Re: question on cipher suites

This is a multipart message in MIME format.
--=_alternative 007FB66A80257027_=
Content-Type: text/plain; charset="US-ASCII"

I would say that an FTP/TLS server implementation (I.E. code) which does 
not allow the user/administrator to change CipherSuites, is broken.

The choice of CipherSuites should be a security decision, based around a 
risk analysis from the Business.  How a developer can assume that that 
analysis can have only one outcome is beyond understanding.

I would be against defining a 'minimum CipherSuite' or some-such in AS3 (I 
have not defined one in the FTP/TLS document); as yesterday's 'minimum' is 
tomorrow's 'trivially broken' and providing a proscriptive method to force 
insecurity seems wrong to me.

If two peers wish to share business data, the security controls around 
that data should be mutually decided and not left to a technical 
specification.  The basic way that this mutual decision is embodied, is in 
the TLS Handshake's CipherSuite negotiation; I see no compelling reason to 
augment or restrict that in a higher level protocol document.

If anything, AS3 should mandate that the allowed CipherSuite values MUST 
be configurable on both Client and Server.  IIRC, AS2 is silent on this - 
https (if used) has the same problem.

Paul

--
Paul Ford-Hutchinson, CISSP :  eCommerce application security
e: [email protected]
e: [email protected]
p: MPT-6, IBM , PO Box 31, Birmingham Rd, Warwick, CV34 5JL 
t: +44 (0)1926 462005
w: http://www.ford-hutchinson.com/~fh-1-pfh/ftps-ext.html




"Kyle Meadors" <[email protected]> 
Sent by: [email protected]
21/06/2005 19:58

To
<[email protected]>
cc

Subject
question on cipher suites






An issue was recently brought up in regards to using TLS in AS3. Within 
the TLS handshaking, the connecting AS3 application only uses one cipher, 
3DES, in the handshaking. In this case, the FTP server receiving the 
connection does not support 3DES but does support others. Since the AS3 
app does not support anything but 3DES, it can not work through the 
handshaking to find a cipher both agree on.
 
Would it be necessary to state something within the AS3 draft about 
supporting a specific set of ciphers. Or, is this outside the scope of AS3 
since it may lie only with the FTP server be beyond the control of the AS3 
application.
 
Kyle Meadors
Principal, Test Process
Drummond Group Inc.
615.384.5006
 

--
No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.323 / Virus Database: 267.7.8/22 - Release Date: 6/17/2005

--=_alternative 007FB66A80257027_=
Content-Type: text/html; charset="US-ASCII"


<br><font size=2 face="sans-serif">I would say that an FTP/TLS server implementation
(I.E. code) which does not allow the user/administrator to change CipherSuites,
is broken.</font>
<br>
<br><font size=2 face="sans-serif">The choice of CipherSuites should be
a security decision, based around a risk analysis from the Business. &nbsp;How
a developer can assume that that analysis can have only one outcome is
beyond understanding.</font>
<br>
<br><font size=2 face="sans-serif">I would be against defining a 'minimum
CipherSuite' or some-such in AS3 (I have not defined one in the FTP/TLS
document); as yesterday's 'minimum' is tomorrow's 'trivially broken' and
providing a proscriptive method to force insecurity seems wrong to me.</font>
<br>
<br><font size=2 face="sans-serif">If two peers wish to share business
data, the security controls around that data should be mutually decided
and not left to a technical specification. &nbsp;The basic way that this
mutual decision is embodied, is in the TLS Handshake's CipherSuite negotiation;
I see no compelling reason to augment or restrict that in a higher level
protocol document.</font>
<br>
<br><font size=2 face="sans-serif">If anything, AS3 should mandate that
the allowed CipherSuite values MUST be configurable on both Client and
Server. &nbsp;IIRC, AS2 is silent on this - https (if used) has the same
problem.</font>
<br>
<br><font size=2 face="sans-serif">Paul</font>
<br>
<br><font size=2 face="sans-serif">--<br>
Paul Ford-Hutchinson, CISSP : &nbsp;eCommerce application security<br>
e: [email protected]<br>
e: [email protected]<br>
p: MPT-6, IBM , PO Box 31, Birmingham Rd, Warwick, CV34 5JL <br>
t: +44 (0)1926 462005<br>
w: http://www.ford-hutchinson.com/~fh-1-pfh/ftps-ext.html<br>
</font>
<br>
<br>
<br>
<table width=100%>
<tr valign=top>
<td width=40%><font size=1 face="sans-serif"><b>&quot;Kyle Meadors&quot;
&lt;[email protected]&gt;</b> </font>
<br><font size=1 face="sans-serif">Sent by: [email protected]</font>
<p><font size=1 face="sans-serif">21/06/2005 19:58</font>
<td width=59%>
<table width=100%>
<tr>
<td>
<div align=right><font size=1 face="sans-serif">To</font></div>
<td valign=top><font size=1 face="sans-serif">&lt;[email protected]&gt;</font>
<tr>
<td>
<div align=right><font size=1 face="sans-serif">cc</font></div>
<td valign=top>
<tr>
<td>
<div align=right><font size=1 face="sans-serif">Subject</font></div>
<td valign=top><font size=1 face="sans-serif">question on cipher suites</font></table>
<br>
<table>
<tr valign=top>
<td>
<td></table>
<br></table>
<br>
<br>
<br><font size=2 face="Arial">An issue was recently brought up in regards
to using TLS in AS3. Within the TLS handshaking, the connecting AS3 application
only uses one cipher, 3DES, in the handshaking. In this case, the FTP server
receiving the connection does not support 3DES but does support others.
Since the AS3 app does not support anything but 3DES, it can not work through
the handshaking to find a cipher both agree on.</font>
<br><font size=2 face="Arial">&nbsp;</font>
<br><font size=2 face="Arial">Would it be necessary to state something
within the AS3 draft about supporting a specific set of ciphers. Or, is
this outside the scope of AS3 since it may lie only with the FTP server
be beyond the control of the AS3 application.</font>
<br><font size=2 face="Arial">&nbsp;</font>
<br><font size=2 face="Arial">Kyle Meadors</font>
<br><font size=2 face="Arial">Principal, Test Process</font>
<br><font size=2 face="Arial">Drummond Group Inc.</font>
<br><font size=2 face="Arial">615.384.5006</font>
<br><font size=3 face="Times New Roman">&nbsp;</font>
<p>
<p><font size=2>--<br>
No virus found in this outgoing message.<br>
Checked by AVG Anti-Virus.<br>
Version: 7.0.323 / Virus Database: 267.7.8/22 - Release Date: 6/17/2005</font>
<p>
--=_alternative 007FB66A80257027_=--