CRYPTOGRAPHY AND NETWORK
SECURITY- E-Mail Security
Dr R Jegadeesan Prof-CSE
Jyothishmathi Institute of Technology and Science,
karimnagar

SYLLABUS
.
E-Mail Security: Pretty Good Privacy, S/MIME IP Security: IP Security overview,
IP Security architecture, Authentication Header, Encapsulating security
payload, Combining security associations, Internet Key Exchange Case Studies
on Cryptography and security: Secure Multiparty Calculation, Virtual Elections,
Single sign On, Secure Inter-branch Payment Transactions, Cross site Scripting
Vulnerability.

:
Topic Name : E-Mail Security: Pretty Good Privacy, S/MIME IP Security: IP Security overview, IP
Security architecture
Topic : Introduction to E-Mail Security. S/MIME IP Security
Aim & Objective : Illustrate E-mail Security and IP security
Application With Example :E-Mail security Applications
Limitations If Any:
Reference Links :
Cryptography and Network Security: C K Shyamala, N Harini, Dr T R Padmanabhan, Wiley India, 1st
Edition.
Cryptography and Network Security : Forouzan Mukhopadhyay, Mc Graw Hill, 3rd Edition
https://www.slideshare.net/ayyakathir/unit-5-76965230?qid=1f7b88c5-e066-4449-9e89-
602464a7e397&v=&b=&from_search=5
https://www.tutorialspoint.com/internet_technologies/e_mail_security.htm
https://www.minigranth.com
https://www.igi-global.com ›
•Video Link details
•https://www.youtube.com/results?search_query=email+security+in+cryptography+and+network+s
ecurity
https://www.youtube.com/watch?v=lcBpC1RsWsw

Universities & Important Questions :
•What is transport mode and tunnel mode in IP sec?
•Give a brief note on Virtual Elections.
•Briefly explain the scenario of IP security and its Policy
•Explain IP security architecture and also explain basic combinations
of security associations with a neat diagram.
•List and explain the PGP services and explain how PGP message
generation is done with a neat diagram

247
E-MAIL, IP & WEB SECURITY
• E-mail Security: Security Services for E-mail-attacks possible through
E-mail – establishing keys privacy-authentication of the source-
Message Integrity-Non-repudiation-Pretty Good Privacy-S/MIME.
IPSecurity: Overview of IPSec – IP and IPv6-Authentication Header-
Encapsulation Security
• Payload (ESP)-InternetKeyExchange (Phasesof like
ISAKMP/IKE Encoding).
•WebSecurity:SSL/TLS basic Protocol-computing the keys- client
authentication-PKI as deployed by SSLAttacks fixed in v3- Exportability-
Encoding-Secure Electronic Transaction (SET).

Email Security
248
• email is one of the most widely used and
regarded network services
Example
From : abc @gmail.com To :
cdf@yahoo.com Sub: Hi
How are you?

Single mail to group of recipients
• Remote Exploder
mail mail
mail
• Local Exploder mail
Req list
reterive list
mail mail mail
Sender
Recipient 2
Recipient 1
Distribution
site
Recipient 3
Sender
Distribution list
maintenance
Recipient 3
Recipient 1 Recipient 2

250
Email Security Services
from
1. Confidentiality- protection
disclosure
2. Authentication of the source
3. message integrity -protection from
modification
4. non-repudiation of origin -protection
from denial by sender

251
5. Proof of submission
6. Proof of delivery
7. Message flow confidentiality-intruder unable to know
whether msg sent/not between users
8. Anonymity- Hiding the sender details
9. Containment- ability to maintain n/w security
10. Audit
11. Self Destruct
12. Message Sequence

252
Pretty Good Privacy (PGP)
• widely used de facto secure email
• developed by Phil Zimmermann
• selected best available crypto algs to use
integrated into a single program
• available on Unix, PC, Macintosh and Amiga
systems
• originally free, now have commercial versions available also

253
PGP Services
1. Authentication
2. Confidentiality
3. Compression
4. 4.E-mail Compatibility 5.Segmentation &
Reassembly

255
PGP Operation – Authentication
1. sender creates a message
2. SHA-1 used to generate 160-bit hash code of message
3. hash code is encrypted with RSA using the sender's private
key, and result is attached to message
4. receiver uses RSA or DSS with sender's public key to decrypt
and recover hash code
5. receiver generates new hash code for message and compares
with decrypted hash code, if match, message is accepted as
authentic

256
PGP Operation – Confidentiality
•sender generates message and random 128-bit number to be used as session key
for this message only
•message is encrypted, using CAST-128 / IDEA/3DES with session key
•session key is encrypted using RSA with recipient's
public key, then attached to message
•receiver uses RSA with its private key to decrypt and recover session key
•session key is used to decrypt message

257
PGP Operation – Confidentiality &
Authentication
• uses both services on same message
–create signature & attach to message
–encrypt both message & signature
–attach RSA encrypted session key

258
PGP Operation – Compression
• by default PGP compresses message
after signing but before encrypting
– One can store uncompressed message &
signature for later verification
– & because compression is non deterministic
• uses ZIP compression algorithm

259
PGP Operation – Email
Compatibility
• when using PGP will have binary data to
send (encrypted message etc)
• however email was designed only for text
• hence PGP must encode raw binary data
into printable ASCII characters
• uses radix-64 algorithm
– maps 3 bytes to 4 printable chars(ASCII)
– also appends a CRC
• PGP also segments messages if too big

260
PGP Operation

261
Segmentation & Reassembly
• PGP subdivides the
original message(if
length >50000
octects) which is too
large to small
enough to send via
mail.
• On the receiving
end, PGP
reassemble entire
original block

262
PGP Session Keys
• need a session key for each message
– of varying sizes: 56-bit DES, 128-bit CAST or IDEA, 168-bit
Triple-DES
• generated using ANSI X12.17 mode
• uses random inputs taken from previous uses and from
keystroke timing of user

PGP Public & Private Keys
• since many public/private keys may be in use, need
to identify which is actually used to encrypt session
key in a message
– could send full public-key with every message
– but this is inefficient
• rather use a key identifier based on key
– is least significant 64-bits of the key
– will very likely be unique
• also use key ID in signatures

264
PGP Key Rings
• each PGP user has a pair of keyrings:
– public-key ring contains all the public-keys of other PGP users
known to this user, indexed by key ID
– private-key ring contains the public/private key pair(s) for this
user, indexed by key ID & encrypted keyed from a hashed
passphrase

PGP Key Management
• rather than relying on certificate authorities
• in PGP every user is own CA
– can sign keys for users they know directly
• forms a “web of trust”
– trust keys have signed
– can trust keys others have signed if have a chain of
signatures to them
• key ring includes trust indicators
• users can also revoke their keys

S/MIME (Secure/Multipurpose Internet Mail Extensions)
• security enhancement to MIME email
– original Internet RFC822 email was text only
– MIME provided support for varying content types and multi-part
messages
– with encoding of binary data to textual form
– S/MIME added security enhancements
• have S/MIME support in various modern mail agents: MS
Outlook, Netscape etc

S/MIME Functions
• enveloped data
– encrypted content and associated keys
• signed data
– encoded message + signed digest
• clear-signed data
– cleartext message + encoded signed
digest
• signed & enveloped data
– nesting of signed & encrypted entities

S/MIME Cryptographic Algorithms
• hash functions: SHA-1 & MD5
• digital signatures: DSS & RSA
• session key encryption: ElGamal & RSA
• message encryption: Triple-DES, RC2/40
and others
• have a procedure to decide which
algorithms to use

S/MIME Certificate Processing
• S/MIME uses X.509 v3 certificates
• managed using a hybrid of a strict X.509 CA hierarchy & PGP’s
web of trust
• each client has a list of trusted CA’s certs
• and own public/private key pairs & certs
• certificates must be signed by trusted CA’s

IP Security
• have a range of application specific security
mechanisms
– eg. S/MIME, PGP,Kerberos, SSL/HTTPS
• however there are security concerns that cut
across protocol layers
• would like security implemented by the
network for all applications

• general IP Security mechanisms
• provides
– authentication
– confidentiality
– key management
• applicable to use over LANs, across
public & private WANs, &
for the Internet
• need identified in 1994 report
– need authentication, encryption in IPv4 &
IPv6

IP Security Uses

Benefits of IPSec
• in a firewall/router provides strong security to
all traffic crossing the perimeter
• in a firewall/router is resistant to bypass
• is below transport layer, hence transparent to
applications
• can be transparent to end users
• can provide security for individual users
• secures routing architecture

IPSecurity Specification
The IPSec specification has become quite complex. key management. The totality of the IPsec
specification is scattered across dozens of RFCs and draft IETF documents, making this the
most complex and difficult to grasp of all IETF specifications. The best way to keep track of
and get a handle on this body of work is to consult the latest version of the IPsec
document roadmap. The documents can be categorized into the following groups:
• Architecture: Covers the general concepts, security requirements, definitions, and
mechanisms defining IPsec technology, RFC 4301, Security Architecture for the Internet
Protocol.
• Authentication Header (AH): AH is an extension header for message authentication, now
deprecated.
• Encapsulating Security Payload (ESP): ESP consists of an encapsulating header and trailer
used
to provide encryption or combined encryption/authentication. .
• Internet Key Exchange (IKE): a collection of documents describing the key management
schemes for use with IPsec
• Cryptographic algorithms: a large set of documents that define and describe cryptographic
algorithms for encryption, message authentication, pseudorandom functions (PRFs), and
cryptographic key exchange.
• Other: There are a variety of other IPsec-related RFCs, including those dealing with security
policy and management information base (MIB) content.

IPSec Services
• Access control
• Connectionless integrity
• Data origin authentication
• Rejection of replayed packets
– a form of partial sequence integrity
• Confidentiality (encryption)
• Limited traffic flow confidentiality

Transport and Tunnel Modes
• Transport Mode
– to encrypt & optionally authenticate IP data
– can do traffic analysis but is efficient
– good for ESP host to host traffic
• Tunnel Mode
– encrypts entire IP packet
– add new header for next hop
– no routers on way can examine inner IP header
– good for VPNs, gateway to gateway security

Transport and
Tunnel Modes

Transport and
Tunnel Mode
Protocols

Security Associations
• a one-way relationship between sender &
receiver that affords security for traffic flow
• defined by 3 parameters:
– Security Parameters Index (SPI)
– IP Destination Address
– Security Protocol Identifier
• has a number of other parameters
– seq no, AH & EH info, lifetime etc
• have a database of Security Associations

Security Policy Database
• relates IP traffic to specific SAs
– match subset of IP traffic to relevant SA
– use selectors to filter outgoing traffic to map
– based on: local & remote IP addresses, next
layer protocol, name, local & remote ports

Transport Mode SA Tunnel Mode SA
AH Authenticates IP payload
and selected portions of IP
header and IPv6 extension
headers
Authenticates entire inner
IP packet plus selected
portions of outer IP header
ESP Encrypts IP payload and any
IPv6 extesion header
Encrypts inner IPpacket
ESP with
authentication
Encrypts IP payload and any
IPv6 extesion header.
Authenticates IPpayload
but no IP header
Encrypts inner IPpacket.
Authenticates innerIP
packet.

Before applying AH
Transport Mode (AH Authentication

Tunnel Mode (AH
Authentication

Encapsulating Security Payload

Encryption & Authentication
Algorithms & Padding
• ESP can encrypt payload data, padding, pad
length, and next header fields
– if needed have IV at start of payload data
• ESP can have optional ICV for integrity
– is computed after encryption is performed
• ESP uses padding
– to expand plaintext to required length
– to align pad length and next header fields
– to provide partial traffic flow confidentiality

Anti-Replay Service
• replay is when attacker resends a
copy of an
authenticated packet
• use sequence number to thwart this
attack
• sender initializes sequence number to 0
when a new SA is established
– increment for each packet
– must not exceed limit of 232 – 1
• receiver then accepts packets with seq
no within window of (N –W+1)

IPSec Key Management
• handles key generation & distribution
• typically need 2 pairs of keys
– 2 per direction for AH & ESP
• manual key management
– Sys-admin manually configures every system
• automated key management
– automated system for on demand creation of keys for
SA’s in large systems
– has Oakley & ISAKMP elements

• a key exchange protocol
• based on Diffie-Hellman key exchange
• adds features to address weaknesses
– no info on parties, man-in-middle attack, cost
– so adds cookies, groups (global params), nonces,
DH key exchange with authentication
• can use arithmetic in prime fields or elliptic
curve fields

ISAKMP
•Internet Security Association and Key Management
Protocol
•provides framework for key management defines procedures and
packet formats to establish, negotiate, modify, & delete SAs
•independent of key exchange protocol, encryption alg, &
authentication method
•IKEv2 no longer uses Oakley& ISAKMP terms,
but basic functionality is same

CASE STUDIES ON CRYPTOGRAPY
AND SECURITY

Secure Inter-branch Payment Transactions
 Points for classroom discussions
1.What is the technology to achieve non-repudiation? How is this guaranteed?
2.How is the problem of key distribution resolved in PKI?
3.Why are cryptographic toolkits required?
4.How can smart cards be used in cryptography?
General Bank Of India (GBI) has implemented an Electronic Payment System called as
EPS in about 1200 branches across the country. This system transfers payment
instructions between two computerized branches of GBI. A central server is maintained
at the EPS office located in Mumbai. The branch offices connect to the Local VSAT of a
private network by using dial-up connection. The local VSAT has a connectivity
established with the EPS office. GBI utilizes its proprietary messaging service called as
GBI-Transfer to exchange payment instructions. Currently, EPS has minimal data
security. As the system operates in a closed network, the current security infrastructure
may suffice the need. The data moving across the network is in encrypted format.

• Current EPS Architecture EPS is used to transmit payment details from
the payer branch to the payee branch via the central server in Mumbai
which is also described stepby- step.
• A data-entry person in the Payer Branch enters transaction details
through the EPS interface.
• A Bank Officer checks the validity of the transaction through the EPS
interface.
• After validating the transaction, the Bank Officer authorizes the
transaction. Authorized transaction is stored in a local Payment Master
(PM) database.
• Once the transaction is stored in PM, a copy of the same is encrypted and
stored in a file. This transaction file is stored in OUT directory. The GBI-
Transfer application looks for any pending transactions (i.e. for the
presence of any files in the OUT directory) by a polling mechanism and if
it finds such

• transactions, it sends all these files one-by-one to the EPS central office located in Mumbai
by dialing the local VSAT.
 The local VSAT gets connectivity to the EPS central office and the transaction is transferred
and
• stored in the IN directory at the EPS central office.
 The interface program at the EPS central office collects the file pending in the IN directory
and sends it to the PM application at that office.
 In order to send the Credit Request to PM, the transaction headers are changed. The
transaction with changed headers in encrypted format is then placed in OUT directory of
the EPS central office.
 The GBI-Transfer application at the EPS central office collects the transactions pending in
the OUT directory and sends them to the Payee Bank through the VSAT.
 The transaction is transferred and stored in the IN directory of the Payee Branch.
 The interface program at the Payee Branch collects the transaction and posts it in PM.
 PM marks the credit entry and returns back an acknowledgement of the same. The
acknowledgement is placed in OUT directory of the Payee Branch.
 The acknowledgement is picked by GBI-Transfer at the Payee Branch and sent to the EPS
central office through the VSAT.
 The EPS central office receives the credit acknowledgement and forwards it to Payer
Branch.
 The Payer Branch receives the credit acknowledgement receipt. This completes the transaction.

• Requirements to Enhance EPS As GBI is in the process of
complete automation and setting up connectivity over the Internet or
a private network, they need to ensure stringent security measures,
which demand the usage of a Public Key Infrastructure (PKI)
framework.
• As a part of implementing security, GBI wants the following
aspects to be ensured:
•Non-repudiation (Digital Signatures)
•Encryption – 128-bit (Upgrade to the current 56-bit encryption)
•Smart card support for storing sensitive data & on-card digital
signing

•Closed loop Public Key Infrastructure :
• Proposed Solution Since providing
cryptographic functionalities require the usage
of a
• cryptographic toolkit, it is assumed that GBI
will implement an appropriate Certification
Authority (CA) infrastructure and a PKI
infrastructure offering.

• The transaction will be digitally signed and encrypted/decrypted at the
Payer and Payee branches, as well as at the EPS central office. The
signing operation can be performed on the system or on external
hardware like a smart card. On the server side, a provision of
automated signing without any manual intervention will be provided.
• The transaction flow described earlier would now be split into two
legs:
• The Payer Leg (Payer Branch to the EPS central office)
• The Payee Leg (EPS central office to the Payee Branch)

Cross Site Scripting Vulnerability (CSSV)

Points for classroom discussions
• What is the purpose of scripting technologies on the Internet?
• What can prevent CSSV attacks?
• What sort of testing can the creators of a Web site perform in order to guard against
possible CSSV attacks?
• Cross Site Scripting Vulnerability (CSSV) is a relatively new form of attacks that
exploits inadequate validations on the server-side. The term Cross Server Scripting
Vulnerability (CSSV) is actually not completely correct. However, this term was coined
when the problem was not completely understood and has stuck ever since. Cross-site
scripting happens when malicious tags and/or scripts attack a Web browser via another site’s
dynamically generated Web pages. The attacker’s target is not a Website, but rather its users
(i.e. clients or browsers).
• The idea of CSSV is quite simple to understand and is based on exploiting the
scripting
• technologies, such as JavaScript, VBScript or JScript. Let us understand how this works.
Consider the following Web page containing a form as shown in Fig. 10.9, in which the user
is expected to enter her postal address. Suppose that the URL of the site sending this page is
www.test.com and when the user submits this form, it would be processed by a server-side
program called as address.asp.

• We would typically expect the user to enter the house number,
street name, city, postal code and country, etc. However, imagine
that the user enters the following weird string, instead:
• <SCRIPT>Hello World</SCRIPT>
• As a result, the URL submitted would be something like
www.test.com/address.asp?address=
• <SCRIPT>Hello World </SCRIPT>.
• Now suppose that the server-side program address.asp does not
validate the input sent by the user and simply sends the value of
the field address to the next Web page. What would this translate
to? It would mean that the next Web page would receive the value
of address as
• <SCRIPT>Hello World</ SCRIPT>.
• As we know, this would most likely treat the value of the address
field as a script, which would be executed as if it is written in a
scripting language, such as JavaScript etc on the Web browser.
Therefore, the user would get to see Hello World.

Virtual Elections
• Points for classroom discussions
• Is it technically possible to have elections on the Internet?
How? What sort of infrastructure would be needed for
this?
• What would be the main concerns in such a virtual
election?
• What would be the use of digital signatures and
encryption in virtual elections?
• Another situation where cryptography is useful is virtual
elections. Computerized voting would become quite
common in the next few decades. As such, it is important
that the protocol for ritual

They must come
together; combine their
respective secrets (S and
R) to generate the
original secret (P).
Bob gets
only R
Alice
gets only
S
Combine
d secret
R
XOR
Random
Number
≈
Origina
l secret
P
S
Fig:Secret splitting

• elections should protect individual privacy and
should also disallow cheating. Consider the
following protocol in order that voters can
send their votes electronically to the Election
authority (EA).

• Each voter casts the vote and encrypts it with the public key of the EA.
• Each voter sends the encrypted vote to the EA.
• The EA decrypts all the votes to retrieve the original vote, tabulates all the
votes and announces the result of the election.
• Is this protocol secure and does it provide comfort both to the voters as well
as to the EA? Not at all!
• There are following problems in this scheme:
• The EA does not know whether the authorized voters have voted or it has
received fake (bogus) votes.
• Secondly, there is no mechanism to prevent duplicate voting.
• What is the advantage of this protocol? Clearly, no one would be able to
change another voter’s vote, because it is first encrypted with the EA’s
public key and is then sent to the EA.
• However, if we observe this scheme carefully, an attacker need not change
someone’s vote at all. The attacker can simply send duplicate votes!
• How can we improve upon this protocol to make it more robust? Let us
rewrite it, as follows:
• Each voter casts the vote and signs it with her private key.

• Each voter then encrypts the signed vote with the public key
of the EA.
• Each voter sends the vote to the EA.
• The EA decrypts the voter with its private key and verifies
the signature of the voter with the help of the voter’s
public key.
• The EA then tabulates all the votes and announces the result
of the election. This protocol would now ensure that duplicate
voting is disallowed. Because the voter has signed the vote (with
her private key) in Step 1, this can be checked. Similarly, no one
can change another voter’s vote. This is because a vote is
digitally signed and any changes to it will be detected and
exposed in the signature verification process.

• Although this protocol is a lot better, the trouble
with this scheme is that the EA would come to
• know who voted for whom, leading to privacy
concerns. We shall leave it to the reader to figure
out how this problem can be solved.

Thank you