IPSec

IPSec.1

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Applications of IPSec

•

IPSec provides the capability to secure communications across a LAN, across private and public WANs, and across the Internet. Examples of its use include:

•

Secure branch office connectivity over the Internet

•

Secure remote access over the Internet

IPSec.2

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IPSec Explained

With thanks to William Stallings and, IPSec.3

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Applications of IPSec

•

Establishment of extranet and intranet connectivity with partners

• •

Enhancement of electronic commerce security encrypt or authenticate all traffic at the IP level

IPSec.4

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Applications of IPSec

IPSec.5

•

Using IPSec all distributed applications can be secured,

– Remote logon, – client/server, – e-mail, – file transfer, – Web access – etc.

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Applications of IPSec

IPSec.6

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Where can IPSec be used

•

These protocols can operate in

– networking devices, •

such as a router or firewall

– or they may operate directly on the workstation or server.

IPSec.7

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IPSec.8

How can IPSec be used

•

Secure Communications between devices

– Workstation to Workstation – Protection against data changes •

Accidental or Intentional

– Contents can be hidden •

Secure communicatoins through IPSec tunnels

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IPSec.9

Benefits of IPSec

•

The benefits of IPSec include:

– Strong security that can be applied to all traffic crossing the perimeter. – Transparent to applications.

– No need to change software on a user or server system •

When IPSec is implemented in a router or firewall

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Benefits of IPSec

•

The benefits of IPSec include:

– IPSec can be transparent to end users.

– There is no need to train users on security mechanisms – PSec can provide security for individual IPSec.10

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Is IPSec the Right Choice?

IPSec.11

• •

For transport level (personal) services IPSec must be a part of the network code deployed on all participating platforms.

Individual protocols may implement their own security:

– E-Mail: PGP – Web: SSL – E-Commerce: SET – Etc.

•

As a tunnel protocol it is available to all services on the network.

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IPSec.12

The Scope of IPSec

•

IPSec provides three main facilities

– An authentication-only function, •

Referred to as

Authentication Header

(AH)

– Acombined authentication/ encryption function •

Called

Encapsulating Security Payload

– A key exchange function. •

IKE (ISAKMP / Oakley) (ESP)

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The Scope of IPSec

IPSec.13

•

Both authentication and encryption are generally desired,

– (1) assure that unauthorized users do not penetrate the virtual private network – (2) assure that eavesdroppers on the Internet cannot read messages sent over the virtual private network. •

Because both features are generally desirable, most implementations are likely to use ESP rather than AH.

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Security Associations (SA)

IPSec.14

• • •

Used for both the authentication (AH) and confidentiality (ESP) A one-way relationship between a sender and a receiver that affords security services to the traffic carried on it.

– If a peer relationship is needed, for two-way secure exchange, then two security associations are required.

Security services are afforded to an SA for the use of AH or ESP, but not both.

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Security Associations (SA)

•

Each SA is uniquely identified by three parameters:

–

Security Parameters Index

–

IP destination address

–

Security protocol identifier

(SPI) IPSec.15

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Security Associations (SA)

•

Security Parameters Index

(SPI)

– The SPI is a bit string assigned to the SA that has local significance only. – The SPI is carried in AH and ESP headers to enable the receiving system to select the SA under which a received packet will be processed. IPSec.16

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IPSec.17

Security Associations (SA)

•

IP destination address

– The IP address of the destination endpoint of the SA • •

May be an end-user system Or, a network system such as a firewall or router.

– Currently, only unicast addresses are allowed

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Security Associations (SA)

•

Security Protocol Identifier

– Indicates which IPSec protocol is in use on the SA •

AH (Authentication only)

•

ESP (complete encryption and possibly Authentication)

IPSec.18

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Security Associations (SA)

•

For any IP packet, the security association is uniquely identified by

• •

the destination address SPI in the enclosed extension header (AH or ESP).

IPSec.19

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Security Associations (SA)

• •

IPSec includes a security association database The database defines the parameters associated with each SA

IPSec.20

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Security Associations (SA)

IPSec.21

•

Each SA is defined by (contains):

– – – – – – – –

Sequence number counter Sequence counter overflow Anti-replay window AH information ESP information Lifetime of this security association IPSec protocol mode Path MTU CEENet ‘2000 - Understanding and using Remote Access and VPN services

Security Associations (SA)

IPSec.22

• • •

Sequence number counter

– A 32-bit value used to generate the sequence number field in AH or ESP headers

Sequence counter overflow

– A flag indicating whether overflow of the sequence number counter should generate an auditable event and prevent further transmission of packets on this SA

Anti-replay window

– Used to determine whether an inbound AH or ESP packet is a replay, by defining a sliding window within which the sequence number must fall

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Security Associations (SA)

IPSec.23

•

AH information

– Authentication algorithm, keys, key lifetimes, and related parameters being used with AH •

ESP information

– Encryption and authentication algorithm, keys, initialization values, key lifetimes, and related parameters being used with ESP •

IPSec protocol mode

– Tunnel, transport, or wildcard (required for all implementations)

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Security Associations (SA)

IPSec.24

• •

Lifetime of this security association

– A time interval or byte count after which an SA must be replaced with a new SA (and new SPI) or terminated, plus an indication of which of these actions should occur

Path MTU

– Any observed path maximum transmission unit (maximum size of a packet that can be transmitted without fragmentation) and aging variables (required for all implementations)

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Security Associations (SA)

• •

The key management mechanism that is used to distribute keys is coupled to the authentication and privacy mechanisms only by way of the Security Parameters Index. Therefore, authentication and privacy are specified independent of any specific key management mechanism.

IPSec.25

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Authentication Header (AH)

IPSec.26

•

Provides support for data integrity and authentication of IP packets

•

Ensures that content changes of a packet in transit can be detected

•

Enables an end system or network device to authenticate the user or application and filter traffic accordingly

• •

Prevents the address spoofing attacks Guards against the replay attack

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IPSec Authentication Header

IPSec.27

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Authentication Header (AH)

IPSec.28

•

Authentication is based on the use of a

Message Authentication Code

(MAC)

• •

The two parties must share a secret key. Uses the following elements to guarantee data integrity

– Payload length – SPI – Sequence number – Integrity Check Value (ICV) or

Message Authentication Code (MAC) CEENet ‘2000 - Understanding and using Remote Access and VPN services

IPSec.29

Anti-Replay Service

• • •

A replay attack is one in which an attacker obtains a copy of an authenticated packet and later transmits it to the intended destination. The receipt of duplicate, authenticated IP packets may disrupt service in some way or may have some other undesired consequence. The

Sequence Number

field is designed to thwart such attacks.

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IPSec.30

Anti-Replay Service

• • • •

When a new SA is established, the

sender

initializes a sequence number counter to 0 Each time that a packet is sent on this SA, the sender increments the counter and places the value in the Sequence Number field

– Thus, the first value to be used is 1

If anti-replay is enabled (the default), the sender must not allow the sequence number to cycle past 2 32 – 1 back to zero

– Otherwise, there would be multiple valid packets with the same sequence number

If the limit of 2 32 – 1 is reached, the sender should terminate this SA, and negotiate a new SA with a new key

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IPSec.31

Anti-Replay Service

• • •

Because IP is a connectionless, unreliable service, the protocol does not guarantee that packets will be delivered in order and does not guarantee that all packets will be delivered Therefore, the IPSec authentication document dictates that the

receiver

should implement a window of size W, with a default of W = 64 The protocol describes means to determine that a sequence number is correct in respect to it's position in or above the window

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IPSec.32

Message Authentication Code

• • •

Uses an algorithm known as HMAC

•

HMAC takes as input a portion of the message and a secret key and produces a MAC as output

•

This MAC value is stored in the Authentication Data field of the AH header The calculation takes place over the entire enclosed TCP segment plus the authentication header When this IP packet is received at the destination, the same calculation is performed using the same key

– If the calculated MAC equals the value of the received MAC, then the packet is assumed to be authentic

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Message Authentication Code

IPSec.33

•

The authentication data field is calculated over:

– IP header fields that either do not change in transit (immutable) or that are predictable in value upon arrival at the endpoint for the AH SA – The AH header other than the Authentication Data field – The entire upper-level protocol data, which is assumed to be immutable in transit (for instance, a TCP segment or an inner IP packet in tunnel mode)

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Message Authentication Code

• •

For IPv4, examples of immutable fields are

–

Internet Header Length

–

Source Address

.

An example of a mutable but predictable field is the Destination Address

IPSec.34

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Encapsulating Security Payload (ESP)

•

Provides confidentiality service, including

– message contents and limited traffic flow confidentiality – As an optional feature, ESP can also provide a authentication services like AH IPSec.35

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IPSec ESP Format

IPSec.36

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IPSec.37

Encapsulating Security Payload (ESP)

• • • • • • •

Security Prameters Index

(32bits)

Sequence Number

(32 bits)

Payload Data

(variable) Padding (0 –255 bytes)

Pad Length

(8 bits)

Next Header

(8 bits)

Authentication Data

(variable

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IPSec.38

Encapsulating Security Payload (ESP)

• • •

Security Prameters Index

(32bits)

– Identifies a security association

Sequence Number

(32 bits)

– A monotonically increasing counter value.

Payload Data

(variable)

– A transport-level segment (transport mode) or IP packet (tunnel mode) that is protected by encryption.

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IPSec.39

Encapsulating Security Payload (ESP)

•

Padding (0 –255 bytes)

– Extra bytes that may be required if the encryption algorithm requires the plaintext to be a multiple of some number of octets •

Pad Length

(8 bits)

– Indicates the number of pad bytes immediately preceding this field

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Encapsulating Security Payload (ESP)

IPSec.40

• •

Next Header

(8 bits)

– Identifies the type of data contained in the payload data field by identifying the first header in that payload (for example, an upper-layer protocol such as TCP)

Authentication Data

–

(variable)

A variable-length field (must be an integral number of 32-bit words) that contains the integrity check value computed over the ESP packet minus the Authentication Data field

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Encryption and Authentication Algorithms

• •

The Payload Data, Padding, Pad Length, and Next Header fields are encrypted by the ESP service.

The current IPSec specification dictates that a compliant implementation must support the

Data Encryption Standard

(DES).

IPSec.41

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Encryption and Authentication Algorithms

IPSec.42

•

A number of other algorithms have been assigned identifiers and could, therefore, be used for encryption;

•

These include

– Three-key Triple DES – RC5 – International Data Encryption Algorithm (IDEA) – Three-key Triple IDEA – CAST – Blowfish

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Transport and Tunnel Modes

•

AH and ESP each support two modes of use

–

Transport mode

–

Tunnel mode

IPSec.43

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IPSec.44

Transport and Tunnel Modes

IPv4 Orig IP Header TCP Original IP Packet Data IPv4 Orig IP Header ESP Hdr Authenticated Encrypted TCP Data Transport Mode IPv4 New IP Header ESP Hdr Authenticated Encrypted TCP Data Tunnel Mode ESP Trlr ESP Auth ESP Trlr ESP Auth

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IPSec.45

Transport Mode

•

Provides protection primarily for upper-layer protocols.

– Extends to the

payload

• • •

TCP UDP (ICMP), etc.

of an IP packet.

IPv4 Orig IP Header ESP Hdr TCP Authenticated Encrypted Data Transport Mode ESP Trlr ESP Auth

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IPSec.46

Transport Mode

• •

Typically used for end-to-end communication between two hosts

– for example, between a

workstation

or between two servers and a server,

When a host runs AH or ESP over IPv4, the payload is the data that normally follows the IP header Authenticated Encrypted IPv4 Orig IP Header ESP Hdr TCP Data ESP Trlr ESP Auth Transport Mode

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Transport Mode

IPSec.47

• •

ESP in transport mode encrypts and optionally authenticates the IP payload

but not the IP header

AH in transport mode authenticates the IP payload and

selected portions

of the IP header IPv4 Orig IP Header ESP Hdr TCP Authenticated Encrypted Data Transport Mode ESP Trlr ESP Auth

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IPSec.48

Transport Mode

•

As an example, consider a Telnet session within an ESP packet in transport mode

– The IP header would contain 51 in the

Next Header

field – In the ESP header, the Next Header field would be

6

for TCP – Within the TCP header, Telnet would be identified as port

23

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IPSec.49

Transport Mode

•

Transport mode operation may be summarized for ESP as follows:

– At the source, the block of data consisting of the ESP trailer plus the entire transport-layer segment is encrypted – The plaintext of this block is replaced with its ciphertext to form the IP packet for transmission – Authentication is added if this option is selected

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IPSec.50

Transport Mode

– The packet is then routed to the destination – Each intermediate router needs to examine and process the IP header plus any plaintext IP extension headers but will not need to examine the ciphertext – The destination node examines and processes the IP header plus any plaintext IP extension headers

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IPSec.51

Transport Mode

– Then, on the basis of the SPI in the ESP header, the destination node decrypts the remainder of the packet to recover the plaintext transport-layer segment – This process is similar for AH, however the payload (upper layer protocol) is not encrypted

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Tunnel Mode

IPSec.52

• •

Tunnel mode encapsulates an

entire

IP packet

within

an IP packet to ensure that

no part

of the original packet is changed as it is moved through a network The

entire original

, or inner, packet travels through a tunnel from one point of an IP network to another

– No routers along the way need to examine the inner IP header

Authenticated Encrypted IPv4 New IP Header ESP Hdr Orig IP Header TCP Data ESP Trlr ESP Auth Tunnel Mode

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Tunnel Mode

•

Tunnel mode is used when one or both

ends

of an SA is a security gateway, such as a firewall or router that implements IPSec, etc.

IPSec.53

Authenticated Encrypted IPv4 New IP Header ESP Hdr Orig IP Header TCP Data ESP Trlr ESP Auth Tunnel Mode

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Tunnel Mode

IPSec.54

• •

With tunnel mode, a number of hosts on networks behind firewalls may engage in secure communications without implementing IPSec.

•

The unprotected packets generated by such hosts are tunneled through external networks These paths use SAs set up by the IPSec process in the firewall or secure router at the boundary of the local network Authenticated Encrypted IPv4 New IP Header ESP Hdr Orig IP Header TCP Data ESP Trlr ESP Auth Tunnel Mode

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Tunnel Mode

•

Transport mode is suitable for protecting connections between hosts that support the ESP feature

IPSec.55

Authenticated Encrypted IPv4 New IP Header ESP Hdr Orig IP Header TCP Data ESP Trlr ESP Auth Tunnel Mode

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Tunnel Mode

IPSec.56

•

Tunnel mode is useful in a configuration that includes a firewall or other sort of security gateway that protects a trusted network from external networks

– Encryption occurs only between an external host and the security gateway or between two security gateways – This setup relieves hosts on the internal network of the processing burden of encryption and simplifies the key distribution task by reducing the number of needed keys

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Transport and Tunnel Modes

IPSec.57

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IPSec.58

Tunnel Mode

•

The user system prepares an inner IP packet with a destination address of the target host on the internal LAN.

– For a Telnet session, this packet would be a TCP packet with the original SYN flag set with a destination port set to 23. •

This entire IP packet is prefixed by an ESP header; then the packet and ESP trailer are encrypted and Authentication Data may be added.

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IPSec.59

Tunnel Mode

• •

The Next Header field of the ESP header would be decimal 4 for IP-in-IP, indicating that the entire original IP packet is contained as the ìpayload The resulting block is encapsulated with a new IP header whose destination address is the firewall

– This forms the outer IP packet – The Next Header field for this IP packet is 50 for ESP

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Tunnel Mode

•

The outer packet is routed to the destination firewall.

– Each intermediate router needs to examine and process the outer IP header plus any outer IP extension headers but does not need to examine the ciphertext. IPSec.60

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IPSec.61

Tunnel Mode

•

The destination firewall examines and processes the outer IP header plus any outer IP extension headers

– On the basis of the SPI in the ESP header, the gateway decrypts the remainder of the packet to recover the plaintext inner IP packet •

This inner packet (tunnel contents) is then transmitted on the internal network

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Tunnel Mode

IPSec.62

•

The inner packet is routed through zero or more routers in the internal network to the destination host

– The receiver would have no indication that the packet had been encapsulated and protected by the tunnel between the user system and the gateway.

– It would see the packet as a request to start a Telnet session and would respond back with a TCP SYN / ACK

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Tunnel Mode

• •

The return packet would go back to the gateway.

The gateway would encapsulate that packet into an IPSec packet and transport it back to the user system through this tunnel, etc

IPSec.63

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Key Management

•

Manual

: Configures each system with its own keys and with the keys of other communicating systems.

– This is practical for small, relatively static environments. IPSec.64

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IPSec.65

Key Management

•

Automated

: Enables on-demand creation of keys for SAs and facilitates the use of keys in a large distributed system with an evolving configuration.

– An automated system is the most flexible… – But requires more effort to configure and requires more software, so smaller installations are likely to opt for manual key management.

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Key Management

• •

Default automated key management protocol for IPSec is referred to as

Internet Key Exchange

(IKE) IKE provides a standardized method for dynamically authenticating IPSec peers, negotiating security services, and generating shared keys

IPSec.66

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Key Management

•

IKE has evolved from many different protocols and can be thought of as having two distinct capabilities

– ISAKMP (Key Management) – Oakley (Key Distribution) IPSec.67

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Key Management

IPSec.68

•

ISAKMP (Pronounced Ice-Uh-Kamp)

– provides a framework for Internet key management – provides the specific protocol support, including formats, for negotiation of security attributes – Does not dictate a specific key exchange algorithm •

Consists of a set of message types that enable the use of a variety of key exchange algorithms.

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IPSec.69

Key Management

• • •

The actual key exchange mechanism in IKE is derived from Oakley Plus several other key exchange protocols that had been proposed for IPSec Key exchange is based on the use of the Diffie Hellman algorithm

– – But provides added security In particular, Diffie-Hellman alone does not authenticate the two users that are exchanging keys, making the protocol vulnerable to impersonation – IKE includes mechanisms to authenticate the users

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Public Key Certificates

•

An important element of IPSec key management is the use of public key certificates

•

A public key certificate is provided by a trusted

Certificate Authority

(CA) to authenticate a user's public key

•

The essential steps include…

IPSec.70

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IPSec.71

Public Key Certificates

•

Step 1

– Client software creates a pair of keys, one public and one private – The client prepares an unsigned certificate that includes a user ID and the user's public key – The client then sends the unsigned certificate to a CA in a secure manner

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IPSec.72

Public Key Certificates

•

Step 2

– A CA creates a signature by calculating the hash code of the unsigned certificate and encrypting the hash code with the CA's private key •

The encrypted hash code is the signature

– The CA attaches the signature to the unsigned certificate and returns the now signed certificate to the client

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Public Key Certificates

IPSec.73

•

Step 3

– The client may send its signed certificate to any other user – That user may verify that the certificate is valid by •

Calculating the hash code of the certificate (not including the signature)

• •

Decrypting the signature using the CA's public key Comparing the hash code to the decrypted signature.

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Public Key Certificates

IPSec.74

• • •

If all users subscribe to the same CA, then there is a common trust of that CA User certificates can be placed in the directory for access by all users. Or a user can transmit his or her certificate directly to other users.

– In either case, once B is in possession of A's certificate, B has confidence that messages it encrypts with A's public key will be secure from eavesdropping and that messages signed with A's private key are unforgeable

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IPSec.75

Public Key Certificates

•

If there is a large community of users, it may not be practical for all users to subscribe to the same CA

•

Because it is the CA that signs certificates, each participating user must have a copy of the CA's own public key to verify signatures

•

This public key must be provided to each user in an absolutely secure

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IPSec.76

Recommended Web Sites

• •

The IPSec Working Group of the IETF. Charter for the group and latest RFCs and Internet Drafts for IPSec:

– http://ietf.org/html.charters/ipsec-charter.html

IPSec Resources: List of companies implementing IPSec, implementa-tion survey, and other useful material:

– http://web.mit.edu/tytso/www/ipsec/index.html

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