Maximum satisfiability problem: Difference between revisions
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'''Elliptic curve Diffie–Hellman (ECDH)''' is an anonymous [[key agreement]] protocol that allows two parties, each having an elliptic curve public–private key pair, to establish a shared secret over an insecure channel.<ref>NIST, [http://csrc.nist.gov/publications/nistpubs/800-56A/SP800-56A_Revision1_Mar08-2007.pdf Special Publication 800-56A, Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography], March, 2006.</ref><ref>Certicom Research, [http://www.secg.org/download/aid-385/sec1_final.pdf Standards for efficient cryptography, SEC 1: Elliptic Curve Cryptography], Version 1.0, September 20, 2000.</ref><ref>NSA Suite B Cryptography, [http://www.nsa.gov/ia/_files/SuiteB_Implementer_G-113808.pdf Suite B Implementers' Guide to NIST SP 800-56A], July 28, 2009.</ref> This shared secret may be directly used as a key, or better yet, to [[Key derivation function|derive another key]] which can then be used to encrypt subsequent communications using a [[Symmetric-key algorithm|symmetric key cipher]]. It is a variant of the [[Diffie–Hellman key exchange|Diffie–Hellman]] protocol using [[elliptic curve cryptography]]. | |||
==Key establishment protocol== | |||
The following example will illustrate how a key establishment is made. Suppose [[Alice and Bob|Alice]] wants to establish a shared key with [[Alice and Bob|Bob]], but the only channel available for them may be eavesdropped by a third party. Initially, the [[Elliptic curve cryptography#Domain_parameters|domain parameters]] (that is, <math>(p,a,b,G,n,h)</math> in the prime case or <math>(m,f(x),a,b,G,n,h)</math> in the binary case) must be agreed upon. Also, each party must have a key pair suitable for elliptic curve cryptography, consisting of a private key <math>d</math> (a randomly selected integer in the interval <math>[1, n-1]</math>) and a public key <math>Q</math> (where <math>Q = d G</math>, that is, the result of [[Elliptic_curve#The_group_law|adding]] <math>G</math> together <math>d</math> times). Let Alice's key pair be <math>(d_A, Q_A)</math> and Bob's key pair be <math>(d_B, Q_B)</math>. Each party must have the other party's public key (an exchange must occur). | |||
Alice computes <math>(x_k, y_k) = d_A Q_B</math>. Bob computes <math>(x_k, y_k) = d_B Q_A</math>. The shared secret is <math>x_k</math> (the ''x'' coordinate of the point). Most standardized protocols based on ECDH derived a symmetric key from <math>x_k</math> using some hash-based key derivation function. | |||
The shared secret calculated by both parties is equal, because <math>d_A Q_B = d_A d_B G = d_B d_A G = d_B Q_A</math>. | |||
The only information about her private key that Alice initially exposes is her public key. So, no party other than Alice can determine Alice's private key, unless that party can solve the elliptic curve [[discrete logarithm]] problem. Bob's private key is similarly secure. No party other than Alice or Bob can compute the shared secret, unless that party can solve the elliptic curve [[Diffie–Hellman problem]]. | |||
The public keys are either static (and trusted, say via a certificate) or ephemeral. Ephemeral keys are temporary and not necessarily authenticated, so if authentication is desired, authenticity assurances must be obtained by other means. Authentication is necessary to avoid [[man-in-the-middle attack]]s. If one of Alice or Bob's public key is static then man-in-the-middle attacks are thwarted. Static public keys provide neither [[forward secrecy]] nor key-compromise impersonation resilience, among other advanced security properties. Holders of static private keys should validate the other public key, and should apply a secure key derivation function to the raw Diffie–Hellman shared secret to avoid leaking information about the static private key. For schemes with other security properties, see [[ECMQV]] and [[FHMQV]]. | |||
While the shared secret may be used directly as a key, it is often desirable to hash the secret to remove weak bits due to the Diffie–Hellman exchange.<ref>{{cite journal | |||
| last1 = Law | |||
| first1 = Laurie | |||
| author1-link = Laurie Law | |||
| last2 = Menezes | |||
| first2 = Alfred | |||
| author2-link = Alfred Menezes | |||
| last3 = Qu | |||
| first3 = Minghua | |||
| author3-link = Minghua Qu | |||
| last4 = Solinas | |||
| first4 = Jerry | |||
| author4-link = Jerry Solinas | |||
| last5 = Vanstone | |||
| first5 = Scott | |||
| author5-link = Scott Vanstone | |||
| title = An Efficient Protocol for Authenticated Key Agreement | |||
| publisher = Certicom | |||
| date = August 28, 1998 | |||
| url = http://download.certicom.com/pdfs/corr98-05.pdf | |||
| accessdate = January 19, 2012 }}</ref> | |||
==References== | |||
<references/> | |||
{{Cryptography navbox | public-key}} | |||
{{DEFAULTSORT:Elliptic curve Diffie-Hellman}} | |||
[[Category:Key-agreement protocols]] | |||
[[Category:Elliptic curve cryptography]] | |||
[[de:Elliptic Curve Diffie-Hellman]] | |||
Latest revision as of 01:14, 3 February 2014
Elliptic curve Diffie–Hellman (ECDH) is an anonymous key agreement protocol that allows two parties, each having an elliptic curve public–private key pair, to establish a shared secret over an insecure channel.[1][2][3] This shared secret may be directly used as a key, or better yet, to derive another key which can then be used to encrypt subsequent communications using a symmetric key cipher. It is a variant of the Diffie–Hellman protocol using elliptic curve cryptography.
Key establishment protocol
The following example will illustrate how a key establishment is made. Suppose Alice wants to establish a shared key with Bob, but the only channel available for them may be eavesdropped by a third party. Initially, the domain parameters (that is, in the prime case or in the binary case) must be agreed upon. Also, each party must have a key pair suitable for elliptic curve cryptography, consisting of a private key (a randomly selected integer in the interval ) and a public key (where , that is, the result of adding together times). Let Alice's key pair be and Bob's key pair be . Each party must have the other party's public key (an exchange must occur).
Alice computes . Bob computes . The shared secret is (the x coordinate of the point). Most standardized protocols based on ECDH derived a symmetric key from using some hash-based key derivation function.
The shared secret calculated by both parties is equal, because .
The only information about her private key that Alice initially exposes is her public key. So, no party other than Alice can determine Alice's private key, unless that party can solve the elliptic curve discrete logarithm problem. Bob's private key is similarly secure. No party other than Alice or Bob can compute the shared secret, unless that party can solve the elliptic curve Diffie–Hellman problem.
The public keys are either static (and trusted, say via a certificate) or ephemeral. Ephemeral keys are temporary and not necessarily authenticated, so if authentication is desired, authenticity assurances must be obtained by other means. Authentication is necessary to avoid man-in-the-middle attacks. If one of Alice or Bob's public key is static then man-in-the-middle attacks are thwarted. Static public keys provide neither forward secrecy nor key-compromise impersonation resilience, among other advanced security properties. Holders of static private keys should validate the other public key, and should apply a secure key derivation function to the raw Diffie–Hellman shared secret to avoid leaking information about the static private key. For schemes with other security properties, see ECMQV and FHMQV.
While the shared secret may be used directly as a key, it is often desirable to hash the secret to remove weak bits due to the Diffie–Hellman exchange.[4]
References
- ↑ NIST, Special Publication 800-56A, Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography, March, 2006.
- ↑ Certicom Research, Standards for efficient cryptography, SEC 1: Elliptic Curve Cryptography, Version 1.0, September 20, 2000.
- ↑ NSA Suite B Cryptography, Suite B Implementers' Guide to NIST SP 800-56A, July 28, 2009.
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