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>17.8. Encryption Options</A
></H1
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 to secure
sensitive data such as medical records or financial transactions.
</P
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><DT
>Password Storage Encryption</DT
><DD
><P
> By default, database user passwords are stored as MD5 hashes, so
the administrator cannot 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.
</P
></DD
><DT
>Encryption For Specific Columns</DT
><DD
><P
> The <A
HREF="pgcrypto.html"
>pgcrypto</A
> module 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.
</P
><P
> 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.
</P
></DD
><DT
>Data Partition Encryption</DT
><DD
><P
> On Linux, encryption can be layered on top of a file system
using a <SPAN
CLASS="QUOTE"
>"loopback device"</SPAN
>. This allows an entire
file system partition to be encrypted on disk, and decrypted by the
operating system. On FreeBSD, the equivalent facility is called
GEOM Based Disk Encryption (<ACRONYM
CLASS="ACRONYM"
>gbde</ACRONYM
>), and many
other operating systems support this functionality, including Windows.
</P
><P
> This mechanism prevents unencrypted data from being read from the
drives if the drives or the entire computer is stolen. This does
not protect against attacks while the file system is mounted,
because when mounted, the operating system provides an unencrypted
view of the data. However, to mount the file system, 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.
</P
></DD
><DT
>Encrypting Passwords Across A Network</DT
><DD
><P
> The <TT
CLASS="LITERAL"
>MD5</TT
> 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 using the same encrypted password to
connect to the database server at a later time.
</P
></DD
><DT
>Encrypting Data Across A Network</DT
><DD
><P
> SSL connections encrypt all data sent across the network: the
password, the queries, and the data returned. The
<TT
CLASS="FILENAME"
>pg_hba.conf</TT
> file allows administrators to specify
which hosts can use non-encrypted connections (<TT
CLASS="LITERAL"
>host</TT
>)
and which require SSL-encrypted connections
(<TT
CLASS="LITERAL"
>hostssl</TT
>). Also, clients can specify that they
connect to servers only via SSL. <SPAN
CLASS="APPLICATION"
>Stunnel</SPAN
> or
<SPAN
CLASS="APPLICATION"
>SSH</SPAN
> can also be used to encrypt transmissions.
</P
></DD
><DT
>SSL Host Authentication</DT
><DD
><P
> It is possible for both the client and server to provide SSL
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 prevents a computer from
pretending to be the server just long enough to read the password
sent by the client. It also helps prevent <SPAN
CLASS="QUOTE"
>"man in the middle"</SPAN
>
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.
</P
></DD
><DT
>Client-Side Encryption</DT
><DD
><P
> If the system administrator for the server's machine cannot 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.
</P
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