[SSL-Talk List FAQ] Secure Sockets Layer Discussion List FAQ v1.1.1
From: Shannon Appel (SAppel_at_consensus.com)
Date: 09 Aug 2003 10:12:01 GMT
title="[SSL-Talk List FAQ] Secure Sockets Layer Discussion List FAQ v1.1.1"
Last-modified: Nov 16 12:00:00 PST 1998
Version: 1.1.1 (text) Mon Nov 16 12:00:00 PST 1998
Copyright-Notice: (c) Copyright 1996-1998 by Consensus Development Corporation -- All Rights Reserved
Secure Sockets Layer Discussion List FAQ v1.1.1
Mon Nov 16 12:00:00 PST 1998
FAQ Maintained by:
Shannon Appel <SAppel@consensus.com>
Consensus Development Corporation
The latest edition of this FAQ can always be found at:
Copyright (c) 1996-1998 Consensus Development Corporation - All Rights
Due to the November 15, 1998 dissolution of the SSL-Talk mailing
list, this will be the last version of this FAQ in its current form.
It will be replaced by a more general TLS & SSL FAQ in the near
future that is not tied to any mailing list or newsgroup.
All information contained in this work is provided "as is." All
warranties, expressed, implied or statutory, concerning the accuracy
of the information of the suitability for any particular use are
hereby specifically disclaimed. While every effort has been taken to
ensure the accuracy of the information contained in this work,
the authors assume(s) no responsibility for errors or omissions or
for damages resulting from the use of the information contained
This work may be copied in any printed or electronic form for
non-commercial, personal, or educational purposes if the work is not
modified in any way, provided that the copyright notice, the notices
of any other author included in this work, and this copyright
agreement appear on all copies.
Consensus Development Corporation also grants permission to
distribute this work in electronic form over computer networks for
other purposes, provided that, in addition to the terms and
restrictions set forth above, Consensus Development Corporation
and/or other cited authors are notified and that no fees are charged
for access to the information in excess of normal online charges
that are required for such distribution.
This work may also be mentioned, cited, referred to or described
(but not copied or distributed, except as authorized above) in
printed publications, on-line services, other electronic
communications media, and otherwise, provided that Consensus
Development Corporation and any other cited author receives
Comments about, suggestions about, or corrections to this document
are welcomed. If you would like to ask us to change this document
in some way, the method we appreciate most is for you to actually
make the desired modifications to a copy of the posting, and then to
send us the modified document, or a context diff between the posted
version and your modified version (if you do the latter, make sure
to include in your mail the "Version:" line from the posted
version). Submitting changes in this way makes dealing with them
easier for us and helps to avoid misunderstandings about what you
Many people have in the past provided feedback and corrections; we
thank them for their input.
In particular, many thanks to:
Christopher Allen <ChristopherA@consensus.com>
Shannon Appel <SAppel@consensus.com>
Nelson Bolyard <NelsonB@netscape.com>
Tim Dierks <TimD@consensus.com>
Eric Greenberg <firstname.lastname@example.org>
Charles Neerdaels <email@example.com>
Bruce Schneier <firstname.lastname@example.org>
Tom Weinstein <email@example.com>
Jonathan Zamick <JonathanZ@consensus.com>
Remaining ambiguities, errors, and difficult-to-read passages are
not their fault. :)
1) THE SSL-TALK LIST
2) GENERAL SSL QUESTIONS
3) USING PROXIES, GATEWAYS AND FIREWALLS WITH SSL
4) SSL PROTOCOL QUESTIONS
5) CERTIFICATE RELATED QUESTIONS
6) SSL IMPLEMENTATION QUESTIONS
6.1) NETSCAPE QUESTIONS
6.2) MICROSOFT QUESTIONS
7) SSL TOOLKIT QUESTIONS
7.1) SSLREF QUESTIONS
7.2) SSL PLUS QUESTIONS
7.3) SSLEAY QUESTIONS
1) THE SSL-TALK LIST
This section contains information about the SSL-Talk list.
1.1) What is the SSL-Talk List?
The SSL-Talk List was an email list intended for discussion of the
technical issues of implementing the SSL protocol. It ceased to exist
on November 15, 1998.
Past discussions included issues of software development,
cryptanalysis of the protocol and of its various implementations,
testing, interoperability, the applicability of SSL to additional
TCP-based applications, infrastructure growth questions, etc.
1.1.1) Do archives of the SSL-Talk List exist?
Yes. An archive is maintained at:
It covers the list from 1995-1998 and is filled with useful
We are not aware of any plain text archives of the list.
1.2) What is SSL?
SSL is the Secure Sockets Layer protocol. Version 2.0 originated by
Netscape Development Corporation, and version 3.0 was designed with
public review and input from industry, and is defined at
1.2.1) What is TLS?
TLS is the Transport Layer Security protocol. It is effectively SSL
3.1 and was submitted to the IETF standards committee for change
control in 1996. It should be close to release.
1.3) Has netscape replaced the SSL-Talk mailing list?
Yes. Netscape, the host of the old ssl-talk mailing list, has
replaced it with a newsgroup.
The newsgroup netscape.dev.ssl is now available via two means:
a) from <snews://secnews.netscape.com/>
Note: snews is nntp over SSL. Supported in Communicator 4.x.
1.4) Are there any other SSL mailing lists?
Some people prefer mailing lists to newsgroups. Fortunately, several
other mailing lists exist to discuss SSL.
THE SSL DEVELOPER LIST
This is a mailing list specifically geared toward application
developers who are incorporating SSL or TLS into their products. It
is hosted by Consensus Development, a division of Certicom, who has
helped to develop the TLS specifications. To join, send a message to:
THE SSL LIST
As with the older SSL-Talk mailing list, the purpose of this
mailing list is to discuss any SSL & TLS related issues. It covers
the whole spectrum of issues, from beginners on up and is more
oriented toward users of SSL-enabled applications. To subscribe
simply send e-mail to:
THE SSL-USERS LIST
This is a list concerning SSLeay, a public implementation of SSL. To
subscribe send mail to:
The command "subscribe ssl-users" must appear in the body of the
THE IETF-TLS LIST
This is a mailing list dedicated to the writing of the TLS
specification for the IETF. To subscribe, send a message to:
2) GENERAL SSL QUESTIONS
This section contains general information on SSL and the SSL
2.1) What is the current version of the SSL protocol?
The current version is SSL 3.0, as documented at
Errata to the SSL 3.0 Specification is periodically posted on
the SSL discussion list, and is available at
Netscape has submitted SSL 3.0 to the IETF-TLS Working Group
as an Internet Draft (see the section 4.5 of this FAQ for more
info on TLS). This will be TLS 1.0:
The previous version of SSL, version 2.0 is documented at
2.2) Where can I get a "management overview" of SSL and web security?
There is a brief introduction on how Netscape uses public key
cryptography in the SSL protocol called "Using Public Key
An overview on certificates and VeriSign's Digital IDs is at
General information on Netscape security can be found in a
set of web pages called "Network Security Solutions", at:
2.3) Where can I get a more in-depth look at SSL and web security?
The online version of the technical specifications for the SSL 3.0
protocol is at
A PostScript version is also available at
A FAQ for SSLeay, a freeware implementation which support SSL 2.0,
SSL 3.0, and TLS 1.0, is available at
A rather broad list of public key related documents, with a focus on
certificates and standards can be found at
2.4) What software supports SSL 2.0 and SSL 3.0?
A list of web servers that support SSL 3.0 can be found using the
SSL is not just for web servers and is supported by numerous other
internet clients and servers.
2.5) What are the laws regarding the import and export of cryptography in
There is an impressive "International Law Crypto Survey" of
cryptographic laws and regulations throughout the world at
RSA Data Security, Inc. offers an Acrobat version of their
"Frequently Asked Questions: Export" at
Other information on US export issues can be found on
the Electronic Frontier Foundation's web site at
Canadian export issues are covered at
3) USING PROXIES, GATEWAYS, AND FIREWALLS WITH SSL
This section contains information on how the SSL protocol interacts
with proxy servers, security gateways, and firewalls.
3.1) What is a proxy server?
A proxy server is a computer program that resides on your firewall
and acts as a conduit between your computer and the broader
Internet. In addition to acting as network guardian and logging
traffic, a proxy server can also provide an enterprise cache for
files as well as replication and site-filtering services.
Any application which needs to communicate through a proxy has to
negotiate with the proxy first before continuing through the
firewall. Netscape Navigator works with many different types of
proxies (such as the CERN proxy server and their own Netscape Proxy
Server) and gateways that use the SOCKS protocol.
One problem with SSL-based traffic is that it does not allow
caching and replication with proxy servers. For a proxy server to
support SSL it must either support SOCKS (a protocol independent
proxy mechanism), or use a special SSL Tunneling protocol. The
Netscape Proxy Server supports both SOCKS and the SSL Tunneling
3.2) How does SSL work through (application level) firewalls,
gateways and proxy servers?
SSL was designed to provide security between client and server and
to avoid any kind of 3-way man-in-the-middle attack. Thus SSL cannot
be proxied through traditional application level firewalls (such as
the CERN proxy server) because SSL considers a proxy server to be a
The simplest alternative to this problem is to use a packet
filtering firewall. You set it up to open a reserved and trusted
port for the SSL+HTTP or SSL+NNTP services (443 or 563 respectively)
allowing all traffic on those ports to be passed through
unrestricted. The risk with this solution is that an internal
attacker could attempt to use these trusted ports without using SSL
and there is no way for the firewall to know.
SSL also can work with gateways that support the SOCKS protocol, a
protocol independent proxy mechanism. SOCKS is a generic byte
forwarding gateway between client and server and generally works
at the socket level. If all you want is TCP/UDP restrictions based on
client IP or server IP, SOCKS works fine.
However, most non-SSL HTTP proxies work at the protocol level and
have the ability to understand header information related to the
protocol. This goes beyond SOCKS to allow the firewall administrator
to use the header information for filtering and/or monitoring the
traffic. Also, SOCKS does not offer the firewall administrator
enough information about the request to let it decide whether to
allow it and whether to log the request.
A more secure approach is to use a firewall that supports the SSL
Tunneling CONNECT extension method as described in the Internet
In SSL Tunneling, the client initiates an SSL connection via normal
HTTP then handshakes and creates a secure connection to the server
via a byte-forwarding tunnel. The proxy has access to the
client-proxy request headers, but the session is encrypted. Once
the handshake occurs, the proxy acts just like a SOCKS gateway. This
allows the firewall to monitor the requests, but not the traffic.
The biggest difference between the two methods is that when using
SOCKS, DNS resolution is the responsibility of the client, whereas
when requests are forwarded through a proxy, DNS resolution is the
responsibility of the proxy.
The are three additional things that the SSL Tunneling mechanism
does with the proxy server that do not happen when using SOCKS:
* The client sends a "user agent" message (for example,
* The proxy can send to the client an authorization request
allowing the administrator to use passwords to control external
* The standard is more easily extensible. For example, the client
could, in theory, send the URL being requested (or anything
else) to the firewall. However, there is no standard to support
this behavior and as far as we know there are no products which
The Netscape Proxy Server supports the SSL Tunneling CONNECT
extension method for tunneling SSL, and the use of the proxy is
Another solution, also available using the Netscape Proxy Server, is
that the proxy server can spoof SSL on behalf of the internal client.
The proxy will initiate SSL between itself and other servers on the
Internet, but be unsecure inside the firewall between the proxy
server and the client.
This compromise means that client authentication is not possible;
only server authentication of the remote sites is available.
However, you gain the ability for client authentication between the
client and the proxy. The administrator must decide which is more
important, until such time as a better solution arises. The
description of this feature of the Netscape Proxy Server is at
Reverse proxies are a solution for serving secure content inside
a firewall to outside clients. For the Netscape Proxy Server
this is described at
It is possible for a proxy server to hold both client and server
keys for its internal clients. This allows SSL sessions to be
carried out twice: once between the client and proxy server, and
again between the proxy server and the secure server. Thus, the
proxy server can to listen in on the conversation without having the
private keys of external servers. Clearly this isn't reasonable for
the general internet, but it is a viable solution for corporate
requirements inside a firewall.
Netscape Proxy Server 3.5 supports this feature. It can be used as
described above, or simply to create a secure tunnel between sites
across an insecure network. This is really multiple sessions of SSL,
not an end-to-end secure connection.
This means that 3.5 has full SSL support as opposed to just SSL
tunneling. It can therefore do client authentication and serve
documents like a secure server, or request documents like an
SSL-enabled client. SSL doesn't allow recursive encryption, so by
using it this way you lose the transparency of the proxy and get
multiple segments of secure connections, rather than a single
3.3) Since SSL is supposed to withstand replay attacks, does this
preclude proxy servers from caching the data?
A proxy server must pass SSL directly through without caching.
3.4) What ports does SSL use?
Theoretically SSL can transparently secure any TCP-based protocol
running on any port if both sides know the other side is using SSL.
However, in practice, separate port numbers have been reserved for
each protocol commonly secured by SSL -- this allows packet
filtering firewalls to allow such secure traffic through.
As of October 1998, SSL has the following port numbers reserved
with the Internet Assigned Numbers Authority (IANA), a part of the
Internet Engineering Task Force (IETF):
Keyword Decimal Description
------- ------- -----------
nsiiops 261/tcp IIOP Name Service over TLS/SSL
https 443/tcp http protocol over TLS/SSL
ddm-ssl 448/tcp DDM-SSL
smtps 465/tcp smtp protocol over TLS/SSL
nntps 563/tcp nntp protocol over TLS/SSL
sshell 614/tcp SSLshell
ldaps 636/tcp ldap protocol over TLS/SSL
ftps-data 989/tcp ftp protocol, data, over TLS/SSL
ftps 990/tcp ftp, control, over TLS/SSL
telnets 992/tcp telnet protocol over TLS/SSL
imaps 993/tcp imap4 protocol over TLS/SSL
ircs 994/tcp irc protocol over TLS/SSL
pop3s 995/tcp pop3 protocol over TLS/SSL
A listing of all IANA port assignments can currently be found at
3.5) Do you have any information on sftp?
SSL FTP has been assigned port 990 under the name ftps.
4) SSL PROTOCOL QUESTIONS
This section contains more detailed information on the SSL protocol.
4.1) Does SSL protect users from replay attack by eavesdroppers or
Yes. The client and the server each provide part of the random data
used to generate the keys for a connection. (The random portions of
the connection that initiate a session, drawn from both the client
and the server, are used to generate the master secret associated
with that session.) Additionally, each record is protected with a
MAC (Message Authentication Code) that contains a sequence number for
4.2) Isn't encrypt-only SSL open to "man-in-the-middle" attacks?
Yes, even though SSL 3.0 defines an encrypt-only cipher suite (the
SSL_DH_anon_WITH_DES_CBC_SHA cipher suite), there are many possible
attacks against it, and some recommend against using it. SSL *MUST*
have strong server authentication or it becomes open to some attacks.
Netscape's browser and server products do not presently support
encrypt-only cipher suites for this reason.
4.3) When did MD5 get "disavowed"?
It hasn't been truly "disavowed", but weaknesses have been
discovered such that some people believe that an alternative should
be found. These weaknesses were found by Dr. Hans Dobbertin
<firstname.lastname@example.org> of the German Information Security Agency
in a paper called "Cryptanalysis of MD5 Compress" dated May 2, 1996.
A postscript version of the paper is at
SSL uses MD5 in combination with SHA for all negotiation. It also
uses MD5 alone in most negotiated cipher suites. However, in these
cases it is used with the HMAC construction, which strengthens it
such that there are no known problems with this construction.
It has been proposed with TLS to start phasing out all use of MD5.
4.4) The record protocol sits underneath the other protocols, right?
It appears that information can be sent only in blocks. Does
there have to be a one-to-one mapping between write() calls on the
client/server and SSL records? Is there some other blocking
taking place when user data is being sent?
The record layer takes a data stream from the higher layers and
fragments it into records. If the write is longer than 2^14 bytes
(with headers), the record layer will generate multiple records.
Multiple writes can be condensed into a single record.
4.5) It appears that there is no way in the SSL protocol to
resynchronize blocks if they get out of synch. Is that true?
Yes. SSL relies on an underlying reliable protocol to assure that
bytes are not lost or inserted. There was some discussion of
reengineering the future TLS protocol to work over datagram
protocols such as UDP, however TLS 1.0 does not support this.
4.6) Why does SSL3 have Diffie-Hellman encryption at all? What good is
it? Exchanging random numbers that are encrypted with the server's (or
client's) public key would seem to be an adequate way of getting the
secret bits across. Why have DH as well?
Anonymous DH key exchange doesn't require the use of certificates.
Ephemeral DH allows you to use signing-only certificates, and it
protects the session from future compromise of the server's private
key. Another advantage of DH is that the patent expired in 1997.
4.7) What is TLS? What happened at these meetings? Has anything come
out of them yet?
TLS is the Transport Layer Security Working Group of the IETF
(Internet Engineering Task Force). It is the working group
responsible for moving transport layer protocols such as SSL
through the standards tracks.
IETF working groups do most of their activities through mailing
lists and thrice-annual IETF meetings. The first official IETF-TLS
Working Group meeting was June 1996 in Montreal. (Before then it was
an unofficial BOF "birds of a feather" group.)
The home page for the IETF-TLS Working Group is at
The discussion list for IETF-TLS is at IETF-TLS@CONSENSUS.COM. You
subscribe and unsubscribe by sending to IETF-TLS@CONSENSUS.COM with
subscribe or unsubscribe in the SUBJECT of the message. Archives of
the list are at
Minutes are available for a number of past IETF-TLS meetings.
Not currently available
A number of internet-draft documents have been submitted to the
IETF-TLS Working Group.
The TLS Protocol 1.0 (Current Version 06):
Addition of Kerberos Cipher Suites to Transport Layer
ECC Cipher Suites for TLS
HTTP over SSL:
An Internet AttributeCertificate Profile for Authorization
TLS extensions for AttributeCertificate based authorization
The following internet drafts are expired, but are of historical
Addition of Shared Key Authentication to Transport Layer
(16885 bytes, expires May '97)
Modifications to the SSL protocol for TLS:
<draft-ietf-tls-ssl-mods-00.txt> (9271 bytes, expires May '97)
Secure FTP over SSL:
(14238 bytes, expires June '97)
SSH Transport Layer Protocol (originally
(44411 bytes, expired December '96)
4.8) What is the purpose of pad1 and pad2, and why were the numbers 0x36
and 0x5c chosen?
The purpose of the construction of a "keyed-MAC" in the form of
HASH(K,pad2,HASH(K,pad1,text)). It was proposed by the cryptographer
Hugo Krawczyk of IBM as a much more secure alternative to traditional
MACs. In a paper last year he demonstrated a proof that even if the
hash function was relatively weak (as MD5 has since proven itself to
be) the addition of the secret key in the function makes it
significantly more secure. The particular method proposed by
Krawczyk is now known as an HMAC.
The particular construction that Netscape uses for SSL is based on
the original internet-draft, and since that time it has been revised
such that it XORs the pads rather than appending them -- a nice
consequence of which is that pads are of the same size whether you
use MD5 or SHA; it also allows for long keys and has some
security advantages. This version may now be found as RFC 2104:
In the proposals we've seen for the IETF-TLS Working Group the
scheme SSL 3.0 uses will be replaced by the official RFC HMAC
The particular pad bytes used are the ones defined in Krawczyk's
original HMAC paper. We believe that they are relatively arbitrary.
The salient property is that half the bits differ: the hamming
distance between 0x36 and 0x5c is 4 out of a possible 8. We don't
know if the fact that each of the pads also has a hamming weight of
4 is significant or not.
4.9) Are you aware of any SSL toolkits supporting client authentication?
SSLRef 3.0 and SSL Plus both support SSL 3.0 client authentication.
SSLeay supports SSL 2.0 and 3.0 client authentication as well as the
proposed TLS standard for client authentication.
4.10) What SSL implementations should I test against?
There is no formal conformance testing, but Netscape does currently
offer an interoperability test server that has been used to test
conformance with many other implementations of SSL 3.0. This server
is located at
Another interoperability test server can be found at:
VeriSign also has an "Authentic Site" program listing various sites
that use SSL authentication. Also included is a test page that
requires that you present a valid VeriSign client certificate.
More information on the Authentic Site program is at
Client authentication can be tested at:
4.11) What is the difference between SSL 2.0 and 3.0?
1. SSL 2.0 is vulnerable to a "man-in-the-middle" attack. An
active attacker can invisibly edit the list of ciphersuite
preferences in the hello messages to invisibly force both client and
server to use 40-bit encryption. SSL 3.0 defends against this
attack by having the last handshake message include a hash of all
the previous handshake messages.
2. SSL 2.0 uses a weak MAC construction, although post-encryption
seems to stop attacks. This is fixed in 3.0.
3. SSL 2.0 feeds padding bytes into the MAC in block cipher modes,
but leaves the padding-length field unauthenticated, which could
allow active attackers to delete bytes from the end of messages.
This, too, is fixed in 3.0.
4. In SSL 3.0, the Message Authentication Hash uses a full 128 bits
of keying material, even when using an Export cipher. In SSL 2.0,
Message Authentication used only 40 bits when using an Export
1. In SSL 2.0, the client can only initiate a handshake at the
beginning of the connection. In 3.0, the client can initiate a
handshake routine, even in the middle of an open session. A server
can request that the client start a new handshake. Thus, the
parties can change the algorithms and keys used whenever they want.
2. SSL 3.0 allows the server and client to send chains of
certificates. This allows organizations to use a certificate
hierarchy that is more than two certifications deep.
3. SSL 3.0 has a generalized key exchange protocol. It allows
Diffie-Hellman and Fortezza key exchanges and non-RSA certificates.
4. SSL 3.0 allows for record compression and decompression.
1. SSL 3.0 can recognize an SSL 2.0 client hello and fall back to
SSL 2.0. An SSL 3.0 client can also generate an SSL 2.0 client
hello with the version set to SSL 3.0, so SSL 3.0 servers will
continue the handshake in SSL 3.0, and SSL 2.0 server will cause the
client to fall back to SSL 2.0.
1. SSL 3.0 separates the transport of data from the message layer.
In 2.0, each packet contained only one handshake message. In 3.0, a
record may contain part of a message, a whole message, or several
messages. This requires different logic to process packets into
handshake messages. Therefore, the formatting of the packets had to
be completely changed.
2. Cipher specifications, handshake messages, and other constants
5) CERTIFICATE RELATED QUESTIONS
This section contains information on certificates used by the SSL
5.1) How does Netscape handle client certificates in Communicator 4.X?
Netscape describes their framework for web-based key generation and
certificate issuing on their web pages at
5.2) What is the format of the SSL certificates used by Netscape
Netscape has documented their SSL 2.0 certificate format at
5.3) I am distributing load on several different web servers and I
don't want to have to have a different certificate for each. How can
I do this?
When establishing a secure connection in SSL, many SSL clients
applications, including Netscape's Navigator, check the common name
of the certificate against the name of the site in the URL. If it
doesn't match, the client application warns the user. Thus the
preferred format of a common name of an SSL server is a simple DNS
name like "www.consensus.com".
To support multiple servers you can use a round-robin DNS to send
each request for "www.consensus.com" to different IP addresses. As
Netscape Navigator does not check to see that the IP address matches
the original domain name (reverse-IP), this will work for each
Netscape's Navigator will also allow for some simple pattern
matching. Netscape has documented a number of different possibilities
in their SSL 2.0 Certificate Format web pages at:
Note, however, none of these regular expression/pattern matching
choices are accepted by VeriSign. In the past they have accepted
server certificate common names with regular expressions, but these
are no longer allowed.
Other CAs may have different policies regarding use of regular
expressions in common names.
5.4) When comparing a URL against the common name of the certificate,
why don't you do a reverse-DNS lookup?
DNS is not a secure name service, and trying to treat it like one
could be a security hole. The purpose of checking the common name
against the URL is to make sure that at least the user's expectation
of what site the user is visiting is not compromised.
5.5) Does Netscape require hierarchical naming (that is, distinguished
names) for its certificates?
Yes, Netscape requires distinguished names.
5.6) Where can I get more information on certificates?
PKIX is an IETF working group dedicated to providing standards
for an X509-based PKI. You can find their charter at:
VeriSign, the default CA (Certificate Authority) used by Netscape
and most other WWW browsers has a FAQ at:
Entrust has a primer on Web Security with an emphasis on
Certificate Authorities at:
There is also a good resource of links to a variety of certificate
technical and policy issue sites available at:
5.7) What other CAs exist besides VeriSign?
We know of these CAs:
Numerous other CAs now exist; additional links will be included in
the replacement TLS/SSL FAQ intended for the future.
5.8) How do I set up my own Certificate Authority?
There is some support for creating your own CA in SSLeay; there is
information on how to integrate it with Netscape available at:
Several specific products also exist; additional links will be
included in the replacement TLS/SSL FAQ intended for the future.
5.9) What criteria should I use in deciding between one CA and another?
The purpose of a Certificate Authority is to bind a public key to
the common name of the certificate, and thus assure third parties
that some measure of care was taken to ensure that this binding
is valid. A measure of a Certificate Authority is their "Policy
Statement" which states what measures they take for each class of
certificate they offer to ensure that this binding of identity
with public key is valid.
5.10) What are Attribute Certificates?
Attribute Certificates are a new type of certificate proposed by
Netscape. These are signed objects that assert additional properties
about a particular identity certificate.
An attribute cert has no associated key pair and consequently cannot
be used to establish identity. Informally, one can think of them as
a mechanism for extending the attributes of an identity certificate
without requiring that the identity certificate be reissued.
More details of the proposal are at
6) SSL IMPLEMENTATION ISSUES
This section offers specific implementation details of different SSL
clients and servers that are not specific to the protocol.
6.1) NETSCAPE QUESTIONS
This is not an official statement by Netscape, and Netscape has not
reviewed this for accuracy. For additional information, please see:
6.1.1) I just downloaded a new version of Netscape's browser, and it
doesn't have 128-bit encryption. What version(s) of the browser have
All versions of Netscape Navigator and Communicator, except "Preview
Release" versions, are available in two flavors: a "domestic" flavor
with 128-bit encryption for use in the USA and Canada, and an
"export" or "international" flavor with only 40-bit encryption.
(There is also a third flavor of Communicator 4.x available for use
in France.) Preview releases are only available in the export flavor.
To get 128-bit encryption, you must download the U.S. flavor.
6.1.2) I just downloaded a newly released version of Netscape's browser
and my bank's server tells me my browser does not have adequate
security. What's wrong?
Here are the likely explanations for this:
a) You have downloaded an "export" flavor of the browser with only
40-bit encryption, but your bank requires that you use the "domestic"
flavor of the browser with 128-bit encryption. In this case, you must
download the domestic 128-bit version.
b) Your bank's server keeps a list of the browser versions with which
it will work, and that list has not yet been updated to include the
very latest version(s) of Netscape's browser. In this case, please
ask your bank to add the newest version of Netscape's browser to
their server's list of acceptable versions. Note that many banks will
not accept "Preview Release" versions because they do not contain
domestic (128-bit) encryption.
6.1.3) I downloaded a version of Netscape's browser that is newer than
version 4.05. Now, when I go to certain https web sites that used to
work for me (like my bank) I get an error message telling me that
"Netscape has received bad data from the server." I've been told the
problem is with SSL v3 in my new browser, and that I should disable
SSL v3 in my browser. What's wrong with SSL v3 in these new browsers?
Should I disable it?
Newer versions of Netscape's browsers enforce the legal export
control requirements of the SSL v3 specification and will not work
with servers that violate the export control provisions of the SSL v3
Some SSL servers do not properly follow the SSL v3 specification's
requirements for the U.S. Government's export control regulations.
Netscape's server products, and most other brands of server products,
conform to the specification, but a few others do not.
We strongly advise you to NOT disable SSL v3 in your browser. If you
do disable SSL v3, you lose the extra security protections of SSL v3
with ALL the https web sites you visit. By keeping SSL v2 and v3
enabled in your browser, you get the best protection each site can
Please ask the failing web site to upgrade to conforming servers.
Web sites whose servers violate the specification have several
options at their disposal, including falling back on the less secure
SSL v2, by disabling the non-conformant SSL v3 in their servers, or
replacing their servers with servers that conform to the SSL v3 spec.
6.1.4) Do Netscape's browsers cache data on disk that has been received
Navigator 3.0 and Communicator 4.x have an option to allow on-disk
caching of data fetched over SSL connections. The default setting is
to not cache https data on disk.
In Navigator 2.0, documents fetched using SSL were cached in the same
way as non-SSL documents. You could use the "Pragma: no-cache" HTTP
header to disable caching for a particular page.
In Navigator 1.0, documents fetched with SSL were not cached on disk.
6.1.5) Is the cached data encrypted using some key?
No, Navigator and Communicator do not encrypt documents that are
stored in the cache.
6.1.6) Does Netscape use "regular" RSA libraries (such as BSAFE) or
"custom" RSA code? More specifically, is Netscape using BSAFE 3.0?
Netscape is a BSAFE source licensee. Much of the code in BSAFE 3.0
has been integrated into Netscape's products. However, the BSAFE API
is not available to plugins.
6.1.7) Are the 512-bit RSA keys used by exportable servers generated on
the fly by Netscape's servers? How often are they changed?
Does the Netscape server take care of changing them automatically?
In Netscape's server products, if the server's public key is longer
than 512 bits, the server generates a temporary 512-bit export key at
start-up time. This key is regenerated only when the server is
6.1.8) How can additional root CA certificates be added to the
browser's certificate database?
Root keys for CA (Certificate Authority) certificates may be loaded
using an SSL connection to a previously unknown CA. Please see:
for more information. Also, new releases of the Navigator have added
additional CA root keys.
6.1.9) What X.509v3 certificate extensions are supported by the various
versions of Netscape browsers?
Please see <http://home.netscape.com/eng/security/certs.html>.
6.1.10) The Help Information for Netscape's Enterprise server indicates
that the server supports 6 ciphers for SSL 2.0 and 6 ciphers for SSL
3.0. However, the Encryption|Security Preferences menu in the server
Manager displays only 2 choices for SSL 2.0 and 3 choices for SSL 3.0.
How can I select the other choices?
The Enterprise server is available in two flavors, the "domestic"
flavor with 128-bit encryption for use in the USA and Canada, and the
"export" or "international" flavor with only 40-bit encryption. If
you do not have all the ciphers, then you have the export flavor of
the server. If you want to use the others, you must use the domestic
6.1.11) When will Netscape support SSL sockets for Java browser applets?
There are presently no announced plans to do so.
6.2) MICROSOFT QUESTIONS
The text for sub-section 6.2 was grabbed from various documents
6.2.1) Which of Microsoft's products will support SSL?
Internet Explorer 3.0 provides support for SSL versions 2.0 and 3.0
and for Private Communication Technology (PCT) version 1.0. It will
include support for the Transport Layer Security Protocol (TLS),
which is being considered by IETF.
6.2.2) Which Microsoft products support Client Authentication?
Client authentication as implemented by Microsoft Internet Explorer
3.0 is interoperable with popular Web servers that support secure
sockets layer (SSL) 3.0 client authentication.
Internet Information Server 3.0 supports client authentication using
standards-based X.509 version 3 certificates. Webmasters can easily
add client authentication to their Web sites by creating an Active
Server Pages (ASP) application.
7) SSL TOOKIT QUESTIONS
This section offers specific details of different SSL development
toolkits that are not specific to the protocol.
7.1) SSLREF QUESTIONS
This subsection contains information on SSLRef 3.0 which was
codeveloped by Netscape Communications Corp. of Mountain View,
California <http://home.netscape.com/> and Consensus Development
Corporation of Berkeley, California <http://www.consensus.com/>.
7.1.1) What is SSLRef 3.0?
SSLRef 3.0 is a reference implementation of the SSL (Secure Sockets
Layer) protocol. SSLRef 3.0 is intended to aid and accelerate
developers' efforts to provide security within TCP/IP applications.
It can also be used to qualify other implementations of version 3.0
of the SSL protocol.
SSLRef 3.0 consists of a software library, distributed as ANSI C
source-code, that can be compiled on Windows 95/NT and Solaris
platforms and then linked into TCP/IP application programs. SSLRef
3.0 was also designed to be easily ported to a wide variety of
other platforms and operating systems.
More information on SSLRef can be found at
If you are a US citizen you can download SSLRef 3.0 at
7.1.2) How can I license SSLRef 3.0? What does it cost? With what
The SSLRef 3.0 distribution includes a license for non-commercial
use. For commercial licensing, send mail to <email@example.com>.
The SSLRef 3.0 commercial license is Part Number 70-01128-00 and the
price is $30,000. The license agreement is a flat one-time fee, not
a recurring royalty.
SSLRef 3.0 may not be exported. However, the encryption options in
SSLRef 3.0 can be limited to make exportable products.
SSLRef 3.0 does not include an RSA/BSAFE license for required
cryptographic functions. Most users would use BSAFE.
For BSAFE information contact RSA at
7.2) SSL PLUS QUESTIONS
This sub-section contains information specific to the SSL Plus: SSL
3.0 Integration Suite(tm) software toolkit developed by Consensus
Development Corporation of Berkeley, California
7.2.1) What is the relationship between SSLRef and SSL Plus?
SSLRef 3.0 was written by Netscape Development Corporation and
Consensus Development Corporation. SSL Plus, a derivative of
SSLRef 3.0, is fully supported and offers unique value-added
SSL Plus 2.0 includes numerous updates to SSLRef 3.0, support, a
VeriSign certificate request tool, and a "signer" file format for
storing keys and certificates. It is qualified for additional
platforms, and system integration services are available. When the
TLS spec becomes official, SSL Plus will be upgraded to that new
SSLRef 3.0 offers 4 ciphersuites:
* RSA authenticated, unencrypted, with MD5
* RSA authenticated with exportable RC4 encryption, and MD5
* RSA authenticated with DES encryption, and SHA
* Diffie-Hellman anonymous key exchange with DES encryption,
SSL Plus 2.0 adds support for an additional 6 ciphersuites:
* RSA authenticated, unencrypted, with SHA
* RSA authenticated with non-exportable RC4 encryption, with
MD5 or SHA
(SSL_RSA_WITH_RC4_128_MD5 & SSL_RSA_WITH_RC4_128_SHA)
* RSA authenticated with Triple-DES encryption, with SHA
* Diffie-Hellman anonymous key exchange with RC4 encryption,
* Diffie-Hellman anonymous key exchange with Triple-DES
encryption and SHA
For more information on SSL Plus features see
7.2.2) What is the relationship between SSL Plus and SSLRef 2.0?
There is no relationship between SSLRef 2.0 and SSL Plus -- SSL Plus
was originally based on SSLRef 3.0 which was not based on SSLRef 2.0.
7.2.3) How can I license SSL Plus?
SSL Plus is available for commercial use only. Certicom will work
with you to provide a license tailored to fit your company's business
model, whether you prefer flat yearly licenses, royalty based payment
or even one time buyouts.
Since SSL Plus is a protocol toolkit, SSL Plus customers will also
require a license for one of the standard crytographic libraries such
as Certicom's Security Builder, or RSA's BSAFE.
In addition, you can contact Consensus directly for more information
on SSL Plus, including pricing, technical information, or more
information about cryptographic library tools, at
<mailto:firstname.lastname@example.org> or at 510/649-3300.
7.2.4) Is there any relationship between SSL Plus and Winsock 1.1 or
Winsock 2.0? Which Winsock would you recommend using to test our
SSL? Does it matter if Winsock 1.1 or 2.0 architecture is used?
No -- SSL Plus is designed to be transport independent and work with
both socket and stream styles of I/O. SSL Plus includes some
examples of using WinSock 1.1 in the Win32 builds of our sample
code. However, we recommend that you write your own callback code if
you want better handling of your I/O than what our sample routines
7.2.5) How does the data flow within the application, WinSock, SSL,
TCP/IP stack layers?
The short answer is that you insert SSL Plus between your I/O and
your application code.
Basically, you call SSL Plus instead of your read and write. SSL
Plus does its stuff and calls your callback code to do the I/O. Data
comes through your I/O routines, through SSL Plus, and then finally
to your application. SSL Plus only manages the data flowing through
the connection; it does not handle setting up and tearing down the
underlying network connection; your application should open the
network connection, then hand it off to SSL Plus for SSL handshaking
and data transfer. (This step is not shown in the diagram).
| Application |
| I/O Calls
| WinSock |
| TCP Calls
| Internet |
| Application |
| SSL I/O Calls
------------- I/O Callbacks --------------------
| SSL Plus | <---------------->| Your Callback Code |
| I/O Calls
| WinSock |
| TCP Calls
| Internet |
7.2.6) With the WinSock 2.0 architecture, the application need only chose
an appropriate SSL-enabled service provider. Does SSL Plus support this?
With WinSock 2.0 there is some discussion of functionality that
allows you to create a module that you could add to WinSock 2.0.
At this time we do not believe that this functionality is actually
shipping (as Microsoft was supporting PCT but is now supporting
SSL 3), but we do know that it is part of their plans. See the
MS-ISF (Microsoft Internet Security Framework) description at
We can't speak to when or if Microsoft will add it to their system
software, or if another third-party offers such a module.
Meanwhile, there has been some discussion on what changes might be
required under WinSock 2.0 to do SSL. See:
7.2.7) Does SSL Plus support yielding?
SSL Plus includes support for processor yielding during
cryptographic operations. Because developers provide their own I/O
routines, they can do yielding during I/O. Our sample code include
examples of I/O yielding.
7.2.8) I don't understand the nomenclatures of constants such as
"SSL_RSA_EXPORT_WITH_RC4_40_MD5" -- where are they defined?
They are actually defined by the SSL 3.0 specification, but also see
section 7.2.1 for an overview.
7.2.9) In what order are the cipher suites called?
The default order of the cipher suites is:
7.2.10) Can I change the order of the cipher suites?
Yes. This is easily done with the SSLSetCipherSuites function.
7.2.11) Does SSL Plus support compression?
Not as of 2.0. If there is a specific customer requirement, or if a
compression cipher suite is defined we expect to support it in the
future, but otherwise we have no plans here.
7.2.12) In the function SSLWriteRecord(), the data buffer is
copied, encrypted, then enqueued on the SSL write queue. The function
then returns. What thread services the write queue? How is the
The write queue is serviced by the public function called
SSLServiceWriteQueue(). It is called in a number of places in
ssltrspt.c, including with every call to SSLWrite(). Data to be
written is sent to the I/O layer as you exit out of the write
function (for example, right near the bottom of SSLWrite).
If SSLWrite() returns SSLWouldBlockError, then make a call to
SSLServiceWriteQueue() to service the write queue. (You could
instead make a call to SSLWrite() with more data to be written, but
this is unlikely.)
The write queue is not serviced by a separate execution thread. The
write queue mechanism was designed to support non-blocking I/O
without undue overhead.
7.2.13) When I call SSLRead(), on returning, the length argument should
be replaced with the number of bytes actually read. In practice, this
doesn't seem to be happening. What am I doing wrong?
The difficulty is that it's hard for SSL to precisely emulate the
behavior of Unix-style socket calls.
The problem is that you are using SSL Plus in its blocking mode; if
you return SSLWouldBlock from your I/O Read callback, the library
will return the data it has along with the SSLWouldBlock error.
The best way to solve this is always to know how much data you're
waiting for and to request exactly that much. I know this doesn't
work with a lot of free-form Internet protocols.
Alternatively, you would like the call to block until it gets some
data, then return it to you, even if it's less than 512 bytes.
Ideally, you'd like to do this without busy-looping the CPU waiting
for data. The best way to do this using SSL Plus is to write a
wrapper for SSLRead() which does the following:
* Make a blocking select() call until there is some data
available on the TCP/IP connection over which you're speaking
SSL. This will cause you to block in a friendly way until data
* Call SSLRead(). If zero bytes are returned from the read,
loop and do the select() again. Otherwise, return whatever
* Make your Read() callback non-blocking. The easiest thing to
do is to check how much data is available on the incoming
connection and return SSLWouldBlockErr if you can't completely
fulfill the request. (You can optionally read what data there
is and return it first; this won't affect functionality).
This will result in the following behavior:
1. Your program will block gracefully in the select() call until
something arrives on the connection.
2. You will then ask SSL Plus to read some data.
3. SSL Plus will ask the Read() callback to read the header of the
next record (3 or 5 bytes).
4. The Read() callback will fulfill that, if possible
5. SSL Plus will ask to read the body of the record (whose length
will be equal to how much data was sent by the other side, plus
MAC and encryption padding).
6. The Read() callback will fulfill that, if possible.
7. If the amount of data received is greater than or equal to how
much was requested in 2., the data will be returned
8. Otherwise, go back to 3.
What will happen in practice looks something like this: because the
SSL peer on the other end of the connection generates record layer
records monolithically, and they're relatively small, the header and
content of a record will arrive at your machine all together. Thus,
when your select() call returns, you will be able to successfully
read a header and body without blocking. When SSL Plus goes to read
another one, your Read() callback will see that there's no data
available on the connection (assuming another record hasn't arrived)
and return SSLWouldBlockErr. SSL Plus will then return the data it
has received and the error SSLWouldBlockErr; you can return that
data as a partial completion of the desired read.
If a partial record arrives, your select() will wake up, but SSL
Plus won't be able to decrypt and check a complete record before the
Read() callback returns SSLWouldBlockErr; thus, your read will
return with zero bytes returned. Since this isn't the behavior your
client expects, you should select() again until more data arrives,
hopefully completing the record.
7.2.14) If session cache is stored in a database, can multiple Unix
processes share the same session data?
There is no information stored in the session database which can't
be passed between processes. Specifically, there is no pointer
indirection. Of course, you'll have to figure out how to pass
session database records (and their changes or deletions) between
processes; that is not part of SSL Plus.
7.3) SSLEAY QUESTIONS
This sub-section contains information specific to the SSLeay
toolkit developed by Eric Young <email@example.com>
7.3.1) Where is the SSLeay FAQ?
There is a very complete SSLeay FAQ at:
.. Shannon Appel Consensus Development Corporation ..
.. Research Assistant a subsidiary of Certicom Corporation ..
.. <SAppel@consensus.com> 2930 Shattuck Ave. #206 ..
.. <http://www.consensus.com> Berkeley, CA 94705-1883 ..
.. <http://www.certicom.com> o510/649-3300 f510/649-3301 ..