``The nice thing about standards is that there are so many to choose from.
And if you really don't like all the standards you just have to wait another
year until the one arises you are looking for.''
A. Tanenbaum, ``Introduction to Computer Networks''
Understanding SSL requires an understanding of cryptographic algorithms,
message digest functions (aka. one-way or hash functions), and digital
signatures. These techniques are the subject of entire books (see for instance
[AC96]) and provide the basis for privacy, integrity, and
Suppose Alice wants to send a message to her bank to transfer some money.
Alice would like the message to be private, since it will include information
such as her account number and transfer amount. One solution is to use a
cryptographic algorithm, a technique that would transform her message into an
encrypted form, unreadable except by those it is intended for. Once in this
form, the message may only be interpreted through the use of a secret key.
Without the key the message is useless: good cryptographic algorithms make it
so difficult for intruders to decode the original text that it isn't worth
s an introduction this chapter is aimed at readers who are familiar
with the Web, HTTP, and Apache, but are not security experts. It is not
intended to be a definitive guide to the SSL protocol, nor does it discuss
specific techniques for managing certificates in an organization, or the
important legal issues of patents and import and export restrictions. Rather,
it is intended to provide a common background to mod_ssl users by pulling
together various concepts, definitions, and examples as a starting point for
The presented content is mainly derived, with permission by the author, from
the article Introducing SSL
and Certificates using SSLeay from Frederick J. Hirsch, of The Open
Group Research Institute, which was published in Web Security: A Matter of
Trust, World Wide Web Journal, Volume 2, Issue 3, Summer 1997.
Please send any postive feedback to Frederick Hirsch (the original
article author) and all negative feedback to Ralf S. Engelschall (the mod_ssl
There are two categories of cryptographic algorithms:
conventional and public key.
Although Alice may encrypt her message to make it private, there is still a
concern that someone might modify her original message or substitute
it with a different one, in order to transfer the money to themselves, for
instance. One way of guaranteeing the integrity of Alice's message is to
create a concise summary of her message and send this to the bank as well.
Upon receipt of the message, the bank creates its own summary and compares it
with the one Alice sent. If they agree then the message was received intact.
- Conventional cryptography, also known as symmetric
cryptography, requires the sender and receiver to share a key: a secret
piece of information that may be used to encrypt or decrypt a message.
If this key is secret, then nobody other than the sender or receiver may
read the message. If Alice and the bank know a secret key, then they
may send each other private messages. The task of privately choosing a key
before communicating, however, can be problematic.
- Public key cryptography, also known as asymmetric cryptography,
solves the key exchange problem by defining an algorithm which uses two keys,
each of which may be used to encrypt a message. If one key is used to encrypt
a message then the other must be used to decrypt it. This makes it possible
to receive secure messages by simply publishing one key (the public key) and
keeping the other secret (the private key).
Anyone may encrypt a message using the public key, but only the owner of the
private key will be able to read it. In this way, Alice may send private
messages to the owner of a key-pair (the bank), by encrypting it using their
public key. Only the bank will be able to decrypt it.
A summary such as this is called a message digest, one-way
function or hash function. Message digests are used to create
short, fixed-length representations of longer, variable-length messages.
Digest algorithms are designed to produce unique digests for different
messages. Message digests are designed to make it too difficult to determine
the message from the digest, and also impossible to find two different
messages which create the same digest -- thus eliminating the possibility of
substituting one message for another while maintaining the same digest.
Another challenge that Alice faces is finding a way to send the digest to the
bank securely; when this is achieved, the integrity of the associated message
is assured. One way to to this is to include the digest in a digital
When Alice sends a message to the bank, the bank needs to ensure that the
message is really from her, so an intruder does not request a transaction
involving her account. A digital signature, created by Alice and
included with the message, serves this purpose.
Digital signatures are created by encrypting a digest of the message,
and other information (such as a sequence number) with the sender's
private key. Though anyone may decrypt the signature using the public
key, only the signer knows the private key. This means that only they may
have signed it. Including the digest in the signature means the signature is
only good for that message; it also ensures the integrity of the message since
no one can change the digest and still sign it.
To guard against interception and reuse of the signature by an intruder at a
later date, the signature contains a unique sequence number. This protects
the bank from a fraudulent claim from Alice that she did not send the message
-- only she could have signed it (non-repudiation).
Although Alice could have sent a private message to the bank, signed it, and
ensured the integrity of the message, she still needs to be sure that she is
really communicating with the bank. This means that she needs to be sure that
the public key she is using corresponds to the bank's private key. Similarly,
the bank also needs to verify that the message signature really corresponds to
If each party has a certificate which validates the other's identity, confirms
the public key, and is signed by a trusted agency, then they both will be
assured that they are communicating with whom they think they are. Such a
trusted agency is called a Certificate Authority, and certificates are
used for authentication.
A certificate associates a public key with the real identity of an individual,
server, or other entity, known as the subject. As shown in Table 1, information about the subject includes identifying
information (the distinguished name), and the public key. It also includes
the identification and signature of the Certificate Authority that issued the
certificate, and the period of time during which the certificate is valid. It
may have additional information (or extensions) as well as administrative
information for the Certificate Authority's use, such as a serial number.
Table 1: Certificate Information
||Distinguished Name, Public Key|
||Distinguished Name, Signature|
|Period of Validity:
||Not Before Date, Not After Date|
||Version, Serial Number|
||Basic Contraints, Netscape Flags, etc.|
A distinguished name is used to provide an identity in a specific context --
for instance, an individual might have a personal certificate as well as one
for their identity as an employee. Distinguished names are defined by the
X.509 standard [X509], which defines the fields, field
names, and abbreviations used to refer to the fields
(see Table 2).
Table 2: Distinguished Name Information
||Name being certified||CN=Joe Average|
|Organization or Company||O
||Name is associated with this|
|O=Snake Oil, Ltd.|
||Name is associated with this |
organization unit, such as a department
||Name is located in this City||L=Snake City|
||Name is located in this State/Province||ST=Desert|
||Name is located in this Country (ISO code)||C=XZ|
A Certificate Authority may define a policy specifying which distinguished
field names are optional, and which are required. It may also place
requirements upon the field contents, as may users of certificates. As an
example, a Netscape browser requires that the Common Name for a certificate
representing a server has a name which matches a wildcard pattern for the
domain name of that server, such as
The binary format of a certificate is defined using the ASN.1 notation [ X208] [PKCS]. This notation defines how to
specify the contents, and encoding rules define how this information is
translated into binary form. The binary encoding of the certificate is
defined using Distinguished Encoding Rules (DER), which are based on the more
general Basic Encoding Rules (BER). For those transmissions which cannot
handle binary, the binary form may be translated into an ASCII form by using
Base64 encoding [MIME]. This encoded version is called PEM
encoded (the name comes from "Privacy Enhanced Mail"), when placed between
begin and end delimiter lines as illustrated in Table 3.
By first verifying the information in a certificate request before granting
the certificate, the Certificate Authority assures the identity of the private
key owner of a key-pair. For instance, if Alice requests a personal
certificate, the Certificate Authority must first make sure that Alice really
is the person the certificate request claims.
A Certificate Authority may also issue a certificate for another Certificate
Authority. When examining a certificate, Alice may need to examine the
certificate of the issuer, for each parent Certificate Authority, until
reaching one which she has confidence in. She may decide to trust only
certificates with a limited chain of issuers, to reduce her risk of a "bad"
certificate in the chain.
As noted earlier, each certificate requires an issuer to assert the validity
of the identity of the certificate subject, up to the top-level Certificate
Authority (CA). This presents a problem: Since this is who vouches for the
certificate of the top-level authority, which has no issuer?
In this unique case, the certificate is "self-signed", so the issuer of the
certificate is the same as the subject. As a result, one must exercise extra
care in trusting a self-signed certificate. The wide publication of a public
key by the root authority reduces the risk in trusting this key -- it would be
obvious if someone else publicized a key claiming to be the authority.
Browsers are preconfigured to trust well-known certificate authorities.
Table 3: Example of a PEM-encoded certificate (snakeoil.crt)
A number of companies, such as Thawte and
VeriSign have established themselves as
Certificate Authorities. These companies provide the following services:
- Verifying certificate requests
- Processing certificate requests
- Issuing and managing certificates
It is also possible to create your own Certificate Authority. Although risky
in the Internet environment, it may be useful within an Intranet where the
organization can easily verify the identities of individuals and servers.
Establishing a Certificate Authority is a responsibility which requires a
solid administrative, technical, and management framework.
Certificate Authorities not only issue certificates, they also manage them --
that is, they determine how long certificates are valid, they renew them, and
they keep lists of certificates that have already been issued but are no
longer valid (Certificate Revocation Lists, or CRLs).
Say Alice is entitled to a certificate as an employee of a company. Say too,
that the certificate needs to be revoked when Alice leaves the company. Since
certificates are objects that get passed around, it is impossible to tell from
the certificate alone that it has been revoked.
When examining certificates for validity, therefore, it is necessary to
contact the issuing Certificate Authority to check CRLs -- this is not usually
an automated part of the process.
If you use a Certificate Authority that is not configured into browsers by
default, it is necessary to load the Certificate Authority certificate into
the browser, enabling the browser to validate server certificates signed by
that Certificate Authority. Doing so may be dangerous, since once loaded, the
browser will accept all certificates signed by that Certificate Authority.
The Secure Sockets Layer protocol is a protocol layer which may be placed
between a reliable connection-oriented network layer protocol (e.g. TCP/IP)
and the application protocol layer (e.g. HTTP). SSL provides for secure
communication between client and server by allowing mutual authentication, the
use of digital signatures for integrity, and encryption for privacy.
The protocol is designed to support a range of choices for specific algorithms
used for cryptography, digests, and signatures. This allows algorithm
selection for specific servers to be made based on legal, export or other
concerns, and also enables the protocol to take advantage of new algorithms.
Choices are negotiated between client and server at the start of establishing
a protocol session.
Table 4: Versions of the SSL protocol
||Vendor Standard (from Netscape Corp.) [SSL2]
||First SSL protocol for which implementations exists
||- NS Navigator 1.x/2.x
- MS IE 3.x
||Expired Internet Draft (from Netscape Corp.) [SSL3]
||Revisions to prevent specific security attacks, add non-RSA ciphers, and support for certificate chains
||- NS Navigator 2.x/3.x/4.x
- MS IE 3.x/4.x
||Proposed Internet Standard (from IETF) [TLS1]
||Revision of SSL 3.0 to update the MAC layer to HMAC, add block padding for
block ciphers, message order standardization and more alert messages.
There are a number of versions of the SSL protocol, as shown in Table 4. As noted there, one of the benefits in SSL 3.0 is
that it adds support of certificate chain loading. This feature allows a
server to pass a server certificate along with issuer certificates to the
browser. Chain loading also permits the browser to validate the server
certificate, even if Certificate Authority certificates are not installed for
the intermediate issuers, since they are included in the certificate chain.
SSL 3.0 is the basis for the Transport Layer Security [TLS] protocol standard, currently in development by the
Internet Engineering Task Force (IETF).
The SSL session is established by following a handshake sequence
between client and server, as shown in Figure 1. This
sequence may vary, depending on whether the server is configured to provide a
server certificate or request a client certificate. Though cases exist where
additional handshake steps are required for management of cipher information,
this article summarizes one common scenario: see the SSL specification for the
full range of possibilities.
Once an SSL session has been established it may be reused, thus avoiding the
performance penalty of repeating the many steps needed to start a session.
For this the server assigns each SSL session a unique session identifier which
is cached in the server and which the client can use on forthcoming
connections to reduce the handshake (until the session identifer expires in
the cache of the server).
Figure 1: Simplified SSL Handshake Sequence
The elements of the handshake sequence, as used by the client and server, are
- Negotiate the Cipher Suite to be used during data transfer
- Establish and share a session key between client and server
- Optionally authenticate the server to the client
- Optionally authenticate the client to the server
The first step, Cipher Suite Negotiation, allows the client and server to
choose a Cipher Suite supportable by both of them. The SSL3.0 protocol
specification defines 31 Cipher Suites. A Cipher Suite is defined by the
These three elements are described in the sections that follow.
The key exchange method defines how the shared secret symmetric cryptography
key used for application data transfer will be agreed upon by client and
server. SSL 2.0 uses RSA key exchange only, while SSL 3.0 supports a choice of
key exchange algorithms including the RSA key exchange when certificates are
used, and Diffie-Hellman key exchange for exchanging keys without certificates
and without prior communication between client and server.
- Key Exchange Method
- Cipher for Data Transfer
- Message Digest for creating the Message Authentication Code (MAC)
One variable in the choice of key exchange methods is digital signatures --
whether or not to use them, and if so, what kind of signatures to use.
Signing with a private key provides assurance against a
man-in-the-middle-attack during the information exchange used in generating
the shared key [AC96, p516].
SSL uses the conventional cryptography algorithm (symmetric cryptography)
described earlier for encrypting messages in a session. There are nine
choices, including the choice to perform no encryption:
Here "CBC" refers to Cipher Block Chaining, which means that a portion of the
previously encrypted cipher text is used in the encryption of the current
block. "DES" refers to the Data Encryption Standard [AC96,
ch12], which has a number of variants (including DES40 and 3DES_EDE). "Idea"
is one of the best and cryptographically strongest available algorithms, and
"RC2" is a proprietary algorithm from RSA DSI [AC96,
The choice of digest function determines how a digest is created from a record
unit. SSL supports the following:
- No encryption
- Stream Ciphers
- RC4 with 40-bit keys
- RC4 with 128-bit keys
- CBC Block Ciphers
- RC2 with 40 bit key
- DES with 40 bit key
- DES with 56 bit key
- Triple-DES with 168 bit key
- Idea (128 bit key)
- Fortezza (96 bit key)
The message digest is used to create a Message Authentication Code (MAC) which
is encrypted with the message to provide integrity and to prevent against
The handshake sequence uses three protocols:
- No digest (Null choice)
- MD5, a 128-bit hash
- Secure Hash Algorithm (SHA-1), a 160-bit hash
These protocols, as well as application protocol data, are encapsulated in the
SSL Record Protocol, as shown in Figure 2. An
encapsulated protocol is transferred as data by the lower layer protocol,
which does not examine the data. The encapsulated protocol has no knowledge of
the underlying protocol.
- The SSL Handshake Protocol
for performing the client and server SSL session establishment.
- The SSL Change Cipher Spec Protocol for actually establishing agreement
on the Cipher Suite for the session.
- The SSL Alert Protocol for
conveying SSL error messages between client and server.
Figure 2: SSL Protocol Stack
The encapsulation of SSL control protocols by the record protocol means that
if an active session is renegotiated the control protocols will be transmitted
securely. If there were no session before, then the Null cipher suite is
used, which means there is no encryption and messages have no integrity
digests until the session has been established.
The SSL Record Protocol, shown in Figure 3, is used to
transfer application and SSL Control data between the client and server,
possibly fragmenting this data into smaller units, or combining multiple
higher level protocol data messages into single units. It may compress, attach
digest signatures, and encrypt these units before transmitting them using the
underlying reliable transport protocol (Note: currently all major SSL
implementations lack support for compression).
One common use of SSL is to secure Web HTTP communication between a browser
and a webserver. This case does not preclude the use of non-secured HTTP. The
secure version is mainly plain HTTP over SSL (named HTTPS), but with one major
difference: it uses the URL scheme
Figure 3: SSL Record Protocol
https rather than
http and a different server port (by default 443). This mainly
is what mod_ssl provides to you for the Apache webserver...
[AC96] Bruce Schneier, Applied Cryptography, 2nd Edition, Wiley,
1996. See http://www.counterpane.com/ for
various other materials by Bruce Schneier.
[X208] ITU-T Recommendation X.208, Specification of Abstract Syntax Notation
One (ASN.1), 1988. See for instance
[X509] ITU-T Recommendation X.509, The Directory - Authentication
Framework, 1988. See for instance
[PKCS] Kaliski, Burton S., Jr., An Overview of the PKCS Standards, An RSA
Laboratories Technical Note, revised November 1, 1993.
[MIME] N. Freed, N. Borenstein, Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet Message Bodies, RFC2045.
See for instance
[SSL2] Kipp E.B. Hickman, The SSL Protocol, 1995.
[SSL3] Alan O. Freier, Philip Karlton, Paul C. Kocher, The SSL Protocol
Version 3.0, 1996. See
[TLS1] Tim Dierks, Christopher Allen, The TLS Protocol Version 1.0,