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((Multiple issues|
((More citations needed|date=February 2019))
((Notability|date=December 2020))
))
A '''hash chain''' is the successive application of a [[cryptographic hash function]] to a piece of data. In [[computer security]], a hash chain is a method used to produce many [[One-time password|one-time keys]] from a single [[Key (cryptography)|key]] or [[password]].  For [[non-repudiation]], a hash function can be applied successively to additional pieces of data in order to record the chronology of data's existence.

== Definition ==
A '''hash chain''' is a successive application of a [[cryptographic hash function]] <math>h</math> to a string <math>x</math>.

For example,

<math>h(h(h(h(x))))</math>
gives a hash chain of length 4, often denoted <math>h^{4}(x)</math>

== Applications ==
((main article|S/KEY))

[[Leslie Lamport]]<ref name="lamporthash">L. Lamport, “Password Authentication with Insecure Communication”, Communications of the ACM 24.11 (November 1981), pp 770-772. [https://doi.org/10.1145/358790.358797]</ref> suggested the use of hash chains as a password protection scheme in an insecure environment. A server which needs to provide [[authentication]] may store a hash chain rather than a [[plain text]] password and prevent theft of the password in transmission or theft from the server. For example, a server begins by storing <math>h^{1000}(\mathrm{password})</math> which is provided by the user. When the user wishes to authenticate, they supply <math>h^{999}(\mathrm{password})</math> to the server. The server computes <math>h(h^{999}(\mathrm{password})) = h^{1000}(\mathrm{password})</math> and verifies this matches the hash chain it has stored. It then stores <math>h^{999}(\mathrm{password})</math> for the next time the user wishes to authenticate.

An eavesdropper seeing <math>h^{999}(\mathrm{password})</math> communicated to the server will be unable to re-transmit the same hash chain to the server for [[authentication]] since the server now expects <math>h^{998}(\mathrm{password})</math>. Due to the [[One-way function|one-way property]] of [[Cryptographic hash function|cryptographically secure hash function]]s, it is infeasible for the eavesdropper to reverse the hash function and obtain an earlier piece of the hash chain. In this example, the user could authenticate 1000 times before the hash chain were exhausted. Each time the hash value is different, and thus cannot be duplicated by an attacker.

== Binary hash chains ==
((main article|Merkle tree))

Binary hash chains are commonly used in association with a [[Merkle tree|hash tree]]. A binary hash chain takes two hash values as inputs, concatenates them and applies a hash function to the result, thereby producing a third hash value.

[[File:Hashtreehashchainjux.png|Hash tree and hash chain]]

The above diagram shows a hash tree consisting of eight leaf nodes and the hash chain for the third leaf node. In addition to the hash values themselves the order of concatenation (right or left 1,0) or "order bits" are necessary to complete the hash chain.

== Winternitz chains ==
((vanchor|Winternitz chain|text=Winternitz chains)) (also known as '''function chains'''((sfn|Hülsing|2013b|pp=18-20))) are used in [[hash-based cryptography]]. The chain is parameterized by the ((vanchor|Winternitz parameter|text='''Winternitz parameter''')) ''w'' (number of bits in a "digit" ''d'') and ''security  parameter'' ''n'' (number of bits in the hash value, typically double the [[security strength]],((sfn|Buchmann|Dahmen|Ereth|Hülsing|2011|p=2)) 256 or 512). The chain consists of <math>2^w</math> values that are results of repeated application of a [[One-way function|one-way "chain" function]] ''F'' to a secret key ''sk'': <math>sk, F(sk), F(F(sk)), ..., F^{2^{w-1))(sk)</math>. The chain function is typically based on a standard [[cryptographic hash]], but needs to be parameterized ("randomized"((sfn|Hülsing|2013b))), so it involves few invocations of the underlying hash.<ref>((IETF RFC|8391))</ref> In the Winternitz signature scheme a chain is used to encode one digit of the ''m''-bit message, so the Winternitz signature uses approximately <math>mn/w</math> bits, its calculation takes about <math>2^wm/w</math> applications of the function F.((sfn|Buchmann|Dahmen|Ereth|Hülsing|2011|p=2)) Note that some signature standards (like [[Extended Merkle signature scheme]], XMSS) define ''w'' as the number of possible values in a digit, so <math>w=16</math> in XMSS corresponds to <math>w=4</math> in standards  (like [[Leighton-Micali Signature]], LMS) that define ''w'' in the same way as above - as a number of bits in the digit.<ref>[https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-208.pdf NIST SP 800-208, Recommendation for Stateful Hash-Based Signature Schemes], p. 5</ref>

== Hash chain vs. blockchain ==
A hash chain is similar to a [[blockchain]], as they both utilize a [[cryptographic hash function]] for creating a link between two nodes. However, a blockchain (as used by [[Bitcoin]] and related systems) is generally intended to support distributed agreement around a public ledger (data), and incorporates a set of rules for encapsulation of data and associated data permissions.

== See also==
* [[Challenge–response authentication]]
* [[Hash list]] – In contrast to the recursive structure of hash chains, the elements of a hash list are independent of each other.
* [[One-time password]]
* [[Key stretching]]
* [[Linked timestamping]] – Binary hash chains are a key component in linked timestamping.
* [[X.509]]

== References ==
((Reflist))

== Sources ==
* ((cite book | last1 = Buchmann | first1 = Johannes | last2 = Dahmen | first2 = Erik | last3 = Ereth | first3 = Sarah | last4 = Hülsing | first4 = Andreas | last5 = Rückert | first5 = Markus | title = Progress in Cryptology – AFRICACRYPT 2011 | series = Lecture Notes in Computer Science | chapter = On the Security of the Winternitz One-Time Signature Scheme | date = 2011 | volume = 6737 | pages = 363–378 | publisher = Springer Berlin Heidelberg | issn = 0302-9743 | eissn = 1611-3349 | doi = 10.1007/978-3-642-21969-6_23 | isbn = 978-3-642-21968-9 | chapter-url = https://eprint.iacr.org/2011/191.pdf))
* ((cite thesis |type=PhD |last=Hülsing|first=Andreas  |date=2013b |title=Practical Forward Secure Signatures using Minimal Security Assumptions |publisher=[[TU Darmstadt]] | url=http://tuprints.ulb.tu-darmstadt.de/3651/1/Thesis.pdf))
* ((cite book | title = Progress in Cryptology – AFRICACRYPT 2013 | last1 = Hülsing | first1 = Andreas | chapter = W-OTS+ – Shorter Signatures for Hash-Based Signature Schemes | series = Lecture Notes in Computer Science | date = 2013a | volume = 7918 | pages = 173–188 | publisher = Springer Berlin Heidelberg | issn = 0302-9743 | eissn = 1611-3349 | doi = 10.1007/978-3-642-38553-7_10 | isbn = 978-3-642-38552-0 | chapter-url = https://eprint.iacr.org/2017/965.pdf ))
[[Category:Cryptographic algorithms]]

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