RFC 2634
Enhanced Security Services for S/MIME
Network Working Group P. Hoffman, Editor Request for Comments: 2634 Internet Mail Consortium Category: Standards Track June 1999 Enhanced Security Services for S/MIME Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved.1. Introduction
This document describes four optional security service extensions for S/MIME. The services are: - signed receipts - security labels - secure mailing lists - signing certificates The first three of these services provide functionality that is similar to the Message Security Protocol [MSP4], but are useful in many other environments, particularly business and finance. Signing certificates are useful in any environment where certificates might be transmitted with signed messages. The services described here are extensions to S/MIME version 3 ([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 four 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].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 [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 asContent-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 first 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 memo
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 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 a 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. ESS defines several types of attributes. ContentHints and ContentIdentifier MAY appear in any list of attributes. contentReference, equivalentLabel, eSSSecurityLabel and mlExpansionHistory MUST be carried in a SignedAttributes or AuthAttributes type, and MUST NOT be carried in a UnsignedAttributes, UnauthAttributes or UnprotectedAttributes type. msgSigDigest, receiptRequest and signingCertificate MUST be carried in a SignedAttributes, and MUST NOT be carried in a AuthAttributes, UnsignedAttributes, UnauthAttributes or UnprotectedAttributes type.
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 Attribute and defines
unsignedAttrs as a SET OF Attribute. ESS defines the contentHints,
contentIdentifier, eSSecurityLabel, msgSigDigest, mlExpansionHistory,
receiptRequest, contentReference, equivalentLabels and
signingCertificate 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 those of different senders. Because of this, the same attribute in the two signatures could lead to very different consequences. ContentIdentifier is an attribute (OCTET STRING) used to carry a unique identifier assigned to the message.1.4 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 either omit or copy 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 memo are found in [CMS], [MSG], and [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 memo 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 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.
A receipt request can indicate that receipts be sent to many places,
not just to the sender (in fact, the receipt request might indicate
that the receipts should not even go to the sender). In order to
verify a receipt, the recipient of the receipt must be the originator
or a recipient of the original message. Thus, the sender SHOULD NOT
request that receipts be sent to anyone who does not have an exact
copy of 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 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 to which the receipt request attribute is directly attached. 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, 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 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
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 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]. 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.
Although recipients are not supposed to send more than one signed
receipt, receiving agents SHOULD be able to accept multiple signed
receipts from a recipient.
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 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 ESSVersion,
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}
ESSVersion ::= INTEGER { v1(1) }
The version field defines the syntax version number, which is 1 for
this version of the standard.
2.9 Content Hints
Many applications find it useful to have information that describes the innermost signed content of a multi-layer message available on the outermost signature layer. The contentHints attribute provides such information. Content-hints attribute values have ASN.1 type contentHints. ContentHints ::= SEQUENCE { contentDescription UTF8String (SIZE (1..MAX)) OPTIONAL, contentType ContentType } id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4} The contentDescription field may be used to provide information that the recipient may use to select protected messages for processing, such as a message subject. If this field is set, then the attribute is expected to appear on the signedData object enclosing an envelopedData object and not on the inner signedData object. The (SIZE (1..MAX)) construct constrains the sequence to have at least one entry. MAX indicates the upper bound is unspecified. Implementations are free to choose an upper bound that suits their environment. Messages which contain a signedData object wrapped around an envelopedData object, thus masking the inner content type of the message, SHOULD include a contentHints attribute, except for the case of the data content type. Specific message content types may either force or preclude the inclusion of the contentHints attribute. For example, when a signedData/Receipt is encrypted within an envelopedData object, an outer signedData object MUST be created that encapsulates the envelopedData object and a contentHints attribute with contentType set to the id-ct-receipt object identifier MUST be included in the outer signedData SignerInfo 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 a 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 often describe ranked levels
("secret", "confidential", "restricted", and so on) or are 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 a
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.
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