CISCO
PIX: CONFIGURING SITE TO SITE VPN'S CONT...
IKE Phase Two
Having successfully authenticated each peer and establishing a secure
channel for further communications, IKE Phase One evolves in to Phase
Two.
As mentioned phase two can only occur after the IKE SA negotiation has
completed. Phase Two’s main purpose is to negotiate mutual policies for
non ISAKMP SA’s such as the IPSec SA and to derive the keying material
to be used.
Phase Two only has the one mode, called Quick Mode.
Once the secure tunnel has been setup by Phase One, Phase Two
negotiates the IPSec parameters to be used such as the various
algorithms, Shared Secret keying material etc. It is also used to
re-negotiate the a new SA once the predetermined life-span of the
current IPSec SA has expired.
IPSec has two main protocols, ESP and AH.
**I will cover the encryption and hashing standards first – if they
seem confusing at first it will all become apparent later on when the
VPN gets configured**
Encapsulating Security Payload – ESP
ESP is an extensive protocol that provides antireplay services, data
authentication and data encryption and is primarily responsible for
securelygetting the data from the source to the destination in such a
way that the destination host will know if the data has been tampered
with hence ensuring that your session can not be effectively hijacked.
ESP is versatile enough to be able to encrypt either just the payload
of the packet or the entire data packet and can also authenticate the
sender of the data either in conjunction with Authentication Header or
on its own.
IPv4 Packet Encrypted with ESP:
_________________________________________________________________________
|IP HEADER | ESP HEADER | TCP INFO | DATA | ESP TRAILER | ESP
AUTHENTICATION |
``````````````````` --------- ENCRYPTED BY ESP ----------
``````````````````````````
the ESP Header and all up to and including the ESP Trailer is
Authenticated by ESP
Authentication Header
Authentication Header pretty much does as it names suggests, it
authenticates the information that is in the IP Header, or in other
words it ensures that the data did originate from where it says it does
in the header by means or origin authentication. By doing this is
provides antireplay services and prevents your session being hijacked.
It may seem similar to ESP but the one glaring difference is that AH
does not provide any data encryption, its only purpose is to
authenticate the sender.
One major downfall with AH is that it is not compatible with Network
Address Translation (NAT). Due to the address translation occurring
before the IPSec SA being established, by altering the sending IP
address you would cause the AH hash that confirms the sending source to
fail.
IPv4 Packet with AH
__________________________________________________________
| IP HEADER | AUTHENTICATION HEADER (AH) | TCP | DATA |
``````````````````````````````````````````````````````````
As you can see no encryption takes place, unlike ESP which encrypts the
payload of the packet.
**ESP and NAT is supported by the PIX by way of a fix-up protocol that
inspects ESP on a more granular level**
ESP and AH can be configured to use different algorithms to encrypt and
hash the data.
**An Encryption Algorithm is used to encrypt the data where as a Hash
Algorithm is used to provide data integrity**
The PIX will support the following encryption algorithms:
Data Encryption Standard (DES):
DES is a 56 bit symmetric encryption algorithm. Although it is somewhat
outdated now it is included with the PIX for legacy reasons and should
not be used if there is another option available. Due to US technology
export restrictions it is more commonly used outside the USA.
Triple Data Encryption Standard (3DES):
3DES as it name suggests it three time as strong as DES by way of a 168
bit symmetric cipher which is obtained by encrypting the data three
consecutive times using DES. More specifically the data is first
encrypted using a 56 bit DES key, then decrypted using another 56 bit
DES key and then re-encrypted using yet another 56 bit DES key.
Advanced Encryption Standard (AES)
AES is a symmetric block cipher that encrypts and decrypts the data
using cryptographic keys of 128, 192 or 256 bit lengths. The resulting
encrypted data is then placed into 128 bit blocks which are combined
into cipher block chains.
**Symmetric encryption uses only a single secret key by itself to
encrypt and decrypt the data. Asymmetric encryption uses a key pair --
both a public and a private one -- for encryption and is commonly used
for Certificates. The sending host uses the receiver's public key to
encrypt the data and the receiver uses their private key to decrypt
it.**
Message Hashing
A hash algorithm simply talks the message, puts is through an algorithm
and creates a fixed length Message Digest (MD) from it. This message
digest is then put into another algorithm called a digital signature
algorithm which in turn generates a signature (or hash) for the message
from the message digest, rather than from the actual message, which
reduces the processing time of the message. Should the data be altered
even slightly it will massively throw out the message digest (which
remember is derived from the original data) and as a knock on affect
the signature (or hash) will be invalid.
For the hash to be created at the sending station and then understood
at the receiving station both stations must be using the same
algorithms.
The PIX will support the following hash algorithms:
Secure Hash Algorithm 1 (SHA -1)
SHA -1 produces 160 bit output and is considered more secure than MD5
for this reason
Message Digest 5 (MD5)
MD5 output is 128 bit. It is considered faster that SHA -1 but less
secure.
IPSec Session
Once Phase One and Phase two are established, traffic deemed
interesting for the IPSec session will flow through the tunnel in
encrypted form. The tunnel is not an ‘always on’ means and will time
out after either a predefined time limit is reached or a predefined
amount of data has been sent through it.
One of the main weak points with any encrypted service is the amount of
data sent in the encrypted form that had been encrypted using the same
keysets. The more data that can be collected the easier it may be for
the encryption algorithm to become compromised ( you only have to look
at WEP for a perfect example of this). For this reason the SA’s
established will time out either after a predefined amount of time or a
predefined amount of data has been sent through the IPSEC tunnel. When
the SA times out IKE performs a whole new Phase Two negotiations and if
needs be a new Phase One negotiation. This is done before the current
SA times out to prevent any interruption in the data flow.
Just as a quick summary for those who may not have followed it so far:
IKE Phase One provides a low level set of security services via
policies/standards that are first negotiated and then mutually agreed
upon once common ground has been found. Once this secure channel has
been established IPSec can use this secure channel to set up the IPSec
Security Association (SA) in Phase Two.
**As mentioned earlier without Phase One there would be no way to
verify that the end point in use when you set up the IPSec SA is
actually the correct end point and that in fact you are not setting up
your IPSec tunnel with an attacker of some kind**
IKE Phase Two is where IKE negotiates the IPSec SA’s parameters via the
secure link setup in Phase One and sets up the IPSec ‘tunnel’ between
the two peers. This established IPSec SA is what then protects all the
resulting traffic that flows between the tunnel end points. IPSec uses
IPSec transform sets in the same way IKE uses IKE transform sets. Again
just like IKE, if no common transform set can be agreed upon the
connection is torn down and the IPSec VPN fails.
Most Cisco products support three methods of IPSec peer authentication:
RSA Signatures (Certificates) - This is considered the preferred method
of authentication as it uses digital certificates that are
authenticated by an RAS Signature.
Pre-shared Keys – As the name suggests these are manually configured
case sensitive keys that must exactly match on both peers.
RSA Encrypted Nonce’s - Cisco appliances can use RSA (Rivest Shamir
Adleman) encryption to encrypt a random number (a nonce) that is
generated by the peer along with other pre configured values. The PIX
does not support this type of authentication at this moment in time.
Certificates and Certification Authorities (CA)
Certificate authorities manage certificate requests, issue the
requested certificates once approved and publish Certificate Revocation
Lists (CRL’s, pronounced “Crills”). IKE understands X.509v3
certificates that require public keys.
A digital certificate has a public key, which is available publically
strangely enough, and is used for the automatic authentication of
servers and/or users. The connecting node needs to trust the root CA
that issued the certificate for it to accept it. By default the
connecting node also needs access to the CRL to check if the
certificate has been revoked by the root CA. Some third party CA’s are
already trusted for web browsers etc but the PIX will only trust the
following third party CA’s:
VeriSign
Microsoft Corporation
Baltimore Technologies
Entrust Corporation
Certificates replace the need for pre-shared keys as they already have
a public and private keys as part of the frame work of the certificate.
For certificates to work on the PIX there are four steps to take:
Generate an RSA Key pair
Obtain the CA’s certificate which will have the public key
Using the generated key and the public key obtained by the CA the
firewall will then request a signed certificate from the CA
The CA then verifies the request and offers the signed certificate to
be published (depending on how the CA is configured this sometimes
requires an administrator to manually approve the request)
Pre-Shared Keys
Pre-Shared Keys (PSK) are usually used if you only have a few firewalls
to configure and you control all of the firewalls in the VPN
configuration, as when you change a PSK on one firewall obviously you
will need to change it on the other. PSK’s offer the quickest and
easiest way to configure a VPN. A PSK can be up to 128 bytes long and
uses alphanumeric characters ( A-Z and 0-9)
Original Tutorial
by nokia for TheTAZZone-TAZForum
Originally posted on January 30th, 2007 here
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