Introduction
• Email is one of the most widely used service on internet
• Mail servers are favorite target after web server
• Normally message contents are not secured
• Can be read/edit while on transit from sender to receiver
• Can be read/edit at destination
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Architecture of E-mail
• To explain the architecture of e-mail, we give four
scenarios. We begin with the simplest situation
and add complexity as we proceed. The fourth
scenario is the most common in the exchange of
e-mail.
• First Scenario
• Second Scenario
• Third Scenario
• Fourth Scenario
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First scenario
1
2
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When the sender and the receiver of an e-
mail are on the same mail server,
we need only two user agents.
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Second scenario
1
2 3 4
5
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When the sender and the receiver of an e-
mail are on different mail servers,
we need two UAs and a pair of MTAs (client
and server).
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Third scenario
1
2
3
4
5
6
7
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When the sender is connected to the mail
server via a LAN or a WAN, we
need two UAs and two pairs of MTAs
(client and server).
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Fourth scenario
1
2
3
4
5
6
8
9
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When both sender and receiver are
connected to the mail server via a LAN or a
WAN, we need two UAs, two pairs of MTAs
(client and server), and a pair of MAAs
(client and server). This is the most
common situation today.
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Push versus pull
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USER AGENT
The first component of an electronic mail system is
the user agent (UA). It provides service to the user to
make the process of sending and receiving a
message easier.
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Format of an email
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E-mail address
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Message Transfer Agent
• Mail transfer is done through Message Transfer Agent (MTAs)
• To send mail
• System must have client MTA
• To receive mail
• System must have server MTA
• Note
• The formal protocol that defines the MTA client and server in the internet is
called SMTP
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17
SMTP range
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Commands and responses
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Connection establishment
220 service ready 1
HELO: deanza.edu2
250 OK 3
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Message transfer
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Connection termination
1 QUIT
2221 service closed
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MESSAGE ACCESS AGENT
The first and the second stages of mail delivery use
SMTP. However, SMTP is not involved in the third
stage because SMTP is a push protocol; it pushes
the message from the client to the server. In other
words, the direction of the bulk data (messages) is
from the client to the server. On the other hand, the
third stage needs a pull protocol; the client must pull
messages from the server. The direction of the bulk
data are from the server to the client. The third stage
uses a message access agent.
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Pop3 and IMAP4
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Pop3
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Threats
• Email Confidentiality (protection from disclosure)
• Email Integrity (Protection from modification)
• Email Authentication (verification of sender)
• Lack of non-repudiation ( protection from denial by sender)
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Two systems for Email Security
• PGP
• SMIME
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PGP
• Pretty Good Privacy
• Developed by Phil Zimmermann
• Using best well known crypto algorithms
• Available for free on many platforms with source code
• Not controlled by a governmental or standards organizations
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PGP
• Has 5 components/service
• Authentication
• Confidentiality
• Compression
• Email compatibility
• Segmentation
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PGP - Authentication
• Achieved by digital signature
• The sender creates a message.
• The message is hashed, using SHA-1 algorithm to generate a 150-bit hash code of
the message.
• The hash code is encrypted with the sender’s private key and appended to the
message
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PGP - Authentication
32
H( ). KA( ).-
+ -
H(m )KA(H(m))
-
m
KA
-
Internet
m
KA( ).+
KA
+
KA(H(m))
-
m
H( ). H(m )
compare
Hash the message
Private key of A
Public key of A
Recomputed and compare
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PGP - Confidentiality
• Protection from disclosure
• Achieved by encrypting the message
• Message is encrypted using conventional symmetric shared secret key (DES, CAST-
128, etc)
• Key distribution between sender and receiver is a problem
• In PGP, each key is used only once.
• A new key is generated for each message
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PGP - Confidentiality
34
KS( ).
KB( ).+
+ -
KS(m )
KB(KS )
+
m
KS
KS
KB
+
Internet
KS( ).
KB( ).-
KB
-
KS
m
KS(m )
KB(KS )
+
Sending encrypted email Receiving and decrypting email
Secret key
Encrypt Ks using
B’s public key
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• Sending /encrypting
• Generate random symmetric secret key, Ks
• Encrypts message with Ks
• Encrypts Ks with Bob’s public key
• Send both Ks(m) and Kb(Ks) to Bob
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• Receiving / Decrypting
• Use own private key to decrypt and recover Ks
• Uses Ks to decrypt Ks(m)
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• Confidentiality and Authentication:
Both can be achieved simultaneously
• Sender generates a signature of the plaintext message and attaches it to the
message
• The plaintext message and signature are encrypted using the public key of the
receiver and attached to the message
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PGP - Compression
• PGP compresses message after applying signature but before encryption
• Encrypt the compressed version of message
• Use ZIP as compression algorithm
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PGP – Email compatibility
• PGP must ensure the message transmission format must be the same in
both sender and receiver’s machine
• PGP will encrypt part of the message
• The encrypted part will consists of a stream of arbitrary 8-bit octets
• Thus some of them will be non-printed character
• E.g.: null, space, escape, etc
• While many e-mail system will only permit use of blocks consisting of
ASCII text
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PGP – Email compatibility
• Uses radix-64 algorithm
• Maps 3 bytes (or 8 bit) to 4 printable chars
• PGP provide service to convert raw 8-bit binary stream to a stream of
printable ASCII characters.
• Each group of three octets of binary data is mapped into four Base 64
characters.
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Convert into 8-bit octet
Encode to form 6-bit
Map into four Base64
character
Decimal value
http://en.wikipedia.org/wiki/Base64
http://en.wikipedia.org/wiki/ASCII
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• On receiver side, the incoming block is first converted back to 8-bit octet
binary format.
• Then message is decrypted and verified using the attached keys.
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PGP - Segmentation
• Many email system restricted the maximum message length to 50,000
octets
• Email system will segmentize / divide a long message into smaller
segments
• Each segment is mailed separately.
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• PGP subdivides large message into segments
• Segmentation is done after all other processing (including radix-64
conversion) has been done
• At receiver ends, all email header will be strips off and then reassemble as
the original message.
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PGP Operation Summary
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PGP Message Format
46
Encrypted with
public key
Encrypted and Sign
with private key
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Symmetric vs Asymmetric Key
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Key Rings
• Two key IDs (private and public key) are included in each message
• Provide confidentiality and authentication
• A user often have many public/private key pairs in use
• The keys need to be stored and organized systematically.
• Key IDs also used in signatures
• Key IDs are sent together with messages
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Key Rings
• PGP user has a pair of key rings to store public and private keys
• Public-key ring contains all the public-keys of other PGP users known to
the user
• Indexed by Key ID
• Private-key ring contains the private/public key pairs for the user
• The stored private keys are encrypted using a key derived from a hashed passphrase
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Key Ring - Private
• Each row of the table represents a private/public key pair
owned by the user.
• Key_ID: The least significant 64 bits of the public key (for that
entry)
• Timestamp: The timestamp when this key pair was generated.
• Public_Key : The Public key
• Private_Key: Encrypted private key
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Key Ring – Private
• Private key is encrypted using CAST-128, IDEA, or 3DES algorithms
• Encryption procedure
• Users selects a password to be used for encryption
• System will asks the user for a password before generating a new key pair
• Using SHA-1, a 160 bit hash code is generated from the password.
• System encrypts the private key using CAST-128, and use 128 bits (from the 160-bit
generated) from the hash, as the key.
• Encrypted private key stored in ring
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Key Ring - Public
• Stores the public keys of other users known to this user.
• User ID: Owner of the key.
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PGP Key Management
• PGP uses trust, associates trust with public keys
• Public-key rings has 3 fields:
1. Key legitimacy field (computed by PGP)
“indicate the extent to which PGP will trust the validity of a public key
of any user”
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PGP Key Management
2. Signature trust field:
“indicates the degree to which the user trusts the signer to certify any public keys”
>Key legitimacy field is derived from the collection of signature trust fields.
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3. Owner trust field:
“indicates the degree to which this public key is trusted to sign other public key
certificates”
>Level of trust is assigned by the user.
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• Relation between ‘Signature trust’ and ‘Key legitimacy’ is illustrated by
figure on next slide
• The figure shows the structure of a public-key ring.
• The user has acquired several public-keys.
• Some directly from their owners.
• Some from a third party (key server)
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A public key
ring owned by
“you”
This is calculated
“fully”/”partially” are
assigned by “You”
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• Users D, E, F, and L are always trusted to sign other keys
• legitimate and fully trusted
• Users A and B are partially trusted to sign other keys.
• All keys whose owners are fully or partially trusted are signed by this user
“You”.
• Exception user L: (such a user signature is not always necessary.)
• Both D and L are fully trusted by “You”
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• Key H is deemed legitimate: two partially trusted users are sufficient to
certify a key.
• H is signed by A and B
• A legitimate key user may not be trusted to sign other keys.
• Example: User N signs R’s key but PGP does not consider R’s key legitimate
• S is a detached orphan with two unknown signatures.
• Such key acquired from a key server.
• PGP cannot assume the key to be legitimate.
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Revoking Public Key
• A user might want to revoke his current key due to any reason.
• Key Revocation procedure:
• The owner issues a key revocation certificate.
• the corresponding private key is used to sign the revocation certificate.
• has the same form as normal signature certificate with a revoke indicator.
• disseminated as widely and as quickly as possible.
• receiving nodes updates their rings.
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Thank You
For Your Patience
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Cryptography and Network security # Lecture 7

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