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<concept id="GUID-73491C1D-6C74-5434-B2F3-4AF416AE37B7" xml:lang="en"><title>Overview

of hash algorithms</title><prolog><metadata><keywords/></metadata></prolog><conbody>

<ul>

<li id="GUID-81D29BE4-82D6-5880-8612-1A55415A6730"><p><xref href="GUID-73491C1D-6C74-5434-B2F3-4AF416AE37B7.dita#GUID-73491C1D-6C74-5434-B2F3-4AF416AE37B7/GUID-AEB1ABD2-4977-5A54-86AC-F1D87615C57F">What is a cryptographic hash?</xref>  </p> </li>

<li id="GUID-716D3CA9-5151-5E18-BD61-216A538E037C"><p><xref href="GUID-73491C1D-6C74-5434-B2F3-4AF416AE37B7.dita#GUID-73491C1D-6C74-5434-B2F3-4AF416AE37B7/GUID-1F051CF3-77A8-5408-B14B-A7FD5CFF197F">What is an HMAC?</xref>  </p> </li>

<li id="GUID-C9475D12-31D6-5CF8-BF82-6B37F241B2EF"><p><xref href="GUID-73491C1D-6C74-5434-B2F3-4AF416AE37B7.dita#GUID-73491C1D-6C74-5434-B2F3-4AF416AE37B7/GUID-08E26A60-A8B1-5F54-901D-69787BC8950E">The base class and its derived classes</xref>  </p> </li>

</ul>

<section id="GUID-AEB1ABD2-4977-5A54-86AC-F1D87615C57F"><title>What is a cryptographic

hash?</title> <p>A cryptographic hash algorithm (also known as a message digest,

a one-way function, or simply a hash) takes a variable-length input (the message)

and produce a fixed length output known as the hash (or digest) of the input.

It is often useful to think of the resulting output as a representation, or

fingerprint, of the original input. </p> <p>There are two properties that

are important to cryptographic hashes: </p> <ul>

<li id="GUID-2868C035-9933-5EF2-86E7-E696ADF3A67D"><p>it must be hard to find

collisions, i.e. it is highly unlikely that two distinct strings will hash

to the same output </p> </li>

<li id="GUID-43BF26B8-466F-589B-B8F5-B4F6EF42504F"><p>it is extremely difficult

to determine the original input given only the output. Even very small changes

to the input will give rise to radical changes in the output. </p> </li>

</ul> <p>These properties make hash functions useful in cryptography and other

applications as they allow the representation of objects in a known fixed

size. </p> <p>The hash algorithms supported by the Symbian platform

are: </p> <table id="GUID-D9C86D99-3790-5332-988E-E38FA935DE3E">

<tgroup cols="2"><colspec colname="col0"/><colspec colname="col1"/>

<thead>

<row>

<entry>Hash algorithms</entry>

<entry>Further information</entry>

</row>

</thead>

<tbody>

<row>

<entry><p>MD4 </p> </entry>

<entry><p> <xref href="http://www.ietf.org/rfc/rfc1320.txt" scope="external">RFC

1320</xref>  </p> </entry>

</row>

<row>

<entry><p>MD5 </p> </entry>

<entry><p> <xref href="http://www.ietf.org/rfc/rfc1321.txt" scope="external">RFC

1321</xref>  </p> </entry>

</row>

<row>

<entry><p>SHA-1 </p> </entry>

<entry><p> <xref href="http://www.itl.nist.gov/fipspubs/fip180-1.htm" scope="external">FIPS

180-1</xref> and <xref href="http://www.ietf.org/rfc/rfc3174.txt" scope="external">RFC

3174</xref>  </p> </entry>

</row>

<row>

<entry><p>HMAC </p> </entry>

<entry><p> <xref href="http://www.ietf.org/rfc/rfc2104.txt" scope="external">RFC

2104</xref>  </p> </entry>

</row>

</tbody>

</tgroup>

</table> <p>There is also support for MD2 (see <xref href="http://www.ietf.org/rfc/rfc1319.txt" scope="external">RFC 1319</xref>) and SHA, which is there for backward compatibility,

but it is not to be used in new code. </p> </section>

<section id="GUID-1F051CF3-77A8-5408-B14B-A7FD5CFF197F"><title>What is an

HMAC?</title> <p>Unlike the hashes mentioned above, HMAC (Hashed Message Authentication

Code) is a key-dependant hash. It allows a key to be specified at creation

of the HMAC. Only people with that key can verify the hash. HMACs are useful

when authentication but not secrecy of a message is required. </p> <p>The

sender appends to the message data an authentication tag (which is a function

of the data and the shared key). The recipient recomputes the authentication

tag on the received message using the shared key. The integrity of the message

is deemed valid only if the two authentication tags match. </p> <p>For further

details see <xref href="http://www.ietf.org/rfc/rfc2104.txt" scope="external">RFC

2104</xref>. </p> </section>

<section id="GUID-08E26A60-A8B1-5F54-901D-69787BC8950E"><title>The Hash API</title> <p> <codeph>CMessageDigest</codeph> is

the base class for all hash algorithms. </p> <p>The diagram below show the

main classes used in the hash framework. For information on each class see

the Cryptography API Reference material. </p> <fig id="GUID-08CF0C25-D314-56B0-9667-4CDDA925F786">

<title>              Inheritance diagram for the Hash API            </title>

<image href="GUID-8761F82E-5DC4-5BD5-A422-2EC45A92925B_d0e383456_href.png" placement="inline"/>

</fig> <p>The hash API is used internally by the Security components: AppInst,

Certman (Certificate Management) and Cryptography. Networking/TLS uses a pseudo-random

function (PRF) based on HMAC. Both MD5 and SHA-1 are used in TLS. </p> </section>