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<concept id="GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8" xml:lang="en"><title>Symmetric
ciphers -- guide</title><prolog><metadata><keywords/></metadata></prolog><conbody>
<ul>
<li id="GUID-A66ADA26-979E-5B4A-824F-29C182AFCB24"><p> <xref href="GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8.dita#GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8/GUID-6F996812-1D73-509B-9CED-DD672728D597">What are symmetric ciphers?</xref>  </p> </li>
<li id="GUID-46055BED-B538-5B49-BD43-A2E3AA3368E8"><p> <xref href="GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8.dita#GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8/GUID-BDF6E245-AE19-55D6-89ED-BCBE0FCF006B">Block and stream ciphers</xref>  </p> </li>
<li id="GUID-5B9FF3E3-3732-5A75-B429-23B318C0CCBF"><p> <xref href="GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8.dita#GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8/GUID-A4E43A90-A66F-5868-BD94-DA709B75431B">Types of symmetric algorithms supported</xref>  </p> </li>
<li id="GUID-26930CA6-C2AF-58DB-87AB-803481AA3DED"><p> <xref href="GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8.dita#GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8/GUID-66E8AD5D-A9FF-51E3-897D-D5EC3D66903A">Base classes and their derived classes</xref>  </p> </li>
</ul>
<section id="GUID-6F996812-1D73-509B-9CED-DD672728D597"><title>What are symmetric
ciphers? </title> <p>Symmetric algorithms are much faster than asymmetric
algorithms so are used to encrypt and decrypt large amounts of data. In an
informal setting, symmetric ciphers can be thought of as a mapping of some
plaintext to ciphertext, via some well-known transformation function, dependent
on a single secret key. Symmetric algorithms have the property that if a message
is encrypted under a given key, it can only be decrypted using the same key: </p> <fig id="GUID-FDB090BE-13DA-5941-8403-F42C3DF880BF">
<title> The diagram above shows the encryption and decryption process using:
a symmetric algorithm; a plaintext message, M; a symmetric key, K; and the
ciphertext, K(M).</title>
<image href="GUID-669190F8-3BE9-58FC-B689-00F06FDAD74D_d0e363158_href.png" placement="inline"/>
</fig> <p>So it provides secrecy and also some kind of authentication. If
Alice encrypts a secret using a key known only to her, then only she can access
the secrets. </p><p>It can also be used for communication: if Alice shares
a key with Bob (and only Bob), then she can encrypt her messages with that
key and send it to Bob, and only Bob can decrypt them. In this case the key
is a 'shared secret' enabling private communications. </p><p>There are two
basic types of symmetric ciphers: <xref href="GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8.dita#GUID-712DF59D-FAE1-592E-82A6-4E323676F5B8/GUID-BDF6E245-AE19-55D6-89ED-BCBE0FCF006B">block
ciphers and stream ciphers</xref>. </p> </section>
<section id="GUID-BDF6E245-AE19-55D6-89ED-BCBE0FCF006B"><title>Block and stream
ciphers</title> <ul>
<li id="GUID-D0F3A85C-2DB3-57BE-A12B-102943DB68C9"><p> <b> Stream ciphers</b> are
essentially functions that are initialized with a key, and output a stream
of pseudorandom bits, this 'keystream' is typically XOR-ed with the plaintext
to generate the ciphertext. So they encrypt a bit of plaintext at a time.
They map an <i>n</i> -bit stream of plaintext to a <i>n</i> -bit stream of
ciphertext. </p> </li>
<li id="GUID-25F8F07B-C60E-5B6D-9224-B7E4E89D15A3"><p> <b>Block ciphers</b> encrypt
several bits at once in a fixed-size block. That is, they map <i>m</i>  <i>n</i> -bit
blocks of plaintext to <i>m</i>  <i>n</i> -bit blocks of ciphertext. The cipher
and its mode of operation define the block size: the plaintext is split up
into appropriately-sized blocks and each block is fed into the cipher. </p> <p>There
are two issues here that don't occur with stream ciphers: </p> <ul>
<li id="GUID-2C88F6DF-B8F8-5DF3-8955-03DA02371DEC"><p> <b> padding</b>: the
total size of the input has to be a multiple of the block size, so the plaintext
usually has to be padded to fit (see the class <xref href="GUID-FFD4A90B-CCF7-33D2-802A-A44E2434FAE9.dita"><apiname>CPadding</apiname></xref>).
Optionally, instead of padding out a plaintext message to fit in a block,
block ciphers allow buffering of partial input blocks until the remainder
of the block is given as input. (see <xref href="GUID-CFF1BCCA-5D07-5B8A-9363-AD11EEEAB485.dita#GUID-CFF1BCCA-5D07-5B8A-9363-AD11EEEAB485/GUID-3393A9D6-CB78-5740-B250-F9C1C26C59BD">How
does buffering work within the symmetric cipher framework?</xref> and <xref href="http://www.rsasecurity.com/rsalabs/node.asp?id=2129" scope="external">PKCS#7</xref>). </p> </li>
<li id="GUID-EE6C6890-8F7E-50AD-AEB9-10C43E931DE9"><p> <b>combining blocks</b>:
if you just encrypt each plaintext block with the cipher and then concatenate
the ciphertext blocks, then an attacker who knows something about the structure
can switch the order of the ciphertext blocks to alter the meaning of the
message. For example, if the plaintext includes instructions like 'pay £XXX
to account holder ABC, reference number YYY', then an attacker could replace
the encryption of XXX with some of the encryption of YYY. This leads to other
techniques for combining blocks, called modes, where, for example, the output
of the last block is fed into the input for the next one. So, the output is
not just a function of that plaintext block and key, but is a function of
that plaintext block, the key, and all preceding ciphertext blocks (see <xref href="GUID-CFF1BCCA-5D07-5B8A-9363-AD11EEEAB485.dita#GUID-CFF1BCCA-5D07-5B8A-9363-AD11EEEAB485/GUID-71CD8B41-219D-5D07-8C99-47D68668A880">Symmetric
Modes</xref>). </p> </li>
</ul> </li>
</ul> <p>Both stream ciphers and block ciphers are provided by the API with
a similar interface. Block ciphers usually need padding for non completed
blocks. The API allows the user to choose the padding to be added at the end
of the last encrypted block or to be checked and removed from the last decrypted
block. </p> </section>
<section id="GUID-A4E43A90-A66F-5868-BD94-DA709B75431B"><title>Types of symmetric
algorithms supported</title> <p>The following symmetric algorithms are supported: </p> <table id="GUID-25F43544-4556-544B-B010-76B0E091B80C">
<tgroup cols="3"><colspec colname="col0"/><colspec colname="col1"/><colspec colname="col2"/>
<thead>
<row>
<entry>Symmetric algorithm</entry>
<entry>Type</entry>
<entry>Specified in:</entry>
</row>
</thead>
<tbody>
<row>
<entry><p>AES (Advanced Encryption Standard) </p> </entry>
<entry><p>Block cipher </p> </entry>
<entry><p> <xref href="http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf" scope="external">FIPS-197</xref> </p> </entry>
</row>
<row>
<entry><p>DES (Data Encryption Standard) </p> </entry>
<entry><p>Block cipher </p> </entry>
<entry><p> <xref href="http://csrc.nist.gov/publications/fips/fips46-3/fips46-3.pdf" scope="external">FIPS 46-3</xref>  </p> </entry>
</row>
<row>
<entry><p>3DES (Triple Data Encryption Standard) </p> </entry>
<entry><p>Block cipher </p> </entry>
<entry><p> <xref href="http://csrc.nist.gov/publications/fips/fips46-3/fips46-3.pdf" scope="external">FIPS 46-3</xref>  </p> </entry>
</row>
<row>
<entry><p>RC2-128 </p> </entry>
<entry><p>Block cipher </p> </entry>
<entry><p> <xref href="ftp://ftp.rfc-editor.org/in-notes/rfc2268.txt" scope="external">RFC
2268</xref>  </p> </entry>
</row>
<row>
<entry><p>ARC4 ('alleged' RC4) </p> </entry>
<entry><p>Stream cipher </p> </entry>
<entry><p>the internet and a posting to sci.crypt in 1994. </p> </entry>
</row>
</tbody>
</tgroup>
</table> </section>
<section id="GUID-66E8AD5D-A9FF-51E3-897D-D5EC3D66903A"><title>Base classes
and their derived classes</title> <p>The symmetric cipher API is used by Networking
(TLS/IPSec). </p> <p> <xref href="GUID-F4E08165-A654-3D32-8FED-7ED54BDAD88B.dita"><apiname>CSymmetricCipher</apiname></xref> is the abstract
base class that allows a client to use the supported symmetric algorithms
listed above. It allows clients to encrypt and decrypt without having to know
anything about which encryption algorithm is currently in use. </p> <p>The
PKCS5 API (the <codeph>TPKCS5KDF</codeph> class, also see <xref href="GUID-0C7CDC47-6B42-5A20-BED8-086DA81D272E.dita">Password
Based Encryption</xref>) offers Key Derivation Support that allows the derivation
of deterministic arbitrary length byte streams from an input string. The output
byte stream is generated using multiple iterations of a SHA-1 message digest
(<codeph>CSHA1</codeph>) and is suitable for use as a cryptographic symmetric
key. </p> <p>The diagrams below show the main classes used in symmetric cipher
framework. Blue dotted arrows indicate that a class is contained or used by
another class. The arrows are labelled with the variable(s) through which
the pointed class is accessible. The colour of the boxes indicates the type
of Symbian class, i.e., <codeph>M</codeph>, <codeph>C</codeph>, <codeph>R</codeph> or <codeph>T</codeph> class.
For detailed information on each component see the Cryptography API Reference
material. </p> <p><b><xref href="GUID-F4E08165-A654-3D32-8FED-7ED54BDAD88B.dita"><apiname>CSymmetricCipher</apiname></xref> and derived classes</b> </p> <fig id="GUID-B11D8EE9-F078-57B2-9453-3ABF1FC0AAA1">
<title>The inheritance diagram shows the <codeph>CSymmetricCipher</codeph> abstract
base class and its derived abstract classes <codeph>CBufferedTransformation</codeph> and <codeph>CStreamCipher</codeph> used
for block and stream ciphers respectively. Also shown are the following derived
classes: <codeph>CBufferedDecryptor</codeph>, <codeph>CBufferedEncryptor</codeph>, <codeph>CARC4</codeph>,
and <codeph>CNullCipher</codeph>.</title>
<image href="GUID-5F72210C-1636-584D-9D89-987D25136975_d0e363464_href.png" placement="inline"/>
</fig> <p><b><xref href="GUID-CFDA5321-EE13-3203-8DED-71E69D4469BD.dita"><apiname>CBlockTransformation</apiname></xref> and derived classes</b> </p> <fig id="GUID-EB7BFBBF-D41C-5A3A-AA53-3F5BDA019C2A">
<title>The inheritance diagram above shows the <xref href="GUID-CFDA5321-EE13-3203-8DED-71E69D4469BD.dita"><apiname>CBlockTransformation</apiname></xref> abstract
base class used for block ciphers. Also shown are the following derived classes: <xref href="GUID-436C3EBE-FC60-3760-A3BA-D8DF8FA5B8AF.dita"><apiname>CBlockChainingMode</apiname></xref>, <xref href="GUID-C65A1F55-A8D2-3393-8DC1-35656017E2B8.dita"><apiname>CModeCBCEncryptor</apiname></xref>, <xref href="GUID-7D5FBD2C-83A0-351A-96B3-C5C54FE525E6.dita"><apiname>CModeCBCDecryptor</apiname></xref>, <xref href="GUID-1C18ED2F-085F-3C3E-A93E-5FD37461E440.dita"><apiname>CDES</apiname></xref>, <xref href="GUID-E7401D77-AD0E-3B8F-A7CC-23ADA5151DB6.dita"><apiname>CDESEncryptor</apiname></xref>, <xref href="GUID-249270AC-907B-3E46-AFDB-131FDC6F612A.dita"><apiname>CDESDecryptor</apiname></xref>, <xref href="GUID-90A671B0-756E-3773-8429-6441D1594F4B.dita"><apiname>C3DES</apiname></xref>, <xref href="GUID-B931DBB9-4484-33FD-9E94-6F256ABD5C68.dita"><apiname>C3DESEncryptor</apiname></xref>, <xref href="GUID-C693C22D-04C5-3C9E-8605-0EA90F723AF3.dita"><apiname>C3DESDecryptor</apiname></xref>, <xref href="GUID-BEE18CDE-CFB6-3116-9FAE-046780D4D006.dita"><apiname>CRC2</apiname></xref>, <xref href="GUID-19F20C86-5722-3A35-B17D-3830D18CC7FC.dita"><apiname>CRC2Encryptor</apiname></xref>, <xref href="GUID-E6FD8DF9-258B-39BC-8C39-ABF7434A29B5.dita"><apiname>CRC2Decryptor</apiname></xref>, <xref href="GUID-B0AEE24A-91A6-335E-AD64-C9DDCC1F81A9.dita"><apiname>CRijndael</apiname></xref>, <xref href="GUID-AE1A9AC0-DB79-3C62-AA23-896812F25F14.dita"><apiname>CAESEncryptor</apiname></xref>, and <xref href="GUID-51666543-0256-3E0D-BF5F-8716753C5921.dita"><apiname>CAESDecryptor</apiname></xref>.</title>
<image href="GUID-7461BD6A-F50B-5E14-8995-CF3F2D8F5F14_d0e363544_href.png" placement="inline"/>
</fig> <p id="GUID-A8F9A25F-B83E-5FE7-840F-4DCF246D3D96"><b>CPadding and derived
classes</b> </p> <fig id="GUID-648F7604-A3C6-57EB-9B34-7F8079549D9C">
<title>Above is an inheritance diagram showing the CPadding abstract base
class used with block ciphers. Also shown are the following derived classes: <xref href="GUID-B7090C61-ECC7-3E8A-8D19-75C9170B0135.dita"><apiname>CPaddingPKCS7</apiname></xref>, <xref href="GUID-3150ECC9-CF8E-3890-91CD-87F5EBCE550E.dita"><apiname>CPaddingPKCS1Encryption</apiname></xref>, <xref href="GUID-F845CB30-7ABE-3EB6-B1B9-C72581897D0C.dita"><apiname>CRSAPKCS1v15Encryptor</apiname></xref>, <xref href="GUID-57C40DCA-8340-38B5-9777-2863C8F88B59.dita"><apiname>CPaddingNone</apiname></xref>, <xref href="GUID-723F64D1-01C5-3A03-B987-0FB862EE8EDE.dita"><apiname>CPaddingPKCS1Signature</apiname></xref>,
and <xref href="GUID-CC2347BE-3272-3DDF-8BDD-B44F5C0026B3.dita"><apiname>CPaddingSSLv3</apiname></xref></title>
<image href="GUID-0FD02CAD-B687-50C0-8E44-74ED9B4A936E_d0e363580_href.png" placement="inline"/>
</fig> </section>
</conbody></concept>