Internet Key Exchange 372
The antecedent area declared how arrangement band IPsec protocols action and
which abstracts they use.These protocols use cryptographic algorithms for encryption
and authentication; appropriately some of the best important pieces of abstracts are encryption/
authentication keys. It is accessible to configure these keys manually, but there are
big disadvantages to this approach. First, it is absolute difficult to scale; second, it is
not accessible to renegotiate SAs because they are anchored until manually changed.
Thus, there is a able charge of accoutrement for managing keys and SAs. Key management
includes generation, distribution, storage, and abatement of the keys.The best challenging
phases are the antecedent affidavit of the systems to anniversary added and then
the aegis of the key exchange. Afterwards keys are exchanged, the approach is protected
with these keys and acclimated for ambience up added parameters, including SAs.
The agreement the IETF adopted for assuming these functions is called
Internet Security Association and Key Administration Agreement (ISAKMP), which
is authentic in RFC 2408 and describes the accurate key barter methods
without diving into particularities. ISAKMP has an IANA-assigned UDP port
number of 500.This is a all-encompassing agreement and is not angry to IPsec or any other
key-using protocol. It can be implemented anon over IP or any carriage layer
protocol.When it is accumulated with genitalia of added key administration protocols
called Oakley (RFC 2412) and Defended Key Barter Mechanism (SKEME),
which has no RFC, we end up with a agreement alleged the Internet Key Exchange
(IKE), which is authentic in RFC 2409. Although not carefully correct, the abbreviations
IKE and ISAKMP are generally acclimated interchangeably, alike in Cisco configuration
commands. In fact, on the PIX firewall, all IKE agreement is performed
using the isakmp command.
In IKE, there are two phases of advice exchange; anniversary of them can
operate in one or two modes. IKE Appearance 1 starts back two aeon charge to establish
a defended channel—that is, they do not accept IPsec SAs bare for communication
over IPsec.This appearance includes affidavit of systems by anniversary other,
agreement on encryption and affidavit algorithms acclimated from again on to
protect IKE traffic, assuming a Diffie-Hellman (DH) key exchange, and finally,
establishing an IKE Security Association (IKE SA). IKE SAs are bidirectional;
each IKE affiliation amid aeon has alone one IKE SA associated with it.The
second appearance is centered on negotiating one or added IPsec SAs, which will be
used for the IPsec adit amid these peers. It uses key absolute from IKE
Phase 1 to acquire keys for IPsec. One associate tells the added which cartage it wants to
protect and which encryption/authentication algorithms are supported.The
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Configuring Virtual Private Networking • Chapter 7 341
second associate again agrees on a distinct aegis set for this cartage and establishes
keys bare for this aegis set.
Although implementing altered phases adds some aerial in processing,
there are advantages to this approach:
Trust amid aeon is accustomed in the aboriginal appearance and acclimated in the
second phase.
Key absolute accustomed in the aboriginal appearance can be acclimated in the additional phase.
Renegotiations of the aboriginal appearance can be assisted by the second-phase data.
Let’s accede these two phases in added detail. Appearance 1 has two modes: main
mode and advancing mode. Capital approach uses three exchanges amid peers; each
exchange consists of two messages, a appeal and a reply:
The aboriginal barter in capital approach is acclimated to accommodate the ambit for
protection of the IKE connection.The initiating ancillary sends a proposition
to its counterpart, which includes a set of accessible ambit supported
by the initiator. Anniversary set includes one encryption algorithm (DES,
3DES, etc.) and one of four affidavit algorithms: pre-shared secret,
RSA accessible key encryption with Diffie-Hellman barter accumulation 1 and
2, or accessible key RSA signature (this includes use of certificates).The
other associate again replies by accepting a distinct brace from the offered set,
which it additionally supports. If there is no bout amid the sets supported
by the peers, the IKE adit cannot be established.
The additional barter in capital approach is acclimated to accomplish DH key establishment
between peers. It additionally exchanges two ethics alleged nonces, which
are hashes that alone the added allotment can interpret.This is done in adjustment to
confirm that the bulletin is beatific by the aforementioned hosts as the previous
exchange.
The third and aftermost barter performs absolute affidavit of the peers
using the agreed-on methods: accessible keys signatures, accessible key encryption,
or a pre-shared secret.This barter is additionally adequate by an
encryption adjustment that was called in the aboriginal exchange.
RFC 2408 provides added capacity on the packet architecture and algorithms used.
At the end of the aboriginal phase, anniversary host has an IKE SA, which specifies all parameters
for this IKE tunnel: the affidavit method, the encryption and hashing
algorithm, the Diffie-Hellman accumulation used, the lifetime for this IKE SA, and the
key values.
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342 Chapter 7 • Configuring Virtual Private Networking
Aggressive approach exchanges alone three packets instead of six, so it is faster but
not as secure.The cardinal of packets is decreased because the aboriginal two packets in
this barter accommodate about aggregate in one message; anniversary host sends a proposed
protection set, Diffie-Hellman ethics and affidavit values.The third
packet is beatific alone for acceptance and afterwards the IKE SA is already established.
The weakness in advancing approach is that aggregate campaign on the wire in clear
text and can be eavesdropped or spoofed. However, the alone affair the attacker
can accomplish is to DoS one of the peers, because it is not accessible to ascertain the
keys that are accustomed by the Diffie-Hellman protocol.
The best important approach of Appearance 2 is quick mode. It can be again several
times application the aforementioned IKE SA accustomed in Appearance 1. Anniversary barter in this mode
results in the enactment of two IPsec SAs by anniversary peer. One of these SAs is
used for entering protection, and the added is acclimated for outbound protection.
During the exchange, aeon accede on the IPsec SA ambit and accelerate anniversary other
a new nonce, which is acclimated for anticipation Diffie-Hellman keys from the ones established
in Appearance 1.When the IPsec SA lifetime expires, a new SA is adjourned in
the aforementioned manner. Figure 7.5 summarizes the breeze of the IKE protocol.
Another approach in Appearance 2 is new accumulation mode, which is not accompanying to the
setup of IPsec ambit and is acclimated to change ambit of the Diffie-Hellman
group acclimated in IKE Appearance 1.
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Figure 7.5 IKE Phases and Modes
Start
Main mode
Aggressive
mode
Quick mode
with PFS
Quick mode
without PFS
IPsec tunnel
established
Phase 1
Negotiation
of IKE SA
Phase 2
Negotiation
of the two
IPsec SAs
New IPsec
tunnel or key
renewal
or
or
Configuring Virtual Private Networking • Chapter 7 343
NOTE
It is accessible to appeal that quick approach use Perfect Forward Secrecy
(PFS). PFS dictates that new encryption keys are not acquired from previous
ones, so alike if one key is discovered, alone the cartage adequate by
this key and annihilation abroad will be exposed. PFS is accomplished by performing
a new Diffie-Hellman key enactment in anniversary quick mode.