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[SSL-Talk List FAQ] Secure Sockets Layer Discussion List FAQ v1.1.1

This document is a summary of FAQ (Frequently Asked Questions) found on the SSL-Talk discussion list regarding technical implementation issues of the Secure Sockets Layer protocol, a transport level security protocol used for securing web servers and clients (such as Netscape Navigator) and other internet applications.
Content-type: text/x-usenet-FAQ;
    title="[SSL-Talk List FAQ] Secure Sockets Layer Discussion List FAQ v1.1.1"
Archive-name: computer-security/ssl-talk-faq
Posting-Frequency: monthly
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

                              SSL-Talk FAQ
            Secure Sockets Layer Discussion List FAQ v1.1.1

                      Mon Nov 16 12:00:00 PST 1998

                           FAQ Maintained by:
                  Shannon Appel <>
                    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
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    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
    appropriate attribution.

    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
    are suggesting.

    Many people have in the past provided feedback and corrections; we
    thank them for their input.

    In particular, many thanks to:

        Christopher Allen <>
        Shannon Appel <>
        Nelson Bolyard <>
        Tim Dierks <>
        Eric Greenberg <>
        Charles Neerdaels <>
        Bruce Schneier <>
        Tom Weinstein <>
        Jonathan Zamick <>

    Remaining ambiguities, errors, and difficult-to-read passages are
    not their fault. :)





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 is now available via two means:

    a) from <snews://>
    Note: snews is nntp over SSL. Supported in Communicator 4.x. 

    b) from 

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.


    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:


    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:


    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 

    This is a mailing list dedicated to the writing of the TLS     
    specification for the IETF. To subscribe, send a message to:



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
    Cryptography" at

    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 
    powersearch at:

    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 
various countries?

    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



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
          Internet access.

        * 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
          do it.

    The Netscape Proxy Server supports the SSL Tunneling CONNECT 
    extension method for tunneling SSL, and the use of the proxy is 
    described in 

    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
    end-to-end connection.

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.



This section contains more detailed information on the SSL protocol.

4.1) Does SSL protect users from replay attack by eavesdroppers or
message interceptors?

    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 
    the message.

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
    <> 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.
    August 1998:
	Not currently available
    March 1998:

    December 1997:

    April 1997:

    December 1996:

    June 1996:

    May 1996:

    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
    Security (TLS):

    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
    Security (TLS):
            (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?

    Security improvements:

    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

    Functionality improvements:

    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.

    Backward compatibility:

    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
    are different.



This section contains information on certificates used by the SSL

5.1) How does Netscape handle client certificates in Communicator 4.X? 
Navigator 3.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 "".

    To support multiple servers you can use a round-robin DNS to send
    each request for "" 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
    round-robin server.

    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:

        Thawte Consulting <>
        COST Computer Security Technologies <>
        CompuSource <>
        XCert <>

    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



This section offers specific implementation details of different SSL
clients and servers that are not specific to the protocol.



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
128-bit encryption?

    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
via https?

    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 <>.

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 
    (non-export) flavor.

6.1.11) When will Netscape support SSL sockets for Java browser applets? 

    There are presently no announced plans to do so.



The text for sub-section 6.2 was grabbed from various documents
found at


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.



This section offers specific details of different SSL development
toolkits that are not specific to the protocol.



This subsection contains information on SSLRef 3.0 which was
codeveloped by Netscape Communications Corp. of Mountain View,
California <> and Consensus Development
Corporation of Berkeley, California <>.

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 <>.

    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



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,
        and SHA

    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,
        with MD5
        (SSL_DH_anon_WITH_RC4_128_MD5 &

      * Diffie-Hellman anonymous key exchange with Triple-DES
        encryption and SHA
        (SSL_DH_anon_WITH_RC4_128_MD5 &

    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. 

    You can get more information about SSL Plus at 
    <>, and about Certicom's Security 
    Builder toolkit at <>

    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 
    <> 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    |

    SSL Plus:

        | 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
thread created?

    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
          came back.

        * 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.



This sub-section contains information specific to the SSLeay
toolkit developed by Eric Young <>

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 ..
.. <>                     2930 Shattuck Ave. #206 ..
.. <>                 Berkeley, CA 94705-1883 ..
.. <>             o510/649-3300  f510/649-3301 ..

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