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INTERNET-DRAFT Randomness Requirements for Security 25 March 1993 Expires 24 September 1993 Randomness Requirements for Security ---------- ------------ --- -------- Donald E. Eastlake 3rd, Stephen D. Crocker, & Jeffrey I. Schiller Status of This Document This draft is intended to be submitted to the RFC editor as an Informational RFC. Distribution of this document is unlimited. This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' Please check the 1id- abstracts.txt listing contained in the internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the current status of any Internet Draft. This draft expires 24 September 1993. Abstract At the heart of many security systems is the assumption that it is possible to generate secret quantities that are very hard for an adversary to guess. These include passwords, cryptographic keys, and similar quantities. Choosing such quantities so as to foil a resourceful and motivated adversary is surprisingly difficult. This paper points out many pitfalls in using traditional pseudo-random number generation techniques for choosing such secrets, recommends the use of truly random hardware techniques, provides suggestions to ameliorate the problem when a hardware solution is not available, and gives examples of how large such quantities need to be for some particular applications. Acknowledgements Substantive comments on this draft, or parts thereof, were received from Charlie Kaufman, Dave Balenson, Tim Redmond, and Whitfield Diffie. D. Eastlake, S. Crocker, & J. Schiller [Page 1] INTERNET-DRAFT Randomness Requirements for Security Table of Contents Status of This Document....................................1 Abstract...................................................1 Acknowledgements...........................................1 Table of Contents..........................................2 1. Introduction............................................3 2. Requirements............................................3 3. Traditional Pseudo-Random Sequences.....................5 4. Unpredictability........................................6 4.1 Problems with Clocks and Serial Numbers................6 4.2 Timing External Events.................................7 4.3 The Fallacy of Complex Manipulation....................7 4.4 The Fallacy of Selection from a Large Database.........7 5. Hardware for Randomness.................................8 5.1 Volume Required........................................8 5.2 Sensitivity to Skew....................................9 5.2.1 Using Stream Parity to De-Skew.......................9 5.2.2 Using Transition Mappings to De-Skew................10 6. Recommended Non-Hardware Strategy......................11 6.1 Mixing Functions......................................11 6.1.1 A Trivial Mixing Function...........................12 6.1.2 Stronger Mixing Functions...........................13 6.1.3 Using a Mixing Function to Stretch Random Bits......14 6.1.4 Other Factors in Choosing a Mixing Function.........14 6.2 Non-Hardware Sources of Randomness....................15 6.3 Cryptographically Strong Sequences....................15 7. US DoD Recommendations for Password Generation.........16 8. Examples of Randomness Required........................16 8.1 A Low Security Password...............................16 8.2 A Very High Security Cryptographic Key................17 9. Security Considerations................................19 References................................................20 Authors Addresses.........................................21 Expiration................................................21 D. Eastlake, S. Crocker, & J. Schiller [Page 2] INTERNET-DRAFT Randomness Requirements for Security 1. Introduction Software cryptography is coming into wider use. Systems like Kerberos, PEM, PGP, etc. are maturing and becoming a part of the network landscape. These systems provide substantial protection against snooping and spoofing. However, there is a potential flaw. At the heart of all cryptographic systems is the generation of random numbers. For the present, the lack of generally available facilities for generating unpredictable numbers is an open wound in the design of cryptographic software. For the software developer who wants to build a key or password generation procedure that runs on a wide range of hardware, the only safe strategy is to force the local installation to supply a suitable routine to generate unpredictable numbers. To say the least, this is an awkward, error-prone and unpalatable solution. It is important to keep in mind that the requirement is for data that an adversary has a very low probability of guessing. This will fail if pseudo-random data, which only meets traditional statistical tests for randomness or which is based on guessable range sources, such as clocks, is used. Frequently such random quantities are guessable by an adversary searching through an embarrassingly small space of possibilities. This informational document suggests techniques for producing random quantities that will be resistant to such attack. It recommends that future systems include hardware random number generation, suggests methods for use if such hardware is not available, and gives some estimates of the number of random bits required for some sample applications. 2. Requirements Probably the most commonly encountered randomness requirement is the typical user password character string. Obviously, if a password can be guessed, it does not provide security. (For this particular application it is desirable that users be able to remember the password. This may make it advisable to use pronounceable character strings or phrases composed on ordinary words. But this only affects the format of the password information, not the requirement that the password be hard to guess.) Many other requirements come from the cryptographic arena. Cryptographic techniques can be used to provide a variety of services including confidentiality and authentication. Such services are based on quantities, traditionally called "keys", that are unknown to D. Eastlake, S. Crocker, & J. Schiller [Page 3] INTERNET-DRAFT Randomness Requirements for Security and unguessable by an adversary. In some cases, such as the use of symmetric encryption with the one time pads [CRYPTO*] or the US Data Encryption Standard [DES], the parties who wish to communicate confidentially and/or with authentication must all know the same secret key. In other cases, using what are called asymmetric or "public key" cryptographic techniques, keys come in pairs. One key of the pair is private and must be kept secret by one party, the other is public and can be published to the world. It is computationally infeasible to determine the private key from the public key. [ASYMMETRIC, CRYPTO*] The frequency and volume of the requirement for random quantities differs greatly for different cryptographic systems. Using RSA [CRYPTO*], random quantities are required when the key pair is generated, but thereafter any number of messages can be signed without any further need for randomness. The public key Digital Signature Algorithm that has been proposed by the US National Institute of Standards and Technology requires good random numbers for each signature. And encrypting with a one time pad, in principle the strongest possible encryption technique, requires a volume of randomness equal to all the messages to be processed. In all of these cases, an adversary may try to determine the "secret" key by trial and error as long as the key is enough smaller than the message that the actual key can be uniquely identified. The probability of an adversary succeeding at this must be made acceptably low, depending on the particular application. The size of the space the adversary must search is related to the amount of key "information" present in the information theoretic sense [SHANNON]. This depends on the number of different secret values possible and the probability of each value as follows: ----- \ Bits-of-info = \ - p * log ( p ) / i 2 i / ----- where i varies from 1 to the number of possible secret values and p sub i is the probability of the value numbered i. (Since p sub i is less than one, the log will be negative so each term in the sum will be non-negative.) If there are 2^n different values of equal probability, then n bits of information are present and an adversary would, on the average, have to try half of the values, or 2^(n-1) , before guessing the secret quantity. If the probability of different values is unequal, then there is less information present and fewer guesses will, on D. Eastlake, S. Crocker, & J. Schiller [Page 4] INTERNET-DRAFT Randomness Requirements for Security average, be required by an adversary. In particular, any values that the adversary can know are impossible, or are of low probability, can be ignored by an adversary, who will search through the more probable values first. For example, consider a cryptographic system that uses 56 bit keys. If these 56 bit keys are derived by using a pseudo-random number generator that is seeded with an 8 bit seed, then an attacker needs to search through only 256 keys (by running the pseudo-random number generator with every possible seed), not the 2^56 keys that may at first appear to be the case. Only 8 bits of "information" are in these 56 bit keys. 3. Traditional Pseudo-Random Sequences Most traditional sources of random numbers use deterministic sources of "pseudo-random numbers" . These typically start with a "seed" quantity and use numeric operations to produce a sequence of values. A typical technique is modular arithmetic where the N+1th value is calculated from the Nth value by V = ( V * a + b )(Mod c) N+1 N The goodness of traditional pseudo-random number generator algorithm is measured by statistical tests on this sequence. Carefully chosen values of a, b, and c in even the above simple iteration can produce excellent statistics. These numbers work well in simulations (Monte Carlo experiments) as long as the sequence is orthogonal to the structure of the space being explored. However, such sequences are bad for use in security applications. They are fully predictable if the initial state is known. Depending on the form of the pseudo- random number generator, the sequence may even be determinable from observation of a short portion of the sequence. For example, with the generator above, one can determine V(n+1) given knowledge of V(n). [KNUTH] has a good exposition on pseudo-random numbers. Applications he mentions are simulation of natural phenomena, sampling, numerical analysis, testing computer programs, decision making, and games. None of these have the same characteristics as the sort of security uses we are talking about. Only in the last two could there be an adversary trying to find the random quantity. However, in these cases, the adversary normally has only a single chance to use a guessed value. In guessing passwords or attempting to break an encryption scheme, the adversary normally has many, perhaps unlimited, chances at guessing the correct value and should be assumed to be aided by a computer. D. Eastlake, S. Crocker, & J. Schiller [Page 5] INTERNET-DRAFT Randomness Requirements for Security For testing the "randomness" of numbers, Knuth suggests a variety of measures including statistical and spectral. These tests check things like autocorrelation between different parts of a "random" sequence or distribution of its values. They could be met by a constant stored random sequence, such as the "random" sequence printed in the CRC Standard Mathematical Tables [CRC]. 4. Unpredictability Randomness in the traditional sense described in the previous section is NOT the same as the unpredictability required for security use. For example, use of a widely available constant sequence, such as that from the CRC tables, is very weak against an adversary. Once they learn of or guess it, they can easily break all security, future and past, based on the sequence. [CRC] 4.1 Problems with Clocks and Serial Numbers Computer clocks, or similar operating system or hardware values, may provide significantly fewer real bits of unpredictability than might appear from their specifications. Tests have been done on clocks on numerous systems and it was found that their behavior can vary widely and in unexpected ways. One version of an operating system running on one set of hardware may actually provide, say, microsecond resolution in a clock while a different configuration of the "same" system may always provide the same lower bits and only count in the upper bits at much lower resolution. This means that successive reads on the clock may produce identical values even if enough time has passed that the value "should" change based on the nominal clock resolution. There are also cases where frequently reading a clock can produce artificial sequential values because of extra code that checks for the clock being unchanged between two reads and increases it by one! Designing portable application code to generate unpredictable numbers based on system clocks is particularly challenging because the system designer does not always know the properties of the system clocks that the code will execute on. Use of a hardware serial number such as an ethernet address may also provide fewer bits of uniqueness than one would guess. Such quantities are usually heavily structured and subfields may have only a limited range of possible values or values easily guessable based on approximate date of manufacture or other data. For example, it is likely that most of the ethernet cards installed on Digital Equipment D. Eastlake, S. Crocker, & J. Schiller [Page 6] INTERNET-DRAFT Randomness Requirements for Security Corporation (DEC) hardware within DEC were manufactured by DEC itself, which significantly limits the range of possible serial numbers. Problems such as those described above related to clocks and serial numbers make code to produce unpredictable quantities difficult if the code is to be ported across a variety of computer platforms and systems. 4.2 Timing External Events It is possible to measure the timing of mouse movement, key strokes, and the like. This a reasonable source of unguessable data with two exceptions. On some machines, inputs such as key strokes are buffered. Even though the user's inter-keystroke timing may have sufficient variation and unpredictability, there might not be an easy way to access that variation. The other problem is that no standard method exists to sample timing details. This makes it very hard to build standard software intended for distribution to a large range of machines based on this technique. 4.3 The Fallacy of Complex Manipulation One strategy which may give a misleading appearance of strength is to take a very complex algorithm (or an excellent traditional pseudo- random number generator with good statistical properties) and calculate a cryptographic key by starting with the current value of a computer system clock as the seed. An adversary who knew roughly when the generator was started would have a relatively small number of seed values to test as they would know likely values of the system clock. Large numbers of pseudo-random bits could be generated but the search space an adversary would need to check could be quite small. Thus very strong and/or complex manipulation of data will not help if the adversary can learn what the manipulation is and there is not enough unpredictability in the starting value. 4.4 The Fallacy of Selection from a Large Database Another strategy that can give a misleading appearance of strength is selection of a quantity randomly from a database and the assumption that its strength is related to the total number of bits in the database. For example, typical NNTP servers as of this date process D. Eastlake, S. Crocker, & J. Schiller [Page 7] INTERNET-DRAFT Randomness Requirements for Security over 30 megabytes of information per day. Assume a random quantity was selected by fetching 32 bytes of data from a random starting point in this data. This does not yield 32*8 = 256 bits worth of unguessability. Even after allowing that much of the data is human language and probably has more like 3 bits of information per byte, it doesn't yield 32*3 = 96 bits of unguessability. For an adversary with access to the same 30 megabytes the unguessability rests only on the starting point of the selection. That is, at best, about 25 bits of unguessability in this case. The same argument applies to selecting sequences from the data on a CD ROM or Audio CD recording or any other large public database. If the adversary has access to the same database, this "selection from a large volume of data" step buys very little. However, if a selection can be made from data to which the adversary has no access, such as active system buffers on an active multi-user system, it may be of some help. 5. Hardware for Randomness Is there any hope for strong portable randomness in the future? There might be. All that's needed is a physical source of unpredictable numbers. A thermal noise or radioactive decay source and a fast, free-running oscillator would do the trick. This is a trivial amount of hardware, and could easily be included as a standard part of a computer system's architecture. All that's needed is the common perception among computer vendors that this small addition is necessary and useful. 5.1 Volume Required How much unpredictability is needed? Is it possible to quantify the requirement in, say, number of random bits per second? The answer is not very much is needed. For DES, the key is 56 bits and, as we show in an example below, even the highest security system is unlikely to require a keying material of over 200 bits. Even if a series of keys are needed, they can be generated from a strong random seed using a cryptographically strong sequence as explained in Section 6.3. A couple of hundred random bits generated once a day may well be enough using such techniques. Even if the random bits are generated as slowly as one per second, it should be tolerable in high security applications to wait 200 seconds occasionally. These numbers are trivial to achieve. It could be done by a person repeatedly tossing a coin. Almost any hardware process is likely to D. Eastlake, S. Crocker, & J. Schiller [Page 8] INTERNET-DRAFT Randomness Requirements for Security be much faster. 5.2 Sensitivity to Skew Is there any specific requirement on the shape of the distribution of the random numbers? The good news is the distribution need not be uniform. All that is needed is a conservative estimate of how non- uniform it is to bound performance. Two simple techniques to de-skew the bit stream are given below and stronger techniques are mentioned in Section 6.1.2 below. 5.2.1 Using Stream Parity to De-Skew Consider taking a sufficiently long string of bits and map the string to "zero" or "one". The mapping will not yield a perfectly uniform distribution, but it can be as close as desired. One mapping that serves the purpose is to take the parity of the string. This has the advantages that it is robust across all degrees of skew up to the estimated maximum skew and is absolutely trivial to implement in hardware. The following analysis gives the number of bits that must be sampled: Suppose the ratio of ones to zeros is 0.5 + e : 0.5 - e, where e is between 0 and 0.5 and is a measure of the "eccentricity" of the distribution. Consider the distribution of the parity function of N bit samples. The probabilities that the parity will be one or zero will be the sum of the odd or even terms in the binomial expansion of (p + q)^N, where p = 0.5 + e, the probability of a one, and q = 0.5 - e, the probability of a zero. These sums can be computed easily as 1/2 * [(p + q)^N + (p - q)^N] and 1/2 * [(p + q)^N - (p - q)^N]. (Which one corresponds to the probability the parity will be 1 depends on whether N is odd or even.) Since p + q = 1 and p - q = 2e, these expressions reduce to 1/2 * [1 + (2e)^N] and 1/2 * [1 - (2e)^N]. D. Eastlake, S. Crocker, & J. Schiller [Page 9] INTERNET-DRAFT Randomness Requirements for Security Neither of these will ever be exactly 0.5 unless e is zero, but we can bring them arbitrarily close to 0.5. If we want the probabilities to be within some delta d of 0.5, i.e. then 0.5 + 0.5 * (2e)^N < 0.5 + d. Solving for N yields N > log(2d)/log(2e). (Note that 2e is less than 1, so its log is negative. Division by a negative number reverses the sense of an inequality.) The following table gives the length of the string which must be sampled for various degrees of skew in order to come within 0.001 of a 50/50 distribution. +---------+--------+-------+ | Prob(1) | e | N | +---------+--------+-------+ | 0.5 | 0.00 | 1 | | 0.6 | 0.10 | 4 | | 0.7 | 0.20 | 7 | | 0.8 | 0.30 | 13 | | 0.9 | 0.40 | 28 | | 0.95 | 0.45 | 59 | | 0.99 | 0.49 | 308 | +---------+--------+-------+ The last entry shows that even if the distribution is skewed 99% in favor of ones, the parity of a string of 308 samples will be within 0.001 of a 50/50 distribution. 5.2.2 Using Transition Mappings to De-Skew Another possible technique is to examine a bit stream as a sequence of non-overlapping pairs. You could then discard any 00 or 11 pairs found, interpret 01 as a 0 and 10 as a 1. Assume the probability of a 1 is 0.5+e and the probability of a 0 is 0.5-e where e is the eccentricity of the source and described in the previous section. Then the probability of each pair is as follows: +------+-----------------------------------------+ | pair | probability | +------+-----------------------------------------+ | 00 | (0.5 - e)^2 = 0.25 - e + e^2 | | 01 | (0.5 - e)*(0.5 + e) = 0.25 - e^2 | | 10 | (0.5 + e)*(0.5 - e) = 0.25 - e^2 | | 11 | (0.5 + e)^2 = 0.25 + e + e^2 | +------+-----------------------------------------+ D. Eastlake, S. Crocker, & J. Schiller [Page 10] INTERNET-DRAFT Randomness Requirements for Security This technique will completely eliminate any bias but at the expense of taking an indeterminate number of input bits for any particular desired number of output bits. The probability of any particular pair being discarded is 0.5 + 2e^2 so the expected number of input bits to produce X output bits is X/(0.25 - e^2). This technique assume that the bits are from a stream where each bit has the same probability of being a 0 or 1 as any other bit in the stream and that bits are not correlated, i.e., that the bits are identical independent distributions. If alternate bits were from two different sources, for example, the above analysis breaks down. The above technique provides another illustration of how a simple statistical analysis can mislead if one is not always on the lookout for patterns that could be exploited by an adversary. If the algorithm were mis-read slightly so that overlapping successive bits pairs were used instead of non-overlapping pairs, the statistical analysis given is the same; however, instead of provided an unbiased uncorrelated series of random 1's and 0's, it would instead produce a totally predictable sequence of exactly alternating 1's and 0's. 6. Recommended Non-Hardware Strategy What is the best overall strategy for meeting the requirement for unguessable random numbers in the absence of a reliable hardware source? It is to obtain random input from a large number of uncorrelated sources and to mix them with a strong mixing function. Such a function will preserve the randomness present in any of the sources even if other quantities being combined are fixed or easily guessable. This may be advisable even with a good hardware source as hardware can also fail, though this should be weighed against any increase in the chance of overall failure due to added software complexity. 6.1 Mixing Functions A strong mixing function is one which combines two or more inputs and produces an output where each output bit is a complex non-linear function of all the input bits. On average, changing any input bit will change about half the output bits. But because the relationship is complex and non-linear, no particular output bit is guaranteed to change when any particular input bit is changed. Note that the problem of converting a stream of bits that is skewed towards 0 or 1 to a shorter stream which is more random, as discussed in Section 5.2 above, is simply another case where a strong mixing D. Eastlake, S. Crocker, & J. Schiller [Page 11] INTERNET-DRAFT Randomness Requirements for Security function is desired. The technique given in Section 5.2.1 of using the parity of a number of bits is simply the result of successively xor'ing them which is examined as a trivial example immediately below. Use of stronger mixing functions to extract more of the randomness in a stream of skewed bits is mentioned in 6.1.2 below. 6.1.1 A Trivial Mixing Function A trivial example for single bit inputs is the exclusive or (xor) function, which is equivalent to addition without carry, as show in the table below. This is a degenerate case in which the one output bit always changes for a change in either input bit but it will still provide a useful illustration. +-----------+-----------+----------+ | input 1 | input 2 | output | +-----------+-----------+----------+ | 0 | 0 | 0 | | 0 | 1 | 1 | | 1 | 0 | 1 | | 1 | 1 | 0 | +-----------+-----------+----------+ If inputs 1 and 2 are uncorrelated and combined in this fashion then the output will be an even better (less skewed) random bit than the inputs. If we assume an "eccentricity" e as defined in section 5.2 above, then the output eccentricity relates to the input eccentricity as follows: e = 2 * e * e output input 1 input 2 Since e is never greater than 1/2, the eccentricity is always improved except in the case where one input is a totally skewed constant. This is illustrated in the following table where the top and left side values are the two input eccentricities and the entries are the output eccentricity: +--------+--------+--------+--------+--------+--------+--------+ | e | 0.00 | 0.10 | 0.20 | 0.30 | 0.40 | 0.50 | +--------+--------+--------+--------+--------+--------+--------+ | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | | 0.10 | 0.00 | 0.02 | 0.04 | 0.06 | 0.08 | 0.10 | | 0.20 | 0.00 | 0.04 | 0.08 | 0.12 | 0.16 | 0.20 | | 0.30 | 0.00 | 0.06 | 0.12 | 0.18 | 0.24 | 0.30 | | 0.40 | 0.00 | 0.08 | 0.16 | 0.24 | 0.32 | 0.40 | | 0.50 | 0.00 | 0.10 | 0.20 | 0.30 | 0.40 | 0.50 | +--------+--------+--------+--------+--------+--------+--------+ D. Eastlake, S. Crocker, & J. Schiller [Page 12] INTERNET-DRAFT Randomness Requirements for Security However, keep in mind that the above calculations assume that the inputs are not correlated. If the inputs were, say, the parity of the number of minutes from midnight on two clocks accurate to a few seconds, then each might appear random if sampled at random intervals much longer than a minute. Yet if they were both sampled and combined with xor, the result would normally be a constant zero. 6.1.2 Stronger Mixing Functions The US Government Data Encryption Standard [DES] is a good example of a strong mixing function for multiple bit quantities. It takes up to 120 bits of input (64 bits of "data" and 56 bits of "key") and produces 64 bits of output each of which is dependent on a complex function of all input bits. Another good family of mixing functions are the "message digest" or hashing functions such as MD2, MD4, or MD5 that take an arbitrary amount of input and produce an output, frequently 128 bits, mixing all the input bits. [MD2, MD4, MD5] Although message digest functions like MD5 are designed for variable amounts of input, DES can also be used to combine any number of inputs. If 64 bits of output is adequate, the inputs can be packed into a 64 bit data quantity and successive 56 bit keys, padding with zeros if needed, which are then used to successively encrypt using DES in Electronic Codebook Mode [DES MODES]. If more than 64 bits of output are needed, use more complex mixing. For example, if inputs are packed into three quantities, A, B, and C, use DES to encrypt A with B as a key and then with C as a key to produce the 1st part of the output, then encrypt B with C and then A for more output and, if necessary, encrypt C with A and then B for yet more output. Still more output can be produced by reversing the order of the keys given above to stretch things, but keep in mind that it is impossible to get more bits of "randomness" out than are put in. An example of using a strong mixing function would be to reconsider the case of a string of 308 bits each of which is biased 99% towards zero. The parity technique given in Section 5.2.1 above reduced this to one bit with only a 1/1000 deviance from being equally likely a zero or one. But, applying the equation for information given in Section 2, this 308 bit sequence has 5 bits of information in it. Thus hashing it with MD5 and taking the bottom 5 bits of the result would yield 5 unbiased random bits as opposed to the single bit given by calculating the parity of the string. Other strong encryption functions besides DES and the MD* family should serve well as mixing functions. This is an advantage of Diffie-Hellman exponential key exchange. Diffie-Hellman yields a shared secret between two parties that is a mixture of initial random quantities generated by each of them [D-H]. D. Eastlake, S. Crocker, & J. Schiller [Page 13] INTERNET-DRAFT Randomness Requirements for Security 6.1.3 Using a Mixing Function to Stretch Random Bits While it is not necessary for a mixing function to produce the same or fewer bits than its inputs, mixing bits cannot "stretch" the amount of random unpredictability present in the inputs. Thus four inputs of 32 bits each where there is 12 bits worth of unpredicatability (such as 4,096 equally probable values) in each input cannot produce more than 48 bits worth of unpredictable output. The output can be expanded to hundreds or thousands of bits by, for example, mixing with successive integers, but the clever adversary's search space is still 2^48 possibilities. Furthermore, mixing to fewer bits than are input will tend to strengthen the randomness of the output the way using xor to produce one bit from two did above. The last table in Section 6.1.1 shows that mixing a random bit with a constant bit with xor will produce a random bit. While this is true, it does not provide a way to "stretch" one random bit into more than one. If, for example, a random bit is mixed with a 0 and then with a 1, this produces a two bit sequence but it will always be either 01 or 10. Since there are only two possible values, there is still only the one bit of original randomness. 6.1.4 Other Factors in Choosing a Mixing Function For local use, DES has the advantages that it has been widely tested for flaws, is widely documented, and is widely implemented with hardware and software implementations available all over the world including source code available by anonymous FTP. The MD* family are younger algorithms which has been less tested but there is no particular reason to believe they are flawed. They also have source code available by anonymous FTP [MD2, MD4, MD5]. DES, MD4, and MD5 are royalty free for all purposes but MD2 has been freely licensed only for non-profit use in connection with Privacy Enhanced Mail. (Some people believe that, as with Goldilocks and the Three Bears, MD2 is strong but too slow, MD4 is fast but too weak, and MD5 is just right.) Another advantage of the MD* or similar hashing algorithms is that they are not subject to the regulations imposed by the US Government prohibiting the export or import of encryption/decryption software (or hardware). The same should be true of DES rigged to produce an irreversible hash code but most DES packages are oriented to reversible encryption. D. Eastlake, S. Crocker, & J. Schiller [Page 14] INTERNET-DRAFT Randomness Requirements for Security 6.2 Non-Hardware Sources of Randomness The best source of input for mixing would be a hardware random number generator based on some fundamentally random physical process such as thermal emission or radioactive decay. However, if that is not available, other possibilities include system clocks, system or input/output buffers, user/system/hardware/network serial numbers and/or addresses, user input, and timings of input/output operations. Any of these sources can produce limited or predicatable values under some circumstances. Most of the sources listed above would be quite strong on multi-user system where, in essence, each user of the system is a source of randomness. However, on a small single user system, such as a typical IBM PC or Apple Macintosh, it might be possible for an adversary to assemble a similar configuration. This could give the adversary inputs to the mixing process that were sufficiently correlated to those used originally as to make exhaustive search practical. The use of multiple random inputs with a strong mixing function is recommended and can overcome weakness in any particular input. This strategy may make practical portable code to produce good random numbers for security even when some of the inputs are very weak on some of the target systems. However, even this may fail against a high grade attack on small single user systems if a hardware random source is not available. 6.3 Cryptographically Strong Sequences In cases where a series of random quantities must be generated, an adversary may learn some values in the sequence. In general, they should not be able to predict other values from the ones that they know. The correct technique is to start with a strong random seed and take cryptographically strong steps from that seed [CRYPTO2]. If each value in the sequence can be calculated in a fixed way from the previous value, then when any value is compromised, all future values can be determined. This would be the case, for example, if each value were a constant function of the previous values, even if the function were a very strong, non-invertible message digest function. The best way to achieve a strong sequence is to have the values be produced by successive multiple "encryption" of a random seed under a random key or by hashing the quantities produced by concatenating the seed with successive integers or the like. To predict values of a sequence from others when the sequence was generated by these techniques is equivalent to breaking the cryptosystem or inverting the "non-invertible" hashing involved. D. Eastlake, S. Crocker, & J. Schiller [Page 15] INTERNET-DRAFT Randomness Requirements for Security 7. US DoD Recommendations for Password Generation The United States Department of Defense has specific recommendations for password generation [DoD]. They suggest using the US Data Encryption Standard [DES] in Output Feedback Mode [DES MODES] as follows: use an initialization vector determined from the system clock, system ID, user ID, and date and time; use a key determined from system interrupt registers, system status registers, and system counters; and, as plain text, use an external randomly generated 64 bit quantity such as 8 characters typed in by a system administrator. The password can then be calculated from the 64 bit "cipher text" generated in 64-bit Output Feedback Mode. As many bits as are needed can be taken from these 64 bits and expanded into a pronounceable word, phrase, or other format. 8. Examples of Randomness Required Below are two examples showing rough calculations of needed randomness for security. 8.1 A Low Security Password Assume that user passwords change once a year and a probability of less than one in a thousand that an adversary could guess the password for a particular account is desired. The key question is how often they can try possibilities. Assume that delays have been introduced into a system so that, at most, an adversary can make one password try every six seconds. That's 600 per hour or about 15,000 per day or about 5,000,000 tries in a year. Assuming any sort of monitoring, it is unlikely someone could actually try continuously for a year. In fact, even if log files are only checked monthly, 500,000 tries is more plausible before the attack is noticed and steps taken to change passwords and make it harder to try more passwords. (All this assumes that sending a password to the system is the only way to try a password.) To have a one in a thousand chance of guessing the password in D. Eastlake, S. Crocker, & J. Schiller [Page 16] INTERNET-DRAFT Randomness Requirements for Security 500,000 tries implies a universe of at least 500,000,000 passwords or about 2^29. Thus 29 bits of randomness are needed. This can probably be achieved using the US DoD recommended inputs for password generation as it has 8 inputs which probably average over 5 bits of randomness each. Using a list of 1000 words, the password could be expressed as a three word phrase (1,000,000,000 possibilities) or, using case insensitive letters and digits, six would suffice ((26+10)^6 = 2,176,782,336 possibilities). For a higher security password, the number of bits required goes up. To decrease the probability by 1,000 requires increasing the universe of passwords by the same factor which adds about 12 bits. Thus to have only a one in a million chance of a password being guessed under the above scenario would require 31 bits of randomness and a password that was a four word phrase from a 1000 word list or eight letters/digits. To go to a one in 10^9 chance, 43 bits of randomness are needed implying a five word phrase or ten letter/digit password. 8.2 A Very High Security Cryptographic Key Assume that a very high security key is needed for symmetric encryption/decryption between two parties. Assume an adversary can observe communications and knows the algorithm being used. Within the field of random possibilities, the adversary can exhaustively try key values. How much effort will it take to try each key? For very high security applications it is best to assume a low value of effort. Even if it would clearly take tens of thousands of computer cycles or more to try a single key, there may be some pattern that enables huge blocks of key values to be tested with much less effort per key. Thus it is probably best to assume no more than a hundred cycles per key. (There is no clear lower bound on this as computers operate in parallel on a number of bits and a poor encryption algorithm could allow many keys or even groups of keys to be tested in parallel. However, we need to assume some value and can hope that a reasonably strong algorithm has been chosen for our hypothetical high security task.) If the adversary can command a highly parallel processor or a large network of work stations, 10^10 cycles per second is probably a minimum assumption for availability today. Looking forward just a few years, there should be at least an order of magnitude improvement. Thus assuming 10^9 keys could be checked per second or 3.6*10^11 per hour or 6*10^13 per week or 2.4*10^14 per month is reasonable. This implies a need for a minimum of 48 bits of randomness in keys to be sure they cannot be found in a week. Even then it is possible that, a few years from now, a highly determined D. Eastlake, S. Crocker, & J. Schiller [Page 17] INTERNET-DRAFT Randomness Requirements for Security and resourceful adversary could break the key in 2 weeks (on average they need try only half the keys). Assuming a known plain text attack, where the adversary can force some known plain text to be encrypted or knows some standard part of messages, the structure of the encryption algorithm may allow a "meet in the middle" attack. An oversimplified explanation of this type of attack is as follows: the adversary can half-encrypt the know plain text with all possible first half-keys, sort these, then half-decrypt the encoded text with all the second half-keys. If a match is found, the full key can be assembled from the halves and used to decrypt other parts of the message or other messages. At its best, this type of attack can halve the exponent of the work required by the adversary requiring a doubling of the amount of randomness in the key to a minimum of 96 bits. This assumes that the cryptographic algorithm can be decomposed in this way but we can not rule that out without a deep knowledge of the algorithm. Enormous resources may be required for this sort of attack but they are probably within the range of the national security services of a major nation. Almost all nations spy on other nations government traffic and Some nations are known to spy on commercial traffic and give the information to their domestic companies to assist them against foreign competition. Since we have not even considered the possibilities of special purpose code breaking hardware or just how much of a safety margin we want beyond our assumptions above, probably a good minimum for a very high security cryptographic key is 128 bits of randomness which implies a minimum key length of 128 bits. If the two parties agree on a key by Diffie-Hellman exchange [D-H], then in principle only half of this randomness would have to be supplied by each party. However, there is probably some correlation between their random inputs so it is probably best to assume that each party needs to provide at least 96 bits worth of randomness for very high security. This amount of randomness is probably beyond the limit of that in the inputs recommended by the US DoD for password generation and could require user typing timing, hardware random number generation, or other sources. It should be noted that key length calculations such at those above are controversial and depend on various assumptions about the cryptographic algorithms in use. In some cases, a professional with a deep knowledge of code breaking techniques and of the strength of the algorithm in use could be satisfied with less than half of the key size derived above. D. Eastlake, S. Crocker, & J. Schiller [Page 18] INTERNET-DRAFT Randomness Requirements for Security 9. Security Considerations The entirety of this draft concerns techniques and recommendations for generating "random" quantities for use as passwords, cryptographic keys, and similar security uses. D. Eastlake, S. Crocker, & J. Schiller [Page 19] INTERNET-DRAFT Randomness Requirements for Security References [ASYMMETRIC] - Secure Communications and Asymmetric Cryptosystems, edited by Gustavus J. Simmons, AAAS Selected Symposium 69, Westview Press, Inc. [CRC] - C.R.C. Standard Mathematical Tables, Chemical Rubber Publishing Company. [CRYPTO1] - Cryptography: A Primer, by Alan G. Konheim, A Wiley- Interscience Publication, John Wiley & Sons, 1981, Alan G. Konheim. [CRYPTO2] - Cryptography: A New Dimension in Computer Data Security, A Wiley-Interscience Publication, John Wiley & Sons, 1982, Carl H. Meyer & Stephen M. Matyas. [DES] - Data Encryption Standard, United States of America, Department of Commerce, National Institute of Standards and Technology, Federal Information Processing Standard (FIPS) 46-1. - Data Encryption Algorithm, American National Standards Institute, ANSI X3.92-1981. (See also FIPS 112, Password Usage, which includes FORTRAN code for performing DES.) [DES MODES] - DES Modes of Operation, United States of America, Department of Commerce, National Institute of Standards and Technology, Federal Information Processing Standard (FIPS) 81. - Data Encryption Algorithm - Modes of Operation, American National Standards Institute, ANSI X3.106-1983. [D-H] - New Directions in Cryptography, IEEE Transactions on Information Technology, November, 1976, Whitfield Diffie and Martin E. Hellman. [DoD] - Password Management Guideline, United States of America, Department of Defense, Computer Security Center, CSC-STD-002-85. (See also FIPS 112, Password Usage, which incorporates CSC-S002-85 as one of its appendicies.) [KNUTH] - The Art of Computer Programming, Volume 2: Seminumerical Algorithms, Chapter 3: Random Numbers. Addison Wesley Publishing Company, 1971, Donald E. Knuth. [MD2] - The MD2 Message-Digest Algorithm, RFC1319, April 1992, B. Kaliski [MD4] - The MD4 Message-Digest Algorithm, RFC1320, April 1992, R. Rivest [MD5] - The MD5 Message-Digest Algorithm, RFC1321, April 1992, R. Rivest D. Eastlake, S. Crocker, & J. Schiller [Page 20] INTERNET-DRAFT Randomness Requirements for Security [SHANNON] - The Mathematical Theory of Communication, University of Illinois Press, 1963, Claude E. Shannon. (originally from: Bell System Technical Journal, July and October 1948) Authors Addresses Donald E. Eastlake 3rd Digital Equipment Corporation 30 Porter Road, MS: LJO2/I4 Littleton, MA 01460 Telephone: +1 508 486 2358(w) +1 617 244 2679(h) EMail: dee@ranger.enet.dec.com NIC Handle: [DEE] Stephen D. Crocker Trusted Information Systems, Inc. 3060 Washington Road Glenwood, MD 21738 Telephone: +1 301 854 6889 EMail: crocker@tis.com NIC Handle: [SDC1] Jeffrey I. Schiller Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge, MA 02139 Telephone: +1 617 253 0161 EMail: jis@mit.edu NIC Handle: [JIS] Expiration This draft expires 24 September 1993. D. Eastlake, S. Crocker, & J. Schiller [Page 21]