28

Let E denote the function which takes a block of 32 bits as input and yields a block of 48 bits as output. The 48 bits of output, written as 8 blocks of 6 bits each, are obtained by selecting the bits from the input according to the above table. Thus the first 3 bits of E(R) are the bits in positions 32, 1, and 2 of R while the last 2 bits of E(R) are the bits in positions 32 and 1.

Table 5.2 Permuted choice 1

57	49	41	33	25	17	9

1	58	50	42	34	26	18

10	2	59	51	43	35	27

19	11	3	60	52	44	36

63 55	47	39	31	23	15

7	62	54	46	38	30	22

14	6	61	53	45	37	29

21	13	5	28	20	12	4

The table has been divided into two parts. with the first part determining how the bits of CO are chosen. The bits of KEY are numbered 1 through 64. The bits of CO are respectively bits 57, 49, 41 44 and 36 of KEY, with the bits of DO being bita 63,55,47,... 12 and 4 of KEY.

Table 5.3 Pennuted choice 2

14 17 11 24 1 5

3 28 15 6 21 10

23 19 12 4 26 8

16 7 27 20 13 2

41 52 31 37 47 55

30 40 51 45 33 48

44 49 39 56 34 53

46 42 50 36 29 32

The first bit of Kn is the 14th bit of CnDn, the "econd bit the 17th, and so on with the 47th bit the 29th, and the 48th bit the 32nd.

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Table 5.4 Left Shift Table

Iteration	Number of

Number	left Shifts

1	1

2	1

3	2

4	2

5	2

6	2

7	2

8	2

9	1

10	2

11	2

12	2

13	2

14	2

15	2

16	1

Succ-ive C and D valuee are formed according to the above table. For eumple, C3 and D3 are obtained from C, and D respectively, by two left shifts, and C16 and D16 are obtained from C15 and D15, respectively, by one left shift. In all cases, by a single left shift is meant a rotation of the bits one place to the left, so that after one left shift the bits in the 28 positions are the bits that were previously in positions 2, 3, 28, 1.

6.KEY MANAGEMENT

Management of the cryptographic keys used to protect data is of utmost importance to the security of the data. This chapter will provide guidance on how to generate, distribute, and protect keys.

There are at least three types of keys: data-encrypting keys, key, generating keys, and key encrypting keys. When keys are stored in an encrypted form, the security of the keys is equivalent to the security of the key which was used for the encryption. Keys should be encrypted when stored in a less than fully secure medium and when transmitted over unprotected channels, in any cryptographic key system there has to be at least one unencrypted key. This key is often called a master key. The master key is the sole protector of all the information protected by each of the keys encrypted under the master. Thus, a master key is more valuable than any of the data encrypting keys which it protects.

6.1	Key Generation and Protection

A DES cryptographic key consists of 64 bits, 56 of which are used by the algorithm (forming the active key) and 8 of which are used to detect errors within the key. If the 64 bits are numbered from left to right (1, 2, ..., 64), bits (8, 16, 24, ..., 64) are used for parity checking of each 8-bit byte. The parity bits should be set to the complement of the modulo 2 sum of the previous seven bits. Thus the modulo 2 sum of the entire eight bits is always 1.

Certain fundamental guidelines should be followed in generating keys. Every bit of the active key should be generated or selected at random. Every possible combination of bits in the active key should have equal probability of being selected, and each key should be generated independently of every other key. The security provided by each of the possible 256 keys is the same although, in certain situations, the dual keys noted earlier may be undesirable because of the characteristic of the algorithm which makes the encrypt and decrypt functions identical for these keys. Repeating a short key to make a 56-bit key severely decreases security. A key made by repeating four hexadecimal

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characters, such as 29FB, four times to produce a 16-character DES key (including odd parity) provides only 214,266 = 10-12.64 of the security of a fully independent key. A 56-bit key made from 8 decimal digits, each coded in 7 bits, reduces the security to 108/256 = 10-8/256 = 10-8.85 of its maximum level.

The useful lifetime of a key will depend on the requirements and the environment of the application. A new key should be generated and used when any event occurs that may have compromised the existing key. A new key should also be generated and used periodically in the event that an undetected compromise has occurred. A system with low requirements for security and high costs of key change may change the key monthly. A system with medium security requirements may change the key weekly. High security requirements may dictate the need for changing the key daily or even more often. The method and cost of key distribution must be considered whenever a key management system is designed. Manual techniques and automated techniques are discussed in 6.2.

Unencrypted keys must always be physically protected to prevent unauthorized individuals from gaining knowledge of their values. Encrypted keys may also require physical protection if an unauthorized individual could in some way use an encrypted key to spoof system users. Physical protection of keys is often considered the weak link in the security of a cryptographic system. It is possible to design cryptographic algorithms to meet any specified level of security. This level may be measured in dollars or years of computer time required to recover plaintext. But it is difficult to quantify the effort required to subvert physical security. In many cases, a guard or courier could be bought for much less money than the amount required to mount a cryptographic attack.

6.2	Key Distribution

Key distribution is perhaps the most critical operation in a complex cryptographic system. Generating a "good key" for the DES is a relatively simple task. However, distributing this key to all the authorized users or devices may require the greatest amount of planning in the design and operation of a secure communication system. Since key distribution techniques depend on the particular application, this subsection will treat two basic applications of data encryption separately.

6.2.1 Communication Security

A requirement for communication security based on encryption is to have the decryption key available wherever decryption is authorized. Each authorized user of the key must be authenticated before the key is distributed and the transmitter of the key should be authenticated by the receiver before the previous key is discarded.

Manual methods of key distribution are commonly used where the security requirements allow long lifetimes for keys or where there are only a few devices using the same key. Duplicated key lists are often distributed by certified mail or by courier. These lists usually contain a set of keys to be sequentially used and specifications as to when to use them. In case of a possible compromise of a key, alternative keys are presented. In case of a possible compromise of a list, alternative lists are distributed with instructions for use. Machine readable storage media, such as punched cards, paper or magnetic tape, or magnetic striped cards may be used. Special key loading devices such as electronic memory chips, electronic modules, battery operated key loaders, etc. may also be used. Keys are generally inserted at the designated time into an encryption device by a security officer who physically unlocks the device and manually or electronically enters the key. Cryptographic devices generally have some form of physical protection against theft or tampering.

Automated methods of distributing a key may also be used. In general, a key to be used for a terminal-computer connection or transaction may be generated, distributed to the communicating devices via a secure path, and then destroyed at the conclusion of the connection or transaction. The secure path may be a dedicated path for distributing keys or it may be a path that is established within the network that is protected by a key used only for that purpose. The latter is considered more feasible in a general computer network. The key-encrypting key should be manually distributed or distributed outside the normal communication network.

Specific methods to be used in key distribution must be based on the characteristics of the network being protected. The value of the data being transmitted and the anticipated threats to the data are important factors. It must be emphasized that the protection provided through the use of the DES is no better than the protection provided to the key. It should be assumed that if a particular

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key is lost or compromised that any data protected by that key is also compromised. Provisions should always be made so that the key can be changed in an orderly and timely manner if its compromise is suspected.

6.2.2 File Security

Encryption protection may be provided for data to be stored in files. This protection is called file security. Data files may have many different structures and they may be stored on various storage media. It is very important that the use of encryption be evaluated with respect to the anticipated threats to the data. Only certain types of threats can be prevented or deterred in general. Theft of storage media will not permit the thief to read the plaintext corresponding to the encrypted data. However, unless a backup exists, the data will also be unavailable to the owner. Physically secured computers or computers with secure operating systems are required to protect the plaintext data while it is resident in the computer itself. Encryption does not solve the computer security problem, but it may reduce its magnitude and provide increased flexibility.

The distribution problem for encryption keys in file security applications is different than that in communication security applications. The former requires that only one copy of the key be available when the data is encrypted before it is stored. However, the key used to protect the data must be associated with the data and securely stored until the data is to be used. File security in this application simply reduces the amount of data requiring physically secure storage to the key itself. The key can only be discarded when the data is reencrypted under a new key, when the data is decrypted and no longer requires cryptographic protection, or when the data is no longer needed.

Encryption may also be used in another file security application which is analogous to a secure data vault. The computer facility is used to store data that was encrypted at a terminal and which can only be decrypted at a terminal. The encrypted data cannot be processed within the computer, but encrypted data may be stored and retrieved by location, by surrounding unencrypted data, or by a related unencrypted index. Users may encrypt selected fields of sensitive data at the terminal before it is sent to the computer. The user must store or remember the key used. When the data is to be retrieved and used at the terminal, it is decrypted just before it is printed. The disadvantage of this application is that the computational capability of the computer cannot be fully used because its sensitive data is always encrypted.

6.3	Key Destruction

When keys are no longer needed for encryption or decryption,,they should be destroyed. Even after a key is destroyed the information which it protects often continues to be sensitive. One should always assume that the cipher has been exposed to unauthorized, untrusted, individuals. It is therefore necessary that the remains of the destroyed key contain no information which would aid an adversary in the reconstruction of the key.

7.	TRANSPARENCY IN COMMUNICATIONS PROTOCOLS

A protocol is a procedural standard or a discipline for maintaining order. It is an agreement to follow an established set of rules. A communications protocol is a set of rules for a group of cooperating users which will allow them to communicate effectively. Transparency is an attribute of a communication protocol that describes the flexibility of the protocol for allowing changes which do not affect the rest of the protocol. This chapter presents some of the issues of adding encryption to a communication system.

A computer network can be described in terms of communications protocols, configurations, code sets and operational procedures. A protocol specifies the control procedures of the network (e.g., connection establishment, flow control, error control). The configuration specifies the topology and participating equipment of the network. The code set specifies the bit patterns of the user data and the control information. Finally, the operational procedures specify the administrative aspects of the network: when the network is available, how people will get access to the data and services of the

network, how connections between communicating devices are to be established, etc.

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The fundamental use of encryption in communications has historically been to hide the meaning of messages from the enemy. However, encryption can provide additional benefits. In some communication systems it is desirable to hide the fact that a message is sent at a particular time. This is called traffic flow security. It may also be desirable to assure that a message is received unaltered. A more recent requirement of some communication systems is for the receiver of a message to be able to "prove" to a third party that he did, in fact, receive the message from the transmitter. The protocols of a communication system will depend greatly on the security requirements as well as the physical properties of the system.

7.1	Transparent Use of Encryption

A goal of adding cryptographic protection into an existing data network is to make its use transparent to the other functions of the network. How well this goal is met will depend on the characteristics of the network and at what point in the development of the network cryptographic protection is incorporated. Cryptography should be incorporated into the design phase as soon as possible. The security objective of performing encryption at the place of origination of a message and not performing decryption until the message reaches its ultimate destination often makes complete transparency more difficult to achieve.

Cryptographic devices may generally be placed at the ends of a simple communication link with little difficulty. Transparent operation of the devices can be achieved by encrypting everything that leaves one end and decrypting it upon arrival at the other end. Since there are not any devices in the path between the cryptographic devices that are sensitive to the information being transmitted, control information need not be separated from data, Both synchronous and asynchronous

transmissions may be protected in this way. The only requirement for transparency is that the data entering the encryption device must exit the decryption device at the other end of the communication line with an acceptable delay. All links of a network may use the same key, or different keys may be used for each link. As a rule, network users will not know that the data is encrypted from the operational response of the network.

More complex communication systems make use of network control devices to route data to the intended receiver. Control information for such systems must be in plaintext wherever it is used by a network control device. The control device must be able to differentiate between plaintext and ciphertext if both are contained in the data stream. The differentiation can be implicit or explicit. An example of the former is the separation of data from control by position, and an example of the latter is to reserve special codes for control. It is generally easier to add encryption onto a communication network which implicitly separates data and control information. In either case separation of control information from data is necessary before encrypting in all but the simplest link encryption application. Separating control information from data in order to achieve cryptographic transparency in end-to-end encryption applications is necessary and is often difficult if encryption is performed after the communication control information is added.

Data transparency requires that encrypted data which have the same codes as control characters not be interpreted as legitimate control. In Binary Synchronous Communications (BISYNC) transparent mode, valid control characters are medicated by a two-character sequence consisting of "DLE" followed by the control character. If the cipher results in a "DLE" character, a "DLE DLE" is sent for the single "DLE" and the extra "DLE" is removed before decryption.

Section 8 describes an alternate method of achieving transparency in which control characters are never generated in the encryption process. This method requires special operations for mapping data characters only onto data characters.

Cipher text transparency is generally easy to achieve in packet or message communication protocols because the data is implicitly separated from control information. Control information is typically added after the user data is encrypted. Traffic flow security is generally not provided in such networks but link encryption of data and control may be utilized in those networks where the amount of message traffic is considered sensitive. This requires that the encryption device continues to produce cipher which is transmitted even though no messages are being sent.

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7.2	Nontransparent Use of Encryption

When code transparency is not required in a communication system, adding cryptography to the system is generally easier. The designer need not be concerned about the output of the encryption process since no device in the system that is sensitive to the code exists in the path between the encryption and decryption devices. However, if the encryption device in some way controls the decryption device, then the control must be provided by 1) control signals sent outside the data path; 2) special control codes that are detected by the decrypting device in the decrypted plaintext; or 3) special control codes in the ciphertext.

In any communication application of encryption other than the simplest implementation of link encryption, a certain degree of nontransparency will be unavoidable. Procedures must be established for entering the key at the proper time, errors must be handled in some way, and encrypted data must be recognized.

7.3	Communication Standards Based on the DES

Standards are necessary to assure that terminals and computers which use the DES are able to communicate. It is assumed that for any two devices to communicate in an encrypted mode, they must first be able to communicate in an unencrypted mode. This requirement establishes many of the parameters of communications protocols such as the code, the synchronization mode, the message protocol, the line speed, the channel capacity, the error control and the connection control. The use of DES in communications requires the specification of the following additional parameters:

a. Mode of encryption/decryption

b. Initialization

c. Synchronization

d. Error control

e. Buffering

f. Key management

Federal and American National Standards Institute (ANSI) standards efforts have been initiated to define appropriate specifications for these parameters in several communications protocols. The Federal standards are being drafted in a subcommittee of the Federal Telecommunications Standards Committee. One of the standards being prepared is expected to be issued as Federal Standard 1026. It specifies interoperability and security related requirements for communication security devices implementing the DES. Federal Standard 1027 will be a companion document which specifies the minimum physical and electrical security features of devices implementing the DES. ANSI cryptographic standards efforts at the time of this publication include:

1 .	ANSI X9A3:	Security Standards of Consumer Initiated Electronic Financial Transactions;

2 .	ANSI X9E8:	Financial Message Authentication Standard;

3 .	ANSI X3T1:	Standards for Use of the Data Encryption Algorithm.

In addition, the International Organization for Standardization has established a working group to address data encryption.

8.	USING DES TO MAP A CHARACTER SET ONTO ITSELF

In certain applications it is desirable that only valid plaintext characters appear as cipher. For example, special control characters are often used to designate headers, synchronization bits, and the beginning or ending of ciphertext. If control characters can also appear randomly as cipher, then it is difficult to distinguish between intended plaintext control characters and cipher. One solution is to stuff redundant characters into the transmitted data (to indicate control characters) thereby adding additional overhead. Also, in situations where cipher characters are to be printed, no unprintable characters can be permitted in cipher. A character is defined to be valid if it is not used as a control character and invalid if it may be used for control. For example, a character which indicates a carriage return is invalid. A problem arises since presently defined DES modes of

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operation map K-bit characters onto K-bit characters. If the number of members in a valid plaintext character set is not a power of two, then invalid characters will appear in cipher. A modification is proposed which permits the encryption of a character set of arbitrary size onto itself. Therefore, valid characters are always encrypted to valid characters. The modification is discussed as it applies to specific examples as well as to the general problem.

8.1	Example I (Digits)

In this subsection we will consider a solution for the problem of enciphering digits onto digits. Later subsections will apply the solution to other cases.

Consider DES as used in the Cipher Feedback (CFB) mode. (See fig. 5.6.) K bits of the 64-bit DES output are exclusive OR'ed with a K-bit plaintext character to form cipher. Suppose that one desires to encipher the digits, 0 through 9. Four-bit characters are required to represent the 10 digits, the first 10 character representations correspond to the digits, and the remaining 6 are invalid. (See table 8.1.) Even if only valid plaintext characters are enciphered, DES in the CFB mode will produce cipher characters which may be invalid.

8.1.1	Solution

Let FO denote the 64 bits of the final DES output. Instead of exclusive OR'ing the first four bits of FO with the four-bit plaintext character, add the two values modulo 10 (base 10). The modulo 10 sum of the digits A and B is the remainder of A + B divided by 10. X is congruent to Y modulo 10

(X = Y modulo 10) if and only if X - Y = 10m for some integer m. Thus A + B is congruent to a valid cipher character. For example, suppose that FO = 1101 and that one wished to encipher 0011. Since 0011 + 1101 = 10000 - 0110 modulo 10, 0110 is the resultant cipher. The input register to the DES function will contain exactly 16 valid cipher characters, so 1016 distinct input

register combinations are possible.

8.1.2	Decryption

The decryption algorithm is similar to the encryption algorithm except that the first FO character is subtracted modulo 10 from cipher to form plaintext. Using the values from the previous example, 0110 - 1101 = -0111 - 0011 modulo 10. The algorithms are inverses of each other because the FO generated by the decrypting device is the same as the FO generated by the encrypting device.

Let P be a valid plaintext character and G be the first character of FO. Let C be the

corresponding cipher character.

C	-	(P+G)modulo 10.

C	=	P+G-10m.

P	=	C-G+10m.

P	-	(C - G) modulo 10.

Decryption is unique even though the first character of FO may not be an integer modulo 10 (i.e., a

base 10 digit).

Since G is not necessarily a valid character, there is a bias on cipher which depends on the plaintext. If the plaintext is flat (randomly generated), for example, the cipher is also flat, but if several plaintext zeros are encrypted there is a bias towards zero through five in the cipher. This problem can be overcome by selecting G from FO in a manner which virtually assures that G s evenly distributed over the digits. Subsection 8.5 provides a solution which will render an insignificant bias in most applications.

One might consider encrypting the digits as follows: Exclusive OR(XOR) the first FO character with the plaintext character and then use the result modulo 10. The trouble with this solution is that it does not decrypt correctly. Suppose that FO = 0101 and that plaintext is 1000. 0101 XOR 1000 = 1101 - 0011 modulo 10. Therefore 0011 would be taken as cipher. But 0011 XOR 0101 = 0110 * 1000 modulo 10. Decryption would not produce the correct plaintext.

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8.2	Example II (Alphanumerics)

The USA Standard Code for Information Interchange (ASCII), with b7 as the high-order bit and b1 as the low-order bit, appears in table 8.2. Suppose one desires to encipher the 96 characters whose binary representations range from 0100000 to 1111111. These 96 characters may be mapped into the integers modulo 96 by subtracting 0100000 from their ASCII representations. Let symbolize this mapping. Then

SP = 0100000	0000000	= 0,

= 0100001	0000001	= 1,

DEL = 1111111	1011111 = 95.

If we wish to encipher the character, n, and the first character of FO is ~, then cipher is formed using the following equations.

n = 1101110	1001110 .

=	1111101.

cipher (1001110 + 1111101) modulo 96 - 0001011. cipher = (0001011 + 0100000) = 0101011 =

One must remember to translate the plaintext to an integer modulo 96 before addition and then to translate the result back to a valid character after addition. Nine characters may be held in the 64-bit input register. The number of possible input register settings is, 96~ = 6.92 X 10". Note that since the length of a character (7 bits) does not evenly divide the length of the input register (64 bits) the first bit of the input register is always fixed to zero.

8.3	Example III (General Solution)

The proposed method may be used as a general solution. Suppose one has an N character alphabet. Let K be such that 2K-1 < N < 2K. Then one must be satisfied that N[64/K] (the number of possible input register combinations) is sufficiently large where [X] is the greatest integer < X. For security reasons, it is recommended that N[64/K] be at least 248 = 10 14.4 .

If the characters are contiguous, then a simple translation will map them onto the integers modulo N before addition is performed; and after addition, the inverse will map back to valid characters (as previously discuss- in 8.2). If the characters are not contiguous, then conversion tables can be used to make the transformations to and from the integers modulo N. Consider the USA Standard Code (ASCII) presented in table 8.2. Suppose that the only valid characters are: A, B, C, F, H, I, M, N, O, P, U, V, and Z. In this case N=13 and K=4. The number of possible inputs at each encryption is, 1316 = 6.65 X 1017.

If the set of possible characters is not too large, then for each possible character the conversion table will list its modulo N value, if it is valid, or an invalid indicator, if it is invalid. This table could be used to determine whether or not a character is valid as well as to map it to its corresponding modulo N value. (See table 8.3.)

If the character set is too large other possibilities exist. A conversion table could be made which just covers the range from the first to the last valid character. In this case characters which are found to be less than 1000001 and greater than 1011010 are invalid. For the others, subtract 1000000 and use the result as an index to the table. (See table 8.4.)

Another possibility is to store the binary representations and modulo N values for only valid characters. Searching, hashing, or some other method must be used to find the correct location of the character being looked up. (See table 8.5.)

Once the modulo N sum of the plaintext and K bits of FO have been found, another table the inverse of table 8.5) is required to convert back to the binary representation. This table need only have one entry for each integer modulo N. The integer modulo N is incremented and the result is used as an index to find the corresponding cipher character.

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8.4	Solution for Plaintext Bias

When the ciphertext bias produced by the use of invalid characters from FO is unacceptable, only valid characters should be selected from FO. Consider the example where the digits are to be mapped onto themselves. The first four bits of FO will be valid with probability 10/16. If the first four bits form a valid character they may be used for the addition to plaintext. If they are invalid consider the second four bits. If the second four bits form a valid character they may be added to the plain text to form cipher. Repeat this procedure until either a valid cipher is formed or until all 16 four-bit characters of FO have been examined and each one is found to be invalid. The latter event, called the default condition, will occur with probability (6/16)16 = .000000153. In this case the value to be added to plaintext can be arbitrarily selected as 1001 (9). A new Fo is generated for each character to be enciphered.

If the bits of FO are statistically random then, as Long as the default condition is not employed, the cipher should also be random. The default condition is definiteLy nonrandom, but since it should only occur with probability .000000153 the ciphertext will be near random. In fact, frequency counts would have to be done on very large amounts of data before the slight bias would be detectable. Using a Chi-square test would require data from more than 1013 encryptions before one could expect to detect nonrandomness. Of course, if the plaintext is flat random, no bias will be found on cipher.

In general if one has a character set of N members and K is such that K-1 < N < 2K, then one must be satisfied that ((2K-N)/2K)[64/K] where [X] is the greatest integer <X, is sufficiently small.

Table 8.1 Digit to Character Conversion Table

0	0000

1	0001

2	0010

valid	3	0011	valid

digits	4	0100	characters

5	0101

6	0110

7	0111

8	1000

9	1001

10	1010

11	1011

invalid	12	1100	invalid

digits	13	1101	characters

14	1110

15	1111

37

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Table 8.4 Valid Character Range

Entry	Binary	McduIo N Value

1	1000001(A)	0 (valid character

0 modulo 13)

2	1000010 (B)	1

3	1000011 (C)	2

4	1000100 (D)	17

25	1011001 (Y)	17

26	1011010 (Z)	12

Table 8.5 Valid Characters Only

Entry	Binary	Modulo N Value

1	1000001 (A)	0

2	1000010 (B)	1

3	1000011 (C)	2

4	1000110 (F)	3

5	1001000 (H)	4

13	1011010 (Z)	12

9. REFERENCES

[1] Data Encryption Algorithm (DEA), American National Standards

Institute ANSI X3.92.

[2] Data Encryption Standard, National Bureau of Standards

(U.S.), Federal Information Processing Standards Publication (FIPS PUB) 46, National Technical Information Service, Springfield, VA (1977).

[3] DES Modes of Operation, National Bureau of Standards (U.S.),

Federal Information Processing Standards Publication (FIPS PUB) 81, National Technical Information Service, Springfield, VA (1980).

[4] Feistel, Horst, Cryptography and Computer Privacy, Scientific

American, Vol. 228 No. 5, May 1973, pages 15-23.

[5] Gait, Jason, Validating the Correctness of Hardware Implementations

of the NBS Data Encryption Standard, NBS Special Publication 500-20, Revised September 1980.

[6] Guideline For Automatic Data Processing Risk Analysis,

National Bureau of Standards (U.S.), Federal Information Processing Standards Publication (FIPS PUB) 65, National Technical Information Service, Springfield, VA (1979).

[7] Management and Use of Personal Identification Numbers, ABA

Bank Card Standard, Aids from ABA catalog number 207213 (1979).

[8] National Security Agency Memorandum for the Members, Former

United States Communications Security Board, Serial: N/0817 (7 July 1978).

[9] USA Standard X3.4-1968, Standard Code for Information Interchange,

Federal Information Processing Standards Publication (FIPS PUB) 1, United States of America Standards Institute, 10 East 40th Street, New York, New York 10016 (November 1968).

39

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Technical Notes-Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standard Developed under procedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS administers this program as J supplement to the activities of the private sector standardizing organizations.

Consumer Information Series-Practical information, based on NBS research and experience, covering areas of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order the above NBS publications from: Superintendent of Documents, Government Printing Office, Washington. DC 20402.

Order the following NBS publications- FIPS and NBSIR's-from the National Technical Information Services, Springfield VA 22161

Federal Information Processing Standard Publications (FIPS) PUB-Publications in this series collectively constitute the federal Information Processing Standards Register. The Register serves as the official source of information in the Federal Government regarding standards issued by NBS pursuant to the federal Property and Administrative Services Act of 1949 as amended. Public Law 59-306 (79 Stat. 1127), and as implemented by Executive Order 11717(35 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Report (NBSIR)-A special series of interim or final reports on work performed by NBS for outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Services. Springfield, VA 22161 in paper copy or microfiche form.