different method for deriving key material (mastersecret
and key-block).
Pseudo-random function based on HMAC with MD5 and
SHA-1.
Additional alert codes.
More client certificate types.
Variable length padding (can be used to hide lengths of
short messages and so frustrate traffic analysis).
And more…
22nd April 2011 PV222: Security Architectures: Lec 2 81
SSL/TLS applications
Secure e-commerce using SSL/TLS.
Client authentication not needed until client
decides to buy something.
SSL provides secure channel for sending credit
card information.
Client authenticated using credit card information,
merchant bears (most of) risk.
Very widely used.
22nd April 2011 PV222: Security Architectures: Lec 2 82
SSL/TLS application issues
Secure e-commerce: some issues.
No guarantees about what happens to client data (including
credit card details) after session: may be stored on
insecure server.
Does client understand meaning of certificate expiry and
other security warnings?
Does client software actually check complete certificate
chain?
Does the name in certificate match the URL of e-commerce
site? Does the user check this?
Is the site the one the client thinks it is?
Is the client software proposing appropriate ciphersuites?
22nd April 2011 PV222: Security Architectures: Lec 2 83
SSL/TLS application issues
Secure electronic banking:
Client authentication may be enabled using client
certificates.
Issues of registration, secure storage of private keys,
revocation and re-issue.
Otherwise, SSL provides secure channel for sending
username, password, mother’s maiden name, …
What else does client use same password for?
Does client understand meaning of certificate expiry and
other security warnings?
Is client software proposing appropriate ciphersuites?
Enforce from server.
22nd April 2011 PV222: Security Architectures: Lec 2 84
Standards
All IETF RFCs can be obtained from:
www.ietf.org
The W3C recommendations are available at:
www.w3c.org
For general information about security
standards see: A. W. Dent and C. J. Mitchell:
User’s guide to cryptography and standards
(Artech House, 2004).
http://www.isg.rhul.ac.uk/ugcs
22nd April 2011 PV222: Security Architectures: Lec 2 85
Acknowledgements
SSL/TLS discussion based on (an abbreviated version of) Kenny
Paterson’s lecture for IY5511 course.
Information on the OWASP Top 10 taken from:
http://www.owasp.org/
Information on the basics of HTML taken from: C Musciano & B
Kennedy, “HTML: The Definitive Guide”, O’Reilly.
Plenty of information on HTML available from a number of web
sites, including the W3C web site (lots of useful tutorial
information there).
Basic information on web security taken from: S. Garfinkel and
G. Spafford – “Web Security, Privacy & Commerce”, O’Reilly.
Some additional information taken from original lecture notes by
Chris Mitchell.
22nd April 2011 PV222: Security Architectures: Lec 2 86
Conclusions
After today’s lecture you should:
Have a basic understanding of how the
components that make up the web work.
Understand what are the security problems faced
by clients and servers using the web as an
interface.
Be able to describe a high level overview of how
SSL allows us to build secure connections
between clients and servers.
Be able to appreciate that security of web
applications does not just start and end with SSL.