Electronic Mail Security Lec # 10

7/31/2019 Electronic Mail Security Lec # 10

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Electronic Mail Security

Khalid Haseeb

Computer Science Deptt.Islamia College University, Peshawar


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Electronic Mail.

Electronic mail is used for sending a single

message that includes text, voic, video,

graphics to one or more recipients.

Simple Mail Transfer Protocol (SMTP) is the

standard mechanism for electronic mail in the

internet.


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Sending Mail. To send mail, a user creates mail

that looks very similar to postalmail, it has an envelope andmessage.

Envelope:

It contains the sender address,the receiver address and other

information. Message:

It contains headers and the body.

Header defines the sender,receiver and subject of themessage.

Body contains the actualinformation to be read by therecipient.

Mail From : [email protected]

RECPT To : [email protected]

From : khalid haseeb.

To: Qaisar Khan

Date : 31/05/2007

Subject : Network

Dear Ali.

How Are You and bla bla bla.

Envelope

Message

Header

Body
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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Receiving Mail.

The email system periodically checks themailboxes, it a user has a mail, it informs the userwith a notice.

If a user is ready to read the mail, a list isdisplayed in which each contains a summary ofthe information about a particular message in themailbox.

The summary usually contains the sender emailaddress, the subject, time of mail sent andreceived etc.


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Address.

The address system used by SMTP consists of

two parts:

Local Part:

Local part defines the name of a special file, called the

user mailbox, where all the mails received for a user are

stored for retrieval by the user agent.

Domain Name:

It is a logical name assigned to a host responsible for receiving

and sending emails( host is also called mail exchanger ).

Local Part @ Domain Name

Address of the mailbox

on the local site.

The domain name of

the destination.


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User Agent.

User Agent is a software

package that composes,reads, replies to and

forward messages.

It also handles mailboxes.

Services of UA: Composing Messages.

Reading Messages.

Replying to Messages.

Forwarding Messages.

Handling Mailboxes.

User AgentServices

ComposingMessages

ReadingMessages

Replying toMessages

ForwardingMessages

HandlingMailboxes


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Composing Messages.

A UA is responsible for composing the emailmessage to be sent out.

UA provide a template on the screen to be

filled in by the user. Some have built in editorwith facilities like spell checking, grammarchecking etc.

A user can also use his favorite text editor orword processor to create the message andimport it or cut and paste into the user agent.


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Reading Messages.

UA reads the incoming messages.

When a user invokes a UA, it first checks the mail in theincoming mailbox.

Most UA show a one line summary of each received mail

which contain the following fields: A number field.

A flag that shows if the mail is new, already read but notreplied to, read and replied to and so on.

The size of the message.

The sender.

The subject of the mail.


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Replying to Messages.

After reading the message, a user can use the

UA to reply to a message.

UA sends the reply to the original sender.

The reply message contains the original

message and the new message.


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Forwarding Messages.

Replying means sending a message to the

sender, but forwarding means to send the

message to a third party.

Forwarding allows a user to forward the

message to a third party with or without extra

comments.


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Handling Mailboxes.

UA normally creates two mailboxes: inbox and

outbox.

Each box is a file with special format that can

be handled by the UA.

The inbox keeps all the received emails until

they are deleted by the user.

The outbox keeps all the sent mails until the

user deletes them.


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User Agent Types

Two types of UA:

Command-Driven:

Command driven UA belong to the early days of electronic mail.

The are still present as the underlying UA in servers.

It accepts a one character command form the keyboard to perform itstask like type the character r at the command prompt to reply to the

sender etc.

Examples of command driven UA are mail, pine and elm etc.

GUI-Based:

Modern UA are GUI-based.

They contian GUI that allow the user to interact with the software by

using both the keyboard and mouse.

They have graphical components like icons, menu bars and windows that

make the services easy to access.

Examples are Eudora, Microsoft's outlook and Netscape.


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Typical Diagram.

Sender

Local Mail Server

SMTP

Remote Mail Server

SMTP

Receiver

POP3


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Electronic Mail Security.

In all distributed environments, electronic mail

is the most heavily used network based

application.

With the growing reliance on electronic mail

for every conceivable purpose, there grows a

demand for authentication and confidentiality

services.

Two schemes are used for this purpose:

PGP ( Pretty Good Privacy ).

S/MIME ( Secure Multipurpose Internet Mailing Extension ).


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Pretty Good Privacy

PGP is the result of Phil Zimermann efforts.

PGP provides confidentiality and

authentication service for electronic mail and

file storage application.

PGP has grown explosively and is now widely

used. A number of reasons can be sited for

this growth.


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Reasons.

It is available free worldwide in versions that run on a verity of

platforms including Windows, UNIX, Macintosh and many

more.

It is based on algorithms that have survived extensive public

review and are considered extremely secure. Specifically, the

package includes RSA, DSS, Diffie-Hellman, 3DES and SHA-1.

It has a wide range of applicability, from corporations that

wish to select and enforce a standardized scheme for

encrypting files and messages to individuals who wish tocommunicate securely with others worldwide over the

internet.

PGP is now on an Internet standards track ( RFC 3156 ).


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Operational Description.

The actual operations of PGP consists of five

services:

Authentication.

Confidentiality.

Compression.

E-mail compatibility.

Segmentation.


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Authentication.

Authentication is carried out as:

Sender creates the message.

SHA-1 is used to generate a 160-bits hash code of themessage.

The hash code is encrypted with RSA using the sendersprivate key and the result is prepended to the message.

The receiver uses RSA with the senders public key todecrypt and recover the hash code.

The receiver generates a new hash code for the messageand compares it with the decrypted hash code. If the twomatches the message is accepted as authentic.


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Authentication only.

M

H EP

KRa

|| ZM

EKRa[H(M)]

DP

KUa

HCompare

Z-1


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Confidentiality.

Confidentiality is provided by encryptingmessages to be transmitted or to be storedlocally as files.

In both cases the symmetric encryptionalgorithm ( 3DES or others ) is used.

In PGP each symmetric key is used only once.

It is commonly called session key. Session key is selected by sender and is bound

to the message and transmitted with it.

To protect the key it is encrypted with thereceivers public key.


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Confidentiality Steps.

The sender generates a message and a 128-bitnumber to be used as a session key for this messageonly.

The message is encrypted using ( 3DES or others )with the session key.

The session key is encrypted with RSA, using therecipient's public key and is prepeneded to the

message. The receiver uses RSA with it private key to decrypt

and recover the session key.

The session key is used to decrypt the message.


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Confidentiality only.

M||ECZ

Ks

Z-1DC

DP

EP

KUb EKUb[Ks] KRb

M

Ks


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Confidentiality and Authentication.

1. A signature is generated for the plaintext and encrypted

using RSA using senders private key. Signature is then

perpended to the message. This step provides the message

authentication

2. Plaintext message plus signature is then compressed.

3. The compressed version is then encrypted using 3DES or

other symmetric encryption algorithm and session key. This

step provides the message confidentiality.

4. Session key is encrypted using RSA and receivers public key

and prepended with the encrypted version of plaintext

message plus signature. This step provides the session key

confidentiality.


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Confidentiality and Authentication.

M||ECZ||

EPH

KRa

Ks

H

DC

DPEP

KUb[Ks]

KRb

DP

Z-1

M

KUa

Compare


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Compression.

PGP compresses the message after applying the signature but beforeencryption. This has the benefit of saving both space for E-mailtransmission and for file storage.

The signature is generated before compression for two reasons:

It is preferable to sign an uncompressed message so that one can store only

the uncompressed message together with the signature for future verification. PGP compression algorithm is not deterministic:

Various PGP compression algorithm achieve different tradeoff in running speedversus compression ratio and as a result produce different compressed forms.

However different compression algorithms are interoperable.

Therefore applying hash function signature after compression would require the

same PGP compression algorithm implementation. Message encryption is applied after compression to strengthen

cryptographic security. Because the compressed message has lessredundancy than the original message, cryptanalysis is more difficult.


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E-mail Compatibility.

After encryption the part or all of the resulting blockconsists of a stream of arbitrary 8-bit octate.

Many electronic mail systems (SMTP) only permitsthe use of blocks consisting of ASCII text.

Therefore PGP has to convert 8-bit binary streaminto a stream of printable ASCII characters.

The scheme used for this purpose is radix-64

conversion. It will convert a group of three octets ofbinary data into four ASCII characters.


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Radix-64/Base64 Encoding

11001100 10000001 00111001

Base64

110011 001000 000100 111001

(51) (8) (4) (57)

01111010 01001001 01000101 00110101

Z I E 5


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E-mail Compatibility.

Radix-64 expands a message by 33%.

The compression should be more than enough to

compensate for the radix-64 expansion.

Another advantage of radix-64 algorithm is that itblindly convert the input stream to radix-64 format

regardless of content, even it the input happens to

be ASCII text. If message is signed but not encrypted

and the conversion is applied to the entire block, the

output will be unreadable to the casual observer

which provides a certain level of confidentiality.


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Generic transmission diagram

(form A)X -- File

Signature Required

CompressX Z(X)

Confidentiality Required

Convert to radix-64

X R64[X]

YesGeneric Signature

X Signature || X

No

Encrypt key, X

X EKUb[Ks] || EKs[X]

Yes

No


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Generic reception diagram ( to B ).

Convert from radix 64X R64-1[X]

Confidentiality Required

Decompress

X Z-1(X)

Signature Required

Yes Decrypt key, X

K DKRb [ EKUb [Ks]];

K DK [X]

No

Strip signature form X

Verify Signature

Yes

No


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Segmentation and Reassembly.

Email facilities are often restricted to a maximum message length e.gInternet imposes a maximum length of 50,000 octets.

Any message longer than that must be broken up into smaller segments,each of which is mailed separately.

PGP automatically subdivides a message that is too larger into segments

that are small enough to send via E-mail. The segmentation is done after all of the other processing including the

radix-64 conversion.

Thus the session key component and signature component appear onlyonce, at the beginning of the first segment.

At the receiving end PGP strip off all of the E-mail headers form the firstsegment and the second segment and reassembles them into the originalblock before performing the steps shown in the previous slide.

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Electronic Mail Security Lec # 10