Authentication Authentication ApplicationsApplications

1.Kerberos 2.Key Management and Distribution3.X.509 Directory Authentication

service4.Public Key Infrastructure5.Electronic Mail Security:

a.Pretty Good Privacy, b.S/MIME.

Kerberos Kerberos .

Kerberos is a computer network authentication protocol which works on the basis of 'tickets' to allow nodes communicating over a non-secure network to prove their identity to one another in a secure manner.

Key Management and Key Management and DistributionDistribution

PUBLIC ANNOUNCEMENTPUBLIC ANNOUNCEMENT

PUBLICLY AVILABLE PUBLICLY AVILABLE DIRECTORYDIRECTORY

Public key AuthorityPublic key Authority

Public Key CertificatesPublic Key Certificates

One scheme has become universally accepted One scheme has become universally accepted for formatting public-key certificatesfor formatting public-key certificates

the X.509 standardthe X.509 standard.. X.5applications, including IP X.5applications, including IP security, transport layer security, transport layer security (TLS), and security (TLS), and S/MIME,X.509 certificates are S/MIME,X.509 certificates are used in most network used in most network securitysecurity

X.509 Authentication X.509 Authentication Service Service part of CCITT X.500 directory service

standards◦ distributed servers maintaining some info database

defines framework for authentication services ◦ directory may store public-key certificates◦ with public key of user◦ signed by certification authority

also defines authentication protocols uses public-key crypto & digital signatures

◦ algorithms not standardised, but RSA recommended

X.509 CertificatesX.509 Certificates issued by a Certification Authority (CA),

containing: ◦ version (1, 2, or 3) ◦ serial number (unique within CA) identifying certificate ◦ signature algorithm identifier ◦ issuer X.500 name (CA) ◦ period of validity (from - to dates) ◦ subject X.500 name (name of owner) ◦ subject public-key info (algorithm, parameters, key) ◦ issuer unique identifier (v2+) ◦ subject unique identifier (v2+) ◦ extension fields (v3) ◦ signature (of hash of all fields in certificate)

notation CA<<A>> denotes certificate for A signed by CA

X.509 CertificatesX.509 Certificates

Obtaining a Obtaining a Certificate Certificate any user with access to CA can

get any certificate from it only the CA can modify a

certificate because cannot be forged,

certificates can be placed in a public directory

CA Hierarchy CA Hierarchy if both users share a common CA then

they are assumed to know its public key otherwise CA's must form a hierarchy use certificates linking members of

hierarchy to validate other CA's ◦ each CA has certificates for clients (forward)

and parent (backward) each client trusts parents certificates enable verification of any certificate

from one CA by users of all other CAs in hierarchy

CA Hierarchy UseCA Hierarchy Use

Certificate RevocationCertificate Revocation certificates have a period of validity may need to revoke before expiry, eg:

1. user's private key is compromised2. user is no longer certified by this CA3. CA's certificate is compromised

CA’s maintain list of revoked certificates

◦ the Certificate Revocation List (CRL) users should check certs with CA’s

CRL

Authentication ProceduresAuthentication ProceduresX.509 includes three alternative

authentication procedures: One-Way Authentication Two-Way Authentication Three-Way Authentication all use public-key signatures

One-Way AuthenticationOne-Way Authentication1 message ( A->B) used to

establish ◦the identity of A and that message is

from A ◦message was intended for B ◦integrity & originality of message

message must include timestamp, nonce, B's identity and is signed by A

Two-Way AuthenticationTwo-Way Authentication2 messages (A->B, B->A) which

also establishes in addition:◦the identity of B and that reply is

from B ◦that reply is intended for A ◦integrity & originality of reply

reply includes original nonce from A, also timestamp and nonce from B

Three-Way AuthenticationThree-Way Authentication3 messages (A->B, B->A, A->B)

which enables above authentication without synchronized clocks

has reply from A back to B containing signed copy of nonce from B

means that timestamps need not be checked or relied upon

Public Key Public Key InfrastructureInfrastructure

RFC 2822 (Internet Security Glossary) defines public-key infrastructure (PKI) as the set of hardware, software, people, policies, and procedures needed to create, manage, store, distribute, and revoke digital certificates based on asymmetric cryptography.

The principal objective for developing a PKI is to enable secure, convenient , and efficient acquisition of public keys

PKIX Management protocols PKIX Management protocols

RFC 2510 defines the certificate management protocols (CMP)

RFC 2797 defines certificate management messages over CMS (CMC), where CMS refers to RFC 2630, cryptographic message syntax.

PGPPGPPhil ZimmermannPhilip R. "Phil" Zimmermann, is the creator of Pretty Good Privacy,

the most widely used email encryption software in the world. He is also known for his work in VoIP encryption protocols, notably ZRTP and Zfone.

Born: February 12, 1954 (age 61), Camden, New Jersey, United States

Education: Florida Atlantic University

Organizations founded: Silent Circle

Books: PGP source code and internals, High tension, The official PGP user's guide

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Email SecurityEmail Securityemail is one of the most widely

used and regarded network services

currently message contents are not secure ◦may be inspected either in transit ◦or by suitably privileged users on

destination system

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Email Security Email Security EnhancementsEnhancementsconfidentiality

◦protection from disclosureauthentication

◦of sender of messagemessage integrity

◦protection from modification non-repudiation of origin

◦protection from denial by sender

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Pretty Good Privacy (PGP)Pretty Good Privacy (PGP)Open source, freely available

software package for secure e-mailde facto standard for secure emaildeveloped by Phil Zimmermannselected best available crypto algs to

useRuns on a variety of platforms like

Unix, PC, Macintosh and other systems

originally free (now also have commercial versions available)

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PGP Operation – PGP Operation – AuthenticationAuthentication1. sender creates message2. Generates a digital signature for the

message3. use SHA-1 to generate 160-bit hash of

message 4. signed hash with RSA using sender's

private key, and is attached to message5. receiver uses RSA with sender's public

key to decrypt and recover hash code6. receiver verifies received message using

hash of it and compares with decrypted hash code

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PGP Operation – PGP Operation – ConfidentialityConfidentiality1. sender generates a message and

encrypts it.2. Generates a128-bit random number as

session key3. Encrypts the message using CAST-128 /

IDEA / 3DES in CBC mode with session key4. session key encrypted using RSA with

recipient's public key and attached to the msg

5. receiver uses RSA with private key to decrypt and recover session key

6. session key is used to decrypt message

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PGP Operation – Confidentiality PGP Operation – Confidentiality & Authentication & Authentication can use both services on the same

message◦ create signature & attach it to the message◦ encrypt both message & signature◦ attach RSA/ElGamal encrypted session key

This sequence is preferred because --one can store the plaintext message/file

and its signature--no need to decrypt the message/file again

and again

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PGP Operation – PGP Operation – CompressionCompressionPGP compresses messages to save

space for e-mail transmission and storage

by default PGP compresses message after signing but before encrypting◦ so can store uncompressed message &

signature for later verification◦ Encryption after compression strengthens

security (because compression has less redundancy)

uses ZIP compression algorithm

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PGP Operation – Email PGP Operation – Email CompatibilityCompatibilitywhen using PGP will have binary data (8-bit

octets) to send (encrypted message, etc)however email was designed only for texthence PGP must encode raw binary data

into printable ASCII charactersuses radix-64 algorithm

◦ maps 3 bytes to 4 printable chars◦ also appends a CRC

PGP also segments messages if too big (maximum length 50,000 octets)

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PGP Operation – SummaryPGP Operation – Summary

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PGP Session KeysPGP Session Keysneed a session key for each

message◦of varying sizes: 56-bit DES, 128-bit

CAST or IDEA, 168-bit Triple-DESuses random inputs taken from -actual keys hit -keystroke timing of a user

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PGP Public & Private KeysPGP Public & Private Keyssince 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

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PGP Message FormatPGP Message Format

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PGP Key RingsPGP Key Ringseach 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

security of private keys thus depends on the pass-phrase security

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PGP Message GenerationPGP Message Generation

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PGP Message ReceptionPGP Message Reception

S/MIMES/MIMESecure/Multipurpose Internet Mail ExtensionsA standard way for email encryption and

signingIETF effort (RFCs 2632, 2633 – for version 3.0;

RFCs 3850, 3851 for version 3.1; 5750, 5751 for version 3.2)

Industry supportNot a standalone software, a system that is to

be supported by email clients◦ such as MS Outlook and Thunderbird

S/MIME handles digital signatures◦ Also provides encryption

Quick E-mail HistoryQuick E-mail HistorySMTP and RFC 822

◦ only ASCII messages (7-bit)MIME (Multipurpose Internet Mail Extensions)

◦ content type Almost any type of information can appear in an email

message◦ transfer encoding

specifies how the message body is encoded into textual form (radix64 is common)

S/MIME: Secure MIME◦ new content types, like signature, encrypted data

S/MIME FunctionsS/MIME Functionsenveloped data

◦ encrypted content and associated keyssigned data

◦ encoded message + encoded signed message digest

clear-signed data◦ cleartext message + encoded signed

message digestsigned and enveloped data

◦ Nested signed and encrypted entities

S/MIME Cryptographic S/MIME Cryptographic AlgorithmsAlgorithms

hash functions: SHA-1 & MD5digital signatures: DSS & RSAsession key encryption: ElGamal & RSAmessage encryption: Triple-DES, AES and

otherssender should know the capabilities of the

receiving entity (public announcement or previously received messages from receiver)◦ otherwise sender takes a risk

Scope of S/MIME Security Scope of S/MIME Security S/MIME secures a MIME entity

◦ a MIME entity is entire message except the headers

◦ so the header is not securedFirst MIME message is preparedThis message and other security related

data (algorithm identifiers, certificates, etc.) are processed by S/MIME

and packed as one of the S/MIME content type

S/MIME Content TypesS/MIME Content Types

EnvelopedDataEnvelopedDataFor message encryptionSimilar to PGP

◦ create a random session key, encrypt the message with that key and a conventional crypto, encrypt the session key with recipient’s public key

Unlike PGP, recipient’s public key comes from an X.509 certificate◦ trust management is different

SignedDataSignedDataFor signed message

◦ both message and signature are encoded so that the recipient only sees some ASCII characters if he does not use an email client with S/MIME support

Similar to PGP◦ first message is hashed, then the hash is encrypted

using sender’s private keyMessage, signature, identifiers of algorithms

and the sender’s certificate are packed together◦ again difference between S/MIME and PGP in trust

management

Clear SigningClear SigningAnother mechanism for signature

◦but the message is not encoded, so an email client with no S/MIME support could also view the message of course the signature will not be verified

and will be seen as a meaningless attachmentmultipart/signed content type

◦2 parts Clear text message Signature

◦Let’s see an example

S/MIME Certificate ProcessingS/MIME Certificate ProcessingS/MIME uses X.509 v3 certificates

◦ Certification Authorities (CAs) issue certificates◦ unlike PGP, a user cannot be a CA

each client has a list of trusted CA certificates◦ actually that list comes with e-mail client software or OS

and own public/private key pairs and certsOur textbook says “S/MIME key management is a

hybrid of a strict X.509 CA hierarchy and PGP’s web of trust”◦ but I do not believe that this is the case, because it is

very hard for an average user to maintain the list of trusted CAs

S/MIME Certificate Processing S/MIME Certificate Processing and CAsand CAs

One should obtain a certificate from a CA in order to send signed messages

Certificates classes (common practice by most CAs) ◦ Class 1◦ Class 2◦ Class 3

CA certification policies (Certificate Practice Statement)◦ ID-control practices

Class 1: only email address check Class 2: class1 + against third party database / fax documents Class 3: class1 + apply in person and submit picture IDs and/or paper

documents

Stronger identity validation

Easier to issue