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Glossary for the Linux FreeS/WAN project

- -

Entries are in alphabetical order. Some entries are only one line or one -paragraph long. Others run to several paragraphs. I have tried to put the -essential information in the first paragraph so you can skip the other -paragraphs if that seems appropriate.

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Jump to a letter in the glossary

- - -numeric A B C D E F G H I J K L M N O P Q R S T U V W X Y Z -

- -

Other glossaries

- -

Other glossaries which overlap this one include:

The VPN Consortium's glossary of VPN terms.
glossary portion of the Cryptography FAQ
an extensive crytographic glossary on Terry Ritter's - page.
The NSA's glossary of - computer security on the SANS - Institute site.
a small glossary for Internet Security at - PC magazine
The glossary - from Richard Smith's book Internet Cryptography

- -

Several Internet glossaries are available as RFCs:

- -

More general glossary or dictionary information:

Free Online Dictionary of Computing (FOLDOC) -
- North America
- Europe
- Japan
-
There are many more mirrors of this dictionary.
-
The Jargon File, the definitive resource for hacker slang and folklore -
- North America
- Holland
- home page
-
There are also many mirrors of this. See the home page for a list.
-
A general technology - glossary
An online dictionary resource - page with pointers to many dictionaries for many languages
A search engine that accesses - several hundred online dictionaries
O'Reilly Dictionary - of PC Hardware and Data Communications Terms
Connected Internet - encyclopedia
whatis.com

- -

Definitions

0

3DES (Triple DES)

Using three DES encryptions on a single data - block, with at least two different keys, to get higher security than is - available from a single DES pass. The three-key version of 3DES is the - default encryption algorithm for Linux - FreeS/WAN. -

IPsec always does 3DES with three different - keys, as required by RFC 2451. For an explanation of the two-key - variant, see two key triple DES. Both use an EDE encrypt-decrypt-encrpyt sequence of operations.

Single DES is insecure.

Double DES is ineffective. Using two 56-bit keys, one might expect - an attacker to have to do 2¹¹² work to break it. In fact, - only 2⁵⁷ work is required with a meet-in-the-middle attack, though a large amount of - memory is also required. Triple DES is vulnerable to a similar attack, - but that just reduces the work factor from the 2¹⁶⁸ one - might expect to 2¹¹². That provides adequate protection - against brute force attacks, and no better attack - is known.

3DES can be somewhat slow compared to other ciphers. It requires - three DES encryptions per block. DES was designed for hardware - implementation and includes some operations which are difficult in - software. However, the speed we get is quite acceptable for many uses. - See our performance document for - details.

A

Active attack

An attack in which the attacker does not merely eavesdrop (see passive attack) but takes action to change, delete, - reroute, add, forge or divert data. Perhaps the best-known active - attack is man-in-the-middle. In general, authentication is a useful defense against - active attacks.

AES

The Advanced Encryption Standard -- a new block cipher standard to replace DES -- developed by NIST, the US National Institute of Standards and - Technology. DES used 64-bit blocks and a 56-bit key. AES ciphers use a - 128-bit block and 128, 192 or 256-bit keys. The larger block size helps - resist birthday attacks while the large key - size prevents brute force attacks. -

Fifteen proposals meeting NIST's basic criteria were submitted in - 1998 and subjected to intense discussion and analysis, "round one" - evaluation. In August 1999, NIST narrowed the field to five "round two" - candidates:

Mars - from IBM
RC6 from RSA
Rijndael - from two Belgian researchers
Serpent, a - British-Norwegian-Israeli collaboration
Twofish - from the consulting firm Counterpane

Three of the five finalists -- Rijndael, Serpent and Twofish -- have - completely open licenses.

In October 2000, NIST announced the winner -- Rijndael.

For more information, see:

NIST's AES home - page
the Block Cipher Lounge AES page
Brian Gladman's code - and benchmarks
Helger Lipmaa's survey of - implementations

AES will be added to a future release of Linux - FreeS/WAN. Likely we will add all three of the finalists with good - licenses. User-written AES patches are - already available.

Adding AES may also require adding stronger hashes, SHA-256, SHA-384 and SHA-512.

AH

The IPsec Authentication Header, - added after the IP header. For details, see our IPsec document and/or RFC 2402.

Alice and Bob

A and B, the standard example users in writing on cryptography and - coding theory. Carol and Dave join them for protocols which require - more players. -

Bruce Schneier extends these with many others such as Eve the - Eavesdropper and Victor the Verifier. His extensions seem to be in the - process of becoming standard as well. See page 23 of Applied Cryptography

Alice and Bob have an amusing biography on the - web.

ARPA

see DARPA

ASIO

Australian Security Intelligence Organisation.

Asymmetric cryptography

See public key cryptography.

Authentication

Ensuring that a message originated from the expected sender and has - not been altered on route. IPsec uses - authentication in two places: -

peer authentication, authenticating the players in IKE's Diffie-Hellman key - exchanges to prevent man-in-the-middle - attacks. This can be done in a number of ways. The methods - supported by FreeS/WAN are discussed in our advanced configuration - document.
packet authentication, authenticating packets on an established - SA, either with a separate authentication header or with the optional - authentication in the ESP protocol. In either - case, packet authentication uses a hashed message - athentication code technique.

Outside IPsec, passwords are perhaps the most common authentication - mechanism. Their function is essentially to authenticate the person's - identity to the system. Passwords are generally only as secure as the - network they travel over. If you send a cleartext password over a - tapped phone line or over a network with a packet sniffer on it, the - security provided by that password becomes zero. Sending an encrypted - password is no better; the attacker merely records it and reuses it at - his convenience. This is called a replay - attack.

A common solution to this problem is a challenge-response system. This defeats simple - eavesdropping and replay attacks. Of course an attacker might still try - to break the cryptographic algorithm used, or the random number generator.

Automatic keying

A mode in which keys are automatically generated at connection - establisment and new keys automaically created periodically thereafter. - Contrast with manual keying in which a single - stored key is used. -

IPsec uses the Diffie-Hellman key exchange - protocol to create keys. An authentication mechansim is required for - this. FreeS/WAN normally uses RSA for this. Other - methods supported are discussed in our advanced configuration document.

Having an attacker break the authentication is emphatically not a - good idea. An attacker that breaks authentication, and manages to - subvert some other network entities (DNS, routers or gateways), can use - a man-in-the middle attack to break the security - of your IPsec connections.

However, having an attacker break the authentication in automatic - keying is not quite as bad as losing the key in manual keying.

An attacker who reads /etc/ipsec.conf and gets the keys for a - manually keyed connection can, without further effort, read all - messages encrypted with those keys, including any old messages he - may have archived.
Automatic keying has a property called perfect - forward secrecy. An attacker who breaks the authentication gets - none of the automatically generated keys and cannot immediately - read any messages. He has to mount a successful man-in-the-middle attack in real time before he - can read anything. He cannot read old archived messages at all and - will not be able to read any future messages not caught by - man-in-the-middle tricks.

That said, the secrets used for authentication, stored in ipsec.secrets(5), should - still be protected as tightly as cryptographic keys.

B

Bay Networks

A vendor of routers, hubs and related products, now a subsidiary of - Nortel. Interoperation between their IPsec products and Linux FreeS/WAN - was problematic at last report; see our interoperation section.

benchmarks

Our default block cipher, triple DES, is slower - than many alternate ciphers that might be used. Speeds achieved, - however, seem adequate for many purposes. For example, the assembler - code from the LIBDES library we use encrypts 1.6 - megabytes per second on a Pentium 200, according to the test program - supplied with the library. -

For more detail, see our document on FreeS/WAN performance.

BIND

Berkeley Internet Name Daemon, a widely - used implementation of DNS (Domain Name Service). - See our bibliography for a useful reference. See the - BIND home page for more - information and the latest version.

Birthday attack

A cryptographic attack based on the mathematics exemplified by the birthday paradox. This math turns up whenever the - question of two cryptographic operations producing the same result - becomes an issue: -

collisions in message - digest functions.
identical output blocks from a block - cipher
repetition of a challenge in a challenge-response system

Resisting such attacks is part of the motivation for:

hash algorithms such as SHA and RIPEMD-160 giving a 160-bit result rather than - the 128 bits of MD4, MD5 and - RIPEMD-128.
AES block ciphers using a 128-bit block - instead of the 64-bit block of most current ciphers
IPsec using a 32-bit counter for packets - sent on an automatically keyed SA and requiring that the connection always be - rekeyed before the counter overflows.

Birthday paradox

Not really a paradox, just a rather counter-intuitive mathematical - fact. In a group of 23 people, the chance of a least one pair having - the same birthday is over 50%. -

The second person has 1 chance in 365 (ignoring leap years) of - matching the first. If they don't match, the third person's chances of - matching one of them are 2/365. The 4th, 3/365, and so on. The total of - these chances grows more quickly than one might guess.

Block cipher

A symmetric cipher which operates on - fixed-size blocks of plaintext, giving a block of ciphertext for each. - Contrast with stream cipher. Block ciphers can - be used in various modes when multiple block are to - be encrypted. -

DES is among the the best known and widely used - block ciphers, but is now obsolete. Its 56-bit key size makes it highly insecure today. Triple - DES is the default block cipher for Linux - FreeS/WAN.

The current generation of block ciphers -- such as Blowfish, CAST-128 and IDEA -- all use 64-bit blocks and 128-bit keys. The - next generation, AES, uses 128-bit blocks and - supports key sizes up to 256 bits.

The Block Cipher - Lounge web site has more information.

Blowfish

A block cipher using 64-bit blocks and keys of - up to 448 bits, designed by Bruce Schneier and - used in several products. -

This is not required by the IPsec RFCs and not - currently used in Linux FreeS/WAN.

Brute force attack (exhaustive search)

Breaking a cipher by trying all possible keys. This is always - possible in theory (except against a one-time pad), - but it becomes practical only if the key size is inadequate. For an - important example, see our document on the insecurity of DES with its 56-bit key. For an - analysis of key sizes required to resist plausible brute force attacks, - see this paper. -

Longer keys protect against brute force attacks. Each extra bit in - the key doubles the number of possible keys and therefore doubles the - work a brute force attack must do. A large enough key defeats - any brute force attack.

For example, the EFF's DES Cracker searches a - 56-bit key space in an average of a few days. Let us assume an attacker - that can find a 64-bit key (256 times harder) by brute force search in - a second (a few hundred thousand times faster). For a 96-bit key, that - attacker needs 2³² seconds, about 135 years. Against a - 128-bit key, he needs 2³² times that, over 500,000,000,000 - years. Your data is then obviously secure against brute force attacks. - Even if our estimate of the attacker's speed is off by a factor of a - million, it still takes him over 500,000 years to crack a message.

This is why

single DES is now considered dangerously insecure
all of the current generation of block - ciphers use a 128-bit or longer key
AES ciphers support keysizes 128, 192 and 256 - bits
any cipher we add to Linux FreeS/WAN will have at least - a 128-bit key

Cautions:
- Inadequate keylength always indicates a weak cipher but it is - important to note that adequate keylength does not necessarily - indicate a strong cipher. There are many attacks other than brute - force, and adequate keylength only guarantees resistance to - brute force. Any cipher, whatever its key size, will be weak if design - or implementation flaws allow other attacks.

Also, once you have adequate keylength (somewhere around 90 - or 100 bits), adding more key bits make no practical - difference, even against brute force. Consider our 128-bit example - above that takes 500,000,000,000 years to break by brute force. We - really don't care how many zeroes there are on the end of that, as long - as the number remains ridiculously large. That is, we don't care - exactly how large the key is as long as it is large enough.

There may be reasons of convenience in the design of the cipher to - support larger keys. For example Blowfish - allows up to 448 bits and RC4 up to 2048, but beyond - 100-odd bits it makes no difference to practical security.

Bureau of Export Administration

see BXA

BXA

The US Commerce Department's Bureau of Export - Administration which administers the EAR - Export Administration Regulations controling the export of, among other - things, cryptography.

C

CA

Certification Authority, an entity in a public key infrastructure that can certify keys by - signing them. Usually CAs form a hierarchy. The top of this hierarchy - is called the root CA. -

See Web of Trust for an alternate model.

CAST-128

A block cipher using 64-bit blocks and 128-bit - keys, described in RFC 2144 and used in products such as Entrust and recent versions of PGP. -

This is not required by the IPsec RFCs and not - currently used in Linux FreeS/WAN.

CAST-256

Entrust's candidate cipher for the AES standard, largely based on the CAST-128 design.

CBC mode

Cipher Block Chaining mode, - a method of using a block cipher in which for each - block except the first, the result of the previous encryption is XORed - into the new block before it is encrypted. CBC is the mode used in IPsec. -

An initialisation vector (IV) must be provided. It - is XORed into the first block before encryption. The IV need not be - secret but should be different for each message and unpredictable.

CIDR

Classless Inter-Domain Routing, - an addressing scheme used to describe networks not - restricted to the old Class A, B, and C sizes. - A CIDR block is written - address/mask, where address is - a 32-bit Internet address. - The first mask bits of address - are part of the gateway address, while the remaining bits designate - other host addresses. - For example, the CIDR block 192.0.2.96/27 describes a network with - gateway - 192.0.2.96, hosts 192.0.2.96 through 192.0.2.126 and broadcast - 192.0.2.127. -

FreeS/WAN policy group files accept CIDR blocks of the format - address/[mask], where address may - take the form name.domain.tld. An absent mask - is assumed to be /32. -

Certification Authority

see CA

Challenge-response authentication

An authentication system in which one - player generates a random number, encrypts it and - sends the result as a challenge. The other player decrypts and sends - back the result. If the result is correct, that proves to the first - player that the second player knew the appropriate secret, required for - the decryption. Variations on this technique exist using public key or symmetric - cryptography. Some provide two-way authentication, assuring each player - of the other's identity. -

This is more secure than passwords against two simple attacks:

If cleartext passwords are sent across the wire (e.g. for - telnet), an eavesdropper can grab them. The attacker may even be - able to break into other systems if the user has chosen the same - password for them.
If an encrypted password is sent, an attacker can record the - encrypted form and use it later. This is called a replay - attack.

A challenge-response system never sends a password, either cleartext - or encrypted. An attacker cannot record the response to one challenge - and use it as a response to a later challenge. The random number is - different each time.

Of course an attacker might still try to break the cryptographic - algorithm used, or the random number - generator.

Cipher Modes

Different ways of using a block cipher when encrypting multiple - blocks. -

Four standard modes were defined for DES in FIPS 81. They can actually be applied with any block - cipher.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - -

ECB	Electronic CodeBook	encrypt each block independently
CBC	Cipher Block Chaining -	XOR previous block ciphertext into new block plaintext before - encrypting new block
CFB	Cipher FeedBack
OFB	Output FeedBack

IPsec uses CBC mode since - this is only marginally slower than ECB and is more - secure. In ECB mode the same plaintext always encrypts to the same - ciphertext, unless the key is changed. In CBC mode, this does not - occur.

Various other modes are also possible, but none of them are used in - IPsec.

Ciphertext

The encrypted output of a cipher, as opposed to the unencrypted plaintext input.

Cisco

A vendor of routers, hubs and related products. Their IPsec products - interoperate with Linux FreeS/WAN; see our interop section.

Client

This term has at least two distinct uses in discussing IPsec: -

The clients of an IPsec gateway are the machines - it protects, typically on one or more subnets behind the gateway. - In this usage, all the machines on an office network are clients of - that office's IPsec gateway. Laptop or home machines connecting to - the office, however, are not clients of that gateway. They - are remote gateways, running the other end of an IPsec connection. - Each of them is also its own client.
IPsec client software is used to describe - software which runs on various standalone machines to let them - connect to IPsec networks. In this usage, a laptop or home machine - connecting to the office is a client, and the office gateway is the - server.

We generally use the term in the first sense. Vendors of Windows - IPsec solutions often use it in the second. See this discussion.

Common Criteria

A set of international security classifications which are replacing - the old US Rainbow Book standards and similar - standards in other countries. -

Web references include this US - government site and this global home page.

Conventional cryptography

See symmetric cryptography

Collision resistance

The property of a message digest algorithm - which makes it hard for an attacker to find or construct two inputs - which hash to the same output.

Copyleft

see GNU General Public License

CSE

Communications Security - Establishment, the Canadian organisation for signals intelligence.

D

DARPA (sometimes just ARPA)

The US government's Defense Advanced Research - Projects Agency. Projects they have funded over the years - have included the Arpanet which evolved into the Internet, the TCP/IP - protocol suite (as a replacement for the original Arpanet suite), the - Berkeley 4.x BSD Unix projects, and Secure DNS. -

For current information, see their web site.

Denial of service (DoS) attack

An attack that aims at denying some service to legitimate users of a - system, rather than providing a service to the attacker. -

One variant is a flooding attack, overwhelming the system with - too many packets, to much email, or whatever.
A closely related variant is a resource exhaustion attack. For - example, consider a "TCP SYN flood" attack. Setting up a TCP - connection involves a three-packet exchange: -
- Initiator: Connection please (SYN)
- Responder: OK (ACK)
- Initiator: OK here too
-
If the attacker puts bogus source information in the first - packet, such that the second is never delivered, the responder may - wait a long time for the third to come back. If responder has - already allocated memory for the connection data structures, and if - many of these bogus packets arrive, the responder may run out of - memory.
-
Another variant is to feed the system undigestible data, hoping - to make it sick. For example, IP packets are limited in size to 64K - bytes and a fragment carries information on where it starts within - that 64K and how long it is. The "ping of death" delivers fragments - that say, for example, that they start at 60K and are 20K long. - Attempting to re-assemble these without checking for overflow can - be fatal.

The two example attacks discussed were both quite effective when - first discovered, capable of crashing or disabling many operating - systems. They were also well-publicised, and today far fewer systems - are vulnerable to them.

DES

The Data Encryption Standard, a block cipher with 64-bit blocks and a 56-bit key. - Probably the most widely used symmetric cipher - ever devised. DES has been a US government standard for their own use - (only for unclassified data), and for some regulated industries such as - banking, since the late 70's. It is now being replaced by AES. -

DES is seriously insecure - against current attacks.

Linux FreeS/WAN does not include DES, even - though the RFCs specify it. We strongly recommend that single DES - not be used.

See also 3DES and DESX, - stronger ciphers based on DES.

DESX

An improved DES suggested by Ron Rivest of RSA - Data Security. It XORs extra key material into the text before and - after applying the DES cipher. -

This is not required by the IPsec RFCs and not - currently used in Linux FreeS/WAN. DESX would - be the easiest additional transform to add; there would be very little - code to write. It would be much faster than 3DES and almost certainly - more secure than DES. However, since it is not in the RFCs other IPsec - implementations cannot be expected to have it.

DH

see Diffie-Hellman

DHCP

Dynamic Host - Configuration Protocol, a method of - assigning dynamic IP addresses, and providing - additional information such as addresses of DNS servers and of - gateways. See this DHCP resource - page.

Diffie-Hellman (DH) key exchange protocol

A protocol that allows two parties without any initial shared secret - to create one in a manner immune to eavesdropping. Once they have done - this, they can communicate privately by using that shared secret as a - key for a block cipher or as the basis for key exchange. -

The protocol is secure against all passive - attacks, but it is not at all resistant to active man-in-the-middle attacks. If a third party can - impersonate Bob to Alice and vice versa, then no useful secret can be - created. Authentication of the participants is a prerequisite for safe - Diffie-Hellman key exchange. IPsec can use any of several authentication mechanisims. Those supported - by FreeS/WAN are discussed in our configuration section.

The Diffie-Hellman key exchange is based on the discrete logarithm problem and is secure unless - someone finds an efficient solution to that problem.

Given a prime p and generator g (explained - under discrete log below), Alice:

generates a random number a
calculates A = g^a modulo p
sends A to Bob

Meanwhile Bob:

generates a random number b
calculates B = g^b modulo p
sends B to Alice

Now Alice and Bob can both calculate the shared secret s = - g^(ab). Alice knows a and B, so she - calculates s = B^a. Bob knows A and b - so he calculates s = A^b.

An eavesdropper will know p and g since these - are made public, and can intercept A and B but, - short of solving the discrete log problem, these do - not let him or her discover the secret s.

Digital signature

Sender: -

calculates a message digest of a - document
encrypts the digest with his or her private key, using some public key cryptosystem.
attaches the encrypted digest to the document as a signature

Receiver:

calculates a digest of the document (not including the - signature)
decrypts the signature with the signer's public key
verifies that the two results are identical

If the public-key system is secure and the verification succeeds, - then the receiver knows

that the document was not altered between signing and - verification
that the signer had access to the private key

Such an encrypted message digest can be treated as a signature since - it cannot be created without both the document and - the private key which only the sender should possess. The legal issues are complex, but several - countries are moving in the direction of legal recognition for digital - signatures.

discrete logarithm problem

The problem of finding logarithms in a finite field. Given a field - defintion (such definitions always include some operation analogous to - multiplication) and two numbers, a base and a target, find the power - which the base must be raised to in order to yield the target. -

The discrete log problem is the basis of several cryptographic - systems, including the Diffie-Hellman key exchange - used in the IKE protocol. The useful property is - that exponentiation is relatively easy but the inverse operation, - finding the logarithm, is hard. The cryptosystems are designed so that - the user does only easy operations (exponentiation in the field) but an - attacker must solve the hard problem (discrete log) to crack the - system.

There are several variants of the problem for different types of - field. The IKE/Oakley key determination protocol uses two variants, - either over a field modulo a prime or over a field defined by an - elliptic curve. We give an example modulo a prime below. For the - elliptic curve version, consult an advanced text such as Handbook of Applied Cryptography.

Given a prime p, a generator g for the field - modulo that prime, and a number x in the field, the problem - is to find y such that g^y = x.

For example, let p = 13. The field is then the integers from 0 to - 12. Any integer equals one of these modulo 13. That is, the remainder - when any integer is divided by 13 must be one of these.

2 is a generator for this field. That is, the powers of two modulo - 13 run through all the non-zero numbers in the field. Modulo 13 we - have:

          y      x
-        2^0  ==  1
-        2^1  ==  2
-        2^2  ==  4
-        2^3  ==  8
-        2^4  ==  3 that is, the remainder from 16/13 is 3
-        2^5  ==  6          the remainder from 32/13 is 6
-        2^6  == 12 and so on
-        2^7  == 11
-        2^8  ==  9
-        2^9  ==  5
-        2^10 == 10
-        2^11 ==  7
-        2^12 ==  1

Exponentiation in such a field is not difficult. Given, say, - y = 11,calculating x = - 7is straightforward. One method is just to calculate - 2^11 = 2048,then 2048 mod 13 == - 7.When the field is modulo a large prime (say a few 100 - digits) you need a silghtly cleverer method and even that is moderately - expensive in computer time, but the calculation is still not - problematic in any basic way.

The discrete log problem is the reverse. In our example, given - x = 7,find the logarithm y = - 11.When the field is modulo a large prime (or is based on - a suitable elliptic curve), this is indeed problematic. No solution - method that is not catastrophically expensive is known. Quite a few - mathematicians have tackled this problem. No efficient method has been - found and mathematicians do not expect that one will be. It seems - likely no efficient solution to either of the main variants the - discrete log problem exists.

Note, however, that no-one has proven such methods do not exist. If - a solution to either variant were found, the security of any crypto - system using that variant would be destroyed. This is one reason IKE supports two variants. If one is broken, we can - switch to the other.

discretionary access control

access control mechanisms controlled by the user, for example Unix - rwx file permissions. These contrast with mandatory access controls.

DNS

Domain Name Service, a distributed database - through which names are associated with numeric addresses and other - information in the Internet Protocol Suite. See also the DNS background section of our - documentation.

DOS attack

see Denial Of Service attack

dynamic IP address

an IP address which is automatically assigned, either by DHCP or by some protocol such as PPP or PPPoE which the machine - uses to connect to the Internet. This is the opposite of a static IP address, pre-set on the machine - itself.

E

EAR

The US government's Export Administration - Regulations, administered by the Bureau of Export - Administration. These have replaced the earlier ITAR regulations as the controls on export of - cryptography.

ECB mode

Electronic CodeBook mode, the simplest way to - use a block cipher. See Cipher Modes.

EDE

The sequence of operations normally used in either the three-key - variant of triple DES used in IPsec or the two-key variant used - in some other systems. -

The sequence is:

Encrypt with key1
Decrypt with key2
Encrypt with key3

For the two-key version, key1=key3.

The "advantage" of this EDE order of operations is that it makes it - simple to interoperate with older devices offering only single DES. Set - key1=key2=key3 and you have the worst of both worlds, the overhead of - triple DES with the "security" of single DES. Since both the security of single DES and the - overheads of triple DES are seriously inferior to many other ciphers, - this is a spectacularly dubious "advantage".

Entrust

A Canadian company offerring enterprise PKI - products using CAST-128 symmetric crypto, RSA public key and X.509 - directories. Web site

EFF

Electronic Frontier Foundation, an - advocacy group for civil rights in cyberspace.

Encryption

Techniques for converting a readable message (plaintext) into apparently random material (ciphertext) which cannot be read if intercepted. - A key is required to read the message. -

Major variants include symmetric encryption - in which sender and receiver use the same secret key and public key methods in which the sender uses one of a - matched pair of keys and the receiver uses the other. Many current - systems, including IPsec, are hybrids combining the two techniques.

ESP

Encapsulated Security Payload, the IPsec protocol which provides encryption. It can also provide authentication service and may be used with - null encryption (which we do not recommend). For details see our IPsec document and/or RFC 2406.

Extruded subnet

A situation in which something IP sees as one network is actually in - two or more places. -

For example, the Internet may route all traffic for a particular - company to that firm's corporate gateway. It then becomes the company's - problem to get packets to various machines on their subnets in various departments. They may decide to - treat a branch office like a subnet, giving it IP addresses "on" their - corporate net. This becomes an extruded subnet.

Packets bound for it are delivered to the corporate gateway, since - as far as the outside world is concerned, that subnet is part of the - corporate network. However, instead of going onto the corporate LAN (as - they would for, say, the accounting department) they are then - encapsulated and sent back onto the Internet for delivery to the branch - office.

For information on doing this with Linux FreeS/WAN, look in our advanced configuration - section.

Exhaustive search

See brute force attack.

F

FIPS

Federal Information Processing Standard, - the US government's standards for products it buys. These are issued by - NIST. Among other things, DES - and SHA are defined in FIPS documents. NIST have a - FIPS home page.

Free Software Foundation (FSF)

An organisation to promote free software, free in the sense of these - quotes from their web pages

- "Free software" is a matter of liberty, not price. To understand the - concept, you should think of "free speech", not "free beer." -
"Free software" refers to the users' freedom to run, copy, - distribute, study, change and improve the software.
-

FreeS/WAN

see Linux FreeS/WAN

Fullnet

The CIDR block containing all IPs of its IP version. - The IPv4 fullnet is written 0.0.0.0/0. - Also known as "all" and "default", - fullnet may be used in a routing table - to specify a default route, - and in a FreeS/WAN - policy group file to - specify a default IPsec policy.

FSF

see Free software Foundation

G

GCHQ

Government Communications - Headquarters, the British organisation for signals intelligence.

generator of a prime field

see discrete logarithm problem

GILC

Global Internet Liberty Campaign, - an international organisation advocating, among other things, free - availability of cryptography. They have a campaign to remove - cryptographic software from the Wassenaar - Arrangement.

Global Internet Liberty Campaign

see GILC.

Global Trust Register

An attempt to create something like a root CA - for PGP by publishing both as a book and on the - web the fingerprints of a set of verified keys for well-known users - and organisations.

GMP

The GNU Multi-Precision library code, used in Linux FreeS/WAN by Pluto for - public key calculations. See the GMP home page.

GNU

GNU's Not Unix, the Free Software - Foundation's project aimed at creating a free system with at least - the capabilities of Unix. Linux uses GNU utilities - extensively.

GOST

a Soviet government standard block cipher. Applied Cryptography has details.

GPG

see GNU Privacy Guard

GNU General Public License(GPL, copyleft)

The license developed by the Free Software - Foundation under which Linux, Linux FreeS/WAN and many other pieces of software - are distributed. The license allows anyone to redistribute and modify - the code, but forbids anyone from distributing executables without - providing access to source code. For more details see the file COPYING included with GPLed source distributions, - including ours, or the - GNU site's GPL page.

GNU Privacy Guard

An open source implementation of Open PGP as - defined in RFC 2440. See their web - site

GPL

see GNU General Public License.

H

Hash

see message digest

Hashed Message Authentication Code (HMAC)

using keyed message digest functions to - authenticate a message. This differs from other uses of these functions: -

In normal usage, the hash function's internal variable are - initialised in some standard way. Anyone can reproduce the hash to - check that the message has not been altered.
For HMAC usage, you initialise the internal variables from the - key. Only someone with the key can reproduce the hash. A successful - check of the hash indicates not only that the message is unchanged - but also that the creator knew the key.

The exact techniques used in IPsec are defined - in RFC 2104. They are referred to as HMAC-MD5-96 and HMAC-SHA-96 - because they output only 96 bits of the hash. This makes some attacks - on the hash functions harder.

HMAC

see Hashed Message Authentication Code

HMAC-MD5-96

see Hashed Message Authentication Code

HMAC-SHA-96

see Hashed Message Authentication Code

Hybrid cryptosystem

A system using both public key and symmetric cipher techniques. This works well. - Public key methods provide key management and digital signature facilities which are not - readily available using symmetric ciphers. The symmetric cipher, - however, can do the bulk of the encryption work much more efficiently - than public key methods.

I

IAB

Internet Architecture Board.

ICMP

Internet Control - Message Protocol. This is used for - various IP-connected devices to manage the network.

IDEA

International Data Encrypion Algorithm, - developed in Europe as an alternative to exportable American ciphers - such as DES which were too - weak for serious use. IDEA is a block cipher - using 64-bit blocks and 128-bit keys, and is used in products such as - PGP. -

IDEA is not required by the IPsec RFCs and not - currently used in Linux FreeS/WAN.

IDEA is patented and, with strictly limited exceptions for personal - use, using it requires a license from Ascom.

IEEE

Institute of Electrical and Electronic - Engineers, a professional association which, among other things, - sets some technical standards

IESG

Internet Engineering Steering - Group.

IETF

Internet Engineering Task Force, - the umbrella organisation whose various working groups make most of the - technical decisions for the Internet. The IETF IPsec - working group wrote the RFCs we are - implementing.

IKE

Internet Key Exchange, based on the Diffie-Hellman key exchange protocol. For details, see - RFC 2409 and our IPsec document. IKE is - implemented in Linux FreeS/WAN by the Pluto daemon.

IKE v2

A proposed replacement for IKE. There are other - candidates, such as JFK, and at time of writing - (March 2002) the choice between them has not yet been made and does not - appear imminent.

iOE

See Initiate-only opportunistic - encryption.

IP

Internet Protocol.

IP masquerade

A mostly obsolete term for a method of allowing multiple machines to - communicate over the Internet when only one IP address is available for - their use. The more current term is Network Address Translation or NAT.

IPng

"IP the Next Generation", see IPv6.

IPv4

The current version of the Internet protocol - suite.

IPv6 (IPng)

Version six of the Internet protocol suite, - currently being developed. It will replace the current version four. IPv6 has IPsec as a - mandatory component. -

See this web - site for more details, and our compatibility document for information on - FreeS/WAN and the Linux implementation of IPv6.

IPsec or IPSEC

Internet Protocol SECurity, security functions - (authentication and encryption) implemented at the IP level of the - protocol stack. It is optional for IPv4 and - mandatory for IPv6. -

This is the standard Linux FreeS/WAN is - implementing. For more details, see our IPsec - Overview. For the standards, see RFCs listed in our RFCs document.

IPX

Novell's Netware protocol tunnelled over an IP link. Our firewalls document includes an - example of using this through an IPsec tunnel.

ISAKMP

Internet Security Association and Key - Management Protocol, defined in RFC 2408.

ITAR

International Traffic in Arms - Regulations, US regulations administered by the State Department - which until recently limited export of, among other things, - cryptographic technology and software. ITAR still exists, but the - limits on cryptography have now been transferred to the Export Administration Regulations under the Commerce - Department's Bureau of Export Administration.

IV

see Initialisation vector

Initialisation Vector (IV)

Some cipher modes, including the CBC mode which IPsec uses, require some extra data at - the beginning. This data is called the initialisation vector. It need - not be secret, but should be different for each message. Its function - is to prevent messages which begin with the same text from encrypting - to the same ciphertext. That might give an analyst an opening, so it is - best prevented.

Initiate-only opportunistic - encryption (iOE)

A form of - opportunistic encryption (OE) in which - a host proposes opportunistic connections, but lacks the reverse DNS - records necessary to support incoming opportunistic connection requests. - Common among hosts on cable or pppoe connections where the system - administrator does not have write access to the DNS reverse map - for the host's external IP. -

Configuring for initiate-only opportunistic encryption - is described in our - quickstart document.

J

JFK

Just Fast Keying, - a proposed simpler replacement for IKE.

K

Kernel

The basic part of an operating system (e.g. Linux) which controls the - hardware and provides services to all other programs. -

In the Linux release numbering system, an even second digit as in - 2.2.x indicates a stable or production kernel while an - odd number as in 2.3.x indicates an experimental or - development kernel. Most users should run a recent kernel version from - the production series. The development kernels are primarily for people - doing kernel development. Others should consider using development - kernels only if they have an urgent need for some feature not yet - available in production kernels.

Keyed message digest

See HMAC.

Key length

see brute force attack

KLIPS

Kernel IP Security, the Linux FreeS/WAN project's changes to the Linux kernel to support the IPsec protocols.

L

LDAP

Lightweight Directory Access Protocol, - defined in RFCs 1777 and 1778, a method of accessing information - stored in directories. LDAP is used by several PKI - implementations, often with X.501 directories and X.509 certificates. It may also be used by IPsec to obtain key certifications from those PKIs. - This is not yet implemented in Linux - FreeS/WAN.

LIBDES

A publicly available library of DES code, written - by Eric Young, which Linux FreeS/WAN uses in - both KLIPS and Pluto.

Linux

A freely available Unix-like operating system based on a kernel - originally written for the Intel 386 architecture by (then) student - Linus Torvalds. Once his 32-bit kernel was available, the GNU utilities made it a usable system and contributions - from many others led to explosive growth. -

Today Linux is a complete Unix replacement available for several CPU - architectures -- Intel, DEC/Compaq Alpha, Power PC, both 32-bit SPARC - and the 64-bit UltraSPARC, SrongARM, . . . -- with support for multiple - CPUs on some architectures.

Linux FreeS/WAN is intended to run on all - CPUs supported by Linux and is known to work on several. See our compatibility section for a list.

Linux FreeS/WAN

Our implementation of the IPsec protocols, - intended to be freely redistributable source code with a - GNU GPL license and no constraints under US or other export laws. Linux FreeS/WAN is intended - to interoperate with other IPsec implementations. - The name is partly taken, with permission, from the S/WAN multi-vendor IPsec compatability effort. Linux - FreeS/WAN has two major components, KLIPS (KerneL - IPsec Support) and the Pluto daemon which manages - the whole thing. -

See our IPsec section for more detail. For - the code see our primary site or one - of the mirror sites on this list.

Linux Security Modules (LSM)

a project to create an interface in the Linux kernel that supports - plug-in modules for various security policies. -

This allows multiple security projects to take different approaches - to security enhancement without tying the kernel down to one particular - approach. As I understand the history, several projects were pressing - Linus to incorporate their changes, the various sets of changes were - incompatible, and his answer was more-or-less "a plague on all your - houses; I'll give you an interface, but I won't incorporate - anything".

It seems to be working. There is a fairly active LSM - mailing list, and several projects are already using the - interface.

LSM

see Linux Security Modules

M

Mailing list

The Linux FreeS/WAN project has several - public email lists for bug reports and software development - discussions. See our document on mailing - lists.

Man-in-the-middle attack

An active attack in which the attacker - impersonates each of the legitimate players in a protocol to the other. -

For example, if Alice and Bob are - negotiating a key via the Diffie-Hellman key - agreement, and are not using authentication to be certain they are - talking to each other, then an attacker able to insert himself in the - communication path can deceive both players.

Call the attacker Mallory. For Bob, he pretends to be Alice. For - Alice, he pretends to be Bob. Two keys are then negotiated, - Alice-to-Mallory and Bob-to-Mallory. Alice and Bob each think the key - they have is Alice-to-Bob.

A message from Alice to Bob then goes to Mallory who decrypts it, - reads it and/or saves a copy, re-encrypts using the Bob-to-Mallory key - and sends it along to Bob. Bob decrypts successfully and sends a reply - which Mallory decrypts, reads, re-encrypts and forwards to Alice.

To make this attack effective, Mallory must

subvert some part of the network in some way that lets him carry - out the deception
- possible targets: DNS, router, Alice or Bob's machine, mail server, - ...
beat any authentication mechanism Alice and Bob use
- strong authentication defeats the attack entirely; this is why IKE requires authentication
work in real time, delivering messages without introducing a - delay large enough to alert the victims
- not hard if Alice and Bob are using email; quite difficult in some - situations.

If he manages it, however, it is devastating. He not only gets to - read all the messages; he can alter messages, inject his own, forge - anything he likes, . . . In fact, he controls the communication - completely.

mandatory access control

access control mechanisims which are not settable by the user (see discretionary access control), but are - enforced by the system. -

For example, a document labelled "secret, zebra" might be readable - only by someone with secret clearance working on Project Zebra. - Ideally, the system will prevent any transfer outside those boundaries. - For example, even if you can read it, you should not be able to e-mail - it (unless the recipient is appropriately cleared) or print it (unless - certain printers are authorised for that classification).

Mandatory access control is a required feature for some levels of Rainbow Book or Common Criteria - classification, but has not been widely used outside the military and - government. There is a good discussion of the issues in Anderson's Security Engineering.

The Security Enhanced Linux project is adding - mandatory access control to Linux.

Manual keying

An IPsec mode in which the keys are provided by the administrator. In - FreeS/WAN, they are stored in /etc/ipsec.conf. The alternative, automatic keying, is preferred in most cases. See this - discussion.

MD4

Message Digest Algorithm Four from Ron Rivest - of RSA. MD4 was widely used a few years ago, but - is now considered obsolete. It has been replaced by its descendants MD5 and SHA.

MD5

Message Digest Algorithm Five from Ron Rivest - of RSA, an improved variant of his MD4. Like MD4, it produces a 128-bit hash. For details - see RFC 1321. -

MD5 is one of two message digest algorithms available in IPsec. The - other is SHA. SHA produces a longer hash and is - therefore more resistant to birthday attacks, - but this is not a concern for IPsec. The HMAC - method used in IPsec is secure even if the underlying hash is not - particularly strong against this attack.

Hans Dobbertin found a weakness in MD5, and people often ask whether - this means MD5 is unsafe for IPsec. It doesn't. The IPsec RFCs discuss - Dobbertin's attack and conclude that it does not affect MD5 as used for - HMAC in IPsec.

Meet-in-the-middle attack

A divide-and-conquer attack which breaks a cipher into two parts, - works against each separately, and compares results. Probably the best - known example is an attack on double DES. This applies in principle to - any pair of block ciphers, e.g. to an encryption system using, say, - CAST-128 and Blowfish, but we will describe it for double DES. -

Double DES encryption and decryption can be written:

        C = E(k2,E(k1,P))
-        P = D(k1,D(k2,C))

Where C is ciphertext, P is plaintext, E is encryption, D is - decryption, k1 is one key, and k2 is the other key. If we know a P, C - pair, we can try and find the keys with a brute force attack, trying - all possible k1, k2 pairs. Since each key is 56 bits, there are - 2¹¹² such pairs and this attack is painfully inefficient.

The meet-in-the middle attack re-writes the equations to calculate a - middle value M:

        M = E(k1,P)
-        M = D(k2,C)

Now we can try some large number of D(k2,C) decryptions with various - values of k2 and store the results in a table. Then start doing E(k1,P) - encryptions, checking each result to see if it is in the table.

With enough table space, this breaks double DES with - 2⁵⁶ + 2⁵⁶ = 2⁵⁷work. - Against triple DES, you need 2⁵⁶ + 2¹¹² ~= - 2¹¹².

The memory requirements for such attacks can be prohibitive, but - there is a whole body of research literature on methods of reducing - them.

Message Digest Algorithm

An algorithm which takes a message as input and produces a hash or - digest of it, a fixed-length set of bits which depend on the message - contents in some highly complex manner. Design criteria include making - it extremely difficult for anyone to counterfeit a digest or to change - a message without altering its digest. One essential property is collision resistance. The main applications are - in message authentication and digital signature schemes. Widely used algorithms - include MD5 and SHA. In IPsec, - message digests are used for HMAC authentication of - packets.

MTU

Maximum Transmission - Unit, the largest size of packet that can be sent over - a link. This is determined by the underlying network, but must be taken - account of at the IP level. -

IP packets, which can be up to 64K bytes each, must be packaged into - lower-level packets of the appropriate size for the underlying - network(s) and re-assembled on the other end. When a packet must pass - over multiple networks, each with its own MTU, and many of the MTUs are - unknown to the sender, this becomes a fairly complex problem. See path MTU discovery for details.

Often the MTU is a few hundred bytes on serial links and 1500 on - Ethernet. There are, however, serial link protocols which use a larger - MTU to avoid fragmentation at the ethernet/serial boundary, and newer - (especially gigabit) Ethernet networks sometimes support much larger - packets because these are more efficient in some applications.

N

NAI

Network Associates, a conglomerate - formed from PGP Inc., TIS (Trusted Information - Systems, a firewall vendor) and McAfee anti-virus products. Among other - things, they offer an IPsec-based VPN product.

NAT

Network Address Translation, a process by which - firewall machines may change the addresses on packets as they go - through. For discussion, see our background section.

NIST

The US National Institute of Standards - and Technology, responsible for FIPS standards - including DES and its replacement, AES.

Nonce

A random value used in an authentication protocol.

Non-routable IP address

An IP address not normally allowed in the "to" or "from" IP address - field header of IP packets. -

Almost invariably, the phrase "non-routable address" means one of - the addresses reserved by RFC 1918 for private networks:

10.anything
172.x.anything with 16 <= x <= 31
192.168.anything

These addresses are commonly used on private networks, e.g. behind a - Linux machines doing IP masquerade. Machines within - the private network can address each other with these addresses. All - packets going outside that network, however, have these addresses - replaced before they reach the Internet.

If any packets using these addresses do leak out, they do not go - far. Most routers automatically discard all such packets.

Various other addresses -- the 127.0.0.0/8 block reserved for local - use, 0.0.0.0, various broadcast and network addresses -- cannot be - routed over the Internet, but are not normally included in the meaning - when the phrase "non-routable address" is used.

NSA

The US National Security Agency, - the American organisation for signals - intelligence, the protection of US government messages and the - interception and analysis of other messages. For details, see Bamford's - "The Puzzle Palace". -

Some history - of NSA documents were declassified in response to a FOIA (Freedom - of Information Act) request.

O

Oakley

A key determination protocol, defined in RFC 2412.

Oakley groups

The groups used as the basis of Diffie-Hellman key - exchange in the Oakley protocol, and in IKE. Four - were defined in the original RFC, and a fifth has been added since. -

Linux FreeS/WAN currently supports the three groups based on finite - fields modulo a prime (Groups 1, 2 and 5) and does not support the - elliptic curve groups (3 and 4). For a description of the difference of - the types, see discrete logarithms.

One time pad

A cipher in which the key is: -

as long as the total set of messages to be enciphered
absolutely random
never re-used

Given those three conditions, it can easily be proved that the - cipher is perfectly secure, in the sense that an attacker with - intercepted message in hand has no better chance of guessing the - message than an attacker who has not intercepted the message and only - knows the message length. No such proof exists for any other cipher.

There are, however, several problems with this "perfect" cipher.

First, it is wildly impractical for most - applications. Key management is at best difficult, often completely - impossible.

Second, it is extremely fragile. Small changes - which violate the conditions listed above do not just weaken the cipher - liitle. Quite often they destroy its security completely.

Re-using the pad weakens the cipher to the point where it can be - broken with pencil and paper. With a computer, the attack is - trivially easy.
Using anything less than truly random numbers completely invalidates - the security proof.
In particular, using computer-generated pseudo-random numbers may - give an extremely weak cipher. It might also produce a good stream - cipher, if the pseudo-random generator is both well-designed and - properely seeded.

Marketing claims about the "unbreakable" security of various - products which somewhat resemble one-time pads are common. Such claims - are one of the surest signs of cryptographic snake - oil; most systems marketed with such claims are worthless.

Finally, even if the system is implemented and used correctly, it is - highly vulnerable to a substitution attack. If an - attacker knows some plaintext and has an intercepted message, he can - discover the pad.

This does not matter if the attacker is just a passive eavesdropper. It gives him no plaintext - he didn't already know and we don't care that he learns a pad which - we will never re-use.
However, an active attacker who knows the - plaintext can recover the pad, then use it to encode with whatever - he chooses. If he can get his version delivered instead of yours, - this may be a disaster. If you send "attack at dawn", the delivered - message can be anything the same length -- perhaps "retreat to - east" or "shoot generals".
An active attacker with only a reasonable guess at the plaintext - can try the same attack. If the guess is correct, this works and - the attacker's bogus message is delivered. If the guess is wrong, a - garbled message is delivered.

In general then, despite its theoretical perfection, the - one-time-pad has very limited practical application.

See also -Phil -Karn's definition. -

PFS

see Perfect Forward Secrecy

PGP

Pretty Good Privacy, a personal encryption - system for email based on public key technology, written by Phil - Zimmerman. -

The 2.xx versions of PGP used the RSA public key - algorithm and used IDEA as the symmetric cipher. - These versions are described in RFC 1991 and in Garfinkel's book. Since version 5, the products from PGP Inc. have used Diffie-Hellman - public key methods and CAST-128 symmetric - encryption. These can verify signatures from the 2.xx versions, but - cannot exchange encryted messages with them.

An IETF working group has issued RFC 2440 for an - "Open PGP" standard, similar to the 5.x versions. PGP Inc. staff were - among the authors. A free Gnu Privacy Guard based on - that standard is now available.

For more information on PGP, including how to obtain it, see our - cryptography links.

PGP Inc.

A company founded by Zimmerman, the author of PGP, - now a division of NAI. See the corporate website. Zimmerman left in - 2001, and early in 2002 NAI announced that they would no longer sell - PGP.. -

Versions 6.5 and later of the PGP product include PGPnet, an IPsec - client for Macintosh or for Windows 95/98/NT. See our interoperation document.

Photuris

Another key negotiation protocol, an alternative to IKE, described in RFCs 2522 and 2523.

PPP

Point-to-Point Protocol, originally a method of - connecting over modems or serial lines, but see also PPPoE.

PPPoE

PPP over Ethernet, a somewhat odd protocol that - makes Ethernet look like a point-to-point serial link. It is widely - used for cable or ADSL Internet services, apparently mainly because it - lets the providers use access control and address assignmment - mechanisms developed for dialup networks. Roaring Penguin provide a - widely used Linux implementation.

PPTP

Point-to-Point Tunneling Protocol, used - in some Microsoft VPN implementations. Papers discussing weaknesses in - it are on counterpane.com. It - is now largely obsolete, replaced by L2TP.

PKI

Public Key Infrastructure, the things an - organisation or community needs to set up in order to make public key cryptographic technology a standard part - of their operating procedures. -

There are several PKI products on the market. Typically they use a - hierarchy of Certification Authorities (CAs). Often - they use LDAP access to X.509 - directories to implement this.

See Web of Trust for a different sort of - infrastructure.

PKIX

PKI eXchange, an IETF standard that - allows PKIs to talk to each other. -

This is required, for example, when users of a corporate PKI need to - communicate with people at client, supplier or government - organisations, any of which may have a different PKI in place. I should - be able to talk to you securely whenever:

your organisation and mine each have a PKI in place
you and I are each set up to use those PKIs
the two PKIs speak PKIX
the configuration allows the conversation

At time of writing (March 1999), this is not yet widely implemented - but is under quite active development by several groups.

Plaintext

The unencrypted input to a cipher, as opposed to the encrypted ciphertext output.

Pluto

The Linux FreeS/WAN daemon which handles key - exchange via the IKE protocol, connection - negotiation, and other higher-level tasks. Pluto calls the KLIPS kernel code as required. For details, see the - manual page ipsec_pluto(8).

Public Key Cryptography

In public key cryptography, keys are created in matched pairs. - Encrypt with one half of a pair and only the matching other half can - decrypt it. This contrasts with symmetric or - secret key cryptography in which a single key known to both parties - is used for both encryption and decryption. -

One half of each pair, called the public key, is made public. The - other half, called the private key, is kept secret. Messages can then - be sent by anyone who knows the public key to the holder of the private - key. Encrypt with the public key and you know that only someone with - the matching private key can decrypt.

Public key techniques can be used to create digital signatures and to deal with key - management issues, perhaps the hardest part of effective deployment of - symmetric ciphers. The resulting hybrid cryptosystems use public key methods to - manage keys for symmetric ciphers.

Many organisations are currently creating PKIs, - public key infrastructures to make these benefits widely - available.

Public Key Infrastructure

see PKI

Q

R

Rainbow books

A set of US government standards for evaluation of "trusted computer - systems", of which the best known was the Orange - Book. One fairly often hears references to "C2 security" or a - product "evaluated at B1". The Rainbow books define the standards - referred to in those comments. -

See this reference - page.

The Rainbow books are now mainly obsolete, replaced by the - international Common Criteria standards.

Random

A remarkably tricky term, far too much so for me to attempt a - definition here. Quite a few cryptosystems have been broken via attacks - on weak random number generators, even when the rest of the system was - sound. -

See RFC - 1750 for the theory.

See the manual pages for ipsec_ranbits(8) and - ipsec_prng(3) for more on FreeS/WAN's use of randomness. Both depend on - the random(4) device driver..

A couple of years ago, there was extensive mailing list discussion - (archived here)of Linux - /dev/random and FreeS/WAN. Since then, the design of the random(4) - driver has changed considerably. Linux 2.4 kernels have the new - driver..

Raptor

A firewall product for Windows NT offerring IPsec-based VPN services. - Linux FreeS/WAN interoperates with Raptor; see our interop document for details. Raptor - have recently merged with Axent.

RC4

Rivest Cipher four, designed by Ron Rivest of RSA and widely used. Believed highly secure with - adequate key length, but often implemented with inadequate key length - to comply with export restrictions.

RC6

Rivest Cipher six, RSA's AES candidate cipher.

Replay attack

An attack in which the attacker records data and later replays it in - an attempt to deceive the recipient.

Reverse map

In DNS, a table where IP addresses can be used as - the key for lookups which return a system name and/or other - information.

RFC

Request For Comments, an Internet document. Some - RFCs are just informative. Others are standards. -

Our list of IPsec and other security-related - RFCs is here, along with information on - methods of obtaining them.

Rijndael

a block cipher designed by two Belgian - cryptographers, winner of the US government's AES - contest to pick a replacement for DES. See the Rijndael home - page.

RIPEMD

A message digest algorithm. The current version - is RIPEMD-160 which gives a 160-bit hash.

Root CA

The top level Certification Authority in a hierachy - of such authorities.

Routable IP address

Most IP addresses can be used as "to" and "from" addresses in packet - headers. These are the routable addresses; we expect routing to be - possible for them. If we send a packet to one of them, we expect (in - most cases; there are various complications) that it will be delivered - if the address is in use and will cause an ICMP error packet to come back to us if not. -

There are also several classes of non-routable IP addresses.

RSA algorithm

Rivest Shamir Adleman public - key algorithm, named for its three inventors. It is widely used and - likely to become moreso since it became free of patent encumbrances in - September 2000. -

RSA can be used to provide either encryption or digital - signatures. In IPsec, it is used only for signatures. These provide - gateway-to-gateway authentication for IKE negotiations.

For a full explanation of the algorithm, consult one of the standard - references such as Applied - Cryptography. A simple explanation is:

The great 17th century French mathematician Fermat - proved that,

for any prime p and number x, 0 <= x < p:

        x^p == x         modulo p
-        x^(p-1) == 1     modulo p, non-zero x
-

From this it follows that if we have a pair of primes p, q and two - numbers e, d such that:

        ed == 1          modulo lcm( p-1, q-1)
-

- where lcm() is least common multiple, then
- for all x, 0 <= x < pq: -

      x^ed == x           modulo pq
-

So we construct such as set of numbers p, q, e, d and publish the - product N=pq and e as the public key. Using c for ciphertext and i for the input plaintext, encryption is then:

        c = i^e           modulo N
-

An attacker cannot deduce i from the cyphertext c, short of either - factoring N or solving the discrete logarithm - problem for this field. If p, q are large primes (hundreds or thousands - of bits) no efficient solution to either problem is known.

The receiver, knowing the private key (N and d), can readily recover - the plaintext p since:

        c^d == (i^e)^d    modulo N
-            == i^ed       modulo N
-            == i          modulo N
-

This gives an effective public key technique, with only a couple of - problems. It uses a good deal of computer time, since calculations with - large integers are not cheap, and there is no proof it is necessarily - secure since no-one has proven either factoring or discrete log cannot - be done efficiently. Quite a few good mathematicians have tried both - problems, and no-one has announced success, but there is no proof they - are insoluble.

RSA Data Security

A company founded by the inventors of the RSA - public key algorithm.

S

SA

Security Association, the channel negotiated by the - higher levels of an IPsec implementation (IKE) and used by the lower (ESP and - AH). SAs are unidirectional; you need a pair of them - for two-way communication. -

An SA is defined by three things -- the destination, the protocol - (AH orESP) and the SPI, security parameters index. It is used as an index - to look up other things such as session keys and intialisation - vectors.

For more detail, see our section on IPsec - and/or RFC 2401.

SE Linux

Security Enhanced Linux, an NSA-funded project to add mandatory access control to Linux. See the project home page. -

According to their web pages, this work will include extending - mandatory access controls to IPsec tunnels.

Recent versions of SE Linux code use the Linux - Security Module interface.

Secure DNS

A version of the DNS or Domain Name Service - enhanced with authentication services. This is being designed by the IETF DNS security working group. - Check the Internet Software - Consortium for information on implementation progress and for the - latest version of BIND. Another site has more information. -

IPsec can use this plus Diffie-Hellman key exchange to bootstrap itself. This - allows opportunistic encryption. Any pair of - machines which can authenticate each other via DNS can communicate - securely, without either a pre-existing shared secret or a shared PKI.

Secret key cryptography

See symmetric cryptography

Security Association

see SA

Security Enhanced Linux

see SE Linux

Sequence number

A number added to a packet or message which indicates its position in - a sequence of packets or messages. This provides some security against - replay attacks. -

For automatic keying mode, the IPsec RFCs require that the sender generate sequence - numbers for each packet, but leave it optional whether the receiver - does anything with them.

SHA

SHA-1

Secure Hash Algorithm, a message digest algorithm developed by the NSA for use in the Digital Signature standard, FIPS number 186 from NIST. SHA is - an improved variant of MD4 producing a 160-bit hash. -

SHA is one of two message digest algorithms available in IPsec. The - other is MD5. Some people do not trust SHA because - it was developed by the NSA. There is, as far as we - know, no cryptographic evidence that SHA is untrustworthy, but this - does not prevent that view from being strongly held.

The NSA made one small change after the release of the original SHA. - They did not give reasons. Iit may be a defense against some attack - they found and do not wish to disclose. Technically the modified - algorithm should be called SHA-1, but since it has replaced the - original algorithm in nearly all applications, it is generally just - referred to as SHA..

SHA-256

SHA-384

SHA-512

Newer variants of SHA designed to match the strength of the 128, 192 - and 256-bit keys of AES. The work to break an - encryption algorithm's strength by brute force is - 2 -

- -^{keylength -} -

- operations but a birthday attack on a hash - needs only 2 -

- -^{- hashlength - / - 2 - -} -

- , so as a general rule you need a hash twice the size of the key to - get similar strength. SHA-256, SHA-384 and SHA-512 are designed to - match the 128, 192 and 256-bit key sizes of AES, respectively.

Signals intelligence (SIGINT)

Activities of government agencies from various nations aimed at - protecting their own communications and reading those of others. - Cryptography, cryptanalysis, wiretapping, interception and monitoring - of various sorts of signals. The players include the American NSA, British GCHQ and Canadian CSE.

SKIP

Simple Key management for Internet - Protocols, an alternative to IKE developed by - Sun and being marketed by their Internet Commerce Group.

Snake oil

Bogus cryptography. See the - Snake Oil FAQ or this - paper by Schneier.

SPI

Security Parameter Index, an index used within - IPsec to keep connections distinct. A Security Association (SA) is defined by destination, - protocol and SPI. Without the SPI, two connections to the same gateway - using the same protocol could not be distinguished. -

For more detail, see our IPsec section - and/or RFC 2401.

SSH

Secure SHell, an encrypting replacement for the - insecure Berkeley commands whose names begin with "r" for "remote": - rsh, rlogin, etc. -

For more information on SSH, including how to obtain it, see our - cryptography links.

SSH Communications Security

A company founded by the authors of SSH. Offices - are in Finland and California. They have a toolkit for - developers of IPsec applications.

SSL

Secure Sockets Layer, - a set of encryption and authentication services for web browsers, - developed by Netscape. Widely used in Internet commerce. Also known as - TLS.

SSLeay

A free implementation of SSL by Eric Young (eay) - and others. Developed in Australia; not subject to US export - controls.

static IP address

an IP adddress which is pre-set on the machine itself, as opposed to - a dynamic address which is assigned by a DHCP server or obtained as part of the process of - establishing a PPP or PPPoE - connection

Stream cipher

A symmetric cipher which produces a stream - of output which can be combined (often using XOR or bytewise addition) - with the plaintext to produce ciphertext. Contrasts with block cipher. -

IPsec does not use stream ciphers. Their main - application is link-level encryption, for example of voice, video or - data streams on a wire or a radio signal.

subnet

A group of IP addresses which are logically one network, typically - (but not always) assigned to a group of physically connected machines. - The range of addresses in a subnet is described using a subnet mask. - See next entry.

subnet mask

A method of indicating the addresses included in a subnet. Here are - two equivalent examples: -

101.101.101.0/24
101.101.101.0 with mask 255.255.255.0

The '24' is shorthand for a mask with the top 24 bits one and the - rest zero. This is exactly the same as 255.255.255.0 which has three - all-ones bytes and one all-zeros byte.

These indicate that, for this range of addresses, the top 24 bits - are to be treated as naming a network (often referred to as "the - 101.101.101.0/24 subnet") while most combinations of the low 8 bits can - be used to designate machines on that network. Two addresses are - reserved; 101.101.101.0 refers to the subnet rather than a specific - machine while 101.101.101.255 is a broadcast address. 1 to 254 are - available for machines.

It is common to find subnets arranged in a hierarchy. For example, a - large company might have a /16 subnet and allocate /24 subnets within - that to departments. An ISP might have a large subnet and allocate /26 - subnets (64 addresses, 62 usable) to business customers and /29 subnets - (8 addresses, 6 usable) to residential clients.

S/WAN

Secure Wide Area Network, a project involving RSA - Data Security and a number of other companies. The goal was to - ensure that all their IPsec implementations would - interoperate so that their customers can communicate with each other - securely.

Symmetric cryptography

Symmetric cryptography, also referred to as conventional or secret - key cryptography, relies on a shared secret key, identical for - sender and receiver. Sender encrypts with that key, receiver decrypts - with it. The idea is that an eavesdropper without the key be unable to - read the messages. There are two main types of symmetric cipher, block ciphers and stream - ciphers. -

Symmetric cryptography contrasts with public - key or asymmetric systems where the two players use different - keys.

The great difficulty in symmetric cryptography is, of course, key - management. Sender and receiver must have identical keys and - those keys must be kept secret from everyone else. Not too - much of a problem if only two people are involved and they can - conveniently meet privately or employ a trusted courier. Quite a - problem, though, in other circumstances.

It gets much worse if there are many people. An application might be - written to use only one key for communication among 100 people, for - example, but there would be serious problems. Do you actually trust all - of them that much? Do they trust each other that much? Should they? - What is at risk if that key is compromised? How are you going to - distribute that key to everyone without risking its secrecy? What do - you do when one of them leaves the company? Will you even know?

On the other hand, if you need unique keys for every possible - connection between a group of 100, then each user must have 99 keys. - You need either 99*100/2 = 4950 secure key exchanges between - users or a central authority that securely distributes 100 key - packets, each with a different set of 99 keys.

Either of these is possible, though tricky, for 100 users. Either - becomes an administrative nightmare for larger numbers. Moreover, keys - must be changed regularly, so the problem of key distribution - comes up again and again. If you use the same key for many messages - then an attacker has more text to work with in an attempt to crack that - key. Moreover, one successful crack will give him or her the text of - all those messages.

In short, the hardest part of conventional cryptography is key - management. Today the standard solution is to build a hybrid system using public key - techniques to manage keys.

T

TIS

Trusted Information Systems, a firewall vendor now part of NAI. Their Gauntlet product offers IPsec VPN services. - TIS implemented the first version of Secure DNS on - a DARPA contract.

TLS

Transport Layer Security, a newer name for SSL.

TOS field

The Type Of - Service field in an IP header, used to control - qualkity of service routing.

Traffic analysis

Deducing useful intelligence from patterns of message traffic, - without breaking codes or reading the messages. In one case during - World War II, the British guessed an attack was coming because all - German radio traffic stopped. The "radio silence" order, intended to - preserve security, actually gave the game away. -

In an industrial espionage situation, one might deduce something - interesting just by knowing that company A and company B were talking, - especially if one were able to tell which departments were involved, or - if one already knew that A was looking for acquisitions and B was - seeking funds for expansion.

In general, traffic analysis by itself is not very useful. However, - in the context of a larger intelligence effort where quite a bit is - already known, it can be very useful. When you are solving a complex - puzzle, every little bit helps.

IPsec itself does not defend against traffic - analysis, but carefully thought out systems using IPsec can provide at - least partial protection. In particular, one might want to encrypt more - traffic than was strictly necessary, route things in odd ways, or even - encrypt dummy packets, to confuse the analyst. We discuss this here.

Transport mode

An IPsec application in which the IPsec gateway is the destination of - the protected packets, a machine acts as its own gateway. Contrast with - tunnel mode.

Triple DES

see 3DES

TTL

Time To Live, used - to control DNS caching. Servers discard cached - records whose TTL expires

Tunnel mode

An IPsec application in which an IPsec gateway provides protection - for packets to and from other systems. Contrast with transport mode.

Two-key Triple DES

A variant of triple DES or 3DES in which only two - keys are used. As in the three-key version, the order of operations is - EDE or encrypt-decrypt-encrypt, but in the two-key - variant the first and third keys are the same. -

3DES with three keys has 3*56 = 168 bits of key but has only 112-bit - strength against a meet-in-the-middle attack, so it - is possible that the two key version is just as strong. Last I looked, - this was an open question in the research literature.

RFC 2451 defines triple DES for IPsec as the - three-key variant. The two-key variant should not be used and is not - implemented directly in Linux FreeS/WAN. It - cannot be used in automatically keyed mode without major fiddles in the - source code. For manually keyed connections, you could make Linux - FreeS/WAN talk to a two-key implementation by setting two keys the same - in /etc/ipsec.conf.

U

V

Virtual Interface

A Linux feature which allows one physical - network interface to have two or more IP addresses. See the Linux - Network Administrator's Guide in book form or on the web for - details.

Virtual Private Network

see VPN

VPN

Virtual Private Network, a network which can - safely be used as if it were private, even though some of its - communication uses insecure connections. All traffic on those - connections is encrypted. -

IPsec is not the only technique available for - building VPNs, but it is the only method defined by RFCs and supported by many vendors. VPNs are by no - means the only thing you can do with IPsec, but they may be the most - important application for many users.

VPNC

Virtual Private Network Consortium, - an association of vendors of VPN products.

W

Wassenaar Arrangement

An international agreement restricting export of munitions and other - tools of war. Unfortunately, cryptographic software is also restricted - under the current version of the agreement. Discussion.

Web of Trust

PGP's method of certifying keys. Any user can sign - a key; you decide which signatures or combinations of signatures to - accept as certification. This contrasts with the hierarchy of CAs (Certification Authorities) used in many PKIs (Public Key Infrastructures). -

See Global Trust Register for an interesting - addition to the web of trust.

WEP (Wired Equivalent Privacy)

The cryptographic part of the IEEE standard for - wireless LANs. As the name suggests, this is designed to be only as - secure as a normal wired ethernet. Anyone with a network conection can - tap it. Its advocates would claim this is good design, refusing to - build in complex features beyond the actual requirements. -

Critics refer to WEP as "Wiretap Equivalent Privacy", and - consider it a horribly flawed design based on bogus "requirements". You - do not control radio waves as you might control your wires, so the - metaphor in the rationale is utterly inapplicable. A security policy - that chooses not to invest resources in protecting against certain - attacks which can only be conducted by people physically plugged into - your LAN may or may not be reasonable. The same policy is completely - unreasonable when someone can "plug in" from a laptop half a block - away..

There has been considerable analysis indicating that WEP is - seriously flawed. A FAQ on attacks against WEP is available. Part of it - reads:

- -

- ... attacks are practical to mount using only inexpensive - off-the-shelf equipment. We recommend that anyone using an 802.11 - wireless network not rely on WEP for security, and employ other - security measures to protect their wireless network. Note that our - attacks apply to both 40-bit and the so-called 128-bit versions of - WEP equally well.

WEP appears to be yet another instance of governments, and - unfortunately some vendors and standards bodies, deliberately promoting - hopelessly flawed "security" products, apparently mainly for the - benefit of eavesdropping agencies. See this discussion.

X

X.509

A standard from the ITU (International - Telecommunication Union), for hierarchical directories with - authentication services, used in many PKI - implementations. -

Use of X.509 services, via the LDAP protocol, - for certification of keys is allowed but not required by the IPsec RFCs. It is not yet implemented in Linux FreeS/WAN.

Xedia

A vendor of router and Internet access products, now part of Lucent. - Their QVPN products interoperate with Linux FreeS/WAN; see our interop document.

Y

Z

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