service-model.lyx

#LyX 2.0 created this file. For more info see http://www.lyx.org/
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\begin_body

\begin_layout Standard
***TODO: Brief discussion on Homomorphic Encryption***
\end_layout

\begin_layout Part*
Secure Service Model
\end_layout

\begin_layout Section*
Actors
\end_layout

\begin_layout Itemize
The user.
\end_layout

\begin_layout Itemize
The user's computer, or similar device (the 
\begin_inset Quotes eld
\end_inset

client
\begin_inset Quotes erd
\end_inset

).
\end_layout

\begin_layout Itemize
Intermediaries (ISPs, and other systems between the client and server).
\end_layout

\begin_layout Itemize
The service provider's computer (the 
\begin_inset Quotes eld
\end_inset

server
\begin_inset Quotes erd
\end_inset

).
\end_layout

\begin_layout Section*
Connection
\end_layout

\begin_layout Itemize
All communications between the client and server are assumed to take place
 over a secured connection (e.g.
 HTTPS).
\end_layout

\begin_layout Itemize
Any intermediaries are assumed not to be able to decrypt data sent between
 the client and server (due to the use of HTTPS).
 There is the possibility that they will collect meta-data about the transmissio
n, but attempting to avoid that is outside the scope of this project.
\end_layout

\begin_layout Section*
Software/Website Model
\end_layout

\begin_layout Itemize
We will model our service as a typical multi-page HTML+JavaScript-based
 website, accessed via a web-browser.
\end_layout

\begin_layout Section*
Encryption Scheme
\end_layout

\begin_layout Standard
Our security model uses a moderately complex encryption scheme, visualised
 in Figure 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Encryption Scheme"

\end_inset

.
\end_layout

\begin_layout Standard
All encryption described under this scheme is performed on the client (e.g.
 in a web-browser).
\end_layout

\begin_layout Standard
The entities involved in this encryption scheme are:
\end_layout

\begin_layout Itemize
The User.
\end_layout

\begin_layout Itemize
The User's Password.
\end_layout

\begin_layout Itemize
A symmetric Master Key.
\end_layout

\begin_layout Itemize
An asymmetric Public/Private key-pair.
\end_layout

\begin_layout Itemize
Many symmetric Local Keys.
\end_layout

\begin_layout Itemize
Many instances of distinct private data.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\noindent
\align center
\begin_inset Graphics
	filename Diagrams/Encrypt.eps
	lyxscale 50
	scale 40

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Diagram of encryption scheme dependencies.
\begin_inset CommandInset label
LatexCommand label
name "fig:Encryption Scheme"

\end_inset


\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
Under our model, the User provides their password to the encryption software
 (e.g.
 via a password input in the browser).
 The password is then hashed, and this resulting hash may then be used to
 encrypt/decrypt the Master Key.
\end_layout

\begin_layout Standard
The Master key is a symmetric key, which is used to encrypt/decrypt the
 Private key, and all Local Keys.
 
\end_layout

\begin_layout Standard
The Local Keys are symmetric keys, which correspond one-to-one with an instance
 of private data, which they may be used to encrypt/decrypt.
\end_layout

\begin_layout Standard
The Public/Private key-pair may be used to encrypt/decrypt information that
 the other key in the pair encrypts/decrypts, as is typical under asymmetric
 encryption schemes.
 The Public key is never encrypted, since it is as the name suggests intended
 to be publicly available.
\end_layout

\begin_layout Standard
The Public/Private keys are necessary in order to send private information
 to other users.
 The local keys are necessary in order to only send specific data, as opposed
 to sending more data than is desired.
\end_layout

\begin_layout Standard
The Master key could potentially be replaced by the Public/Private pair.
 However, asymmetric encryption schemes tend to be much slower than symmetric
 ones, so it is preferable to minimise the use of the asymmetric pair to
 only when they are actually needed.
\end_layout

\begin_layout Section*
Common Elements
\end_layout

\begin_layout Standard
All pages and procedures are assumed to have the following steps in common:
\end_layout

\begin_layout Enumerate
The client uses a JavaScript-enabled web-browser or similar program to make
 an HTTP request to the server for the appropriate page.
\end_layout

\begin_layout Enumerate
The server responds, sending the corresponding HTML.
\end_layout

\begin_layout Enumerate
The client interprets the HTML, and makes additional requests to the server
 as appropriate for required resources (Images, CSS, JavaScript etc).
\end_layout

\begin_layout Enumerate
The server responds by sending the appropriate resources.
\end_layout

\begin_deeper
\begin_layout Enumerate
Among the JavaScript sent to the client is code for performing the cryptographic
 operations, described below.
\end_layout

\end_deeper
\begin_layout Standard
\begin_inset Float figure
wide false
sideways true
status collapsed

\begin_layout Plain Layout
\noindent
\align center
\begin_inset Graphics
	filename Diagrams/common.eps
	scale 45

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Common Elements
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Section*
User Signup and Key Generation
\end_layout

\begin_layout Enumerate
The client requests and receives a sign-up page.
\end_layout

\begin_layout Enumerate
On the client, a public-private key-pair is generated (e.g.
 by use of the RSA algorithm).
\end_layout

\begin_layout Enumerate
On the client, a randomly generated symmetric 
\begin_inset Quotes eld
\end_inset

master key
\begin_inset Quotes erd
\end_inset

 is created.
\end_layout

\begin_layout Enumerate
On the client, the private key is encrypted using this master key.
\end_layout

\begin_layout Enumerate
On the client, the user fills out a web-form, supplying at minimum a unique
 identifier (e.g.
 username, email) and password.
\end_layout

\begin_deeper
\begin_layout Enumerate
This may happen concurrent to steps 5-7.
\end_layout

\end_deeper
\begin_layout Enumerate
The master key is encrypted using a hash derived from the password.
\end_layout

\begin_layout Enumerate
The public key, the encrypted private key, and the encrypted master key
 and are all transmitted to the server, along with any other signup information.
 These are then stored on the server.
\end_layout

\begin_deeper
\begin_layout Enumerate
The unencrypted private key, the unencrypted master key, and the password
 (or any derivation of it) are 
\emph on
not
\emph default
 transmitted to the server.
\end_layout

\end_deeper
\begin_layout Standard
\begin_inset Float figure
wide false
sideways true
status collapsed

\begin_layout Plain Layout
\noindent
\align center
\begin_inset Graphics
	filename Diagrams/signup.eps
	scale 45

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
User Signup and Key Generation
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Section*
Authentication and Log-In
\end_layout

\begin_layout Enumerate
The client requests and receives a page containing a login form.
\end_layout

\begin_layout Enumerate
The user fills out the log-in form with their unique identifier and password.
\end_layout

\begin_layout Enumerate
The user submits the form, which causes the client to send the unique identifier
 to the server.
\end_layout

\begin_deeper
\begin_layout Enumerate
But 
\emph on
not
\emph default
 the password.
\end_layout

\end_deeper
\begin_layout Enumerate
The server responds with the encrypted private key, encrypted master key
 and public key corresponding to the unique identifier, and a random session
 token which has been encrypted using the public key of the corresponding
 unique identifier.
\end_layout

\begin_layout Enumerate
The client decrypts the master key using a hash of the supplied password,
 then decrypts private key using the master key, and finally decrypts the
 session key using the private key.
\end_layout

\begin_layout Enumerate
The decrypted session key is then sent to the server, and the user is considered
 authenticated if the key matches the original on the server.
\end_layout

\begin_layout Enumerate
The session key is then set as a cookie, so that the session may be tracked
 across pages.
\end_layout

\begin_layout Enumerate
The decrypted private key, master key and public key are saved to the JavaScript
 
\family typewriter
localStorage
\family default
 object.
\end_layout

\begin_deeper
\begin_layout Enumerate
This is so that they persist across pages, but in a manner not opaquely
 available to the server (in contrast with cookies).
\end_layout

\end_deeper
\begin_layout Standard
\begin_inset Float figure
wide false
sideways true
status collapsed

\begin_layout Plain Layout
\noindent
\align center
\begin_inset Graphics
	filename Diagrams/login.eps
	scale 45

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Authentication and Log-In
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Section*
Storing Private Information
\end_layout

\begin_layout Enumerate
The client requests and receives a page which may be used to supply and
 encrypt private information.
\end_layout

\begin_layout Enumerate
The user supplies the private information they wish encrypted and stored
 to the JavaScript runtime, in some manner applicable to the type on information.
\end_layout

\begin_deeper
\begin_layout Enumerate
Different types of information may use different forms of input.
 The specifics are not relevant, other than that it reaches the JavaScript
 runtime in a useable fashion.
\end_layout

\end_deeper
\begin_layout Enumerate
On the client, randomly generated symmetric keys (the 
\begin_inset Quotes eld
\end_inset

local keys
\begin_inset Quotes erd
\end_inset

) are created for each distinct piece of information to be encrypted.
\end_layout

\begin_layout Enumerate
The JavaScript runtime encrypts the information using the local keys.
\end_layout

\begin_layout Enumerate
The encrypted information is transmitted to the server.
\end_layout

\begin_deeper
\begin_layout Enumerate
The unencrypted information is 
\emph on
not
\emph default
 transmitted to the server.
\end_layout

\end_deeper
\begin_layout Enumerate
The local key(s) are encrypted with the master key, and transmitted to the
 server.
\end_layout

\begin_deeper
\begin_layout Enumerate
The unencrypted local keys are 
\emph on
not
\emph default
 transmitted to the server.
\end_layout

\end_deeper
\begin_layout Standard
\begin_inset Float figure
wide false
sideways true
status collapsed

\begin_layout Plain Layout
\noindent
\align center
\begin_inset Graphics
	filename Diagrams/saveInfo.eps
	scale 45

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Storing Private Information
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Section*
Retrieving Private Information
\begin_inset CommandInset label
LatexCommand label
name "sec:Retrieving-Private-Information"

\end_inset


\end_layout

\begin_layout Enumerate
The client requests and receives a page which may contain encrypted private
 information.
\end_layout

\begin_layout Enumerate
This may contain encrypted private information, including the encrypted
 local key(s) corresponding to said information.
\end_layout

\begin_layout Enumerate
In place of, or in addition to the encrypted information sent in step 2,
 the client requests and receives encrypted private information, including
 the encrypted local key(s) corresponding to said information.
\end_layout

\begin_layout Enumerate
The encrypted local keys are decrypted using the master key.
 The encrypted information is then decrypted using the local keys.
\end_layout

\begin_layout Enumerate
The information is made available to the user in whatever format is appropriate
 to the information.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways true
status collapsed

\begin_layout Plain Layout
\noindent
\align center
\begin_inset Graphics
	filename Diagrams/retrieveInfo.eps
	scale 45

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Retrieving Private Information
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Section*
Making Private Information Available to Other Users
\end_layout

\begin_layout Enumerate
The client requests and receives a page which facilitates making private
 information available to other users.
\end_layout

\begin_layout Enumerate
The user indicates the specific information they would like to make available
 to other users.
\end_layout

\begin_layout Enumerate
The user indicates the specific user(s) they would like to make the information
 available to (using the unique identifier associated with each user).
\end_layout

\begin_layout Enumerate
The public key corresponding to each of the specified unique identifiers
 is requested from, and supplied by the server.
\begin_inset Foot
status collapsed

\begin_layout Plain Layout
There is obvious potential for a man-in-the-middle attack here, which will
 be addressed later.
\end_layout

\end_inset


\end_layout

\begin_layout Enumerate
The local keys corresponding to each of the specified pieces of information
 are retrieved by the server, and decrypted using the user's master key.
\end_layout

\begin_layout Enumerate
The local keys are then re-encrypted using the retrieved public keys.
\end_layout

\begin_layout Enumerate
These re-encrypted version are then sent to the server.
\end_layout

\begin_layout Enumerate
On the receiving end, the process works as described under the 
\begin_inset Quotes eld
\end_inset


\begin_inset CommandInset ref
LatexCommand nameref
reference "sec:Retrieving-Private-Information"

\end_inset


\begin_inset Quotes erd
\end_inset

 section.
\end_layout

\begin_layout Standard

\lang english
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wide false
sideways true
status collapsed

\begin_layout Plain Layout
\align center

\lang english
\begin_inset Graphics
	filename Diagrams/shareInfo.eps
	scale 45

\end_inset


\end_layout

\begin_layout Plain Layout

\lang english
\begin_inset Caption

\begin_layout Plain Layout

\lang english
Making Private Information Available to Others
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\end_body
\end_document