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.
- 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 2
112. 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: 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: 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:
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:
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 A
dministration 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 C
onfiguration 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 ==  1Exponentiation 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 R
egulations, 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. 
 - See also GNU, GNU General Public
 License, and the FSF site. 
- 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 M
essage 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 R
egulations, 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 2
112 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 U
nit, 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 the one
 time pad FAQ. 
- Opportunistic encryption (OE)
- A situation in which any two IPsec-aware machines can secure their
 communications, without a pre-shared secret and without a common
 PKI or previous exchange of public keys. This is one of the goals
 of the Linux FreeS/WAN project, discussed in our
 introduction section.
Setting up for opportunistic encryption is described in our
 quickstart document. 
- Opportunistic responder
- A host which accepts, but does not initiate, requests for
 opportunistic encryption (OE). An opportunistic responder has
 enabled OE in its passive form (pOE) only. A
 web server or file server may be usefully set up as an opportunistic
 responder.
Configuring passive OE is described in our
 policy groups document. 
- Orange book
- the most basic and best known of the US government's
 Rainbow Book series of computer security standards.
- P
- P1363 standard
- An IEEE standard for public key cryptography.
 Web page.
- pOE
- See Passive opportunistic encryption.
- Passive attack
- An attack in which the attacker only eavesdrops and attempts to
 analyse intercepted messages, as opposed to an active
 attack in which he diverts messages or generates his own.
- Passive opportunistic encryption (pOE)
- A form of opportunistic encryption (OE) in
 which the host will accept opportunistic connection requests, but will
 not initiate such requests. A host which runs OE in its passive form
 only is known as an opportunistic responder.
Configuring passive OE is described in our
 policy groups document. 
- Path MTU discovery
- The process of discovering the largest packet size which all links
 on a path can handle without fragmentation -- that is, without any
 router having to break the packet up into smaller pieces to match the
 MTU of its outgoing link.
This is done as follows: 
- originator sends the largest packets allowed by MTU
 of the first link, setting the DF (don't f
ragment) bit in the packet header
- any router which cannot send the packet on (outgoing MTU is too
 small for it, and DF prevents fragmenting it to match) sends back an
 ICMP packet reporting the problem
- originator looks at ICMP message and tries a smaller size
- eventually, you settle on a size that can pass all routers
- thereafter, originator just sends that size and no-one has to
 fragment
 Since this requires co-operation of many systems, and since the next
 packet may travel a different path, this is one of the trickier areas
 of IP programming. Bugs that have shown up over the years have
 included: 
- malformed ICMP messages
- hosts that ignore or mishandle these ICMP messages
- firewalls blocking the ICMP messages so host does not see them
 Since IPsec adds a header, it increases packet size and may require
 fragmentation even where incoming and outgoing MTU are equal. 
- Perfect forward secrecy (PFS)
- A property of systems such as Diffie-Hellman key
 exchange which use a long-term key (such as the shared secret in IKE)
 and generate short-term keys as required. If an attacker who acquires
 the long-term key provably can
- neither read previous messages which he may have archived
- nor read future messages without performing additional
 successful attacks
 then the system has PFS. The attacker needs the short-term keys in
 order to read the trafiic and merely having the long-term key does not
 allow him to infer those. Of course, it may allow him to conduct
 another attack (such as man-in-the-middle) which
 gives him some short-term keys, but he does not automatically get them
 just by acquiring the long-term key. 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(null)>(null)>(null)> operations but a
 birthday attack on a hash needs only 2
 hashlength(null)>
 /(null)>
 2(null)>(null)>(null)>(null)> , 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 P
rotocols, 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 S
ervice 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