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提交 89517a2b 编写于 作者: B Bruce Momjian
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Improve wording of new documentation section on encryption, and move it 

a few sections up.
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doc/src/sgml/runtime.sgml
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<!--
$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.316 2005/05/08 03:29:06 momjian Exp $
$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.317 2005/05/09 17:13:04 momjian Exp $
-->
<chapter Id="runtime">
......@@ -4954,6 +4954,161 @@ $ <userinput>kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid`</userinput
</important>
</sect1>
<sect1 id="encryption-approaches">
<title>Use of Encryption in <productname>PostgreSQL</productname></title>
<indexterm zone="encryption-approaches">
<primary>encryption</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname> offers encryption at several
levels, and provides flexibility in protecting data from disclosure
due to database server theft, unscrupulous administrators, and
insecure networks. Encryption might also be required by government
regulation, for example, for medical records or financial
transactions.
</para>
<variablelist>
<varlistentry>
<term>Password Storage Encryption</term>
<listitem>
<para>
By default, database user passwords are stored as MD5 hashes, so
the administrator can not determine the actual password assigned
to the user. If MD5 encryption is used for client authentication,
the unencrypted password is never even temporarily present on the
server because the client MD5 encrypts it before being sent across
the network. MD5 is a one-way encryption --- there is no
decryption algorithm.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Encryption For Specific Columns</term>
<listitem>
<para>
The <filename>/contrib</> function library
<function>pgcrypto</function> allows certain fields to be stored
encrypted. This is useful if only some of the data is sensitive.
The client supplies the decryption key and the data is decrypted
on the server and then sent to the client.
</para>
<para>
The decrypted data and the decryption key are present on the
server for a brief time while it is being decrypted and
communicated between the client and server. This presents a brief
moment where the data and keys can be intercepted by someone with
complete access to the database server, such as the system
administrator.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Data Partition Encryption</term>
<listitem>
<para>
On Linux, encryption can be layered on top of a filesystem mount
using a <quote>loopback device</quote>. This allows an entire
filesystem partition be encrypted on disk, and decrypted by the
operating system. On FreeBSD, the equivalent facility is called
GEOM Based Disk Encryption, or <acronym>gbde</acronym>.
</para>
<para>
This mechanism prevents unecrypted data from being read from the
drives if the drives or the entire computer is stolen. This
mechanism does nothing to protect against attacks while the
filesystem is mounted, because when mounted, the operating system
provides a unencrypted view of the data. However, to mount the
filesystem, you need some way for the encryption key to be passed
to the operating system, and sometimes the key is stored somewhere
on the host that mounts the disk.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Encrypting Passwords Across A Network</term>
<listitem>
<para>
The <literal>MD5</> authentication method double-encrypts the
password on the client before sending it to the server. It first
MD5 encrypts it based on the user name, and then encrypts it
based on a random salt sent by the server when the database
connection was made. It is this double-encrypted value that is
sent over the network to the server. Double-encryption not only
prevents the password from being discovered, it also prevents
another connection from replaying the same double-encryption
value in a later connection.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Encrypting Data Across A Network</term>
<listitem>
<para>
SSL connections encrypt all data sent across the network: the
password, the queries, and the data returned. The
<filename>pg_hba.conf</> file allows administrators to specify
which hosts can use non-encrypted connections (<literal>host</>)
and which require SSL-encrypted connections
(<literal>hostssl</>). Also, clients can specify that they
connect to servers only via SSL. <application>Stunnel</> or
<application>SSH</> can also be used to encrypt transmissions.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>SSL Host Authentication</term>
<listitem>
<para>
It is possible for both the client and server to provide SSL keys
or certificates to each other. It takes some extra configuration
on each side, but this provides stronger verification of identity
than the mere use of passwords. It prevent a computer from
pretending to be the server just long enough to read the password
send by the client. It also helps prevent 'man in the middle"
attacks where a computer between the client and server pretends to
be the server and reads and passes all data between the client and
server.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Client-Side Encryption</term>
<listitem>
<para>
If the system administrator can not be trusted, it is necessary
for the client to encrypt the data; this way, unencrypted data
never appears on the database server. Data is encrypted on the
client before being sent to the server, and database results have
to be decrypted on the client before being used. Peter Wayner's
book, <citation>Translucent Databases</citation>, discusses how to
do this in considerable detail.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="ssl-tcp">
<title>Secure TCP/IP Connections with SSL</title>
......@@ -5109,132 +5264,6 @@ psql -h localhost -p 3333 template1
</sect1>
<sect1 id="encryption-approaches">
<title>Use of Encryption in <productname>PostgreSQL</productname></title>
<indexterm zone="encryption-approaches">
<primary>encryption</primary>
</indexterm>
<para> There is increasing interest in having verifiable mechanisms
to maintain the privacy of data in databases. In the United
States, legislation called <acronym>HIPAA</acronym> (Health
Insurance Portability and Accountability Act) requires that
personal health information is handled securely. The European
Union has similarly been developing directives as to how personal
data is to be managed there.</para>
<para> Questions frequently come up as to what functionality
<productname>PostgreSQL</productname> offers with regard to
supporting the use of data encryption. It uses and provides use of
encryption tools in several ways that may be useful to provide
protection against certain classes of attacks.</para>
<itemizedlist>
<listitem><para> Passwords stored in MD5 form </para>
<para> Passwords are normally not stored in
<quote>plaintext</quote> form in the database; they are hashed
using the built-in MD5 function, and <emphasis>that</emphasis> is
what is stored in the database. </para>
<programlisting>
sample=# alter user foo password 'some dumb value';
ALTER USER
sample=# select usename, passwd from pg_shadow where usename = 'foo';
usename | passwd
---------+-------------------------------------
foo | md5740daa4aaa084d85eb97648084a43bbb
(1 row)
</programlisting>
</listitem>
<listitem><para> Connections protected using SSL</para>
<para> There are various options to control how mandatory it is
to use SSL to protect data connections. At the most
<quote>paranoid</quote> end of the spectrum, you can configure
<filename>pg_hba.conf</filename> to have the database reject
connections that do <emphasis>not</emphasis> come in via
SSL.</para>
<para> The use of SSL, alone, is useful for protecting
communications against interception. It may not be necessary
for connections that take place across a carefully controlled
network; if connections are coming in from less controlled
sources, its use is highly recommended.</para></listitem>
<listitem><para> Connections authenticated using SSL</para>
<para> It is possible for both the client and server to provide
to one another SSL keys or certificates. It takes some extra
configuration on each side where these are used, but this likely
provides stronger verification of identity than the mere use of a
text password. </para></listitem>
<listitem><para> Using OS level encryption for entire database
partitions</para>
<para> On Linux, encryption can be layered on top of a filesystem
mount using what is called a <quote>loopback device;</quote> this
permits having a whole filesystem partition be encrypted on disk,
decrypted by the operating system. On FreeBSD, the equivalent
facility is called GEOM Based Disk Encryption, or
<acronym>gbde</acronym>.</para>
<para> This mechanism may be expected to be useful for protecting
against the threat that someone might pull disk drives out and
try to install them somewhere else to draw data off of them.
</para>
<para> In contrast, this mechanism does nothing to protect
against attacks when the filesystem is mounted, because when
mounted, the OS provides a <quote>view</quote> of the filesystem
accessible in plain text form. Furthermore, you need some way
for the encryption key to be passed to the operating system in
order to mount the filesystems, which encourages having the key
accessible somewhere on the host that mounts the disk.
</para></listitem>
<listitem><para> Using the contrib function library
<function>pgcrypto</function> so the database engine manages
encryption of certain fields.</para>
<para>If much of the data can be in plain text form, and only a
subset is particularly sensitive, this mechanism supports
treating them differently. The encrypted data is only ever
presented in <quote>unencrypted</quote> form while it is being
communicated between client and server, and the use of an SSL
layer of <quote>superencryption</quote> alleviates that
problem.</para>
<para> Unfortunately, in this approach, the encryption keys need
to be present on the server, even if only for a moment, which
presents the possibility of them being intercepted by someone
with access to the database server. As a result, this mechanism
is not suitable for storage of data that is too sensitive for
system administrators to have access to it. </para></listitem>
<listitem><para> Using cryptographic tools on the client </para>
<para> If it is not safe to trust the system administrators at
least somewhat, you may find it necessary to encrypt data at the
client level such that unencrypted data never appears on the
database server. This sort of <quote>paranoia</quote> is quite
appropriate for applications where it would be damaging for data
to be seen by inappropriate readers that might generally be
considered trustworthy, as can be the case with
medical and legal records.</para>
<para> Peter Wayner's book, <citation>Translucent
Databases</citation>, discusses how to do this in considerable
detail.</para></listitem>
</itemizedlist>
</sect1>
</chapter>
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