RFC 2634
Enhanced Security Services for S/MIME
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)), utf8String UTF8String (SIZE (1..MAX)) } ub-privacy-mark-length INTEGER ::= 128 SecurityCategories ::= SET SIZE (1..ub-security-categories) OF SecurityCategory ub-security-categories INTEGER ::= 64 SecurityCategory ::= SEQUENCE { type [0] OBJECT IDENTIFIER, value [1] ANY DEFINED BY type -- 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 executives3.3.3 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, 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 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 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
nMLA 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 a signed mlExpansionHistory attribute, the MLA checks for an expansion loop as described in the "Detecting Mail List Expansion Loops" section, then go to step 3. If the outermost SignedData layer does not include a signed mlExpansionHistory attribute, the MLA signs the whole message (including this outermost SignedData layer that doesn't have an mlExpansionHistory attribute), and delivers the updated message to mail list members to complete MLA processing. 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 -> Sign message (including outermost SignedData that
doesn't have mlExpansionHistory), deliver it, STOP.
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 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,
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 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.
Authorization information can be used as part of a signature
verification process. This information can be carried in either
attribute certificates and other public key certificates. The signer
needs to have the ability to restrict the set of certificates used in
the signature verification process, and information needs to be
encoded so that is covered by the signature on the SignedData object.
The methods in this section allow for the set of authorization
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 replacing 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 attack 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, authorization information from attribute certificates and other public key certificates or 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 Authorization Information
Some applications require that authorization information found in attribute certificates and/or other public key certificates be validated. This validation requires that the application be able to find the correct certificates to perform the verification process; however there is no list of the certificates to used in a SignerInfo object. The sender has the ability to include a set of attribute certificates and public key certificates in a SignedData object. The receiver has the ability to retrieve attribute certificates and public key certificates from a directory service. There are some circumstances where the signer may wish to limit the set of certificates that may be used in verifying a signature. It is useful to be able to list the set of 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 authorization 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) 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 include the issuerSerial field. If other constraints ensure that issuerAndSerialNumber will be present in the SignerInfo, the issuerSerial field MAY be omitted. 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 authorization certificates that are used during signature validation. Authorization certificates can be either attribute certificates or normal certificates. The issuerSerial field (in the ESSCertID structure) SHOULD be present for these certificates, unless the client who is validating the signature is expected to have easy access to all the certificates requred for validation. If only the signing certificate is present in the sequence, there are no restrictions on the set of authorization 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. [CERT] 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 and additional public key certificates containing authorization information do not have an issuer/serial number pair represented anywhere in a SignerInfo object. When an attribute certificate or an additional public key 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, these certificates SHOULD be 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
All security considerations from [CMS] and [SMIME3] apply to applications that use procedures described in this document. As stated in Section 2.3, a recipient of a receipt request must not send back a reply if it cannot validate the signature. Similarly, if there conflicting receipt requests in a message, the recipient must not send back receipts, since an attacker may have inserted the conflicting request. Sending a signed receipt to an unvalidated sender can expose information about the recipient that it may not want to expose to unknown senders. Senders of receipts should consider encrypting the receipts to prevent a passive attacker from gleaning information in the receipts. Senders must not rely on recipients' processing software to correctly process security labels. That is, the sender cannot assume that adding a security label to a message will prevent recipients from viewing messages the sender doesn't want them to view. It is expected that there will be many S/MIME clients that will not understand security labels but will still display a labelled message to a recipient. A receiving agent that processes security labels must handle the content of the messages carefully. If the agent decides not to show the message to the intended recipient after processing the security label, the agent must take care that the recipient does not accidentally see the content at a later time. For example, if an error response sent to the originator contains the content that was hidden from the recipient, and that error response bounces back to the sender due to addressing errors, the original recipient can possibly see the content since it is unlikely that the bounce message will have the proper security labels. A man-in-the-middle attack can cause a recipient to send receipts to an attacker if that attacker has a signature that can be validated by the recipient. The attack consists of intercepting the original message and adding a mLData attribute that says that a receipt should be sent to the attacker in addition to whoever else was going to get the receipt.
Mailing lists that encrypt their content may be targets for denial- of-service attacks if they do not use the mailing list management described in Section 4. Using simple RFC822 header spoofing, it is quite easy to subscribe one encrypted mailing list to another, thereby setting up an infinite loop. Mailing List Agents need to be aware that they can be used as oracles for the the adaptive chosen ciphertext attack described in [CMS]. MLAs should notify an administrator if a large number of undecryptable messages are received. When verifying a signature using certificates that come with a [CMS] message, the recipient should only verify using certificates previously known to be valid, or certificates that have come from a signed SigningCertificate attribute. Otherwise, the attacks described in Section 5 can cause the receiver to possibly think a signature is valid when it is not.
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 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) internet(1) security(5) mechanisms(5) pkix(7)id-mod(0) id-pkix1-implicit-88(2)} -- X.509 GeneralNames, 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 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) }
-- Section 2.9
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}
-- 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)),
utf8String UTF8String (SIZE (1..MAX))
}
ub-privacy-mark-length INTEGER ::= 128
SecurityCategories ::= SET SIZE (1..ub-security-categories) OF
SecurityCategory
ub-security-categories INTEGER ::= 64
SecurityCategory ::= SEQUENCE {
type [0] OBJECT IDENTIFIER,
value [1] ANY DEFINED BY type -- 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,
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) 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] Ramsdell, B., Editor, "S/MIME Version 3 Certificate Handling", RFC 2632, June 1999. [CMS] Housley, R., "Cryptographic Message Syntax", RFC 2630, June 1999. [MSG] Ramsdell, B., Editor, "S/MIME Version 3 Message Specification", RFC 2633, June 1999. [MUSTSHOULD] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [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] Kaliski, B., "PKCS #7: Cryptographic Message Syntax", RFC 2315, March 1998. [SMIME2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L. and L. Repka"S/MIME Version 2 Message Specification", RFC 2311, March 1998, and Dusse, S., Hoffman, P. and B. Ramsdell,"S/MIME Version 2 Certificate Handling", RFC 2312, March 1998. [UTF8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998.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 memo, including: Andrew Farrell Bancroft Scott Bengt Ackzell Bill Flanigan Blake Ramsdell Carlisle Adams Darren Harter David Kemp Denis Pinkas Francois Rousseau Jim Schaad Russ Housley Scott Hollenbeck Steve DusseEditor's Address
Paul Hoffman Internet Mail Consortium 127 Segre Place Santa Cruz, CA 95060 EMail: phoffman@imc.org
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