Internet Draft                              Editor: Paul Hoffman
draft-ietf-smime-ess-09.txt
draft-ietf-smime-ess-10.txt                 Internet Mail Consortium
October 19,
November 12, 1998
Expires in six months

             Enhanced Security Services for S/MIME

Status of this memo

This document is an Internet-Draft. Internet-Drafts are working documents
of the Internet Engineering Task Force (IETF), its areas, and its working
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Coast).

1. Introduction

This document describes three optional security service extensions for
S/MIME. These services provide functionality that is similar to the Message
Security Protocol [MSP4], 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], ([MSG] and
[CERT]), 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.

This document describes both the procedures and the attributes needed for
the three services. Note that some of the attributes described in this
document are quite useful in other contexts and should be considered when
extending S/MIME or other CMS applications.

The format of the messages are described in ASN.1:1988 [ASN1-1988].

The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
'SHOULD', 'SHOULD NOT', 'RECOMMENDED',  'MAY', and 'OPTIONAL' in this
document are to be interpreted as described in [MUSTSHOULD].

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 signed attributes
bound to the encrypted body. Outer attributes may be added or removed by
the message originator or intermediate agents, and may be signed 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 signed 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:

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

2. 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.

3. Sign the result of step 2 (the inner MIME headers and the original
content). The SignedData encapContentInfo eContentType object identifier
MUST be id-data. If the structure you create in step 4 is multipart/signed,
then the SignedData encapContentInfo eContent MUST be absent. If the
structure you create in step 4 is application/pkcs7-mime, then the
SignedData encapContentInfo eContent MUST contain the result of step 2
above. The SignedData structure is encapsulated by a ContentInfo SEQUENCE
with a contentType of id-signedData.

4. Add an appropriate MIME construct to the signed message from step 3 as
defined in [SMIME3]. [MSG]. 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.

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

6. 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.

7. 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

8. 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] inner SignedData block (eContent is missing)    | )
[step 4]                                                 | )
[step 4] --innerboundary--                               | )
[step 8]
[step 8] --outerboundary
[step 8] Content-type: application/pkcs7-signature
[step 8]
[step 7] outer SignedData block (eContent is missing)
[step 8]
[step 8] --outerboundary--

% = 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 both signatures is:

[step 8] Content-type: application/pkcs7-mime;
[step 8]    smime-type=signed-data
[step 8]
[step 7] outer SignedData block (eContent is present)        O
[step 6] Content-type: application/pkcs7-mime;             ) O
[step 6]    smime-type=enveloped-data;                     ) O
[step 6]                                                   ) O
[step 4] Content-type: application/pkcs7-mime;           | ) O
[step 4]    smime-type=signed-data                       | ) O
[step 4]                                                 | ) O
[step 3] inner SignedData block (eContent is present)  I | ) O
[step 2] Content-type: text/plain                      I | ) O
[step 2]                                               I | ) O
[step 1] Original content                              I | ) O

I = These lines are the inner SignedData block, which 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.
O = These lines are the outer SignedData block, which 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 signed 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 integrity of the original signer's
security label that is
on in 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 signed 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 signedAttributes block. An
eSSSecurityLabel attribute MUST NOT be used in an EnvelopedData or unsigned
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 mail list
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
signed, while signing is optional for others, and some attributes must not
be signed. The following table summarizes the recommendation of this
profile. In the OID column, [ESS] indicates that the attribute is defined
in this document.

                  |                              |Inner or  |
Attribute         |OID                           |outer     |Signed
------------------|----------------------------- |----------|--------
contentHints      |id-aa-contentHint [ESS]       |either    |MAY
contentIdentifier |id-aa-contentIdentifier [ESS] |either    |MAY
contentReference  |id-aa-contentReference [ESS]  |either    |MUST
contentType       |id-contentType [CMS]          |either    |MUST
counterSignature  |id-countersignature [CMS]     |either    |MUST NOT
equivalentLabel   |id-aa-equivalentLabels [ESS]  |either    |MUST
eSSSecurityLabel  |id-aa-securityLabel [ESS]     |either    |MUST
messageDigest     |id-messageDigest [CMS]        |either    |MUST
msgSigDigest      |id-aa-msgSigDigest [ESS]      |inner only|MUST
mlExpansionHistory|id-aa-mlExpandHistory [ESS]   |outer only|MUST
receiptRequest    |id-aa-receiptRequest [ESS]    |inner only|MUST
signingCertificate|id-aa-signingCertificate [ESS]|either    |MUST
signingTime       |id-signingTime [CMS]          |either    |MUST
smimeCapabilities |sMIMECapabilities [MSG]       |either    |MUST
sMIMEEncryption-
  KeyPreference   |id-aa-encrypKeyPref [MSG]     |either    |MUST

CMS defines signedAttrs as a SET OF Attributes and defines
unsignedAttributes as a SET OF Attributes. ESS defines the contentHints,
contentIdentifier, eSSecurityLabel, msgSigDigest, mlExpansionHistory,
receiptRequest, contentReference and equivalentLabels attribute types. A
signerInfo MUST NOT include multiple instances of any of the attribute
types defined in ESS. Later sections of ESS specify further restrictions
that apply to the receiptRequest, mlExpansionHistory and eSSecurityLabel
attribute types.

CMS defines the syntax for the signed and unsigned attributes as
"attrValues SET OF AttributeValue". For all of the attribute types defined
in ESS, if the attribute type is present in a signerInfo, then it MUST only
include a single instance of AttributeValue. In other words, there MUST NOT
be zero or multiple instances of AttributeValue present in the attrValues
SET OF AttributeValue.

If a counterSignature attribute is present, then it MUST be included in the
unsigned attributes. It MUST NOT be included in the signed attributes. The
only attributes that are allowed in a counterSignature attribute are
counterSignature, messageDigest, signingTime, and signingCertificate.

Note that the inner and outer signatures are usually for those of different
senders.
The Because of this, the same attribute in the two signatures could
lead to very different consequences.

The macValue attribute defined in [CMS] is only used in authenticatedData,
never in signedData.

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

1.4 Required and Optional Attributes

Some security gateways sign messages that pass through them. If the message
is any type other than a signedData type, the gateway has only one way to
sign the message: by wrapping it with a signedData block and MIME headers.
If the message to be signed by the gateway is a signedData message already,
the gateway can sign the message by inserting a signerInfo into the
signedData block.

The main advantage of a gateway adding a signerInfo instead of wrapping the
message in a new signature is that the message doesn't grow as much as if
the gateway wrapped the message. The main disadvantage is that the gateway
must check for the presence of certain attributes in the other signerInfos
and duplicate those attributes.

If a gateway or other processor adds a signerInfo to an existing signedData
block, it MUST copy the mlExpansionHistory and eSSSecurityLabel attributes
from other signerInfos. This helps ensure that the recipient will process
those attributes in a signerInfo that it can verify.

Note that someone may in the future define an attribute that must be
present in each signerInfo of a signedData block in order for the signature
to be processed. If that happens, a gateway that inserts signerInfos and
doesn't copy that attribute will cause every message with that attribute to
fail when processed by the recipient. For this reason, it is safer to wrap
messages with new signatures than to insert signerInfos.

1.5 Object Identifiers

The object identifiers for many of the objects described in this draft are
found in [CMS} [CMS], [MSG], and [SMIME3]. [CERT]. 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.

2. 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 signedAttributes 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:

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

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

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

4. Recipient creates a signed receipt (Section 2.4).

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

6. 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 a sending 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 signedAttributes 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 receiving agent MUST NOT request a signed receipt for a
signed receipt.

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

1. A receiptRequest data structure is created.

2. 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.

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

4. 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).

5. The completed receiptRequest attribute is placed in the signedAttributes
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 signedAttributes. 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 to which the
receipt request attribute is directly attached. Processing Receiving software SHOULD
examine the signedAttributes 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. object,
such as requests made from different people who signed the object in
parallel.

Before processing a receiptRequest signedAttribute, the receiving agent
MUST verify the signature of the SignerInfo which covers the receiptRequest
attribute. A recipient MUST NOT process a receiptRequest attribute that has
not been verified. 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 attribute
that it encounters in a SignerInfo that it verifies, and it then ensures
that all other receiptRequest attributes in signerInfos that it verifies
are identical to the first one encountered. If there are verified
ReceiptRequest attributes which conflict, are not the same, then the processing
software MUST NOT return any signed receipt. A signed receipt SHOULD be
returned if any signerInfo containing a receiptRequest attribute can be
validated, even if other signerInfos containing the same receiptRequest
attribute cannot be validated because they are signed using an algorithm
not supported by the receiving agent.

If a receiptRequest attribute is absent from the signed 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
signed 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.

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

1. 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.

2. 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 in an outer signedData block:

        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.

3. 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 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:

0. Receipt Request attribute present?
       YES -> 1.
       NO  -> STOP
1. Has mlExpansionHistory in outer signedData?
       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. 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.
2. 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 in the outer signedData block?
       YES -> 2.2.1.
       NO  -> 2.2.2.
2.2.1. STOP.
2.2.2. Create a receipt, then -> STOP.
3. 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:

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

    1.1. The 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
	signedAttributes 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 signedAttributes (including messageDigest,
	receiptRequest and, possibly, other signed 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.

2. A Receipt structure is created.

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

    2.2. The object identifier from the contentType attribute included in
	the original signedData signerInfo that includes the receiptRequest
	attribute is copied into the Receipt contentType.

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

    2.4. The signature value from the original signedData signerInfo that
	includes the receiptRequest attribute is copied into the Receipt
	originatorSignatureValue.

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

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

5. 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 signedAttributes of the signerInfo which will
eventually contain the signedData/Receipt signature value.

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

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

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

9. 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 eContentType. 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.

10. The signedData/Receipt is then put in an application/pkcs7-mime MIME
wrapper with the smime-type parameter set to "signed-receipt". This will
allow for identification of signed receipts without having to crack the
ASN.1 body. The smime-type parameter would still be set as normal in any
layer wrapped around this message.

11. 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 signedAttributes. 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.

All sending agents that support the generation of ESS signed receipts MUST
provide the ability to send encrypted signed receipts (that is, a
signedData/Receipt encapsulated within an envelopedData). The sending agent
MAY send an encrypted signed receipt in response to an
envelopedData-encapsulated signedData requesting a signed receipt. It is a
matter of local policy regarding whether or not the signed receipt should
be encrypted.  The ESS signed receipt includes the message digest value
calculated for the original signedData object that requested the signed
receipt. If the original signedData object was sent encrypted within an
envelopedData object and the ESS signed receipt is sent unencrypted, then
the message digest value calculated for the original encrypted signedData
object is sent unencrypted. The responder should consider this when
deciding whether or not to encrypt the ESS signed receipt.

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 signedAttributes 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.

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

2. 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.

3. 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:

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

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

3. 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 signedAttributes as described in
	[CMS].

4. The message signature digest value calculated by the sender is then
compared with the value of the msgSigDigest signedAttribute 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 signedAttributes 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.

5. 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.

6. The Receipt content digest value calculated by the sender is then
compared with the value of the messageDigest signedAttribute 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.

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

8. 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 signedAttributes and authenticates the
identity of the signer of the signedData/Receipt signerInfo. Note that the
signedAttributes 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 signedAttributes (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 signed 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.

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) (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) (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 signedAttributes.

2.10  Message Signature Digest Attribute

The msgSigDigest attribute can only be used in the signed attributes of a
signed receipt. It contains the digest of the ASN.1 DER encoded
signedAttributes included in the original signedData that requested the
signed receipt. Only one msgSigDigest attribute can appear in an signed
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}

2.11 Signed Content Reference Attribute

The contentReference attribute is a link from one SignedData to another. It
may be used to link a reply to the original message to which it refers, or
to incorporate by reference one SignedData into another. The first
SignedData MUST include a contentIdentifier signed attribute, which SHOULD
be constructed as specified in section 2.7. The second SignedData links to
the first by including a ContentReference signed attribute containing the
content type, content identifier, and signature value from the first
SignedData.

ContentReference ::= SEQUENCE {
  contentType ContentType,
  signedContentIdentifier ContentIdentifier,
  originatorSignatureValue OCTET STRING }

id-aa-contentReference   OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 }

3. 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 signed
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 signedAttributes field of a SignerInfo block. The security
label attribute MUST NOT be included in the unsigned attributes. Integrity
and authentication security services MUST be applied to the security label,
therefore it MUST be included as an signed 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 signed 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 signed attribute may be included in a
signedAttributes 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 signed 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 signed attribute may also be included in a
signedAttributes 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 signedAttributes. Therefore, a single SignedData
object may include multiple eSSSecurityLabels, 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. If any of the SignerInfos included in a
SignedData object include an eSSSecurityLabel attribute, then all of the
SignerInfos in that SignedData object MUST include an eSSSecurityLabel
attribute and the value of each MUST be identical.

3.1.2 Processing Security Labels

Before processing an eSSSecurityLabel signedAttribute, the receiving agent
MUST verify the signature of the SignerInfo which covers the
eSSSecurityLabel attribute. A recipient MUST NOT process an
eSSSecurityLabel attribute that has not been verified.

A receiving agent 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
eSSSecurityLabels included in a single SignedData object. Because all
eSSSecurityLabels in a SignedData object must be identical, the receiving
agent processes (such as performing access control) on the first
eSSSecurityLabel that it encounters in a SignerInfo that it verifies, and
then ensures that all other eSSSecurityLabels in signerInfos that it
verifies are identical to the first one encountered. If the
eSSSecurityLabels in the signerInfos that it verifies are not all
identical, then the receiving agent MUST warn the user of this condition.

Receiving agents SHOULD have a local policy regarding whether or not to
show the inner content of a signedData object that includes an
eSSSecurityLabel security-policy-identifier that the processing software
does not recognize. If the receiving agent does not recognize the
eSSSecurityLabel security-policy-identifier value, then 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].

ESSSecurityLabel ::= SET {
  security-policy-identifier SecurityPolicyIdentifier,
  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), (SIZE (1..ub-privacy-mark-length)),
    utf8String   UTF8String SIZE (1..MAX) (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 -- 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.

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:
10 -- anyone
15 -- Morgan Corporation and its contractors
20 -- Morgan Corporation employees
25 -- Morgan Corporation board of directors

An example of a security policy that uses part of the X.411 hierarchy might
be:
0 -- unmarked
1 -- unclassified, can be read by everyone
2 -- restricted to Timberwolf Productions staff
6 -- 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.

3.4  Equivalent Security Labels

Because organizations are allowed to define their own security policies,
many different security policies will exist. Some organizations may wish to
create equivalencies between their security policies with the security
policies of other organizations. For example, the Acme Company and the
Widget Corporation may reach a bilateral agreement that the "Acme private"
security-classification value is equivalent to the "Widget sensitive"
security-classification value.

Receiving agents MUST NOT process an equivalentLabels attribute in a
message if the agent does not trust the signer of that attribute to
translate the original eSSSecurityLabel values to the security policy
included in the equivalentLabels attribute. Receiving agents have the
option to process equivalentLabels attributes but do not have to. It is
acceptable for a receiving agent to only process eSSSecurityLabels. All
receiving agents SHOULD recognize equivalentLabels attributes even if they
do not process them.

3.4.1 Creating Equivalent Labels

The EquivalentLabels signed attribute is defined as:

EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel

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

As stated earlier, the ESSSecurityLabel contains the sensitivity values
selected by the original signer of the signedData. If an ESSSecurityLabel
is present in a signerInfo, all signerInfos in the signedData MUST contain
an ESSSecurityLabel and they MUST all be identical. In addition to an
ESSSecurityLabel, a signerInfo MAY also include an equivalentLabels signed
attribute. If present, the equivalentLabels attribute MUST include one or
more security labels that are believed by the signer to be semantically
equivalent to the ESSSecurityLabel attribute included in the same
signerInfo.

All security-policy object identifiers MUST be unique in the set of
ESSSecurityLabel and EquivalentLabels security labels. Before using an
EquivalentLabels attribute, an a receiving agent MUST ensure that all
security-policy OIDs are unique in the security label or labels included in
the EquivalentLabels. Once the receiving agent selects the security label
(within the EquivalentLabels) to be used for processing, then the
security-policy OID of the selected EquivalentLabels security label MUST be
compared with the ESSSecurityLabel security-policy OID to ensure that they
are unique.

In the case that an ESSSecurityLabel attribute is not included in a
signerInfo, then an EquivalentLabels attribute may still be included. For
example, in the Acme security policy, the absence of an ESSSecurityLabel
could be defined to equate to a security label composed of the Acme
security-policy OID and the "unmarked" security-classification.

Note that equivalentLabels MUST NOT be used to convey security labels that
are semantically different from the ESSSecurityLabel included in the
signerInfos in the signedData. If an entity needs to apply a security label
that is semantically different from the ESSSecurityLabel, then it MUST
include the sematically different security label in an outer signedData
object that encapsulates the signedData object that includes the
ESSSecurityLabel.

If present, the equivalentLabels attribute MUST be an signed attribute; it
MUST NOT be an unsigned attribute. CMS [CMS] defines signedAttributes as a SET
OF Attribute. A signerInfo MUST NOT include multiple instances of the
equivalentLabels attribute. CMS defines the ASN.1 syntax for the signed
attributes to include attrValues SET OF AttributeValue. A equivalentLabels
attribute MUST only include a single instance of AttributeValue. There MUST
NOT be zero or multiple instances of AttributeValue present in the
attrValues SET OF AttributeValue.

3.4.2 Processing Equivalent Labels

A receiving agent SHOULD process the ESSSecurityLabel before processing any
EquivalentLabels. If the policy in the ESSSecurityLabel is understood by
the receiving agent, it MUST process that label and MUST ignore all
EquivalentLabels.

When processing an EquivalentLabels attribute, the receiving agent MUST
validate the signature on the EquivalentLabels attribute. A receiving agent
MUST NOT act on an equivalentLabels attribute for which the signature could
not be validated, and MUST NOT act on an equivalentLabels attribute unless
that attribute is signed by an entity trusted to translate the original
eSSSecurityLabel values to the security policy included in the
equivalentLabels attribute. Determining who is allowed to specify
equivalence mappings is a local policy. If a message has more than one
EquivalentLabels attribute, the receiving agent SHOULD process the first
one that it reads and validates that contains the security policy of
interest to the receiving agent.

4. 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 signed attributes of the MLA's
SignerInfo block. The MLA creates or updates the signed 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
signedAttributes 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 signedAttributes. Therefore, a single SignedData
object may include multiple SignerInfos, each SignerInfo having a
mlExpansionHistory attribute. For example, an MLA can send a signed message
with two SignerInfos, one containing a DSS signature, the other containing
an RSA signature.

If an MLA creates a SignerInfo that includes an mlExpansionHistory
attribute, then all of the SignerInfos created by the MLA for that
SignedData object MUST include an mlExpansionHistory attribute, and the
value of each MUST be identical. Note that other agents might later add
SignerInfo attributes to the SignedData block, and those additional
SignerInfos might not include mlExpansionHistory attributes.

A recipient MUST verify the signature of the SignerInfo which covers the
mlExpansionHistory attribute before processing the mlExpansionHistory, and
MUST NOT process the mlExpansionHistory attribute unless the signature over
it has been verified. If a SignedData object has more than one SignerInfo
that has an mlExpansionHistory attribute, the recipient MUST compare the
mlExpansionHistory attributes in all the SignerInfos that it has verified,
and MUST NOT process the mlExpansionHistory attribute unless every verified
mlExpansionHistory attribute in the SignedData block is identical. If the
mlExpansionHistory attributes in the verified signerInfos are not all
identical, then the receiving agent MUST stop processing the message and
SHOULD notify the user or MLA administrator of this error condition. In the
mlExpansionHistory processing, SignerInfos that do not have an
mlExpansionHistory attribute are ignored.

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

The first few paragraphs of this section provide a high-level description
of MLA processing. The rest of the section provides a detailed description
of MLA processing.

MLA message processing depends on the structure of the S/MIME layers in the
message sent to the MLA for expansion. In addition to sending triple
wrapped messages to an MLA, an entity can send other types of messages to
an MLA, such as:
 - a single wrapped signedData or envelopedData message
 - a double wrapped message (such as signed and enveloped, enveloped and
   signed, or signed and signed, and so on)
 - a quadruple-wrapped message (such as if a well-formed triple wrapped
   message was sent through a gateway that added an outer SignedData layer)

In all cases, the MLA MUST parse all layers of the received message to
determine if there are any signedData layers that include an
eSSSecurityLabel signedAttribute. This may include decrypting an
EnvelopedData layer to determine if an encapsulated SignedData layer
includes an eSSSecurityLabel attribute. The MLA MUST fully process each
eSSSecurityLabel attribute found in the various signedData layers,
including performing access control checks, before distributing the message
to the ML members. The details of the access control checks are beyond the
scope of this document. The MLA MUST verify the signature of the signerInfo
including the eSSSecurityLabel attribute before using it.

In all cases, the MLA MUST sign the message to be sent to the ML members in
a new "outer" signedData layer. The MLA MUST add or update an
mlExpansionHistory attribute in the "outer" signedData that it creates to
document MLA processing. If there was an "outer" signedData layer included
in the original message received by the MLA, then the MLA-created "outer"
signedData layer MUST include each signed attribute present in the
original "outer" signedData layer, unless the MLA explicitly replaces an
attribute (such as signingTime or mlExpansionHistory) with a new value.

When an S/MIME message is received by the MLA, the MLA MUST first determine
which received signedData layer, if any, is the "outer" signedData layer.
To identify the received "outer" signedData layer, the MLA MUST verify the
signature and fully process the signedAttributes in each of the
outer signedData layers (working from the outside in) to determine if any
of them either include an mlExpansionHistory attribute or encapsulate an
envelopedData object.

The MLA's search for the "outer" signedData layer is completed when it
finds one of the following:
 - the "outer" signedData layer that includes an mlExpansionHistory
   attribute or encapsulates an envelopedData object
 - an envelopedData layer
 - the original content (that is, a layer that is neither envelopedData nor
   signedData).

If the MLA finds an "outer" signedData layer, then the MLA MUST perform
the following steps:

1. Strip off all of the signedData layers that encapsulated the "outer"
signedData layer

2. Strip off the "outer" signedData layer itself (after remembering the
included signedAttributes)

3. Expand the envelopedData (if present)

4. Sign the message to be sent to the ML members in a new "outer"
signedData layer that includes the signedAttributes (unless explicitly
replaced) from the original, received "outer" signedData layer.

If the MLA finds an "outer" signedData layer that includes an
mlExpansionHistory attribute AND the MLA subsequently finds an
envelopedData layer buried deeper with the layers of the received message,
then the MLA MUST strip off all of the signedData layers down to the
envelopedData layer (including stripping off the original "outer"
signedData layer) and MUST sign the expanded envelopedData in a new "outer"
signedData layer that includes the signedAttributes (unless explicitly
replaced) from the original, received "outer" signedData layer.

If the MLA does not find an "outer" signedData layer AND does not find an
envelopedData layer, then the MLA MUST sign the original, received message
in a new "outer" signedData layer. If the MLA does not find an "outer"
signedData AND does find an envelopedData layer then it MUST expand the
envelopedData layer, if present, and sign it in a new "outer" signedData
layer.

4.2.1 Examples of Rule Processing

The following examples help explain the rules above:

1) A message (S1(Original Content)) (where S = SignedData) is sent to the
MLA in which the signedData layer does not include an MLExpansionHistory
attribute. The MLA verifies and fully processes the signedAttributes in S1.
The MLA decides that there is not an original, received "outer" signedData
layer since it finds the original content, but never finds an envelopedData
and never finds an mlExpansionHistory attribute. The MLA calculates a new
signedData layer, S2, resulting in the following message sent to the ML
recipients: (S2(S1(Original Content))). The MLA includes an
mlExpansionHistory attribute in S2.

2) A message (S3(S2(S1(Original Content)))) is sent to the MLA in which
none of the signedData layers includes an MLExpansionHistory attribute. The
MLA verifies and fully processes the signedAttributes in S3, S2 and S1. The
MLA decides that there is not an original, received "outer" signedData
layer since it finds the original content, but never finds an envelopedData
and never finds an mlExpansionHistory attribute. The MLA calculates a new
signedData layer, S4, resulting in the following message sent to the ML
recipients: (S4(S3(S2(S1(Original Content))))). The MLA includes an
mlExpansionHistory attribute in S4.

3) A message (E1(S1(Original Content))) (where E = envelopedData) is sent
to the MLA in which S1 does not include an MLExpansionHistory attribute.
The MLA decides that there is not an original, received "outer" signedData
layer since it finds the E1 as the outer layer. The MLA expands the
recipientInformation in E1. The MLA calculates a new signedData layer, S2,
resulting in the following message sent to the ML recipients:
(S2(E1(S1(Original Content)))). The MLA includes an mlExpansionHistory
attribute in S2.

4) A message (S2(E1(S1(Original Content)))) is sent to the MLA in which S2
includes an MLExpansionHistory attribute. The MLA verifies the signature
and fully processes the signedAttributes in S2. The MLA finds the
mlExpansionHistory attribute in S2, so it decides that S2 is the "outer"
signedData. The MLA remembers the signedAttributes included in S2 for later
inclusion in the new outer signedData that it applies to the message. The
MLA strips off S2. The MLA then expands the recipientInformation in E1
(this invalidates the signature in S2 which is why it was stripped). The
MLA calculates a new signedData layer, S3, resulting in the following
message sent to the ML recipients: (S3(E1(S1(Original Content)))). The MLA
includes in S3 the attributes from S2 (unless it specifically replaces an
attribute value) including an updated mlExpansionHistory attribute.

5) A message (S3(S2(E1(S1(Original Content))))) is sent to the MLA in which
none of the signedData layers include an MLExpansionHistory attribute. The
MLA verifies the signature and fully processes the signedAttributes in S3
and S2. When the MLA encounters E1, then it decides that S2 is the "outer"
signedData since S2 encapsulates E1. The MLA remembers the signedAttributes
included in S2 for later inclusion in the new outer signedData that it
applies to the message. The MLA strips off S3 and S2. The MLA then expands
the recipientInformation in E1 (this invalidates the signatures in S3 and
S2 which is why they were stripped). The MLA calculates a new signedData
layer, S4, resulting in the following message sent to the ML recipients:
(S4(E1(S1(Original Content)))). The MLA includes in S4 the attributes from
S2 (unless it specifically replaces an attribute value) and includes a new
mlExpansionHistory attribute.

6) A message (S3(S2(E1(S1(Original Content))))) is sent to the MLA in which
S3 includes an MLExpansionHistory attribute. In this case, the MLA verifies
the signature and fully processes the signedAttributes in S3. The MLA finds
the mlExpansionHistory in S3, so it decides that S3 is the "outer"
signedData. The MLA remembers the signedAttributes included in S3 for later
inclusion in the new outer signedData that it applies to the message. The
MLA keeps on parsing encapsulated layers because it must determine if there
are any eSSSecurityLabel attributes contained within. The MLA verifies the
signature and fully processes the signedAttributes in S2. When the MLA
encounters E1, then it strips off S3 and S2. The MLA then expands the
recipientInformation in E1 (this invalidates the signatures in S3 and S2
which is why they were stripped). The MLA calculates a new signedData
layer, S4, resulting in the following message sent to the ML recipients:
(S4(E1(S1(Original Content)))). The MLA includes in S4 the attributes from
S3 (unless it specifically replaces an attribute value) including an
updated mlExpansionHistory attribute.

4.2.3 Processing Choices

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

4.2.3.1 Processing for EnvelopedData

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

2. 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.

3. 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.

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

4.2.3.2 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.

1. 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.

2. If the outermost SignedData layer includes an signed mlExpansionHistory
attribute
attribute, the MLA checks for an expansion loop as described in the
"Detecting Mail List Expansion Loops" section. section, then go to step 3. If the
outermost SignedData layer does not include an signed mlExpansionHistory
attribute, go directly to step 4.

3. 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
		signed 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
		signed attribute list for the new outermost signedData layer which
		MUST include each signed 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 signed 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.

4. 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:

1. Has a valid signature?
       YES -> 2.
       NO  -> STOP.
2. Does outermost SignedData layer contain mlExpansionHistory?
       YES -> Check it, then -> 3.
       NO  -> 3. 4.
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 signed 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.
4. Sign message, deliver it, STOP.

4.2.3.3 Processing for data

1. 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.

2. 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 receiving 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 }

EntityIdentifier ::= CHOICE {
  issuerAndSerialNumber IssuerAndSerialNumber,
  subjectKeyIdentifier SubjectKeyIdentifier }

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. In
the event that a ML's signed receipt policy supersedes the originator's
request for signed receipts, such that the originator will not receive any
signed receipts, then the MLA MAY inform the originator of that fact.

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 }

5. Signing Certificate Attribute

Concerns have been raised over the fact that the certificate which the
signer of a [CMS] CMS SignedData object desired to be bound into the verification
process of the SignedData object is not cryptographically bound into the
signature itself. This section addresses this issue by creating a new
attribute to be placed in the signed attributes section of a SignerInfo
object.

This section also presents a description of a set of possible attacks
dealing with the substitution of one certificate to verify the signature
for the desired certificate. A set of ways for preventing or addressing
these attacks is presented to deal with the simplest of the attacks.

Attribute certificates can be used as part of a signature verification
process. There is no way in CMS to include the list of attribute
certificates to be used in the verification process. The set of attribute
certificates used in the signature verification process needs to have the
ability for the signer to restrict the set of certificates. This
information needs to be encoded in a manner that is covered by the
signature on the SignedData object. The methods in this section allows for
the set of attribute certificates to be listed as part of the signing
certificate attribute.

Explicit certificate policies can also be used as part of a signature
verification process. If a signer desires to state an explicit certificate
policy that should be used when validating the signature, that policy needs
to be cryptographically bound into the signing process. The methods
described in this section allows for a set of certificate policy statements
to be listed as part of the signing certificate attribute.

5.1. Attack Descriptions

At least three different attacks can be launched against a possible
signature verification process by changing the certificate or certficates
used in the signature verification process.

5.1.1 Substitution Attack Description

The first attack deals with simple substitution of one certificate for
another certificate. In this attack, the issuer and serial number in the
SignerInfo is modified to refer to a new certificate. This new certificate
is used during the signature verification process.

The first version of this attack is a simple denial of service attack where
an invalid certificate is substituted for the valid certificate. This
renders the message unverifiable, as the public key in the certificate no
longer matches the private key used to sign the message.

The second version is a substitution of one valid certificate for the
original valid certificate where the public keys in the certificates match.
This allows the signature to be validated under potentially different
certificate constraints than the originator of the message intended.

5.1.2 Reissue of Certificate Description

The second attack deals with a certificate authority (CA) re-issuing the
signing certificate (or potentially one of its certificates). This attack
may start becoming more frequent as Certificate Authorities reissue their
own root certificates, or as certificate authorities change policies in the
certificate while reissuing their root certificates. This problem also
occurs when cross certificates (with potentially different restrictions)
are used in the process of verifying a signature.

5.1.3 Rogue Duplicate CA Description

The third attack deals with a rogue entity setting up a certificate
authority that attempts to duplicate the structure of an existing CA.
Specifically, the rogue entity issues a new certificate with the same
public keys as the signer used, but signed by the rogue entity's private
key.

5.2 Attack Responses

This document does not attempt to solve all of the above attacks; however,
a brief description of responses to each of the attacks is given in this
section.

5.2.1 Substitution Attack Response

The denial of service attack cannot be prevented. After the certificate
identifier has been modified in transit, no verification of the signature
is possible. There is also no way to automatically identify the attack
because it is indistinguishable from a message corruption.

The substitution of a valid certificate can be responded to in two
different manners. The first is to make a blanket statement that the use of
the same public key in two different certificates is bad practice and has
to be avoided. In practice, there is no practical way to prevent users from
getting new certificates with the same public keys, and it should be
assumed that they will do this. Section 5.4 provides a new attribute that
can be included in the SignerInfo signed attributes. This binds the correct
certificate identifier into the signature. This will convert the attack
from a potentially successful one to simply a denial of service attack.

5.2.2 Reissue of Certificate Response

A CA should never reissue a certificate with different attributes.
Certificate Authorities that do so are following poor practices and cannot
be relied on. Using the hash of the certificate as the reference to the
certificate prevents this attach for end-entity certificates.

Preventing the attack based on reissuing of CA certificates would require a
substantial change to the usage of the signingCertificate attribute
presented in section 5.4. It would require that ESSCertIDs would need to be
included in the attribute to represent the issuer certificates in the
signer's certification path. This presents problems when the relying party
is using a cross-certificate as part of its authentication process, and
this certificate does not appear on the list of certificates. The problems
outside of a closed PKI make the addition of this information prone to
error, possibly causing the rejection of valid chains.

5.2.3 Rogue Duplicate CA Response

The best method of preventing this attack is to avoid trusting the rogue
CA. The use of the hash to identify certificates prevents the use of
end-entity certificates from the rogue authority. However the only true way
to prevent this attack is to never trust the rogue CA.

5.3 Related Signature Verification Context

Some applications require that additional information be used as part of
the signature validation process. In particular, attribute certificates and
policy identifiers provide additional information about the abilities and
intent of the signer. The signing certificate attribute described in
Section 5.4 provides the ability to bind this context information as part
of the signature.

5.3.1 Attribute Certificates

Some applications require that attribute certificates be validated. This
validation requires that the application be able to find the correct
attribute certificates to perform the verification process; however there
is no list of attribute certificates in a SignerInfo object. The sender has
the ability to include a set of attribute certificates in a SignedData
object. The receiver has the ability to retrieve attribute certificates
from a directory service. There are some circumstances where the signer may
wish to limit the set of attribute certificates that may be used in
verifying a signature. It is useful to be able to list the set of attribute
certificates the signer wants the recipient to use in validating the
signature.

5.3.2 Policy Information

A related aspect of the certificate binding is the issue of multiple
certification paths. In some instances, the semantics of a certificate in
its use with a message may depend on the Certificate Authorities and
policies that apply. To address this issue, the signer may also wish to
bind that context under the signature. While this could be done by either
signing the complete certification path or a policy ID, only a binding for
the policy ID is described here.

5.4 Signing Certificate Attribute Definition

The signing certificate attribute is designed to prevent the simple
substitution and re-issue attacks, and to allow for a restricted set of
attribute certificates to be used in verifying a signature.

The definition of SigningCertificate is

SigningCertificate ::=  SEQUENCE {
    certs        SEQUENCE OF ESSCertID,
    policies     SEQUENCE OF PolicyInformation OPTIONAL
}

id-aa-signingCertificate OBJECT IDENTIFIER ::= { iso(1)
    member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
    smime(16) id-aa(2) <TBD> 12 }

The first certificate identified in the sequence of certificate identifiers
MUST be the certificate used to verify the signature. The encoding of the
ESSCertID for this certificate SHOULD NOT include the issuerSerial because
the issuerAndSerialNumber is already present in the SignerInfo. The
certificate identified is used during the signature verification process.
If the hash of the certificate does not match the certificate used to
verify the signature, the signature MUST be considered invalid.

If more than one certificate is present in the sequence of ESSCertIDs, the
certificates after the first one limit the set of attribute certificates
that are used during signature validation. The issuerSerial SHOULD be
present in these certificates, unless the client who is validating the
signature is expected to have easy access to all the certificates required
for validation. If only the signing certificate is present in the sequence.
there are no restrictions on the set of attribute certificates used in
validating the signature.

The sequence of policy information terms identifies those certificate
policies that the signer asserts apply to the certificate, and under which
the certificate should be relied upon. This value suggests a policy value
to be used in the relying party's certification path validation.

If present, the SigningCertificate attribute MUST be a signed attribute; it
MUST NOT be an unsigned attribute. CMS defines SignedAttributes as a SET OF
Attribute. A SignerInfo MUST NOT include multiple instances of the
SigningCertificate attribute. CMS defines the ASN.1 syntax for the signed
attributes to include attrValues SET OF AttributeValue. A
SigningCertificate attribute MUST include only a single instance of
AttributeValue. There MUST NOT be zero or multiple instances of
AttributeValue present in the attrValues SET OF AttributeValue.

5.4.1 Certificate Identification

The best way to identify certificates is an often-discussed issue. [CMS] CMS has
imposed a restriction for SignedData objects that the issuer DN must be
present in all signing certificates. The issuer/serial number pair is
therefore sufficient to identify the correct signing certificate. This
information is already present, as part of the SignerInfo object, and
duplication of this information would be unfortunate. A hash of the entire
certificate serves the same function (allowing the receiver to verify that
the same certificate is being used as when the message was signed), is
smaller, and permits a detection of the simple substitution attacks.

Attribute certificates do not have an issuer/serial number pair represented
anywhere in a SignerInfo object. When an attribute certificate is not
included in the SignedData object, it becomes much more difficult to get
the correct set of certificates based only on a hash of the certificate.
For this reason, attribute certificates are identified by the IssuerSerial
object.

This document defines a certificate identifier as:

ESSCertID ::=  SEQUENCE {
     certHash                 Hash,
     issuerSerial             IssuerSerial OPTIONAL
}

Hash ::= OCTET STRING -- SHA1 hash of entire certificate

IssuerSerial ::= SEQUENCE {
     issuer                   GeneralNames,
     serialNumber             CertificateSerialNumber
}

When creating an ESSCertID, the certHash is computed over the entire DER
encoded certificate including the signature. The issuerSerial would
normally be present unless the value can be inferred from other
information.

When encoding IssuerSerial, serialNumber is the serial number that uniquely
identifies the certificate. For non-attribute certificates, the issuer MUST
contain only the issuer name from the certificate encoded in the
directoryName choice of GeneralNames. For attribute certificates, the
issuer MUST contain the issuer name field from the attribute certificate.

6. 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

IMPORTS

-- Cryptographic Message Syntax (CMS)
    ContentType, IssuerAndSerialNumber, SubjectKeyIdentifier, Version
    FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840)
    rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1)}

-- PKIX Certificate and CRL Profile, Sec A.2 Implicitly Tagged Module,
--  1988 Syntax
    PolicyInformation FROM PKIX1Implicit88 {iso(1
    identified-organization(3)dod(6) {iso(1)
    identified-organization(3) dod(6) internet(1) security(5)
    mechanisms(5) pkix(7)id-mod(0) id-pkix1-implicit-88(2)}

-- X.509
    GeneralNames, CertificateSerialNumber, CertificateSerialNumber 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.

UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING
    -- The contents are formatted as described in [UTF8]

-- 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) (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}

-- 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 2.11

ContentReference ::= SEQUENCE {
  contentType ContentType,
  signedContentIdentifier ContentIdentifier,
  originatorSignatureValue OCTET STRING }

id-aa-contentReference   OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 }

-- Section 3.2

ESSSecurityLabel ::= SET {
  security-policy-identifier SecurityPolicyIdentifier,
  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), (SIZE (1..ub-privacy-mark-length)),
    utf8String   UTF8String SIZE (1..MAX) (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 -- 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 3.4

EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel

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

-- 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 }

EntityIdentifier ::= CHOICE {
  issuerAndSerialNumber IssuerAndSerialNumber,
  subjectKeyIdentifier SubjectKeyIdentifier }

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

-- Section 5.4

SigningCertificate ::=  SEQUENCE {
    certs        SEQUENCE OF ESSCertID,
    policies     SEQUENCE OF PolicyInformation OPTIONAL
}

id-aa-signingCertificate OBJECT IDENTIFIER ::= { iso(1)
    member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
    smime(16) id-aa(2) <TBD> 12 }

ESSCertID ::=  SEQUENCE {
     certHash                 Hash,
     issuerSerial             IssuerSerial OPTIONAL
}

Hash ::= OCTET STRING -- SHA1 hash of entire certificate

IssuerSerial ::= SEQUENCE {
     issuer                   GeneralNames,
     serialNumber             CertificateSerialNumber
}

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)"

[CERT] "S/MIME Version 3 Certificate Handling", Internet Draft
draft-ietf-smime-cert-xx.

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

[MSG] "S/MIME Version 3 Message Specification", Internet Draft
draft-ietf-smime-msg-xx.

[MUSTSHOULD] "Key Words for Use in RFCs to Indicate Requirement Levels",
RFC 2119.

[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", RFC 2315.

[SMIME2] "S/MIME Version 2 Message Specification", RFC 2311, and
"S/MIME Version 2 Certificate Handling", RFC 2312.

[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. Acknowledgments

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:
Andrew Farrell
Bancroft Scott
Bengt Ackzell
Bill Flanigan
Blake Ramsdell
Carlisle Adams
David Kemp
Denis Pinkas
Jim Schaad
Russ Housley
Scott Hollenbeck
Steve Dusse

D. Changes from draft-ietf-smime-ess-08 to draft-ietf-smime-ess-09

5: Many to draft-ietf-smime-ess-10

Numerous small changes from John Pawling. See
<http://www.imc.org/ietf-smime/mail-archive/2206.html>. clarifications throughout.

Changed the [SMIME3] reference to [CERT] and [MSG].

1: Added paragraph about usefulness of attributes for other
purposes. Also added reference to [MUSTSHOULD].

1.3.4: Added note about [ESS].

2.3: Added explanation at the end of the first paragraph about why
you might get more than one receipt request. Also changed
CertID "conflict"
to ESSCertID.

A: Copied "are not the new ASN.1 from 5 (whoops!). Put same" in an import of
PolicyInformation from PKIX, and an import the second paragraph.

2.9, 3.2: Added parentheses around SIZE declarations. Also in Appendix
A.

4.2.3.2: Changed step 2 to say that if mlExpansionHistory isn't found,
skip to step 4. Also updated the flow chart.

5.4: Filled in the TBD of CertificateSerialNumber from the CertificateExtensions module. id-aa-signingCertificate OID. Also in
Appendix A.

A: Fixed some bugs in the headers caused by typos.

E. Editor's Address

Paul Hoffman
Internet Mail Consortium
127 Segre Place
Santa Cruz, CA  95060
(831) 426-9827
phoffman@imc.org