7/31/2019 ch-16(CNS)

1/54

Security at theApplication Layer:

PGPandS/MIME

BKM/SIT

7/31/2019 ch-16(CNS)

2/54

E-mail Architecture

BKM/SIT

7/31/2019 ch-16(CNS)

3/54

E-mail Security

Cryptographic Algorithms

In e-mail security, the sender of the message needs

to include the name or identifiersof the algorithms used in the message.

Note

CertificatesIt is obvious that some public-key algorithms must be

used for e-mail security.

BKM/SIT

7/31/2019 ch-16(CNS)

4/54

Cryptographic Secrets

In e-mail security, the encryption/decryption is

done using a symmetric-key algorithm,

but the secret key to decrypt the message is

encrypted with the public key of thereceiver and is sent with the message.

Note

BKM/SIT

7/31/2019 ch-16(CNS)

5/54

PGP

Pretty Good Privacy (PGP) can be used to create a

secure e-mail message or to store a file securely for

future retrieval.

Invented by Phil Zimmermann.

BKM/SIT

7/31/2019 ch-16(CNS)

6/54

Message Integrity

An authenticated message

BKM/SIT

7/31/2019 ch-16(CNS)

7/54

Confidentiality with One-Time Session Key

BKM/SIT

7/31/2019 ch-16(CNS)

8/54

Continued

Code Conversion Another service provided by PGP is code

conversion.

PGP uses Radix-64 conversion for non-ASCII data.

Segmentation

PGP allows segmentation to get uniform size unit of

the message after applying Radix-64 .

BKM/SIT

7/31/2019 ch-16(CNS)

9/54

Key Rings in PGP

Assuming that a person may send message to many

persons, PGP defines the concept of key rings.

BKM/SIT

7/31/2019 ch-16(CNS)

10/54

Key Rings in PGP-Contd

Note that Alice has several pair of public/private keys

belonging to other people. Alice sending a message

She uses her private key to sign the digest.

She uses the receivers public key to encrypt a

newly created session key.

She encrypts the message and signed digest with the

session key created.

Alice receiving a message She uses her private key to decrypt the session key.

She uses the session key to decrypt the message &

digest.

She uses her public key to verify the digest.BKM/SIT

7/31/2019 ch-16(CNS)

11/54

PGP Algorithms

BKM/SIT

7/31/2019 ch-16(CNS)

12/54

Continued

BKM/SIT

7/31/2019 ch-16(CNS)

13/54

Continued

BKM/SIT

7/31/2019 ch-16(CNS)

14/54

Continued

BKM/SIT

7/31/2019 ch-16(CNS)

15/54

PGP vs X.509 Certificates

PGP uses certificates to authenticate public keys. Unlike X.509, there is no need of CAs.

Anyone in the ring can sign a certificate for anyone

else in the ring.

Protocols that use X.509 certificates depend on thehierarchical structure of the trust.

That means a single path from the fully trusted

authority to any certificate.

But, in PGP there can be multiple paths from fullyor partially trusted authorities to any certificate.

BKM/SIT

7/31/2019 ch-16(CNS)

16/54

Private key ring table maintained by each entity

Used ID is usually the email-id of the user

Key ID is the first(LSB) 64 bits of the public key.

Public Key column lists the public key belonging to a

particular private key/public key pair. Encrypted private key column keeps the private

keys of the key ring in encrypted form.

Timestamp holds the date and time of the key pair

creation.BKM/SIT

7/31/2019 ch-16(CNS)

17/54

Public key ring table maintained by each entity

Used ID Here also the user-id is usually the e-mail id of the entity.

Key ID Here also the key id is first 64 bits of the public key.

Public Key This the public key of the entity. Producer Trust defines the trust level of the producer: full, partial

or none.

Certificate(s) It holds the certificates signed by other entities for this

entity.

Certificate Trust(s) It represents the certificate trust value of anentity, which is taken from Producer Trust value after receiving a

certificate from another entity.

Key Legitimacy This value is calculated by PGP based on the value

of the certificate trust and the weight for each certificate trust.BKM/SIT

7/31/2019 ch-16(CNS)

18/54

A series of steps will show how a public key ring table is

formed for Alice.

Example

BKM/SIT

7/31/2019 ch-16(CNS)

19/54

Continued

BKM/SIT

7/31/2019 ch-16(CNS)

20/54

Continued

BKM/SIT

d l

7/31/2019 ch-16(CNS)

21/54

Trust Model in PGP

Following shows a model of trust for Alice at

some point in time.

The diagram may change with changes in thepublic key ring

BKM/SIT

K R i

7/31/2019 ch-16(CNS)

22/54

Key Revocation

It may become necessary for an entity to

revoke(withdraw) his or her public key from thering.

This may happen if the owner of the key feels that

the key is compromised (stolen, for example) or just

too old to be safe.

BKM/SIT

E i I f i f Ri

7/31/2019 ch-16(CNS)

23/54

Extracting Information from RingsExtracting information at the Sender site

Assume that Alice is sending an Email to Bob.

For that, she needs 5 pieces of information from PGP asfollows:

The key id of the public key she wants to use

Her own private key

The session key Bob s public key ID

Bobs public key To get these, she has to input 4 pieces of information to

PGP as follows:

Her user ID

Her password

Key strokes

Bobs user IDBKM/SIT

E i I f i f Ri

7/31/2019 ch-16(CNS)

24/54

Extracting Information from RingsExtracting information at the Sender site

BKM/SIT

C i d

7/31/2019 ch-16(CNS)

25/54

ContinuedExtracting information at theReceiversite

BKM/SIT

PGP P k t

7/31/2019 ch-16(CNS)

26/54

PGP Packets

Format of packet header

BKM/SIT

PGP M

7/31/2019 ch-16(CNS)

27/54

PGP Messages

A PGP message is a combination of sequenced

and/or nested packets. Some examples are:

Encrypted Message

Signed Message Certificate Message

BKM/SIT

Lit l d t k t

7/31/2019 ch-16(CNS)

28/54

Literal data packet

It is the packet that carries the actual data to be

transmitted.

BKM/SIT

C d d t k t

7/31/2019 ch-16(CNS)

29/54

Compressed data packet

This packet carries compressed data.

The compression method used are ZIP(code:1) and

ZLIP(code:2)

BKM/SIT

E t d d t k t

7/31/2019 ch-16(CNS)

30/54

Encrypted data packet

This packet carries data from one/many packet that

have been encrypted using symmetric key algorithm.

But, a session packet carrying one time session key

must be sent before this.

BKM/SIT

Si t k t

7/31/2019 ch-16(CNS)

31/54

Signature packet

It protects the integrity of the data.

BKM/SIT

Continued

7/31/2019 ch-16(CNS)

32/54

Continued

BKM/SIT

Session key packet

7/31/2019 ch-16(CNS)

33/54

Session-key packet

This packet is used to send the session key encrypted

with the receivers public key

BKM/SIT

Public key packet

7/31/2019 ch-16(CNS)

34/54

Public-key packet

This packet contains the public key of the sender.

BKM/SIT

User ID packet

7/31/2019 ch-16(CNS)

35/54

User ID packet

This packet identifies a user and associate the used

ID contents with a public key of the sender.

BKM/SIT

PGP Message Formats

7/31/2019 ch-16(CNS)

36/54

PGP Message Formats

Encrypted message

BKM/SIT

Signed message

7/31/2019 ch-16(CNS)

37/54

Signed message

BKM/SIT

Certificate message

7/31/2019 ch-16(CNS)

38/54

Certificate message

BKM/SIT

7/31/2019 ch-16(CNS)

39/54

S/MIME

Secure/Multipurpose Internet Mail Extension

(S/MIME).

This is an enhancement of the supplementary protocol

called MIME which has been integrated to the email

system. MIME helps to transmit non-ASCII data(which were

not allowed earlier) to be sent over the same email.

MIME transforms non-ASCII data at the sender site to

NVT ASCII data and delivers it to the client MTA to besent through the Internet.

At the receiving site, it is again transformed back to

original data.BKM/SIT

MIME

7/31/2019 ch-16(CNS)

40/54

MIME

BKM/SIT

Continued

7/31/2019 ch-16(CNS)

41/54

Continued

BKM/SIT

Continued

7/31/2019 ch-16(CNS)

42/54

Continued

Text

BKM/SIT

Continued

7/31/2019 ch-16(CNS)

43/54

Continued

BKM/SIT

Radix-64 conversion

7/31/2019 ch-16(CNS)

44/54

Radix-64 conversion

This is a solution for sending data bytes when the

highest bit is not necessarily zero.

It divides the binary data into 24-bit blocks.

Each block is then divided into four sections, 6bits each.

BKM/SIT

Continued

7/31/2019 ch-16(CNS)

45/54

Continued

BKM/SIT

Quoted-printable

7/31/2019 ch-16(CNS)

46/54

Quoted printable

This encoding can be suitable if the data consists of

mostly of ASCII characters with a small non-ASCII

portions. If a character is ASCII, it is sent as it is, but if it is

non-ASCII, it is sent as three characters.

First character is a =sign , then next two are the

hexadecimal representations of the byte.

BKM/SIT

S/MIME

7/31/2019 ch-16(CNS)

47/54

S/MIME

S/MIME adds some new content types to include

security services to the MIME. All of these new types include the parameter

application/pkcs7-mime, in which pkcs defines

Public Key Cryptography Specification.

Cryptographic Message Syntax (CMS)

To define how security services, such as

confidentiality or integrity, can be added to MIME

content types, S/MIME has defined Cryptographic

Message Syntax (CMS).

The syntax in each case defines the exact encoding

scheme for each content type.BKM/SIT

Example

7/31/2019 ch-16(CNS)

48/54

The following shows an example of an enveloped-datain which a small message is encrypted using triple DES.

Example

BKM/SIT

Signed-data content type

7/31/2019 ch-16(CNS)

49/54

Signed data content type

This type provides only integrity of data.

It contains any type and zero or more signature values.

BKM/SIT

Enveloped-data content type

7/31/2019 ch-16(CNS)

50/54

Enveloped data content type This type is used to provide privacy for the message.

It contains any type and zero or more encrypted keys and

certificates.

BKM/SIT

Digest-data content type

7/31/2019 ch-16(CNS)

51/54

Digest data content type

This type is also used to provide integrity for the

message

BKM/SIT

Authenticated-data content type

7/31/2019 ch-16(CNS)

52/54

Authenticated data content type

This type is used to provide authentication of the data

BKM/SIT

Key Management

7/31/2019 ch-16(CNS)

53/54

Key Management

The key management in S/MIME is a combination

of key management used by X.509 and PGP.

BKM/SIT

Cryptographic Algorithms

7/31/2019 ch-16(CNS)

54/54

C yp g p g

S/MIME defines several cryptographic algorithms.

The term must means an absolute requirement; theterm should means recommendation.