Internet Draft                              Editor: Paul Hoffman
draft-ietf-smime-ess-03.txt                 Internet Mail Consortium
March 1, 1998
Expires in six months

             Enhanced Security Services for S/MIME


Status of this memo

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. Introduction

This document describes three optional security service extensions for
S/MIME. These services provide functionality that is similar to the Message
Security Protocol [MSP], but are useful in many other environments,
particularly business and finance. The services are:
- signed receipts
- security labels
- secure mailing lists

The services described here are extensions to S/MIME version 3 [SMIME3],
and some of them can also be added to S/MIME version 2 [SMIME2]. The
extensions described here will not cause an S/MIME version 3 recipient to
be unable to read messages from an S/MIME version 2 sender. However, some
of the extensions will cause messages created by an S/MIME version 3 sender
to be unreadable by an S/MIME version 2 recipient.

The format of the messages are described in ASN.1:1988 [ASN1-1988] with the
modification that UTF8String from ASN.1:1994 [ASN1-1994] is used in the
descriptions.

This draft is being discussed on the "ietf-smime" mailing list. To
subscribe, send a message to:
     ietf-smime-request@imc.org
with the single word
     subscribe
in the body of the message. There is a Web site for the mailing list at
<http://www.imc.org/ietf-smime/>.

1.1 Triple Wrapping

Some of the features of each service use the concept of a "triple wrapped"
message. A triple wrapped message is one that has been signed, then
encrypted, then signed again. The signers of the inner and outer signatures
may be different entities or the same entity. Note that the S/MIME
specification does not limit the number of nested encapsulations, so there
may be more than three wrappings.

1.1.1 Purpose of Triple Wrapping

Not all messages need to be triple wrapped. Triple wrapping is used when a
message must be signed, then encrypted, and then have authenticated
attributes bound to the encrypted body. Outer attributes may be added or
removed by the message originator or intermediate agents, and may be
authenticated by intermediate agents or the final recipient.

The inside signature is used for content integrity, non-repudiation with
proof of origin, and binding attributes (such as a security label) to the

original content. These attributes go from the originator to the recipient,
regardless of the number of intermediate entities such as mail list agents
that process the message. The authenticated attributes can be used for
access control to the inner body. Requests for signed receipts by the
originator are carried in the inside signature as well.

The encrypted body provides confidentiality, including confidentiality of
the attributes that are carried in the inside signature.

The outside signature provides authentication and integrity for information
that is processed hop-by-hop, where each hop is an intermediate entity such
as a mail list agent. The outer signature binds attributes (such as a
security label) to the encrypted body. These attributes can be used for
access control and routing decisions.

1.1.2 Steps for Triple Wrapping
The steps to create a triple wrapped message are:

. Start with a message body, called the "original content".

. Encapsulate the original content with the appropriate MIME Content-type
headers, such as "Content-type: text/plain". An exception to this MIME
encapsulation rule is that a signed receipt is not put in MIME headers.

. Sign the result of step 2 (the inner MIME headers and the original
content). The SignedData structure is encapsulated by a ContentInfo
SEQUENCE with a contentType of data. If the structure you create in step 4
is multipart/signed, the eContent is missing; if the structure you create
in step 4 is application/pkcs7-mime, the eContent is the result of step 2
above.

. Add an appropriate MIME construct to the signed message from step 3 as
defined in [SMIME3]. The resulting message is called the "inside
signature".

- If you are signing using multipart/signed, the MIME construct added
consists of a Content-type of multipart/signed with parameters, the
boundary, the result of step 2 above, the boundary, a Content-type of
application/pkcs7-signature, optional MIME headers (such as
Content-transfer-encoding and Content-disposition), and a body part that is
the result of step 3 above.

- If you are instead signing using application/pkcs7-mime, the MIME
construct added consists of a Content-type of application/pkcs7-mime with
parameters, optional MIME headers (such as Content-transfer-encoding and
Content-disposition), and the result of step 3 above.

. Encrypt the result of step 4 as a single block, turning it into an
application/pkcs7-mime object. The EnvelopedData structure is encapsulated
by a ContentInfo SEQUENCE with a contentType of data. This is called the
"encrypted body".

. Add the appropriate MIME headers: a Content-type of
application/pkcs7-mime with parameters, and optional MIME headers such as
Content-transfer-encoding and Content-disposition.

. Using the same logic as in step 3 above, sign the result of step 6 (the
MIME headers and the encrypted body) as a single block

. Using the same logic as in step 4 above, add an appropriate MIME
construct to the signed message from step 7. The resulting message is
called the "outside signature", and is also the triple wrapped message.


1.2 Format of a Triple Wrapped Message

A triple wrapped message has many layers of encapsulation. The structure
differs based on the choice of format for the signed portions of the
message. Because of the way that MIME encapsulates data, the layers do not
appear in order, and the notion of "layers" becomes vague.

There is no need to use the multipart/signed format in an inner signature
because it is known that the recipient is able to process S/MIME messages
(because they decrypted the middle wrapper). A sending agent might choose
to use the multipart/signed format in the outer layer so that a non-S/MIME
agent could see that the next inner layer is encrypted; however, this is
not of great value, since all it shows the recipient is that the rest of
the message is unreadable. Because many sending agents always use
multipart/signed structures, all receiving agents MUST be able to interpret
either multipart/signed or application/pkcs7-mime signature structures.

The format of a triple wrapped message that uses multipart/signed for
both signatures is:

[step 8] Content-type: multipart/signed;
[step 8]    protocol="application/pkcs7-signature";
[step 8]    boundary=outerboundary
[step 8]
[step 8] --outerboundary
[step 6] Content-type: application/pkcs7-mime;          )
[step 6]    smime-type=enveloped-data                   )
[step 6]                                                )
[step 4] Content-type: multipart/signed;              | )
[step 4]    protocol="application/pkcs7-signature";   | )
[step 4]    boundary=innerboundary                    | )
[step 4]                                              | )
[step 4] --innerboundary                              | )
[step 2] Content-type: text/plain                   % | )
[step 2]                                            % | )
[step 1] Original content                           % | )
[step 4]                                              | )
[step 4] --innerboundary                              | )
[step 4] Content-type: application/pkcs7-signature    | )
[step 4]                                              | )
[step 3] SignedData block (eContent is missing)       | )
[step 6]
[step 8] --outerboundary
[step 8] Content-type: application/pkcs7-signature
[step 8]
[step 7] SignedData block

% = These lines are what the inner signature is computed over.
| = These lines are what is encrypted in step 5. This encrypted result
    is opaque and is a part of an EnvelopedData block.
) = These lines are what the outer signature is computed over.

The format of a triple wrapped message that uses application/pkcs7-mime for
the outer signature is:

[step 8] Content-type: application/pkcs7-mime;
[step 8]    smime-type=signed-data
[step 8]
[step 6] Content-type: application/pkcs7-mime;          )
[step 6]    smime-type=enveloped-data;                  )
[step 6]                                                )
[step 4] Content-type: application/pkcs7-mime;        | )
[step 4]    smime-type=signed-data                    | )
[step 4]                                              | )
[step 3] SignedData block (eContent is present)     $ | )
[step 6]

[step 7] SignedData block                                 !

$ = This SignedData block is opaque and contains the ASN.1 encoded result
    of step 2 as well as control information.
| = These lines are what is encrypted in step 5. This encrypted result
    is opaque and is a part of an EnvelopedData block.
) = These lines are what the outer signature is computed over.
! = This SignedData block is opaque and contains the ASN.1 encoded result
    of step 6 as well as control information.

1.3 Security Services and Triple Wrapping

The three security services described in this document are used with triple
wrapped messages in different ways. This section briefly describes the
relationship of each service with triple wrapping; the other sections of
the document go into greater detail.

1.3.1 Signed Receipts and Triple Wrapping

A signed receipt may be requested in any SignedData object. However, if a
signed receipt is requested for a triple wrapped message, the receipt
request MUST be in the inside signature, not in the outside signature. A
secure mailing list agent may change the receipt policy in the outside
signature of a triple wrapped message when that message is processed by the
mailing list.

Note: the signed receipts and receipt requests described in this draft
differ from those described in the work done by the IETF Receipt
Notification Working Group. The output of that Working Group, when
finished, is not expected to work well with triple wrapped messages as
described in this document.

1.3.2 Security Labels and Triple Wrapping

A security label may be included in the authenticated attributes of any
SignedData object. A security label attribute may be included in either the
inner signature, outer signature, or both.

The inner security label is used for access control decisions related to
the plaintext original content. The inner signature provides authentication
and cryptographically protects the original signer's security label that is
on the inside body. This strategy facilitates the forwarding of messages
because the original signer's security label is included in the SignedData
block which can be forwarded to a third party that can verify the inner
signature which will cover the inner security label. The confidentiality
security service can be applied to the inner security label by encrypting
the entire inner SignedData block within an EnvelopedData block.

A security label may also be included in the authenticated attributes of
the outer SignedData block which will include the sensitivities of the
encrypted message. The outer security label is used for access control and
routing decisions related to the encrypted message. Note that a security
label attribute can only be used in an authenticatedAttributes block. An
eSSSecurityLabel attribute MUST NOT be used in an EnvelopedData or
unauthenticated attributes.

1.3.3 Secure Mailing Lists and Triple Wrapping

Secure mail list message processing depends on the structure of S/MIME
layers present in the message sent to the mail list agent. The agent never
changes the data that was hashed to form the inner signature, if such a
signature is present. If an outer signature is present, then the agent will

modify the data that was hashed to form that outer signature. In all cases,
the agent adds or updates an mlExpansionHistory attribute to document the
agent's processing, and ultimately adds or replaces the outer signature on
the message to be distributed.

1.3.4 Placement of Attributes

Certain attributes should be placed in the inner or outer SignedData
message; some attributes can be in either. Further, some attributes must be
authenticated, while authentication is optional for others. The following
table summarizes the recommendation of this profile.

                  |                             |Inner or  |MUST be
Attribute         |OID                          |outer     |authenticated
------------------|-----------------------------|----------|-------------
contentHints      |id-aa-contentHint [ESS]      |either    |no
contentIdentifier |id-aa-contentIdentifier [ESS]|either    |no
contentType       |id-contentType [CMS]         |either    |yes
counterSignature  |id-countersignature [CMS]    |either    |MUST NOT
eSSSecurityLabel  |id-aa-securityLabel [ESS]    |either    |yes
messageDigest     |id-messageDigest [CMS]       |either    |yes
msgSigDigest      |id-aa-msgSigDigest [ESS]     |inner only|yes
mlExpansionHistory|id-aa-mlExpandHistory [ESS]  |outer only|yes
receiptRequest    |id-aa-receiptRequest [ESS]   |inner only|yes
signingTime       |id-signingTime [CMS]         |either    |yes
smimeCapabilities |sMIMECapabilities [MSG]      |either    |yes

If a counterSignature attribute is present, then it MUST be included in the
unauthenticated attributes. It MUST NOT be included in the authenticated
attributes.

Note that the inner and outer signatures are for different senders, so that
the same attribute in the two signatures could lead to very different
consequences.

ContentIdentifier is an attribute (OCTET STRING) used to carry a unique
identifier assigned to the message.

1.4 Object Identifiers

The object identifiers for many of the objects described in this draft are
found in [CMS} and [SMIME3]. Other object identifiers used in S/MIME can be
found in the registry kept at <http://www.imc.org/ietf-smime/oids.html>.
When this draft moves to standards track within the IETF, it is intended
that the IANA will maintain this registry.

1.5 Criticality of Attributes

Authenticated attributes can be marked as critical. In this specification,
the only attribute which MUST be marked as critical is eSSSecurityLabel.

Note that marking any attribute as critical will make the message
unreadable to S/MIME v2 recipients. Because of this, a sending agent should
only mark attributes critical if necessary for the agent's application, and
at the risk of preventing an S/MIME v2 recipient from verifying (or
possibly even being able to read) the message.


. Signed Receipts

Returning a signed receipt provides to the originator proof of delivery of
a message, and allows the originator to demonstrate to a third party that
the recipient was able to verify the signature of the original message.
This receipt is bound to the original message through the signature;
consequently, this service may be requested only if a message is signed.

The receipt sender may optionally also encrypt a receipt to provide
confidentiality between the receipt sender and the receipt recipient.

2.1 Signed Receipt Concepts

The originator of a message may request a signed receipt from the message's
recipients. The request is indicated by adding a receiptRequest attribute
to the authenticatedAttributes field of the SignerInfo object for which the
receipt is requested. The receiving user agent software SHOULD
automatically create a signed receipt when requested to do so, and return
the receipt in accordance with mailing list expansion options, local
security policies, and configuration options.

Because receipts involve the interaction of two parties, the terminology
can sometimes be confusing. In this section, the "sender" is the agent that
sent the original message that included a request for a receipt. The
"receiver" is the party that received that message and generated the
receipt.

The steps in a typical transaction are:

. Sender creates a signed message including a receipt request attribute
(Section 2.2).

. Sender transmits the resulting message to the recipient or recipients.

. Recipient receives message and determines if there is a valid signature
and receipt request in the message (Section 2.3).

. Recipient creates a signed receipt (Section 2.4).

. Recipient transmits the resulting signed receipt message to the sender
(Section 2.5).

. Sender receives the message and validates that it contains a signed
receipt for the original message (Section 2.6). This validation relies on
the sender having retained either a copy of the original message or
information extracted from the original message.

The ASN.1 syntax for the receipt request is given in Section 2.7; the ASN.1
syntax for the receipt is given in Section 2.8.

Note that an agent SHOULD remember when it has sent a receipt so that it
can avoid re-sending a receipt each time it processes the message.

2.2 Receipt Request Creation

Multi-layer S/MIME messages may contain multiple SignedData layers.
However, receipts may be requested only for the innermost SignedData layer
in a multi-layer S/MIME message, such as a triple wrapped message. Only one
receiptRequest attribute can be included in the authenticatedAttributes of
a SignerInfo.

A ReceiptRequest attribute MUST NOT be included in the attributes of a
SignerInfo in a SignedData object that encapsulates a Receipt content. In
other words, the user agent MUST NOT request a signed receipt for a signed
receipt.

A sender requests receipts by placing a receiptRequest attribute in the
authenticated attributes of a signerInfo as follows:

. A receiptRequest data structure is created.

. A signed content identifier for the message is created and assigned to
the signedContentIdentifier field. The signedContentIdentifier is used to
associate the signed receipt with the message requesting the signed
receipt.

. The entities requested to return a signed receipt are noted in the
receiptsFrom field.

. The message originator MUST populate the receiptsTo field with a
GeneralNames for each entity to whom the recipient should send the signed

receipt. If the message originator wants the recipient to send the signed
receipt to the originator, then the originator MUST include a GeneralNames
for itself in the receiptsTo field. GeneralNames is a SEQUENCE OF
GeneralName. receiptsTo is a SEQUENCE OF GeneralNames in which each
GeneralNames represents an entity. There may be multiple GeneralName
instances in each GeneralNames. At a minimum, the message originator MUST
populate each entity's GeneralNames with the address to which the signed
receipt should be sent. Optionally, the message originator MAY also
populate each entity's GeneralNames with other GeneralName instances (such
as directoryName).

. The completed receiptRequest attribute is placed in the
authenticatedAttributes field of the SignerInfo object.

2.2.1 Multiple Receipt Requests

There can be multiple SignerInfos within a SignedData object, and each
SignerInfo may include authenticatedAttributes. Therefore, a single
SignedData object may include multiple SignerInfos, each SignerInfo having
a receiptRequest attribute. For example, an originator can send a signed
message with two SignerInfos, one containing a DSS signature, the other
containing an RSA signature.

Each recipient SHOULD return only one signed receipt.

Not all of the SignerInfos need to include receipt requests, but in all of
the SignerInfos that do contain receipt requests, the receipt requests MUST
be identical.

2.2.2 Information Needed to Validate Signed Receipts

The sending agent MUST retain one or both of the following items to support
the validation of signed receipts returned by the recipients.

- the original signedData object requesting the signed receipt

- the message signature digest value used to generate the original
   signedData signerInfo signature value and the digest value of the
   Receipt content containing values included in the original signedData
   object.  If signed receipts are requested from multiple recipients, then
   retaining these digest values is a performance enhancement because the
   sending agent can reuse the saved values when verifying each returned
   signed receipt.

2.3 Receipt Request Processing

A receiptRequest is associated only with the SignerInfo object in which the
receipt request attribute is directly attached. Processing software SHOULD
examine the authenticatedAttributes field of each of the SignerInfos for
which it verifies a signature in the innermost signedData object to
determine if a receipt is requested. This may result in the receiving agent
processing multiple receiptRequest attributes included in a single
SignedData object.

Because all receiptRequest attributes in a SignedData object must be
identical, the receiving application fully processes (as described in the
following paragraphs) the first receiptRequest that it encounters in a
SignerInfo that it can verify, and it then ensures that all other
receiptRequests are identical to the first one encountered. If
ReceiptRequests which conflict are present, then the processing software
MUST NOT return any receipt.

If a receiptRequest attribute is absent from the authenticated attributes,
then a signed receipt has not been requested from any of the message

recipients and MUST NOT be created. If a receiptRequest attribute is
present in the authenticated attributes, then a signed receipt has been
requested from some or all of the message recipients. Note that in some
cases, a receiving agent might receive two almost-identical messages, one
with a receipt request and the other without one. In this case, the
receiving agent SHOULD send a signed receipt for the message that requests
a signed receipt. A receipt SHOULD be returned if any signature containing
a receipt request can be validated, even if other signatures containing the
same receipt request cannot be validated.

If a receiptRequest attribute is present in the authenticated attributes,
the following process SHOULD be used to determine if a message recipient
has been requested to return a signed receipt.

. If an mlExpansionHistory attribute is present in the outermost
signedData block, do one of the following two steps, based on the absence
or presence of mlReceiptPolicy:

    1.1. If an mlReceiptPolicy value is absent from the last MLData
    element, a Mail List receipt policy has not been specified and the
    processing software SHOULD examine the receiptRequest attribute value
    to determine if a receipt should be created and returned.

    1.2. If an mlReceiptPolicy value is present in the last MLData element,
    do one of the following two steps, based on the value of
    mlReceiptPolicy:

        1.2.1. If the mlReceiptPolicy value is none, then the receipt
        policy of the Mail List supersedes the originator's request for a
        signed receipt and a signed receipt MUST NOT be created.

        1.2.2. If the mlReceiptPolicy value is insteadOf or inAdditionTo,
        the processing software SHOULD examine the receiptsFrom value from
        the receiptRequest attribute to determine if a receipt should be
        created and returned. If a receipt is created, the insteadOf and
        inAdditionTo fields identify entities that SHOULD be sent the
        receipt instead of or in addition to the originator.

. If the receiptsFrom value of the receiptRequest attribute is
allOrFirstTier, do one of the following two steps based on the value of
allOrFirstTier.

    2.1. If the value of allOrFirstTier is allReceipts, then a signed
    receipt SHOULD be created.

    2.2. If the value of allOrFirstTier is firstTierRecipients, do one of
    the following two steps based on the presence of an mlExpansionHistory
    attribute:

        2.2.1. If an mlExpansionHistory attribute is present, then this
        recipient is not a first tier recipient and a signed receipt MUST
        NOT be created.

        2.2.2. If an mlExpansionHistory attribute is not present, then a
        signed receipt SHOULD be created.

. If the receiptsFrom value of the receiptRequest attribute is a
receiptList:

    3.1. If receiptList contains one of the GeneralNames of the recipient,
    then a signed receipt should be created.

    3.2. If receiptList does not contain one of the GeneralNames of the
    recipient, then a signed receipt MUST NOT be created.


A flow chart for the above steps to be executed for each signerInfo for
which the receiving agent verifies the signature would be:

. Receipt Request attribute present?
       YES -> 1.
       NO  -> STOP
. Has mlExpansionHistory?
       YES -> 1.1.
       NO  -> 2.
1.1. mlReceiptPolicy absent?
       YES -> 2.
       NO  -> 1.2.
1.2. Pick based on value of mlReceiptPolicy.
       none -> 1.2.1.
       insteadOf or inAdditionTo -> 1.2.2.
1.2.1. Use ML's policy, then -> STOP
1.2.2. Examine receiptsFrom to determine if a receipt should be created,
    create it if required, send it to recipients designated by
    mlReceiptPolicy, then -> STOP.
. Is value of receiptsFrom allOrFirstTier?
       YES -> Pick based on value of allOrFirstTier.
             allReceipts -> 2.1.
             firstTierRecipients -> 2.2.
       NO  -> 3.
2.1. Create a receipt, then -> STOP.
2.2. Has mlExpansionHistory?
       YES -> 2.2.1.
       NO  -> 2.2.2.
2.2.1. STOP.
2.2.2. Create a receipt, then -> STOP.
. Is receiptsFrom value of receiptRequest a receiptList?
       YES -> 3.1.
       NO  -> STOP.
3.1. Does receiptList contain the recipient?
       YES -> Create a receipt, then -> STOP.
       NO  -> 3.2.
3.2. STOP.

2.4 Signed Receipt Creation

A signed receipt is a signedData object encapsulating a Receipt content
(also called a "signedData/Receipt"). Signed receipts are created as
follows:

. The signature of the original signedData signerInfo that includes the
receiptRequest authenticated attribute MUST be successfully verified before
creating the signedData/Receipt.

    1.1. The ASN.1 DER encoded content of the original signedData object is
    digested as described in [CMS]. The resulting digest value is then
    compared with the value of the messageDigest attribute included in the
    authenticatedAttributes of the original signedData signerInfo. If these
    digest values are different, then the signature verification process
    fails and the signedData/Receipt MUST NOT be created.

    1.2. The ASN.1 DER encoded authenticatedAttributes (including
    messageDigest, receiptRequest and, possibly, other authenticated
    attributes) in the original signedData signerInfo are digested as
    described in [CMS]. The resulting digest value, called msgSigDigest, is
    then used to verify the signature of the original signedData
    signerInfo.  If the signature verification fails, then the
    signedData/Receipt MUST NOT be created.

. A Receipt structure is created.

    2.1. The value of the Receipt version field is set to 1.

    2.2. The original signedData encapContentInfo eContentType object
    identifier is copied into the Receipt contentType.

    2.3. The original signedData signerInfo receiptRequest
    signedContentIdentifier is copied into the Receipt
    signedContentIdentifier.

. The Receipt structure is ASN.1 DER encoded to produce a data stream, D1.

. D1 is digested. The resulting digest value is included as the
messageDigest attribute in the authenticatedAttributes of the signerInfo
which will eventually contain the signedData/Receipt signature value.


. The digest value (msgSigDigest) calculated in Step 1 to verify the
signature of the original signedData signerInfo is included as the
msgSigDigest attribute in the authenticatedAttributes of the signerInfo
which will eventually contain the signedData/Receipt signature value.

. A contentType attribute including the id-ct-receipt object identifier
MUST be created and added to the authenticated attributes of the signerInfo
which will eventually contain the signedData/Receipt signature value.

. A signingTime attribute indicating the time that the signedData/Receipt
is signed SHOULD be created and added to the authenticated attributes of
the signerInfo which will eventually contain the signedData/Receipt
signature value. Other attributes (except receiptRequest) may be added to
the authenticatedAttributes of the signerInfo.

. The authenticatedAttributes (messageDigest, msgSigDigest, contentType
and, possibly, others) of the signerInfo are ASN.1 DER encoded and digested
as described in CMS, Section 5.3.  The resulting digest value is used to
calculate the signature value which is then included in the
signedData/Receipt signerInfo.

. The ASN.1 DER encoded Receipt content MUST be directly encoded within
the signedData encapContentInfo eContent OCTET STRING defined in [CMS]. The
id-ct-receipt object identifier MUST be included in the signedData
encapContentInfo eContent. This results in a single ASN.1 encoded object
composed of a signedData including the Receipt content. The Data content
type MUST NOT be used. The Receipt content MUST NOT be encapsulated in a
MIME header or any other header prior to being encoded as part of the
signedData object.

. If the signedData/Receipt is to be encrypted within an envelopedData
object, then an outer signedData object MUST be created that encapsulates
the envelopedData object, and a contentHints attribute with contentType set
to the id-ct-receipt object identifier MUST be included in the outer
signedData SignerInfo authenticatedAttributes. When a receiving agent
processes the outer signedData object, the presence of the id-ct-receipt
OID in the contentHints contentType indicates that a signedData/Receipt is
encrypted within the envelopedData object encapsulated by the outer
signedData.

2.4.1 MLExpansionHistory Attributes and Receipts

An MLExpansionHistory attribute MUST NOT be included in the attributes of a
SignerInfo in a SignedData object that encapsulates a Receipt content. This
is true because when a SignedData/Receipt is sent to an MLA for
distribution, then the MLA must always encapsulate the received
SignedData/Receipt in an outer SignedData in which the MLA will include the
MLExpansionHistory attribute. The MLA cannot change the
authenticatedAttributes of the received SignedData/Receipt object, so it
can't add the MLExpansionHistory to the SignedData/Receipt.

2.5 Determining the Recipients of the Signed Receipt

If a signed receipt was created by the process described in the sections
above, then the software MUST use the following process to determine to
whom the signed receipt should be sent.


. The receiptsTo field must be present in the receiptRequest attribute.
The software initiates the sequence of recipients with the value(s) of
receiptsTo.

. If the MlExpansionHistory attribute is present in the outer SignedData
block, and the last MLData contains an MLReceiptPolicy value of insteadOf,
then the software replaces the sequence of recipients with the value(s) of
insteadOf.

. If the MlExpansionHistory attribute is present in the outer SignedData
block and the last MLData contains an MLReceiptPolicy value of
inAdditionTo, then the software adds the value(s) of inAdditionTo to the
sequence of recipients.

2.6.  Signed Receipt Validation

A signed receipt is communicated as a single ASN.1 encoded object composed
of a signedData object directly including a Receipt content. It is
identified by the presence of the id-ct-receipt object identifier in the
encapContentInfo eContentType value of the signedData object including the
Receipt content.

A signedData/Receipt is validated as follows:

. ASN.1 decode the signedData object including the Receipt content.

. Extract the contentType, signedContentIdentifier, and
originatorSignatureValue from the decoded Receipt structure to identify the
original signedData signerInfo that requested the signedData/Receipt.

. Acquire the message signature digest value calculated by the sender to
generate the signature value included in the original signedData signerInfo
that requested the signedData/Receipt.

    3.1. If the sender-calculated message signature digest value has been
    saved locally by the sender, it must be located and retrieved.

    3.2. If it has not been saved, then it must be re-calculated based on
    the original signedData content and authenticatedAttributes as
    described in [CMS].

. The message signature digest value calculated by the sender is then
compared with the value of the msgSigDigest authenticatedAttribute included
in the signedData/Receipt signerInfo. If these digest values are identical,
then that proves that the message signature digest value calculated by the
recipient based on the received original signedData object is the same as
that calculated by the sender. This proves that the recipient received
exactly the same original signedData content and authenticatedAttributes as
sent by the sender because that is the only way that the recipient could
have calculated the same message signature digest value as calculated by
the sender. If the digest values are different, then the signedData/Receipt
signature verification process fails.

. Acquire the digest value calculated by the sender for the Receipt
content constructed by the sender (including the contentType,
signedContentIdentifier, and signature value that were included in the
original signedData signerInfo that requested the signedData/Receipt).

    5.1. If the sender-calculated Receipt content digest value has been
    saved locally by the sender, it must be located and retrieved.

    5.2. If it has not been saved, then it must be re-calculated.  As
    described in section 2.4 above, step 2, create a Receipt structure
    including the contentType, signedContentIdentifier and signature value

    that were included in the original signedData signerInfo that requested
    the signed receipt. The Receipt structure is then ASN.1 DER encoded to
    produce a data stream which is then digested to produce the Receipt
    content digest value.

. The Receipt content digest value calculated by the sender is then
compared with the value of the messageDigest authenticatedAttribute
included in the signedData/Receipt signerInfo. If these digest values are
identical, then that proves that the values included in the Receipt content
by the recipient are identical to those that were included in the original
signedData signerInfo that requested the signedData/Receipt. This proves
that the recipient received the original signedData signed by the sender,
because that is the only way that the recipient could have obtained the
original signedData signerInfo signature value for inclusion in the Receipt
content. If the digest values are different, then the signedData/Receipt
signature verification process fails.

. The ASN.1 DER encoded authenticatedAttributes of the signedData/Receipt
signerInfo are digested as described in [CMS].

. The resulting digest value is then used to verify the signature value
included in the signedData/Receipt signerInfo. If the signature
verification is successful, then that proves the integrity of the
signedData/receipt signerInfo authenticatedAttributes and authenticates the
identity of the signer of the signedData/Receipt signerInfo. Note that the
authenticatedAttributes include the recipient-calculated Receipt content
digest value (messageDigest attribute) and recipient-calculated message
signature digest value (msgSigDigest attribute). Therefore, the
aforementioned comparison of the sender-generated and recipient-generated
digest values combined with the successful signedData/Receipt signature
verification proves that the recipient received the exact original
signedData content and authenticatedAttributes (proven by msgSigDigest
attribute) that were signed by the sender of the original signedData object
(proven by messageDigest attribute). If the signature verification fails,
then the signedData/Receipt signature verification process fails.

The signature verification process for each signature algorithm that is
used in conjunction with the CMS protocol is specific to the algorithm.
These processes are described in documents specific to the algorithms.

2.7 Receipt Request Syntax

A receiptRequest attribute value has ASN.1 type ReceiptRequest. Use the
receiptRequest attribute only within the authenticated attributes
associated with a signed message.

ReceiptRequest ::= SEQUENCE {
  signedContentIdentifier ContentIdentifier,
  receiptsFrom ReceiptsFrom,
  receiptsTo SEQUENCE SIZE (1..ub-receiptsTo)) OF GeneralNames }

ub-receiptsTo INTEGER ::= 16

id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1}

ContentIdentifier ::= OCTET STRING

id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7}


A signedContentIdentifier MUST be created by the message originator when
creating a receipt request. To ensure global uniqueness, the minimal
signedContentIdentifier SHOULD contain a concatenation of user-specific
identification information (such as a user name or public keying material
identification information), a GeneralizedTime string, and a random number.

A contentType attribute including the id-ct-receipt object identifier
must be part of the receipt request.

The receiptsFrom field is used by the originator to specify the recipients
requested to return a signed receipt. A CHOICE is provided to allow
specification of:
- receipts from all recipients are requested
- receipts from first tier (recipients that did not receive the
   message as members of a mailing list) recipients are requested
- receipts from a specific list of recipients are requested

ReceiptsFrom ::= CHOICE {
  allOrFirstTier [0] AllOrFirstTier,
  -- formerly "allOrNone [0]AllOrNone"
  receiptList [1] SEQUENCE OF GeneralNames }

AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone
  allReceipts (0),
  firstTierRecipients (1) }

The receiptsTo field is used by the originator to identify the user(s) to
whom the identified recipient should send signed receipts. The message
originator MUST populate the receiptsTo field with a GeneralNames for each
entity to whom the recipient should send the signed receipt. If the message
originator wants the recipient to send the signed receipt to the
originator, then the originator MUST include a GeneralNames for itself in
the receiptsTo field.

2.8 Receipt Syntax

Receipts are represented using a new content type, Receipt. The Receipt
content type shall have ASN.1 type Receipt. Receipts must be encapsulated
within a SignedData message.

Receipt ::= SEQUENCE {
  version Version,  -- Version is imported from [CMS]
  contentType ContentType,
  signedContentIdentifier ContentIdentifier,
  originatorSignatureValue OCTET STRING }

id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
   rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1}

The version field defines the syntax version number, which is 1 for this
version of the standard.

2.9 Content Hints
Many applications find it useful to have information that describes the
innermost signed content of a multi-layer message available on the
outermost signature layer. The contentHints attribute provides such
information.

Content-hints attribute values have ASN.1 type contentHints.

ContentHints ::= SEQUENCE {
  contentDescription UTF8String SIZE (1..MAX) OPTIONAL,
  contentType ContentType }

id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
    rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4}

The contentDescription field may be used to provide information that the
recipient may use to select protected messages for processing, such as a
message subject. If this field is set, then the attribute is expected to
appear on the signedData object enclosing an envelopedData object and not
on the inner signedData object. The SIZE (1..MAX) construct constrains the
sequence to have at least one entry. MAX indicates the upper bound is

unspecified. Implementations are free to choose an upper bound that suits
their environment.

Messages which contain a signedData object wrapped around an envelopedData
object, thus masking the inner content type of the message, SHOULD include
a contentHints attribute, except for the case of the data content type.
Specific message content types may either force or preclude the inclusion
of the contentHints attribute. For example, when a signedData/Receipt is
encrypted within an envelopedData object, an outer signedData object MUST
be created that encapsulates the envelopedData object and a contentHints
attribute with contentType set to the id-ct-receipt object identifier MUST
be included in the outer signedData SignerInfo authenticatedAttributes.

2.10  Message Signature Digest Attribute

The msgSigDigest attribute can only be used in the authenticated attributes
of a signed receipt. It contains the digest of the ASN.1 DER encoded
authenticatedAttributes included in the original signedData that requested
the signed receipt. Only one msgSigDigest attribute can appear in an
authenticated attributes set. It is defined as follows:

msgSigDigest ::= OCTET STRING

id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5}


. Security Labels

This section describes the syntax to be used for security labels that can
optionally be associated with S/MIME encapsulated data. A security label is
a set of security information regarding the sensitivity of the content that
is protected by S/MIME encapsulation.

"Authorization" is the act of granting rights and/or privileges to users
permitting them access to an object. "Access control" is a means of
enforcing these authorizations. The sensitivity information in a security
label can be compared with a user's authorizations to determine if the user
is allowed to access the content that is protected by S/MIME encapsulation.

Security labels may be used for other purposes such as a source of routing
information. The labels are often priority based ("secret", "confidential",
"restricted", and so on) or role-based, describing which kind of people can
see the information ("patient's health-care team", "medical billing
agents", "unrestricted", and so on).

3.1 Security Label Processing Rules

A sending agent may include a security label attribute in the authenticated
attributes of a signedData object. A receiving agent examines the security
label on a received message and determines whether or not the recipient is
allowed to see the contents of the message.

3.1.1 Adding Security Labels

A sending agent that is using security labels MUST put the security label
attribute in the authenticatedAttributes field of a SignerInfo block. The
security label attribute MUST NOT be included in the unauthenticated
attributes. Integrity and authentication security services MUST be applied
to the security label, therefore it MUST be included as an authenticated
attribute, if used. This causes the security label attribute to be part of
the data that is hashed to form the SignerInfo signature value. A

SignerInfo block MUST NOT have more than one security label authenticated
attribute.

When there are multiple SignedData blocks applied to a message, a security
label attribute may be included in either the inner signature, outer
signature, or both. A security label authenticated attribute may be
included in a authenticatedAttributes field within the inner SignedData
block. The inner security label will include the sensitivities of the
original content and will be used for access control decisions related to
the plaintext encapsulated content. The inner signature provides
authentication of the inner security label and cryptographically protects
the original signer's inner security label of the original content.

When the originator signs the plaintext content and authenticated
attributes, the inner security label is bound to the plaintext content. An
intermediate entity cannot change the inner security label without
invalidating the inner signature. The confidentiality security service can
be applied to the inner security label by encrypting the entire inner
signedData object within an EnvelopedData block.

A security label authenticated attribute may also be included in a
authenticatedAttributes field within the outer SignedData block. The outer
security label will include the sensitivities of the encrypted message and
will be used for access control decisions related to the encrypted message
and for routing decisions. The outer signature provides authentication of
the outer security label (as well as for the encapsulated content which may
include nested S/MIME messages).

There can be multiple SignerInfos within a SignedData object, and each
SignerInfo may include authenticatedAttributes. Therefore, a single
SignedData object may include multiple security labels, each SignerInfo
having an eSSSecurityLabel attribute. For example, an originator can send a
signed message with two SignerInfos, one containing a DSS signature, the
other containing an RSA signature. Not all of the SignerInfos need to
include security labels, but in all of the SignerInfos that do contain
security labels, the security labels MUST be identical.

A recipient SHOULD process an eSSSecurityLabel attribute only if the
recipient can verify the signature of the SignerInfo which covers the
eSSSecurityLabel attribute. A recipient SHOULD NOT use a security label
that the recipient cannot authenticate. Local security policies should
exist for handling of messages which contain an eSSSecurityLabel attribute
only in SignerInfos which cannot be validated, or which contain multiple
valid SignerInfos where the security labels are not identical.

3.1.2 Processing Security Labels

A receiving agent that processes security labels MUST process the
eSSSecurityLabel attribute, if present, in each SignerInfo in the
SignedData object for which it verifies the signature. This may result in
the receiving agent processing multiple security labels included in a
single SignedData object. Because all security labels in a SignedData
object must be identical, the receiving application processes (such as
performing access control) on the first eSSSecurityLabel that it encounters

in a SignerInfo that it can verify, and then ensures that all other
eSSSecurityLabels are identical to the first one encountered.

A receiving agent that processes security labels SHOULD have a local policy
about whether or not to show the inner content of an incoming messages that
has a security label with a security policy identifier that the processing
software does not recognize. If the receiving agent does not recognize the
eSSSecurityLabel security-policy-identifier value, it SHOULD stop
processing the message and indicate an error.

3.2 Syntax of eSSSecurityLabel

The eSSSecurityLabel syntax is derived directly from [MTSABS] ASN.1 module.
(The MTSAbstractService module begins with "DEFINITIONS IMPLICIT TAGS
::=".) Further, the eSSSecurityLabel syntax is compatible with that used in
[MSP4].

The eSSSecurityLabel MUST be marked as critical.

ESSSecurityLabel ::= SET {
  security-policy-identifier SecurityPolicyIdentifier OPTIONAL,
  security-classification SecurityClassification OPTIONAL,
  privacy-mark ESSPrivacyMark OPTIONAL,
  security-categories SecurityCategories OPTIONAL }

id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2}

SecurityPolicyIdentifier ::= OBJECT IDENTIFIER

SecurityClassification ::= INTEGER {
  unmarked (0),
  unclassified (1),
  restricted (2),
  confidential (3),
  secret (4),
  top-secret (5) } (0..ub-integer-options)

ub-integer-options INTEGER ::= 256

ESSPrivacyMark ::= CHOICE {
    pString      PrintableString (SIZE (1..ub-privacy-mark-length)),
    utf8String   UTF8String SIZE (1..MAX)
}

ub-privacy-mark-length INTEGER ::= 128

SecurityCategories ::= SET SIZE (1..ub-security-categories) OF
        SecurityCategory

ub-security-categories INTEGER ::= 64

SecurityCategory ::= SEQUENCE {
  type  [0] OBJECT IDENTIFIER,
  value [1] ANY -- defined by type
}

--Note: The aforementioned SecurityCategory syntax produces identical
--hex encodings as the following SecurityCategory syntax that is
--documented in the X.411 specification:
--
--SecurityCategory ::= SEQUENCE {
--     type  [0]  SECURITY-CATEGORY,
--     value [1]  ANY DEFINED BY type }
--
--SECURITY-CATEGORY MACRO ::=
--BEGIN
--TYPE NOTATION ::= type | empty
--VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)
--END

3.3  Security Label Components

This section gives more detail on the the various components of the
eSSSecurityLabel syntax.

3.3.1 Security Policy Identifier

A security policy is a set of criteria for the provision of security
services. The eSSSecurityLabel security-policy-identifier is used to
identify the security policy in force to which the security label relates.
It indicates the semantics of the other security label components. Even
though the eSSSecurityLabel security-policy-identifier is an optional
field, all security labels used with S/MIME messages MUST include the
security-policy-identifier.

3.3.2 Security Classification

This specification defines the use of the Security Classification field
exactly as is specified in the X.411 Recommendation, which states in part:


    If present, a security-classification may have one of a hierarchical
list of values. The basic security-classification hierarchy is defined
in this Recommendation, but the use of these values is defined by the
security-policy in force. Additional values of security-classification,
and their position in the hierarchy, may also be defined by a
security-policy as a local matter or by bilateral agreement. The basic
security-classification hierarchy is, in ascending order: unmarked,
unclassified, restricted, confidential, secret, top-secret.

This means that the security policy in force (identified by the
eSSSecurityLabel security-policy-identifier) defines the
SecurityClassification integer values and their meanings.

An organization can develop its own security policy that defines the
SecurityClassification INTEGER values and their meanings. However, the
general interpretation of the X.411 specification is that the values of 0
through 5 are reserved for the "basic hierarchy" values of unmarked,
unclassified, restricted, confidential, secret, and top-secret. Note that
X.411 does not provide the rules for how these values are used to label
data and how access control is performed using these values.

There is no universal definition of the rules for using these "basic
hierarchy" values. Each organization (or group of organizations) will
define a security policy which documents how the "basic hierarchy" values
are used (if at all) and how access control is enforced (if at all) within
their domain.

Therefore, the security-classification value MUST be accompanied by a
security-policy-identifier value to define the rules for its use. For
example, a company's "secret" classification may convey a different meaning
than the US Government "secret" classification. In summary, a security
policy SHOULD NOT use integers 0 through 5 for other than their X.411
meanings, and SHOULD instead use other values in a hierarchical fashion.

Note that the set of valid security-classification values MUST be
hierarchical, but these values do not necessarily need to be in ascending
numerical order. Further, the values do not need to be contiguous.

For example, in the Defense Message System 1.0 security policy, the
security-classification value of 11 indicates Sensitive-But-Unclassified
and 5 indicates top-secret. The hierarchy of sensitivity ranks top-secret
as more sensitive than Sensitive-But-Unclassified even though the numerical
value of top-secret is less than Sensitive-But-Unclassified.

(Of course, if security-classification values are both hierarchical and in
ascending order, a casual reader of the security policy is more likely to
understand it.)

An example of a security policy that does not use any of the X.411 values
might be:
 -- anyone
 -- Morgan Corporation and its contractors
 -- Morgan Corporation employees
 -- Morgan Corporation board of directors

An example of a security policy that uses part of the X.411 hierarchy might
be:
 -- unmarked
 -- unclassified, can be read by everyone
 -- restricted to Timberwolf Productions staff
 -- can only be read to Timberwolf Productions executives


3.3.3 Privacy Mark

If present, the eSSSecurityLabel privacy-mark is not used for access
control. The content of the eSSSecurityLabel privacy-mark may be defined by
the security policy in force (identified by the eSSSecurityLabel
security-policy-identifier) which may define a list of values to be used.
Alternately, the value may be determined by the originator of the
security-label.

3.3.4 Security Categories

If present, the eSSSecurityLabel security-categories provide further
granularity for the sensitivity of the message. The security policy in
force (identified by the eSSSecurityLabel security-policy-identifier) is
used to indicate the syntaxes that are allowed to be present in the
eSSSecurityLabel security-categories. Alternately, the security-categories
and their values may be defined by bilateral agreement.

. Mail List Management

Sending agents must create recipient-specific data structures for each
recipient of an encrypted message. This process can impair performance for
messages sent to a large number of recipients. Thus, Mail List Agents
(MLAs) that can take a single message and perform the recipient-specific
encryption for every recipient are often desired.

An MLA appears to the message originator as a normal message recipient, but
the MLA acts as a message expansion point for a Mail List (ML). The sender
of a message directs the message to the MLA, which then redistributes the
message to the members of the ML. This process offloads the per-recipient
processing from individual user agents and allows for more efficient
management of large MLs. MLs are true message recipients served by MLAs
that provide cryptographic and expansion services for the mailing list.

In addition to cryptographic handling of messages, secure mailing lists
also have to prevent mail loops. A mail loop is where one mailing list is a
member of a second mailing list, and the second mailing list is a member of
the first. A message will go from one list to the other in a
rapidly-cascading succession of mail that will be distributed to all other
members of both lists.

To prevent mail loops, MLAs use the mlExpansionHistory attribute of the
outer signature of a triple wrapped message. The mlExpansionHistory
attribute is essentially a list of every MLA that has processed the
message. If an MLA sees its own unique entity identifier in the list, it
knows that a loop has been formed, and does not send the message to the
list again.

4.1 Mail List Expansion
Mail list expansion processing is noted in the value of the
mlExpansionHistory attribute, located in the authenticated attributes of
the MLA's SignerInfo block. The MLA creates or updates the authenticated
mlExpansionHistory attribute value each time the MLA expands and signs a
message for members of a mail list.

The MLA MUST add an MLData record containing the MLA's identification
information, date and time of expansion, and optional receipt policy to the
end of the mail list expansion history sequence. If the mlExpansionHistory
attribute is absent, then the MLA MUST add the attribute and the current
expansion becomes the first element of the sequence. If the

mlExpansionHistory attribute is present, then the MLA MUST add the current
expansion information to the end of the existing MLExpansionHistory
sequence. Only one mlExpansionHistory attribute can be included in the
authenticatedAttributes of a SignerInfo.

Note that if the mlExpansionHistory attribute is absent, then the recipient
is a first tier message recipient.

There can be multiple SignerInfos within a SignedData object, and each
SignerInfo may include authenticatedAttributes. Therefore, a single
SignedData object may include multiple SignerInfos, each SignerInfo having
a mlExpansionHistory attribute. For example, an originator can send a
signed message with two SignerInfos, one containing a DSS signature, the
other containing an RSA signature. Not all of the SignerInfos need to
include mlExpansionHistory attributes, but in all of the SignerInfos that
do contain mlExpansionHistory attributes, the mlExpansionHistory attributes
MUST be identical.

A recipient SHOULD only process an mlExpansionHistory attribute if the
recipient can verify the signature of the SignerInfo which covers the
attribute. A recipient SHOULD NOT use an mlExpansionHistory attribute which
the recipient cannot authenticate.

When receiving a message that includes an outer SignedData object, a
receiving agent that processes mlExpansionHistory attributes MUST process
the mlExpansionHistory attribute, if present, in each SignerInfo in the
SignedData object for which it verifies the signature. This may result in
the receiving agent processing multiple mlExpansionHistory attributes
included in a single SignedData object. Because all mlExpansionHistory
attributes must be identical, the receiving application processes the first
mlExpansionHistory attribute that it encounters in a SignerInfo that it can
verify, and then ensures that all other mlExpansionHistory attributes are
identical to the first one encountered.

4.1.1 Detecting Mail List Expansion Loops

Prior to expanding a message, the MLA examines the value of any existing
mail list expansion history attribute to detect an expansion loop. An
expansion loop exists when a message expanded by a specific MLA for a
specific mail list is redelivered to the same MLA for the same mail list.

Expansion loops are detected by examining the mailListIdentifier field of
each MLData entry found in the mail list expansion history. If an MLA finds
its own identification information, then the MLA must discontinue expansion
processing and should provide warning of an expansion loop to a human mail
list administrator. The mail list administrator is responsible for
correcting the loop condition.

4.2 Mail List Agent Processing

MLA message processing depends on the structure of S/MIME layers found in
the processed message. In all cases, the MLA ultimately signs the message
and adds or updates an mlExpansionHistory attribute to document MLA
processing. In all cases, the MLA may need to perform access control before
distributing the message to mail list members if the message contains a
SignedData block and an associated eSSSecurityLabel attribute. If a
eSSSecurityLabel authenticated attribute is used for access control, then

the signature of the signerInfo block including the eSSSecurityLabel
authenticated attribute MUST be verified before using the security label.
The MLA should continue parsing the MIME-encapsulated message to determine
if there is a security label associated with an encapsulated SignedData
object. This may include decrypting an EnvelopedData object to determine if
an encapsulated SignedData object includes a eSSSecurityLabel attribute.

Each MLA that processes the message creates its own mlExpansionHistory and
adds it to the sequence of mlExpansionHistory attributes already in the
message. An MLA MUST NOT modify the mlExpansionHistory created by a MLA
that previously processed the message. Each MLA copies the sequence of
mlExpansionHistory attributes created by the MLAs that previously processed
the message into the newly constructed expanded message, and adds its own
mlExpansionHistory as the last element of the sequence. Section 4.3
provides more details regarding adding information to an existing
mLExpansionHistory attribute.

The processing used depends on the type of the outermost layer of the
message. There are three cases for the type of the outermost data:
- EnvelopedData
- SignedData
- data

4.2.1 Processing for EnvelopedData

. The MLA locates its own RecipientInfo and uses the information it
contains to obtain the message key.

. The MLA removes the existing recipientInfos field and replaces it with a
new recipientInfos value built from RecipientInfo structures created for
each member of the mailing list. The MLA also removes the existing
originatorInfo field and replaces it with a new originatorInfo value built
from information describing the MLA.

. The MLA encapsulates the expanded encrypted message in a SignedData
block, adding an mlExpansionHistory attribute as described in the "Mail
List Expansion" section to document the expansion.

. The MLA signs the new message and delivers the updated message to mail
list members to complete MLA processing.

4.2.2 Processing for SignedData

MLA processing of multi-layer messages depends on the type of data in each
of the layers. Step 3 below specifies that different processing will take
place depending on the type of CMS message that has been signed. That
is, it needs to know the type of data at the next inner layer, which may or
may not be the innermost layer.

. The MLA verifies the signature value found in the outermost SignedData
layer associated with the signed data. MLA processing of the message
terminates if the message signature is invalid.

. If the outermost SignedData layer includes an authenticated
mlExpansionHistory attribute the MLA checks for an expansion loop as
described in the "Detecting Mail List Expansion Loops" section.

. Determine the type of the data that has been signed. That is, look at
the type of data on the layer just below the SignedData, which may or may
not be the "innermost" layer. Based on the type of data, perform either
step 3.1 (EnvelopedData), step 3.2 (SignedData), or step 3.3 (all other
types).

    3.1. If the signed data is EnvelopedData, the MLA performs expansion
processing of the encrypted message as described previously. Note that

this process invalidates the signature value in the outermost
SignedData layer associated with the original encrypted message.
Proceed to section 3.2 with the result of the expansion.

    3.2. If the signed data is SignedData, or is the result of expanding an
EnvelopedData block in step 3.1:

        3.2.1. The MLA strips the existing outermost SignedData layer after
        remembering the value of the mlExpansionHistory and all other
        authenticated attributes in that layer, if present.

        3.2.2. If the signed data is EnvelopedData (from step 3.1), the MLA
encapsulates the expanded encrypted message in a new outermost
SignedData layer. On the other hand, if the signed data is
SignedData (from step 3.2), the MLA encapsulates the signed data in
a new outermost SignedData layer.

        3.2.3. The outermost signedData layer created by the MLA replaces
        the original outermost signedData layer. The MLA MUST create an
        authenticated attribute list for the new outermost signedData layer
        which MUST include each authenticated attribute present in the
        original outermost signedData layer, unless the MLA explicitly
        replaces one or more particular attributes with new value. A
        special case is the mlExpansionHistory attribute. The MLA MUST add
        an mlExpansionHistory authenticated attribute to the outer
        signedData layer as follows:

            3.2.3.1. If the original outermost SignedData layer included an
mlExpansionHistory attribute, the attribute's value is copied
and updated with the current ML expansion information as
described in the "Mail List Expansion" section.

            3.2.3.2. If the original outermost SignedData layer did not
include an mlExpansionHistory attribute, a new attribute value
is created with the current ML expansion information as
described in the "Mail List Expansion" section.

    3.3. If the signed data is not EnvelopedData or SignedData:

        3.3.1. The MLA encapsulates the received signedData object in an
outer SignedData object, and adds an mlExpansionHistory attribute
to the outer SignedData object containing the current ML expansion
information as described in the "Mail List Expansion" section.

. The MLA signs the new message and delivers the updated message to mail
list members to complete MLA processing.

A flow chart for the above steps would be:

. Has a valid signature?
       YES -> 2.
       NO  -> STOP.
. Does outermost SignedData layer
       contain mlExpansionHistory?
       YES -> Check it, then -> 3.
       NO  -> 3.
. Check type of data just below outermost
       SignedData.
       EnvelopedData -> 3.1.
       SignedData -> 3.2.
       all others -> 3.3.
3.1. Expand the encrypted message, then -> 3.2.
3.2. -> 3.2.1.
3.2.1. Strip outermost SignedData layer, note value of mlExpansionHistory
       and other authenticated attributes, then -> 3.2.2.
3.2.2. Encapsulate in new signature, then -> 3.2.3.
3.2.3. Create new signedData layer. Was there an old mlExpansionHistory?
       YES -> copy the old mlExpansionHistory values, then -> 4.

       NO  -> create new mlExpansionHistory value, then -> 4.
3.3. Encapsulate in a SignedData layer and add an mlExpansionHistory
       attribute, then -> 4.
. Sign message, deliver it, STOP.

4.2.3 Processing for data

. The MLA encapsulates the message in a SignedData layer, and adds an
mlExpansionHistory attribute containing the current ML expansion
information as described in the "Mail List Expansion" section.

. The MLA signs the new message and delivers the updated message to mail
list members to complete MLA processing.

4.3 Mail List Agent Signed Receipt Policy Processing

If a mailing list (B) is a member of another mailing list (A), list B often
needs to propagate forward the mailing list receipt policy of A. As a
general rule, a mailing list should be conservative in propagating forward
the mailing list receipt policy because the ultimate recipient need only
process the last item in the ML expansion history. The MLA builds the
expansion history to meet this requirement.

The following table describes the outcome of the union of mailing list A's
policy (the rows in the table) and mailing list B's policy (the columns in
the table).

             |                    B's policy
A's policy   | none   insteadOf        inAdditionTo        missing
-------------------------------------------------------------------------
none         | none   none             none                none
insteadOf    | none   insteadOf(B)     *1                  insteadOf(A)
inAdditionTo | none   insteadOf(B)     *2                  inAdditionTo(A)
missing      | none   insteadOf(B)     inAdditionTo(B)     missing

*1 = insteadOf(insteadOf(A) + inAdditionTo(B))
*2 = inAdditionTo(inAdditionTo(A) + inAdditionTo(B))

4.4 Mail List Expansion History Syntax

An mlExpansionHistory attribute value has ASN.1 type MLExpansionHistory. If
there are more than ub-ml-expansion-history mailing lists in the sequence,
the processing agent should provide notification of the error to a human
mail list administrator. The mail list administrator is responsible for
correcting the overflow condition.

MLExpansionHistory ::= SEQUENCE
        SIZE (1..ub-ml-expansion-history) OF MLData

id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3}

ub-ml-expansion-history INTEGER ::= 64

MLData contains the expansion history describing each MLA that has
processed a message. As an MLA distributes a message to members of an ML,
the MLA records its unique identifier, date and time of expansion, and
receipt policy in an MLData structure.

MLData ::= SEQUENCE {
  mailListIdentifier EntityIdentifier,
        -- EntityIdentifier is imported from [CMS]
  expansionTime GeneralizedTime,
  mlReceiptPolicy MLReceiptPolicy OPTIONAL }

The receipt policy of the ML can withdraw the originator's request for
the return of a signed receipt. However, if the originator of the
message has not requested a signed receipt, the MLA cannot request a
signed receipt.

When present, the mlReceiptPolicy specifies a receipt policy that
supersedes the originator's request for signed receipts. The policy

can be one of three possibilities: receipts MUST NOT be returned
(none); receipts should be returned to an alternate list of
recipients, instead of to the originator (insteadOf); or receipts
should be returned to a list of recipients in addition to the
originator (inAdditionTo).

MLReceiptPolicy ::= CHOICE {
  none [0] NULL,
  insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames,
  inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames }


. Security Considerations

This entire document discusses security.


A.  ASN.1 Module

ExtendedSecurityServices
     { iso(1) member-body(2) us(840) rsadsi(113549)
       pkcs(1) pkcs-9(9) smime(16) modules(0) ess(2) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN

-- UNIVERSAL type defined in ASN.1 1997 but required for
-- this specification

UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING
IMPORTS
-- Cryptographic Message Syntax (CMS)
    ContentType, EntityIdentifier, SubjectKeyIdentifier, Version
    FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840)
    rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) }

-- X.509
    GeneralNames FROM CertificateExtensions
    {joint-iso-ccitt ds(5) module(1) certificateExtensions(26) 0};


-- Extended Security Services

-- The construct "SEQUENCE SIZE (1..MAX) OF" appears in several ASN.1
-- constructs in this module. A valid ASN.1 SEQUENCE can have zero or
-- more entries. The SIZE (1..MAX) construct constrains the SEQUENCE to
-- have at least one entry. MAX indicates the upper bound is unspecified.
-- Implementations are free to choose an upper bound that suits their
-- environment.

-- Section 2.7

ReceiptRequest ::= SEQUENCE {
  signedContentIdentifier ContentIdentifier,
  receiptsFrom ReceiptsFrom,
  receiptsTo SEQUENCE SIZE (1..ub-receiptsTo) OF GeneralNames }

ub-receiptsTo INTEGER ::= 16

id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1}

ContentIdentifier ::= OCTET STRING

id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7}

ReceiptsFrom ::= CHOICE {
  allOrFirstTier [0] AllOrFirstTier,
  -- formerly "allOrNone [0]AllOrNone"
  receiptList [1] SEQUENCE OF GeneralNames }

AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone
  allReceipts (0),
  firstTierRecipients (1) }


-- Section 2.8

Receipt ::= SEQUENCE {
  version Version,  -- Version is imported from [CMS]
  contentType ContentType,
  signedContentIdentifier ContentIdentifier,
  originatorSignatureValue OCTET STRING }

id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
   rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1}


-- Section 2.9

ContentHints ::= SEQUENCE {
  contentDescription UTF8String SIZE (1..MAX) OPTIONAL,
  contentType OBJECT IDENTIFIER }

id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
    rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4}


-- Section 2.10

MsgSigDigest ::= OCTET STRING

id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5}



-- Section 3.2

ESSSecurityLabel ::= SET {
  security-policy-identifier SecurityPolicyIdentifier OPTIONAL,
  security-classification SecurityClassification OPTIONAL,
  privacy-mark ESSPrivacyMark OPTIONAL,
  security-categories SecurityCategories OPTIONAL }

id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2}

SecurityPolicyIdentifier ::= OBJECT IDENTIFIER

SecurityClassification ::= INTEGER {
  unmarked (0),
  unclassified (1),
  restricted (2),
  confidential (3),
  secret (4),
  top-secret (5) } (0..ub-integer-options)

ub-integer-options INTEGER ::= 256

ESSPrivacyMark ::= CHOICE {
    pString      PrintableString (SIZE (1..ub-privacy-mark-length)),
    utf8String   UTF8String
}

ub-privacy-mark-length INTEGER ::= 128

SecurityCategories ::= SET SIZE (1..ub-security-categories) OF
        SecurityCategory

ub-security-categories INTEGER ::= 64

SecurityCategory ::= SEQUENCE {
  type  [0] OBJECT IDENTIFIER,
  value [1] ANY -- defined by type
}

--Note: The aforementioned SecurityCategory syntax produces identical
--hex encodings as the following SecurityCategory syntax that is
--documented in the X.411 specification:
--
--SecurityCategory ::= SEQUENCE {
--     type  [0]  SECURITY-CATEGORY,
--     value [1]  ANY DEFINED BY type }
--
--SECURITY-CATEGORY MACRO ::=
--BEGIN
--TYPE NOTATION ::= type | empty
--VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)
--END


-- Section 4.4

MLExpansionHistory ::= SEQUENCE
        SIZE (1..ub-ml-expansion-history) OF MLData

id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3}

ub-ml-expansion-history INTEGER ::= 64

MLData ::= SEQUENCE {
  mailListIdentifier EntityIdentifier,
        -- EntityIdentifier is imported from [CMS]
  expansionTime GeneralizedTime,
  mlReceiptPolicy MLReceiptPolicy OPTIONAL }

MLReceiptPolicy ::= CHOICE {
  none [0] NULL,
  insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames,
  inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames }


END -- of ExtendedSecurityServices


B. References

[ASN1-1988] "Recommendation X.208: Specification of Abstract Syntax
Notation One (ASN.1)"

[ASN1-1994] "Recommendation X.680: Specification of Abstract Syntax
Notation One (ASN.1)"

[CMS] "Cryptographic Message Syntax", Internet Draft
draft-ietf-smime-cms-xx.

[MSP4] "Secure Data Network System (SDNS) Message Security Protocol (MSP)
4.0", Specification SDN.701, Revision A, 1997-02-06.

[MTSABS] "1988 International Telecommunication Union (ITU) Data
Communication Networks Message Handling Systems: Message Transfer System:
Abstract Service Definition and Procedures, Volume VIII, Fascicle VIII.7,
Recommendation X.411"; MTSAbstractService {joint-iso-ccitt mhs-motis(6)
mts(3) modules(0) mts-abstract-service(1)}

[PKCS7-1.5] "PKCS #7: Cryptographic Message Syntax", Internet Draft
draft-hoffman-pkcs-crypt-msg-xx.

[SMIME2] "S/MIME Version 2 Message Specification", Internet Draft
draft-dusse-smime-msg-xx, and  "S/MIME Version 2 Certificate Handling",
Internet Draft draft-dusse-smime-cert-xx.


[SMIME3] "S/MIME Version 3 Message Specification", Internet Draft
draft-ietf-smime-msg-xx, and  "S/MIME Version 3 Certificate Handling",
Internet Draft draft-ietf-smime-cert-xx.

[UTF8] "UTF-8, a transformation format of ISO 10646", RFC 2279.

C. Acknowledgements

The first draft of this work was prepared by David Solo. John Pawling did a
huge amount of very detailed revision work during the many phases of the
document.

Many other people have contributed hard work to this draft, including:
Bengt Ackzell
Blake Ramsdell
Carlisle Adams
Jim Schaad
Phillip Griffin
Russ Housley
Scott Hollenbeck
Steve Dusse


D. Open Issues

Should contentHints move to the CMS draft? If it is useful in other types
of specs, not just ESS, there may be a good reason to have it live in
CMS.

We need to define better how a receiving MLA finds the outer signature that
should have the mlExpansionHistory in it. For instance, a well-formed
triple-wrapped message may pass through a gateway that signs it. We need
wording that says something to the effect of "keep going down through
signatures until you find an mlExpansionHistory attribute, throw away the
higher layers, and process this."

Should mlExpansionHistory have a size of (1..MAX) or should we leave it
as 64?


E. Changes from draft-ietf-smime-ess-02 to draft-ietf-smime-ess-03

Spelled eSSSecurityLabel better in a few places.

Changed "encapsulatedContentType" to "contentType" throughout the document.

Added the OIDs for things that are defined in this draft throughout the text
and added them to Appendix A.

1: Changed the second paragraph to be clearer about how the extensions
affect interoperability.

1.1.1: Replaced the first paragraph with a clearer one.

1.1.2: Completely rewritten.

1.2: Completely rewritten.

1.3.4: In the table, specified contentType MUST be authenticated. Also
added msgSigDigest. Also added the origin of the OID in the table.

1.4: Changed the wording to say where the OIDs are found.

1.5: Added this section on criticality of attributes.

2.2: Removed step 2 and renumbered the rest of the steps.

2.2, bullet 4: Replaced the entire paragraph to describe that the
receiptsTo field MUST be populated.

2.3: In the third paragraph, changed MUST to SHOULD for sending the signed
receipt.

2.4, bullet 2.2 and 2.3: Revised to no longer include contentType in the
Receipt.

2.4, bullet 9: Changed "contentInfo content ANY" with "encapContentInfo
eContent OCTET STRING".

2.5, bullet 1: Removed wording about what to do if there isn't a
receiptsTo.

2.6: Changed "contentInfo content ANY" with "encapContentInfo eContent
OCTET STRING".

2.7: Removed the discussion of what an encapsulatedContentType is. Also
changed the wording about the receiptsTo to show it is mandatory.
Also removed the optional contentType from the ReceiptRequest.

2.8: Made the contentType in the Receipt structure non-optional.

2.9: Made contentDescription a UTF8String with a SIZE. Updated Appendix A
for these changes. Thus, the DirectoryString is deleted.

2.10: Added this entire section.

3.1.1: Added last sentence to last paragraph.


3.2: Changed the definition of ESSPrivacy mark to use UTF8String and to not
include the optional language tag. Also added the length. Also stated
"The eSSSecurityLabel MUST be marked as critical."

4.2: Removed the paragraph that talked about changing attributes.

4.2.1: In step 2, added that the MLA also replaces the originatorInfo.

4.2.2, bullets 3.2.1 and 3.2.3: Replaced the bullets.

4.3: Made clarifications in the chart. Added two lengthy items to the
chart's legend.

4.4: Changed the sizes insteadOf and inAdditionTo. Also described better
what the processing agent does in case of too many lists being listed.

A: Added the definition of UTF8String at the beginning of the module. Added
ContentType to the list of imports from CMS. Added comment near the
beginning about SIZE (1..MAX). Updated all the changes from above.


F. Editor's Address

Paul Hoffman
Internet Mail Consortium
 Segre Place
Santa Cruz, CA  95060
(408) 426-9827
phoffman@imc.org