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In 1997, [[Moni Naor]] and [[Omer Reingold]] described efficient constructions for various [[cryptographic primitive]]s in private key as well as [[public-key cryptography]]. Their result is the construction of an efficient [[pseudorandom function family|pseudorandom function]]. Let ''p'' and ''l'' be [[prime number]]s with ''l'' |''p''-1. Select an element ''g'' ∈ <math> {\mathbb F_p}^*</math> of [[multiplicative order]] ''l''. Then for each n-dimensional [[coordinate vector|vector]] ''a'' = (''a''<sub>''1''</sub>, ..., ''a''<sub>''n''</sub>)∈ <math> (\mathbb F_{l})^{n} </math> they define the function | |||
:<math>f_{a}(x) = g^{a_{1}^{x_{1}} a_{2}^{x_{2}}...a_{n}^{x_{n}}} \in \mathbb F_p </math> | |||
where x = x<sub>''1''</sub> ... x<sub>''n''</sub> is the [[Binary_numeral_system#Representation|bit representation]] of integer x, 0 ≤ x ≤ 2<sup>n-1</sup>, with some extra leading zeros if necessary.<ref name="NaorReingold"/> | |||
==Example== | |||
Let ''p'' = 7, ''p'' – 1 = 6, and ''l'' = 3, ''l'' |''p''-1. Select ''g'' = 4 ∈ <math> {\mathbb F_7}^*</math> of multiplicative order 3 (since 4<sup>3</sup> = 64 ≡ 1 mod 7). For n = 3, a = (1, 2, 1) and x = 5 (the bit representation of 5 is 101), we can compute <math> f_{a}\;(5)</math> as follows: | |||
:<math>f_{a}(x) = g^{a_{1}^{x_{1}} a_{2}^{x_{2}}...a_{n}^{x_{n}}}</math> | |||
:<math>f_{a}(5) = 4^{1^{1} 2^{0} 1^{1}} = 4^{1} = 4 \in \mathbb F_7 </math> | |||
==Efficiency== | |||
The evaluation of function <math> f_{a}\;(x)</math> in the '''Naor-Reingold''' construction can be done very efficiently. Computing the value of the function <math> f_{a}\;(x)</math> at any given point is comparable with one [[modular exponentiation]] and n-modular multiplications. This function can be computed in parallel by threshold circuits of bounded depth and polynomial size. | |||
The '''Naor-Reingold''' function can be used as the basis of many [[cryptography|cryptographic]] schemes including [[Symmetric-key algorithm|symmetric encryption]], [[authentication]] and [[digital signatures]]. | |||
==Security of the Function== | |||
Assume that an attacker sees several outputs of the function, e.g. <math> f_{a}(1) = g^{a_{1}}, f_{a}(2) = g^{a_{2}}, f_{a}(3) = g^{a_{1}a_{2}}</math>, ... <math>f_{a}(k) = g^{a_{1}^{x_{1}} a_{2}^{x_{2}}...a_{n}^{x_{n}}}</math> and wants to compute <math> f_{a}\;(k + 1)</math>. Assume for simplicity that x<sub>1</sub> = 0, then the attacker needs to solve the [[Computational Diffie–Hellman assumption|Computational Diffie-Hellman (CDH)]] between <math> f_a (1)= g^{a_{1}} </math> and <math>f_{a}(k) = g^{a_{2}^{x_{2}} ...a_{n}^{x_{n}}}</math> to get <math>f_{a}(k+1) = g^{a_{1}a_{2}^{x_{2}} \dots a_{n}^{x_{n}}}</math>. In general, moving from ''k'' to ''k'' +1 changes the bit pattern and unless ''k'' + 1 is a power of 2 one can split the exponent in <math> f_{a}\;(k + 1)</math> so that the computation corresponds to computing the [[Diffie-Hellman problem|Diffie-Hellman]] key between two of the earlier results. This attacker wants to predict the next [[sequence]] element. Such an attack would be very bad—but it's also possible to fight it off by working in [[Group (mathematics)|groups]] with a hard [[Diffie-Hellman problem]] (DHP). | |||
'''Example:''' | |||
An attacker sees several outputs of the function e.g. <math>f_{a}(5) = 4^{1^{1} 2^{0} 1^{1}} = 4^{1} = 4 </math>, as in the previous example, and <math>f_{a}(1) = 4^{1^{0} 2^{0} 1^{1}} = 4^{1} = 4 </math>. Then, the attacker wants to predict the next sequence element of this function, <math> f_{a}\;(6)</math>. However, the attacker cannot predict the outcome of <math> f_{a}\;(6) </math> from knowing <math> f_{a}\;(1)</math> and <math> f_{a}\;(5)</math>. | |||
There are other attacks that would be very bad for a [[Pseudorandom Number Generator]]: the user expects to get random numbers from the output, so of course the stream should not be predictable, but even more, it should be indistinguishable from a random string. Let <math> \mathcal{A}^f </math> denote the algorithm <math> \mathcal{A} </math> with access to an oracle for evaluating the function <math> f_{a}\;(x)</math> . Suppose the [[Decisional Diffie-Hellman assumption]] holds for <math> \mathbb F_p </math>, '''Naor and Reingold''' show that for every [[PP (complexity)|probabilistic polynomial time]] algorithm <math> \mathcal{A} </math> and sufficiently large ''n'' | |||
: <math> \text{Pr }[\mathcal{A}^{f_{a}(x)}(p,g) \to 1] - \text{Pr }[\mathcal{A}^{R} (p,g)\to 1] </math> is [[Negligible function|negligible]]. | |||
The first probability is taken over the choice of the seed s = (p, g, a) and the second probability is taken over the random distribution induced on p, g by <math> \mathcal{I}\mathcal{G} (n) </math>, instance generator, and the random choice of the function <math> R_{a}\;(x) </math> among the set of all <math> \{0,1\}^{n} \to \mathbb F_p </math> functions.<ref name="DDHBoneh"/> | |||
==Linear Complexity== | |||
One natural measure of how useful a sequence may be for [[cryptography|cryptographic]] purposes is the size of its '''linear complexity'''. The linear complexity of an ''n''-element sequence W(x), x = 0,1,2,…,n – 1, over a ring <math> \mathcal{R}</math> is the length ''l'' of the shortest linear [[recurrence relation]] W (x + ''l'') = A<sub>''l''-1</sub> W (x +''l''-1) + … + A<sub>0</sub> W(x), x = 0,1,2,…, ''n'' – ''l'' –1 with A<sub>0</sub>, …, A<sub>''l''-1</sub> ∈ <math> \mathcal{R}</math>, which is satisfied by this sequence. | |||
For some <math>\gamma</math> > 0,n ≥ (1+ <math>\gamma</math>) <math>\log l</math>, for any <math>\delta > 0 </math>, sufficiently large ''l'', the linear complexity of the sequence <math> f_{a}\;(x)</math>,0 ≤ x ≤ 2<sup>n-1</sup>, denoted by <math>L_a</math> satisfies | |||
:<math>L_{a} \geqslant \begin{cases} | |||
l^{1-\ \delta\,\!} &\text{, if } \gamma\,\! \geqslant 2\\ | |||
l^{\left (\tfrac{\ \gamma\,\!}{2-\ \delta\,\!}\right )} &\text{, if } \gamma\,\! < 2 | |||
\end{cases}</math> | |||
for all except possibly at most <math>3(l - 1)^{n - \delta}</math> vectors a ∈ <math> (\mathbb F_{l})^{n} </math>.<ref name="ShparlinskiLinearComplexity"/> The bound of this work has disadvantages, namely it does not apply to the very interesting case <math>\log p \approx \log n \approx {n.} </math> | |||
==Uniformity of Distribution== | |||
The statistical distribution of <math> f_{a}\;(x)</math> is exponentially close to [[Uniform distribution (discrete)|uniform distribution]] for almost all vectors a ∈ <math> (\mathbb F_{l})^{n} </math>. | |||
Let <math>{\mathbf D}_a</math> be the [[Inversive congruential generator|discrepancy]] of the set <math>\{f_a (x)| 0 \leq x \leq 2^{n-1}\}</math>. Thus, if <math>n = \log p </math> is the bit length of ''p'' then for all vectors a ∈ <math> (\mathbb F_{l})^{n} </math> the bound <math>{\mathbf D}_a\leq \Delta (l,p)</math> holds, where | |||
<math>\Delta (l,p) = \begin{cases} | |||
p^{\left (\tfrac{1-\ \gamma\,\!}{2}\right )}l^{\left (\tfrac{-1}{2}\right )}\log^{2}p &\text{, if } l \geqslant p^{\gamma\,\!}\\ | |||
p^{\left (\tfrac{1}{2}\right )}l^{-1}\log^{2}p &\text{, if } p^{\gamma\,\!} > l \geqslant p^{\left (\tfrac{2}{3}\right )} \\ | |||
p^{\left (\tfrac{1}{4}\right )}l^{\left (\tfrac{-5}{8}\right )}\log^{2}p &\text{, if } p^{\left (\tfrac{2}{3}\right )} > l \geqslant p^{\left (\tfrac{1}{2}\right )} \\ | |||
p^{\left (\tfrac{1}{8}\right )}l^{\left (\tfrac{-3}{8}\right )}\log^{2}p &\text{, if } p^{\left (\tfrac{1}{2}\right )} > l \geqslant p^{\left (\tfrac{1}{3}\right )} \\ | |||
\end{cases}</math> | |||
and <math>\gamma</math> = 2.5 - <math>\log 3 </math>= 0.9150.... | |||
Although this property does not seem to have any immediate cryptographic implications, the inverse fact, namely non uniform distribution, if true would have disastrous consequences for applications of this function.<ref name="ShparlinskiUniformity"/> | |||
==Sequences in Elliptic Curve== | |||
The [[elliptic curve]] version of this function is of interest as well. In particular, it may help to improve the cryptographic security of the corresponding system. Let ''p'' > 3 be prime and let E be an elliptic curve over <math> \mathbb F_p </math>, then each vector '''a''' defines a [[Sequence#Finite_and_infinite|finite sequence]] in the [[subgroup]] <math>\langle G\rangle</math> as: | |||
:<math>F_{a}(x) = (a_{1}^{x_{1}} a_{2}^{x_{2}}\dots a_{n}^{x_{n}})G </math> | |||
where <math>x = x_1 \dots x_n</math> is the bit representation of integer <math>x, 0 \leq x \leq 2^{n-1}</math>. | |||
The '''Naor-Reingold''' elliptic curve sequence is defined as | |||
: <math> u_{k} = X (f_{a}(k))\; \mbox{where } X (P) \mbox{ is the abscissa of}\; P \in E. </math><ref name="EllipticNaor"/> | |||
If the Decisional Diffie-Hellman assumption holds, the index ''k'' is not enough to compute <math>u_k</math> in polynomial time, even if an attacker performs polynomially many queries to a random oracle. | |||
==See also== | |||
*[[Decisional Diffie-Hellman assumption]] | |||
*[[Finite Field]] | |||
*[[Inversive congruential generator]] | |||
*[[Generalized inversive congruential pseudorandom numbers]] | |||
==Notes== | |||
{{Reflist|refs= | |||
<ref name="NaorReingold">Naor, M., Reingold, O. "Number-theoretic constructions of efficient pseudo-random functions," Proc 38th IEEE Symp. on Foundations of Comp. Sci, (1997), 458-467.</ref> | |||
<ref name="ShparlinskiLinearComplexity">Shparlinski, Igor E. "Linear Complexity of the Naor-Reingold pseudo-random function," Inform. Process Lett, 76 (2000), 95-99.</ref> | |||
<ref name="ShparlinskiUniformity">Shparlinski, Igor E. "On the uniformity of distribution of the Naor-Reingold pseudo-random function," Finite Fields and Their Applications, 7 (2001), 318-326</ref> | |||
<ref name="EllipticNaor">Cruz, M., Gomez, D., Sadornil, D. "On the linear complexity of the Naor-Reingold sequence with elliptic curves," Finite Fields and Their Applications, 16 (2010), 329-333</ref> | |||
<ref name="DDHBoneh">Boneh, Dan. "The Decision Diffie–Hellman Problem,"ANTS-III: Proceedings of the Third International Symposium on Algorithmic Number Theory,1998,48–63.</ref> | |||
}} | |||
==References== | |||
*{{citation | |||
| last=Shparlinski | |||
| first=Igor | |||
| title=Cryptographic Applications of Analytic Number Theory: Complexity Lower Bounds and Pseudorandomness | |||
| year=2003 | |||
| edition=first | |||
| publisher=Birkhäuser Basel | |||
| isbn=978-3-7643-6654-4 | |||
}} | |||
*{{citation | |||
| last=Goldreich | |||
| first=Oded | |||
| title=Modern Cryptography, Probabilistic Proofs and Pseudorandomness | |||
| year=1998 | |||
| edition=first | |||
| publisher=Springer | |||
| isbn=978-3-540-64766-9 | |||
}} | |||
[[Category:Pseudorandom number generators]] | |||
[[Category:Cryptography]] | |||
Revision as of 19:07, 5 August 2013
In 1997, Moni Naor and Omer Reingold described efficient constructions for various cryptographic primitives in private key as well as public-key cryptography. Their result is the construction of an efficient pseudorandom function. Let p and l be prime numbers with l |p-1. Select an element g ∈ of multiplicative order l. Then for each n-dimensional vector a = (a1, ..., an)∈ they define the function
where x = x1 ... xn is the bit representation of integer x, 0 ≤ x ≤ 2n-1, with some extra leading zeros if necessary.[1]
Example
Let p = 7, p – 1 = 6, and l = 3, l |p-1. Select g = 4 ∈ of multiplicative order 3 (since 43 = 64 ≡ 1 mod 7). For n = 3, a = (1, 2, 1) and x = 5 (the bit representation of 5 is 101), we can compute as follows:
Efficiency
The evaluation of function in the Naor-Reingold construction can be done very efficiently. Computing the value of the function at any given point is comparable with one modular exponentiation and n-modular multiplications. This function can be computed in parallel by threshold circuits of bounded depth and polynomial size.
The Naor-Reingold function can be used as the basis of many cryptographic schemes including symmetric encryption, authentication and digital signatures.
Security of the Function
Assume that an attacker sees several outputs of the function, e.g. , ... and wants to compute . Assume for simplicity that x1 = 0, then the attacker needs to solve the Computational Diffie-Hellman (CDH) between and to get . In general, moving from k to k +1 changes the bit pattern and unless k + 1 is a power of 2 one can split the exponent in so that the computation corresponds to computing the Diffie-Hellman key between two of the earlier results. This attacker wants to predict the next sequence element. Such an attack would be very bad—but it's also possible to fight it off by working in groups with a hard Diffie-Hellman problem (DHP).
Example: An attacker sees several outputs of the function e.g. , as in the previous example, and . Then, the attacker wants to predict the next sequence element of this function, . However, the attacker cannot predict the outcome of from knowing and .
There are other attacks that would be very bad for a Pseudorandom Number Generator: the user expects to get random numbers from the output, so of course the stream should not be predictable, but even more, it should be indistinguishable from a random string. Let denote the algorithm with access to an oracle for evaluating the function . Suppose the Decisional Diffie-Hellman assumption holds for , Naor and Reingold show that for every probabilistic polynomial time algorithm and sufficiently large n
- is negligible.
The first probability is taken over the choice of the seed s = (p, g, a) and the second probability is taken over the random distribution induced on p, g by , instance generator, and the random choice of the function among the set of all functions.[2]
Linear Complexity
One natural measure of how useful a sequence may be for cryptographic purposes is the size of its linear complexity. The linear complexity of an n-element sequence W(x), x = 0,1,2,…,n – 1, over a ring is the length l of the shortest linear recurrence relation W (x + l) = Al-1 W (x +l-1) + … + A0 W(x), x = 0,1,2,…, n – l –1 with A0, …, Al-1 ∈ , which is satisfied by this sequence.
For some > 0,n ≥ (1+ ) , for any , sufficiently large l, the linear complexity of the sequence ,0 ≤ x ≤ 2n-1, denoted by satisfies
for all except possibly at most vectors a ∈ .[3] The bound of this work has disadvantages, namely it does not apply to the very interesting case
Uniformity of Distribution
The statistical distribution of is exponentially close to uniform distribution for almost all vectors a ∈ .
Let be the discrepancy of the set . Thus, if is the bit length of p then for all vectors a ∈ the bound holds, where
Although this property does not seem to have any immediate cryptographic implications, the inverse fact, namely non uniform distribution, if true would have disastrous consequences for applications of this function.[4]
Sequences in Elliptic Curve
The elliptic curve version of this function is of interest as well. In particular, it may help to improve the cryptographic security of the corresponding system. Let p > 3 be prime and let E be an elliptic curve over , then each vector a defines a finite sequence in the subgroup as:
where is the bit representation of integer . The Naor-Reingold elliptic curve sequence is defined as
If the Decisional Diffie-Hellman assumption holds, the index k is not enough to compute in polynomial time, even if an attacker performs polynomially many queries to a random oracle.
See also
- Decisional Diffie-Hellman assumption
- Finite Field
- Inversive congruential generator
- Generalized inversive congruential pseudorandom numbers
Notes
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References
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Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
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To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010 - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
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