IPSec protocol analysis;AH and ESP protocals

1)         IPSec protocol is used for creating means for secure exchange of information between systems and networks on the Internet.  IPsec protocol helps users to create “private groups” based on common interest, independently from specifications of the networks they are using.

2)         IPSec protocol includes two protocols: Authentication Header (AH) and Encapsulating Secure Payload (ESP).  AH creates an envelope that ensures authentication of the source of the information, the completeness and protection from double messaging.    This way AH offers layers of ways to protect from malicious users.  With help of AH every packet is authenticated, which makes ineffective programs that are trying to capture the administration of the session.

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Besides this AH protocol tries to ensure the authentication of the headers of IP-packets, not depending on the location of the IP-headers outside of the envelope it is creating.  AH Authentication prevents IP-header line manipulation during the transfer of the packet.  Because of this reason protocol cannot be used in the environment where NAT (Network Address Translation – is a mechanism used for transferring network addresses) is being used.  This is because NAT requires IP-header manipulation, which AH Authentication will not allow. ESP protocol ensures confidentiality of the information and executes functions of the AH protocol protection of the encrypted not authenticated information.

IPSec specification allows ESP protocol use without AH functions.  However, it’s not a good idea to do this, unless you really know what you are doing and why.  In ESP protocol it’s possible to use fictitious encryption, which is equal to AH implementation without IP-header.  This allows using NAT mechanism, because in this method the addresses in the headers can be modified.


ESP and AH protocols are registered in IANA organization (Internet Address Naming Authority) and are entered into the protocol registry under the numbers 50 and 51.  If you or your ISP already implemented basic rules of packet filtration on your gateway routers, then you need to add these two protocols to the list of allowed protocols.  Because of the field “protocol type” of the header of IP-packet will now be identical with IPSec envelope, now the original type of transport protocol fits into the “protocol type” field inside of IPSec header.

IPSec protocol can be used in transport as well as tunnel mode.  In the first case IPSec header is placed between network (IP) and transport (TCP or UDP) headers of the “normal” IP-packet.  Transport mode is made for use on the end systems.  Functioning in this mode will reflect in all applications in the systems and in most cases there will be a need for a reprogramming of the existing applications.

Tunnel mode IPSec requires regular IP-packets to be put inside of the IPSec envelope, and that envelope is placed inside of other IP-packet.  This mode allows quickly implement tunnel IPSec systems across the network.  To provide traffic security between networks configured this way is relatively easy task. At least the manual says so.

IPSec protocol connection is established by one-way agreement – SA (Security Association), therefore for each connection there is a need for two SA agreements.  Each of them defines different configurations of IPSec connection, such as encryption algorithm and authentication algorithm.  These algorithms are used when exchanging information between systems, session keys and etc. that conduct their work.

As we know IPSec is a collection of protocols which uses algorithms of authentication and encryption.  Today there are two algorithms of authentication and seven algorithms of encryption.  In AH and ESP protocols we can find implementation of authentication algorithms such as HMAC-MD5 and HMAC-SHA1.  MD5 (Message Digest version 5, standard RFC #1321) works when both sides use common secret key with length of 128 bit and SHA1 (Secure Hash Algorithms version 1, standard FIPS 180-1) works when both sides use common secret key with length of 160 bit.  Algorithm HMAC (Keyed-Hashing for Message Authentication Code) is defined in RFC 2104.


Which algorithm to choose, except required DES, is up to the developer.  Possibility of algorithm choice gives him additional advantage: now an attacker not only has to break the encryption, but also must find out which encryption he needs to break.  Not to mention the choice of keys that will be another additional factor which will leave little hope to timely decryption of the intercepted encrypted data.

In default mode both sides of IPSec network establish SA-agreements for secure messaging with each other using Oakley protocol.  In fast mode, SA-agreements are set from IPSec protocol or any other service, which is requires information to create keys and/or communicating settings.  Oakley protocol is developed in such a way, that it is not related to IPSec at all.  For instance, to increase the security of the session process, we can use it together with SSL (Secure Sockets Layer) protocol version 4.0 instead of key exchange mechanism of SSL 3.0.

One thing that I like about IPSec – it is implemented on the IP-protocol level.  This structure gives possibility of using TCP or UDP through its tunnels.  And the main disadvantage of the IPSec – possibility to work only in IP-networks is neutralized by combined implementation with L2F (Layer 2 Forwarding, which performs readdressing on the 2nd level) and L2TP (Layer 2 Tunneling Protocol).  These protocols allow encapsulation of the packets of a structure different from one that is implemented in IPSec.

After reading through this particular point various times I would like to add my 5 cents into the pile.  However, I do not intent to follow authors’ path: “Again we stress that we did not perform a full analysis of all the ISAKMP protocols; we expect that there are further undiscovered weaknesses.”  This doesn’t add extra credibility to the authors’.  There is an Old Russian saying: “Measure 7 times, before you cut once”.

Speaking of measuring, for the last couple of years many researchers have analyzed the security of IPSec protocol.  They have not found any “serious holes” in them so far, despite various complete testing.  And those holes, that were found, are related to the misuse of the system or were fixed by developers.  However, theoretical possibility of discovering vulnerabilities in IPSec, PPTP and all other protocols and/or applications exists.

Realization attacks are used the most often.  These kinds of attacks do not require extensive mathematical knowledge.  It’s enough to be a quality programmer and know human factor weaknesses.  There are many examples of incorrect realization, which leads to the attack on the system.  Let’s look at the following choices:

Secret encryption key is kept on the hard drive that has no controlled access.

Cryptographic key, that’s saved in the ROM, doesn’t get deleted after the use.

Open access to the “black lists” of the compromised keys.

Absence of the control of the entirety of the VPN system, which allows a malevolent user to change Exchange Protocol, which is responsible for encryption or sum check of the received packets.

Let’s point out some of the actual vulnerabilities in VPN protocols including IPSec.  For instance, vulnerability of PPTP, that leads to denial of service in Windows NT OS, in network screen WatchGuard Firebox 2, routers Sisco and BinTec.  IPSec has also exposed some of it’s vulnerabilities to the large official and underground public.  For example, in OpenBSD there was a vulnerability that existed because of the incorrect processing of the AH/ESP packets (modes of IPSec).  Windows 2000 as a tradition has attracted with vulnerability in key exchange IKE for IPSec in 2003.

Also, it’s necessary to mention attacks on the VPN equipment.  Very often VPN is implemented using already existing network equipment such as Cisco 1720, or software/hardware of the inter network screens such as CheckPoint VPN-1 on the platform of Nokia IP Security Solutions.  In addition there is specialized equipment to build VPN, such as Continent-K.  And if any of the equipment used to build VPN uses TCP/IP, which means there is always a possibility of “denial of service” attacks that can alter the functioning of the equipment as well as temporary hang the network as a hole.

One of the most common attacks in IPSec based VPN networks as well as all other networks are the attack directed to/through the single user.  It’s important to remember that there is an average user, and he/she is also an element of VPN and also is vulnerable to the attacks as well as other elements of VPN.  User can give a disc with secret keys, or can simply lose the CD with secret keys and not notify the admin/security about the loss until the moment this user needs to use the secret keys.

In some systems user can independently create keys for encryption.  Key generation is based on the passwords, which are chosen by the user.  As we all know, an average user has a limited fantasy when picking keys.  Therefore, this leads to things like “brute force” attack.

In conclusion, it’s very important to keep in mind one rule (this applies to all technologies including VPN): “Security of the entire system equals to the security of the weakest link” (Andre Berthiaume, CNS340 instructor has mentioned this rule in the class, in the beginning of the quarter).  Therefore, it’s extremely important not only choose the strongest encryption algorithm and longest keys, but also to pay close attention to other elements of VPN – programming, equipment used, users, realization and etc.



References used:


1)         http://www.securitylab.ru/32393.html

2)         http://www.securitylab.ru/44613.html

3)         http://www.cisco.com/warp/public/707/cisco-sn-20040415-grppass.shtml

4)         http://support.microsoft.com/default.aspx?scid=kb;EN-US;253169

5)         http://secunia.com/advisories/11324/

6)         http://www.securityfocus.com/archive/1/347351

7)         http://support.microsoft.com/?kbid=324953

8)         http://www.net-security.org/vuln.php?id=3439

9)         http://www.winhackingexposed.com/news.html


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