S/MIME

S/MIME

Secure/Multipurpose Internet Mail Extension (S/MIME) is a security enhancement to the MIME Internet e-mail format standard based on technology from RSA Data Security. Although both PGP and S/MIME are on an IETF standards track, it appears likely that S/MIME will emerge as the industry standard for commercial and organizational use, while PGP will remain the choice for personal e-mail security for many users. S/MIME is defined in a number of documents—most importantly RFCs 3370, 3850, 3851, and 3852.

To understand S/MIME, we need first to have a general understanding of the underlying e-mail format that it uses, namely MIME. But to understand the signifi- cance of MIME, we need to go back to the traditional e-mail format standard, RFC 822, which is still in common use. The most recent version of this format specifica- tion is RFC 5322 (Internet Message Format). Accordingly, this section first provides an introduction to these two earlier standards and then moves on to a discussion of S/MIME.

RFC 5322

RFC 5322 defines a format for text messages that are sent using electronic mail. It has been the standard for Internet-based text mail messages and remains in common use. In the RFC 5322 context, messages are viewed as having an envelope and contents.The envelope contains whatever information is needed to accomplish transmission and delivery. The contents compose the object to be delivered to the recipient. The RFC 5322 standard applies only to the contents. However, the content standard includes a set of header fields that may be used by the mail system to create the envelope, and the standard is intended to facilitate the acquisition of such information by programs.

The overall structure of a message that conforms to RFC 5322 is very simple. A message consists of some number of header lines (the header) followed by unre- stricted text (the body). The header is separated from the body by a blank line. Put differently, a message is ASCII text, and all lines up to the first blank line are assumed to be header lines used by the user agent part of the mail system.

A header line usually consists of a keyword, followed by a colon, followed by the keyword’s arguments; the format allows a long line to be broken up into several lines. The most frequently used keywords are From, To, Subject, and Date. Here is an example message:

Date: October 8, 2009 2:15:49 PM EDT

From: "William Stallings"<ws@shore.net>

Subject: The Syntax in RFC 5322 To: Smith@Other-host.com

Cc: Jones@Yet-Another-Host.com

Hello. This section begins the actual message body, which is delimited from the message heading by a blank line.

Another field that is commonly found in RFC 5322 headers is Message-ID. This field contains a unique identifier associated with this  message.

Multipurpose Internet Mail Extensions

Multipurpose Internet Mail Extension (MIME) is an extension to the RFC 5322 framework that is intended to address some of the problems and limitations of the use of Simple Mail Transfer Protocol (SMTP), defined in RFC 821, or some other mail transfer protocol and RFC 5322 for electronic mail. [PARZ06] lists the follow- ing limitations of the SMTP/5322 scheme.

1.                                       SMTP cannot transmit executable files or other binary objects. A number of schemes are in use for converting binary files into a text form that can be used by SMTP mail systems, including the popular UNIX UUencode/UUdecode scheme. However, none of these is a standard or even a de facto standard.

2.                                       SMTP cannot transmit text data that includes national language characters, because these are represented by 8-bit codes with values of 128 decimal or higher, and SMTP is limited to 7-bit ASCII.

3.                        SMTP servers may reject mail message over a certain  size.

4.                        SMTP gateways that translate between ASCII and the character code EBCDIC do not use a consistent set of mappings, resulting in translation problems.

5.                        SMTP gateways to X.400 electronic mail networks cannot handle nontextual data included in X.400 messages.

6.                        Some SMTP implementations do not adhere completely to the SMTP standards defined in RFC 821. Common problems include:

•                       Deletion, addition, or reordering of carriage return and linefeed

•                       Truncating or wrapping lines longer than 76 characters

•                       Removal of trailing white space (tab and space characters)

•                       Padding of lines in a message to the same length

•                       Conversion of tab characters into multiple space characters

MIME is intended to resolve these problems in a manner that is compatible with existing RFC 5322 implementations. The specification is provided in RFCs 2045 through 2049.

OVERVIEW The MIME specification includes the following elements.

1.                        Five new message header fields are defined, which may be included in an RFC 5322 header. These fields provide information about the body of the message.

2.                        A number of content formats are defined, thus standardizing representations that support multimedia electronic mail.

3.                        Transfer encodings are defined that enable the conversion of any content for- mat into a form that is protected from alteration by the mail system.

In this subsection, we introduce the five message header fields. The next two subsections deal with content formats and transfer encodings.

The five header fields defined in MIME are

•                 MIME-Version: Must have the parameter value 1.0. This field indicates that the message conforms to RFCs 2045 and 2046.

•                 Content-Type: Describes the data contained in the body with sufficient detail that the receiving user agent can pick an appropriate agent or mechanism to represent the data to the user or otherwise deal with the data in an appropri- ate manner.

•                 Content-Transfer-Encoding: Indicates the type of transformation that has been used to represent the body of the message in a way that is acceptable for mail transport.

•                 Content-ID: Used to identify MIME entities uniquely in multiple contexts.

•                 Content-Description: A text description of the object with the body; this is useful when the object is not readable (e.g., audio data).

Any or all of these fields may appear in a normal RFC 5322 header. A compliant implementation must support the MIME-Version, Content-Type, and Content- Transfer-Encoding fields; the Content-ID and Content-Description fields are optional and may be ignored by the recipient implementation.

MIME CONTENT TYPES The bulk of the MIME specification is concerned with the definition of a variety of content types. This reflects the need to provide standardized ways of dealing with a wide variety of information representations in a multimedia environment.

Table 18.3 lists the content types specified in RFC 2046. There are seven different major types of content and a total of 15 subtypes. In general, a content type declares the general type of data, and the subtype specifies a particular format for that type of data.

For the text type of body, no special software is required to get the full meaning of the text aside from support of the indicated character set. The primary subtype is plain text, which is simply a string of ASCII characters or ISO 8859 characters. The enriched subtype allows greater formatting flexibility.

The multipart type indicates that the body contains multiple, independent parts. The Content-Type header field includes a parameter (called a boundary) that defines the delimiter between body parts. This boundary should not appear in any parts of the message. Each boundary starts on a new line and consists of two hyphens followed by the boundary value. The final boundary, which indicates the end of the last part, also has a suffix of two hyphens. Within each part, there may be an optional ordinary MIME header.

Table 18.3    MIME Content Types

Here is a simple example of a multipart message containing two parts—both consisting of simple text (taken from RFC 2046).

From: Nathaniel Borenstein <nsb@bellcore.com> To: Ned Freed <ned@innosoft.com>

Subject: Sample message MIME-Version: 1.0

Content-type: multipart/mixed; boundary="simple boundary"

This is the preamble. It is to be ignored, though it   is a handy place for mail composers to include an explanatory note to non-MIME conformant readers.

—simple boundary

This is implicitly typed plain ASCII text. It does NOT end with a linebreak.

—simple boundary

Content-type: text/plain; charset=us-ascii

This is explicitly typed plain ASCII text. It DOES end with a linebreak.

—simple boundary—

This is the epilogue. It is also to be ignored.

There are four subtypes of the multipart type, all of which have the same over- all syntax. The multipart/mixed subtype is used when there are multiple indepen- dent body parts that need to be bundled in a particular order. For the multipart/parallel subtype, the order of the parts is not significant. If the recipient’s system is appropriate, the multiple parts can be presented in parallel. For example, a picture or text part could be accompanied by a voice commentary that is played while the picture or text is displayed.

For the multipart/alternative subtype, the various parts are different represen- tations of the same information. The following is an example:

From: Nathaniel Borenstein <nsb@bellcore.com> To: Ned Freed <ned@innosoft.com>

Subject: Formatted text mail MIME-Version: 1.0

Content-Type: multipart/alternative; boundary=boundary42

—boundary42

Content-Type: text/plain; charset=us-ascii

...plain text version of message goes here....

—boundary42

Content-Type: text/enriched

.... RFC 1896 text/enriched version of same message goes here ...

—boundary42—

In this subtype, the body parts are ordered in terms of increasing preference. For this example, if the recipient system is capable of displaying the message in the text/enriched format, this is done; otherwise, the plain text format is used.

The multipart/digest subtype is used when each of the body parts is inter- preted as an RFC 5322 message with headers. This subtype enables the construction of a message whose parts are individual messages. For example, the moderator of a group might collect e-mail messages from participants, bundle these messages, and send them out in one encapsulating MIME message.

The message type provides a number of important capabilities in MIME. The message/rfc822 subtype indicates that the body is an entire message, including header and body. Despite the name of this subtype, the encapsulated message may be not only a simple RFC 5322 message but also any MIME message.

The message/partial subtype enables fragmentation of a large message into a number of parts, which must be reassembled at the destination. For this subtype, three parameters are specified in the Content-Type: Message/Partial field: an id common to all fragments of the same message, a sequence number unique to each fragment, and the total number of fragments.

The message/external-body subtype indicates that the actual data to be con- veyed in this message are not contained in the body. Instead, the body contains the information needed to access the data. As with the other message types, the mes- sage/external-body subtype has an outer header and an encapsulated message with its own header. The only necessary field in the outer header is the Content-Type field, which identifies this as a message/external-body subtype. The inner header is the message header for the encapsulated message. The Content-Type field in the outer header must include an access-type parameter, which indicates the method of access, such as FTP (file transfer protocol).

The application type refers to other kinds of data, typically either uninter- preted binary data or information to be processed by a mail-based application.

MIME TRANSFER ENCODINGS The other major component of the MIME specification, in addition to content type specification, is a definition of transfer encodings for message bodies. The objective is to provide reliable delivery across the largest range of environments.

The MIME standard defines two methods of encoding data. The Content- Transfer-Encoding field can actually take on six values, as listed in Table 18.4. However, three of these values (7bit, 8bit, and binary) indicate that no encoding has been done but provide some information about the nature of the data. For SMTP transfer, it is safe to use the 7bit form. The 8bit and binary forms may be usable in other mail transport contexts. Another Content-Transfer-Encoding value is x-token,

which indicates that some other encoding scheme is used for which a name is to be supplied. This could be a vendor-specific or application-specific scheme. The two actual encoding schemes defined are quoted-printable and base64. Two schemes are defined to provide a choice between a transfer technique that is essentially human readable and one that is safe for all types of data in a way that is reasonably compact. The quoted-printable transfer encoding is useful when the data consists largely of octets that correspond to printable ASCII characters. In essence, it represents nonsafe characters by the hexadecimal representation of their code and

introduces reversible (soft) line breaks to limit message lines to 76 characters.

The base64 transfer encoding, also known as radix-64 encoding, is a common one for encoding arbitrary binary data in such a way as to be invulnerable to the processing by mail-transport programs. It is also used in PGP and is described in Appendix 18A.

A MULTIPART EXAMPLE Figure 18.8, taken from RFC 2045, is the outline of a complex multipart message. The message has five parts to be displayed serially: two introductory plain text parts, an embedded multipart message, a richtext part, and a closing encapsulated text message in a non-ASCII character set. The embedded multipart message has two parts to be displayed in parallel: a picture and an audio fragment.

CANONICAL FORM An important concept in MIME and S/MIME is that of canonical form. Canonical form is a format, appropriate to the content type, that is standardized for use between systems. This is in contrast to native form, which is a format that may be peculiar to a particular system. Table 18.5, from RFC 2049, should help clarify this matter.

S/MIME Functionality

In terms of general functionality, S/MIME is very similar to PGP. Both offer the ability to sign and/or encrypt messages. In this subsection, we briefly summarize S/MIME capability. We then look in more detail at this capability by examining mes- sage formats and message preparation.

MIME-Version: 1.0

From: Nathaniel Borenstein <nsb@bellcore.com> To: Ned Freed <ned@innosoft.com>

Subject: A multipart example Content-Type: multipart/mixed;

boundary=unique-boundary-1

This is the preamble area of a multipart message. Mail readers that understand multipart format should ignore this preamble. If you are reading this text, you might want to consider changing to a mail reader that understands how to properly display multipart messages.

--unique-boundary-1

...Some text appears here...

[Note that the preceding blank line means no header fields were given and this is text, with charset US ASCII. It could have been done with explicit typing as in the next part.]

--unique-boundary-1

Content-type: text/plain; charset=US-ASCII

This could have been part of the previous part, but illustrates explicit versus implicit typing of body parts.

--unique-boundary-1

Content-Type: multipart/parallel;  boundary=unique-boundary-2

--unique-boundary-2 Content-Type: audio/basic

Content-Transfer-Encoding: base64

... base64-encoded 8000 Hz single-channel mu-law-format audio data goes here....

--unique-boundary-2 Content-Type: image/jpeg

Content-Transfer-Encoding: base64

... base64-encoded image data goes here....

--unique-boundary-2--

--unique-boundary-1 Content-type: text/enriched

This is <bold><italic>richtext.</italic></bold> <smaller>as defined in RFC 1896</smaller> Isn't it <bigger><bigger>cool?</bigger></bigger>

--unique-boundary-1

Content-Type: message/rfc822

From: (mailbox in US-ASCII) To: (address in US-ASCII) Subject: (subject in US-ASCII)

Content-Type: Text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: Quoted-printable

... Additional text in ISO-8859-1 goes here ...

--unique-boundary-1--

Figure 18.8    Example MIME Message Structure

FUNCTIONS  S/MIME provides the following functions.

•                 Enveloped data: This consists of encrypted content of any type and encrypted- content encryption keys for one or more recipients.

•                 Signed data: A digital signature is formed by taking the message digest of the content to be signed and then encrypting that with the private key of the signer. The content plus signature are then encoded using base64 encoding. A signed data message can only be viewed by a recipient with S/MIME  capability.

•                 Clear-signed data: As with signed data, a digital signature of the content is formed. However, in this case, only the digital signature is encoded using base64. As a result, recipients without S/MIME capability can view the message content, although they cannot verify the signature.

•                 Signed and enveloped data: Signed-only and encrypted-only entities may be nested, so that encrypted data may be signed and signed data or clear-signed data may be encrypted.

CRYPTOGRAPHIC ALGORITHMS Table 18.6 summarizes the cryptographic algorithms used in S/MIME. S/MIME uses the following terminology taken from RFC 2119 (Key Words for use in RFCs to Indicate Requirement Levels) to specify the requirement level:

•                 MUST: The definition is an absolute requirement of the specification. An implementation must include this feature or function to be in conformance with the specification.

•                 SHOULD: There may exist valid reasons in particular circumstances to ignore this feature or function, but it is recommended that an implementation include the feature or function.

S/MIME incorporates three public-key algorithms. The Digital Signature Standard (DSS) described in Chapter 13 is the preferred algorithm for digital signature. S/MIME lists Diffie-Hellman as the preferred algorithm for encrypting session keys; in fact, S/MIME uses a variant of Diffie-Hellman that does provide

Table 18.6     Cryptographic Algorithms Used in S/MIME

encryption/decryption, known as ElGamal (Chapter 10). As an alternative, RSA, described in Chapter 9, can be used for both signatures and session key encryption. These are the same algorithms used in PGP and provide a high level of security. For the hash function used to create the digital signature, the specification requires the 160-bit SHA-1 but recommends receiver support for the 128-bit MD5 for backward compatibility with older versions of S/MIME. As we discussed in Chapter 11, there is justifiable concern about the security of MD5, so SHA-1 is clearly the preferred alternative.

For message encryption, three-key triple DES (tripleDES) is recommended, but compliant implementations must support 40-bit RC2. The latter is a weak encryption algorithm but allows compliance with U.S. export controls.

The S/MIME specification includes a discussion of the procedure for deciding which content encryption algorithm to use. In essence, a sending agent has two deci- sions to make. First, the sending agent must determine if the receiving agent is capa- ble of decrypting using a given encryption algorithm. Second, if the receiving agent  is only capable of accepting weakly encrypted content, the sending agent must decide if it is acceptable to send using weak encryption. To support this decision process, a sending agent may announce its decrypting capabilities in order of prefer- ence for any message that it sends out. A receiving agent may store that information for future use.

The following rules, in the following order, should be followed by a sending agent.

If the sending agent has a list of preferred decrypting capabilities from an intended recipient, it SHOULD choose the first (highest preference) capabil- ity on the list that it is capable of using.

1.                        If the sending agent has no such list of capabilities from an intended recipient but has received one or more messages from the recipient, then the outgoing mes- sage SHOULD use the same encryption algorithm as was used on the last signed and encrypted message received from that intended recipient.

2.                        If the sending agent has no knowledge about the decryption capabilities of the intended recipient and is willing to risk that the recipient may not be able to decrypt the message, then the sending agent SHOULD use triple DES.

3.                        If the sending agent has no knowledge about the decryption capabilities of the intended recipient and is not willing to risk that the recipient may not be able to decrypt the message, then the sending agent MUST use RC2/40.

If a message is to be sent to multiple recipients and a common encryption algorithm cannot be selected for all, then the sending agent will need to send two messages. However, in that case, it is important to note that the security of the mes- sage is made vulnerable by the transmission of one copy with lower security.

S/MIME Messages

S/MIME makes use of a number of new MIME content types, which are shown in Table 18.7. All of the new application types use the designation PKCS. This refers to a set of public-key cryptography specifications issued by RSA Laboratories and made available for the S/MIME effort.

We examine each of these in turn after first looking at the general procedures for S/MIME message preparation.

SECURING A MIME ENTITY S/MIME secures a MIME entity with a signature, encryption, or both. A MIME entity may be an entire message (except for the RFC 5322 headers), or if the MIME content type is multipart, then a MIME entity is one or more of the subparts of the message. The MIME entity is prepared according to the normal rules for MIME message preparation. Then the MIME entity plus some security-related data, such as algorithm identifiers and certificates, are processed by S/MIME to produce what is known as a PKCS object. A PKCS object is then treated as message content and wrapped in MIME (provided with appropriate MIME headers). This process should become clear as we look at specific objects and provide examples.

In all cases, the message to be sent is converted to canonical form. In particu- lar, for a given type and subtype, the appropriate canonical form is used for the mes- sage content. For a multipart message, the appropriate canonical form is used for each subpart.

The use of transfer encoding requires special attention. For most cases, the result of applying the security algorithm will be to produce an object that is partially or totally represented in arbitrary binary data. This will then be wrapped in an outer MIME message, and transfer encoding can be applied at that point, typically base64. However, in the case of a multipart signed message (described in more detail later), the message content in one of the subparts is unchanged by the security process. Unless that content is 7bit, it should be transfer encoded using base64 or quoted-printable so that there is no danger of altering the content to which the signature was applied.

We now look at each of the S/MIME content types.

ENVELOPEDDATA An application/pkcs7-mime subtype is used for one of four categories of S/MIME processing, each with a unique smime-type parameter. In all cases, the resulting entity (referred to as an object) is represented in a form known as Basic Encoding Rules (BER), which is defined in ITU-T   Recommendation

            The BER format consists of arbitrary octet strings and is therefore binary data. Such an object should be transfer encoded with base64 in the outer MIME message. We first look at envelopedData.

The steps for preparing an envelopedData MIME entity are

1.                                               Generate a pseudorandom session key for a particular symmetric encryp- tion algorithm (RC2/40 or triple DES).

2.                                               For each recipient, encrypt the session key with the recipient’s public RSA key.

3.                                               For each recipient, prepare a block known as RecipientInfo that contains an identifier of the recipient’s public-key certificate,4 an identifier of the algo- rithm used to encrypt the session key, and the encrypted session key.

4.                                               Encrypt the message content with the session key.

The RecipientInfo blocks followed by the encrypted content constitute the envelopedData. This information is then encoded into base64. A sample message (excluding the RFC 5322 headers) is

Content-Type:   application/pkcs7-mime; smime-type=enveloped- data; name=smime.p7m

Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename=smime.p7m

rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 0GhIGfHfQbnj756YT64V

To recover the encrypted message, the recipient first strips off the base64 encoding. Then the recipient’s private key is used to recover the session key. Finally, the message content is decrypted with the session   key.

SIGNEDDATA The signedData smime-type can be used with one or more signers. For clarity, we confine our description to the case of a single digital signature. The steps for preparing a signedData MIME entity are

1.                                Select a message digest algorithm (SHA or MD5).

2.                                Compute the message digest (hash function) of the content to be signed.

3.                                Encrypt the message digest with the signer’s private key.

4.                                Prepare a block known as SignerInfo that contains the signer’s public- key certificate, an identifier of the message digest algorithm, an identifier of the algorithm used to encrypt the message digest, and the encrypted mes- sage digest.

The signedData entity consists of a series of blocks, including a message digest algorithm identifier, the message being signed, and SignerInfo. The signedData entity may also include a set of public-key certificates sufficient to constitute a chain from a recognized root or top-level certification authority to the signer. This information is then encoded into base64. A sample message (excluding the RFC 5322 headers) is

Content-Type: application/pkcs7-mime;   smime-type=signed- data; name=smime.p7m

Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename=smime.p7m

567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7 77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh 6YT64V0GhIGfHfQbnj75

To recover the signed message and verify the signature, the recipient first strips off the base64 encoding. Then the signer’s public key is used to decrypt the message digest. The recipient independently computes the message digest and com- pares it to the decrypted message digest to verify the signature.

CLEAR SIGNING Clear signing is achieved using the multipart content type with a signed subtype. As was mentioned, this signing process does not involve transforming the message to be signed, so that the message is sent “in the clear.” Thus, recipients with MIME capability but not S/MIME capability are able to read the incoming message.

A multipart/signed message has two parts. The first part can be any MIME type but must be prepared so that it will not be altered during transfer from source to destination. This means that if the first part is not 7bit, then it needs to be encoded using base64 or quoted-printable. Then this part is processed in the same manner as signedData, but in this case an object with signedData format is created that has an empty message content field. This object is a detached signature. It is then transfer encoded using base64 to become the second part of the multipart/signed message. This second part has a MIME content type of application and a subtype of pkcs7-signature. Here is a sample message:

Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary=boundary42

—boundary42

Content-Type: text/plain

This is a clear-signed message.

—boundary42

Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename=smime.p7s

ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6 4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 7GhIGfHfYT64VQbnj756

—boundary42—

The protocol parameter indicates that this is a two-part clear-signed entity. The micalg parameter indicates the type of message digest used. The receiver can verify the signature by taking the message digest of the first part and comparing this to the message digest recovered from the signature in the second part.

REGISTRATION REQUEST Typically, an application or user will apply to a certification authority for a public-key certificate. The application/pkcs10 S/MIME entity is used to transfer a certification request. The certification request includes certification RequestInfo block, followed by an identifier of the public-key encryption algorithm, followed by the signature of the certificationRequestInfo block made using the sender’s private key. The certificationRequestInfo block includes a name of the certificate subject (the entity whose public key is to be certified) and a bit-string representation of the user’s public key.

CERTIFICATES-ONLY MESSAGE A message containing only certificates or a certificate revocation list (CRL) can be sent in response to a registration request. The message is an application/pkcs7-mime type/subtype with an smime-type parameter of degenerate. The steps involved are the same as those for creating a signedData message, except that there is no message content and the signerInfo field is empty.

S/MIME Certificate Processing

S/MIME uses public-key certificates that conform  to  version  3  of  X.509  (see Chapter 14). The key-management scheme used by S/MIME is in some ways a hybrid between a strict X.509 certification hierarchy and PGP’s web of trust. As with the PGP model, S/MIME managers and/or users must configure each client with a list of trusted keys and with certificate revocation lists. That is, the responsi- bility is local for maintaining the certificates needed to verify incoming signatures and to encrypt outgoing messages. On the other hand, the certificates are signed by certification authorities.

USER AGENT ROLE An S/MIME user has several key-management functions to perform.

•                 Key generation: The user of some related administrative utility (e.g., one asso- ciated with LAN management) MUST be capable of generating separate Diffie-Hellman and DSS key pairs and SHOULD be capable of generating RSA key pairs. Each key pair MUST be generated from a good source of non- deterministic random input and be protected in a secure fashion. A user agent SHOULD generate RSA key pairs with a length in the range of 768 to 1024 bits and MUST NOT generate a length of less than 512 bits.

•                 Registration: A user’s public key must be registered with a certification authority in order to receive an X.509 public-key certificate.

•                 Certificate storage and retrieval: A user requires access to a local list of certifi- cates in order to verify incoming signatures and to encrypt outgoing messages. Such a list could be maintained by the user or by some local administrative entity on behalf of a number of users.

VERISIGN CERTIFICATES There are several companies that provide certification authority (CA) services. For example, Nortel has designed an enterprise CA solution and can provide S/MIME support within an organization. There are a number of Internet-based CAs, including VeriSign, GTE, and the U.S. Postal Service. Of these, the most widely used is the VeriSign CA service, a brief description of which we now provide.

VeriSign provides a CA service that is intended to be compatible with S/MIME and a variety of other applications. VeriSign issues X.509 certificates with the product name VeriSign Digital ID. As of early 1998, over 35,000 commercial Web sites were using VeriSign Server Digital IDs, and over a million consumer Digital IDs had been issued to users of Netscape and Microsoft browsers.

The information contained in a Digital ID depends on the type of Digital ID and its use. At a minimum, each Digital ID contains

•                 Owner’s public key

•                 Owner’s name or alias

•                 Expiration date of the Digital ID

•                 Serial number of the Digital ID

•                 Name of the certification authority that issued the Digital ID

•                 Digital signature of the certification authority that issued the Digital ID

Digital IDs can also contain other user-supplied information, including

•                           Address

•                           E-mail address

•                           Basic registration information (country, zip code, age, and gender)

VeriSign provides three levels, or classes, of security for public-key certificates, as summarized in Table 18.8. A user requests a certificate online at VeriSign’s Web site or other participating Web sites. Class 1 and Class 2 requests are processed on line, and in most cases take only a few seconds to approve. Briefly, the following procedures are used.

•                           For Class 1 Digital IDs, VeriSign confirms the user’s e-mail address by sending a PIN and Digital ID pick-up information to the e-mail address provided in the application.

•                           For Class 2 Digital IDs, VeriSign verifies the information in the application through an automated comparison with a consumer database in addition   to

Table 18.8    Verisign Public-Key Certificate Classes

performing all of the checking associated with a Class 1 Digital ID. Finally, confirmation is sent to the specified postal address alerting the user that a Digital ID has been issued in his or her name.

•                           For Class 3 Digital IDs, VeriSign requires a higher level of identity assurance. An individual must prove his or her identity by providing notarized creden- tials or applying in person.

Enhanced Security Services

As of this writing, three enhanced security services have been proposed in an Internet draft. The details of these may change, and additional services may be added. The three services are

•                           Signed receipts: A signed receipt may be requested in a SignedData object. Returning a signed receipt provides proof of delivery to the originator of a message and allows the originator to demonstrate to a third party that the recipient received the message. In essence, the recipient signs the entire origi- nal message plus the original (sender’s) signature and appends the new signa- ture to form a new S/MIME message.

•                           Security labels: A security label may be included in the authenticated attrib- utes of a SignedData object. A security label is a set of security information regarding the sensitivity of the content that is protected by S/MIME encapsu- lation. The labels may be used for access control, by indicating which users are permitted access to an object. Other uses include priority (secret, confidential, restricted, and so on) or role based, describing which kind of people can see the information (e.g., patient’s health-care team, medical billing agents, etc.).

Secure mailing lists: When a user sends a message to multiple recipients, a cer- tain amount of per-recipient processing is required, including the use of each recipient’s public key. The user can be relieved of this work by employing the services of an S/MIME Mail List Agent (MLA). An MLA can take a single incoming message, perform the recipient-specific encryption for each recipi- ent, and forward the message. The originator of a message need only send the message to the MLA with encryption performed using the MLA’s  public    key.