draft-ietf-smime-domsec-08.txt   draft-ietf-smime-domsec-09.txt 
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draft-ietf-smime-domsec-08.txt W Ottaway
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Domain Security Services using S/MIME
Status of this memo
This document is an Internet-Draft and is in full conformance with all
provisions of section 10 of RFC 2026. Internet-Drafts are working
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Abstract
This document describes how the S/MIME protocol can be processed and
generated by a number of components of a communication system, such as
message transfer agents, guards and gateways to deliver security
services. These services are collectively referred to as 'Domain
Security Services'. The mechanisms described in this document are
designed to solve a number of interoperability problems and technical
limitations that arise when different security domains wish to
communicate securely, for example when two domains use incompatible
messaging technologies such as the X.400 series and SMTP/MIME, or when
a single domain wishes to communicate securely with one of its members
residing on an untrusted domain. The scenarios covered by this document
are domain-to-domain, individual-to-domain and domain-to-individual
communications. This document is also applicable to organisations and
enterprises that have internal PKIs which are not accessible by the
outside world, but wish to interoperate securely using the S/MIME
protocol.
This draft is being discussed on the 'ietf-smime' mailing list. To
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Acknowledgements
Significant comments were made by Luis Barriga, Greg Colla, Trevor
Freeman, Russ Housley, Dave Kemp, Jim Schaad and Michael Zolotarev.
1. Introduction
The S/MIME [1] series of standards define a data encapsulation format
for the provision of a number of security services including data
integrity, confidentiality, and authentication. S/MIME is designed for
use by messaging clients to deliver security services to distributed
messaging applications.
There are many circumstances when it is not desirable or practical to
provide end-to-end (desktop-to-desktop) security services, particularly
between different security domains. An organisation that is considering
providing end-to-end security services will typically have to deal with
some if not all of the following issues:
1) Heterogeneous message access methods: Users are accessing mail using
mechanisms which re-format messages, such as using Web browsers.
Message reformatting in the Message Store makes end-to-end encryption
and signature validation impossible.
2) Message screening and audit: Server-based mechanisms such as
searching for prohibited words or other content, virus scanning, and
audit, are incompatible with end-to-end encryption.
3) PKI deployment issues: There may not be any certificate paths between
two organisations. Or an organisation may be sensitive about aspects
of its PKI and unwilling to expose them to outside access. Also, full
PKI deployment for all employees, may be expensive, not necessary or
impractical for large organisations. For any of these reasons, direct
end-to-end signature validation and encryption are impossible.
4) Heterogeneous message formats: One organisation using X.400 series
protocols wishes to communicate with another using SMTP. Message
reformatting at gateways makes end-to-end encryption and signature
validation impossible.
This document describes an approach to solving these problems by
providing message security services at the level of a domain or an
organisation. This document specifies how these 'domain security
services' can be provided using the S/MIME protocol. Domain security
services may replace or complement mechanisms at the desktop. For
example, a domain may decide to provide desktop-to-desktop signatures
but domain-to-domain encryption services. Or it may allow desktop-to-
desktop services for intra-domain use, but enforce domain-based services
for communication with other domains.
Domain services can also be used by individual members of a corporation
who are geographically remote and who wish to exchange encrypted and/or
signed messages with their base.
Whether or not a domain based service is inherently better or worse than
desktop based solutions is an open question. Some experts believe that
only end-to-end solutions can be truly made secure, while others believe
that the benefits offered by such things as content checking at domain
boundaries offers considerable increase in practical security for many
real systems. The additional service of allowing signature checking at
several points on a communications path is also an extra benefit in many
situations. This debate is outside the scope of this document. What is
offered here is a set of tools that integrators can tailor in different
ways to meet different needs in different circumstances.
Message transfer agents (MTAs), guards, firewalls and protocol
translation gateways all provide domain security services. As with
desktop based solutions, these components must be resilient against a
wide variety of attacks intended to subvert the security services.
Therefore, careful consideration should be given to security of these
components, to make sure that their siting and configuration minimises
the possibility of attack.
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 [2].
2. Overview of Domain Security Services
This section gives an informal overview of the security services that
are provided by S/MIME between different security domains. These
services are provided by a combination of mechanisms in the sender's and
recipient's domains.
Later sections describe definitively how these services map onto
elements of the S/MIME protocol.
The following security mechanisms are specified in this document:
1. Domain signature
2. Review signature
3. Additional attributes signature
4. Domain encryption and decryption
The signature types defined in this document are referred to as DOMSEC
defined signatures.
The term 'security domain' as used in this document is defined as a
collection of hardware and personnel operating under a single security
authority and performing a common business function. Members of a
security domain will of necessity share a high degree of mutual trust,
due to their shared aims and objectives.
A security domain is typically protected from direct outside attack by
physical measures and from indirect (electronic) attack by a combination
of firewalls and guards at network boundaries. The interface between two
security domains is termed a 'security boundary'. One example of a
security domain is an organisational network ('Intranet').
2.1 Domain Signature
A domain signature is an S/MIME signature generated on behalf of a set
of users in a domain. A domain signature can be used to authenticate
information sent between domains or between a certain domain and one of
its individuals, for example, when two 'Intranets' are connected using
the Internet, or when an Intranet is connected to a remote user over the
Internet. It can be used when two domains employ incompatible signature
schemes internally or when there are no certification links between
their PKIs. In both cases messages from the originator's domain are
signed over the original message and signature (if present) using an
algorithm, key, and certificate which can be processed by the
recipient(s) or the recipient(s) domain. A domain signature is sometimes
referred to as an "organisational signature".
2.2 Review Signature
A third party may review messages before they are forwarded to the final
recipient(s) who may be in the same or a different security domain.
Organisational policy and good security practice often require that
messages be reviewed before they are released to external recipients.
Having reviewed a message, an S/MIME signature is added to it - a review
signature. An agent could check the review signature at the domain
boundary, to ensure that only reviewed messages are released.
2.3 Additional Attributes Signature
A third party can add additional attributes to a signed message. An
S/MIME signature is used for this purpose - an additional attributes
signature. An example of an additional attribute is the 'Equivalent
Label' attribute defined in ESS [3].
2.4 Domain Encryption and Decryption
Domain encryption is S/MIME encryption performed on behalf of a
collection of users in a domain. Domain encryption can be used to
protect information between domains, for example, when two 'Intranets'
are connected using the Internet. It can also be used when end users do
not have PKI/encryption capabilities at the desktop, or when two
domains employ incompatible encryption schemes internally. In the latter
case messages from the originator's domain are encrypted (or
re-encrypted) using an algorithm, key, and certificate which can be
decrypted by the recipient(s) or an entity in their domain. This scheme
also applies to protecting information between a single domain and one
of its members when both are connected using an untrusted network,
e.g. the Internet.
3. Mapping of the Signature Services to the S/MIME Protocol
This section describes the S/MIME protocol elements that are used to
provide the security services described above. ESS [3] introduces the
concept of triple-wrapped messages that are first signed, then
encrypted, then signed again. This document also uses this concept of
triple-wrapping. In addition, this document also uses the concept of
'signature encapsulation'. 'Signature encapsulation' denotes a signed
or unsigned message that is wrapped in a signature, this signature
covering both the content and the first (inner) signature, if present.
Signature encapsulation MAY be performed on the inner and/or the outer
signature of a triple-wrapped message.
For example, the originator signs a message which is then encapsulated
with an 'additional attributes' signature. This is then encrypted. A
reviewer then signs this encrypted data, which is then encapsulated by
a domain signature.
There is a possibility that some policies will require signatures to be
added in a specific order. By only allowing signatures to be added by
encapsulation it is possible to determine the order in which the
signatures have been added.
A DOMSEC defined signature MAY encapsulate a message in one of the
following ways:
1) An unsigned message has an empty signature layer added to it (i.e.
the message is wrapped in a signedData that has a signerInfos which
contains no elements). This is to enable backward compatibility with
S/MIME software that does not have a DOMSEC capability. Since the
signerInfos will contain no signers the eContentType, within the
EncapsulatedContentInfo, MUST be id-data as described in CMS [5].
However, the eContent field will contain the unsigned message instead
of being left empty as suggested in section 5.2 in CMS [5]. This is
so that when the DOMSEC defined signature is added, as defined in
method 2) below, the signature will cover the unsigned message.
2) Signature Encapsulation is used to wrap the original signed message
with a DOMSEC defined signature. This is so that the DOMSEC defined
signature covers the message and all the previously added signatures.
Also, it is possible to determine that the DOMSEC defined signature
was added after the signatures that are already there.
3.1 Naming Conventions and Signature Types
An entity receiving an S/MIME signed message would normally expect the
signature to be that of the originator of the message. However, the
message security services defined in this document require the recipient
to be able to accept messages signed by other entities and/or the
originator. When other entities sign the message the name in the
certificate will not match the message sender's name. An S/MIME
compliant implementation would normally flag a warning if there were a
mismatch between the name in the certificate and the message sender's
name. (This check prevents a number of types of masquerade attack.)
In the case of domain security services, this warning condition SHOULD
be suppressed under certain circumstances. These circumstances are
defined by a naming convention that specifies the form that the signers
name SHOULD adhere to. Adherence to this naming convention avoids the
problems of uncontrolled naming and the possible masquerade attacks that
this would produce.
As an assistance to implementation, a signed attribute is defined to be
included in the S/MIME signature - the 'signature type' attribute. On
receiving a message containing this attribute, the naming convention
checks are invoked.
Implementations conforming to this standard MUST support the naming
convention for signature generation and verification. Implementations
conforming to this standard MUST recognise the signature type attribute
for signature verification. Implementations conforming to this standard
MUST support the signature type attribute for signature generation.
3.1.1 Naming Conventions
The following naming conventions are specified for agents generating
signatures specified in this document:
* For a domain signature, an agent generating this signature MUST be
named 'domain-signing-authority'
* For a review signature, an agent generating this signature MUST be
named 'review-authority'.
* For an additional attributes signature, an agent generating this
signature MUST be named 'attribute-authority'.
This name shall appear as the 'common name (CN)' component of the
subject field in the X.509 certificate. There MUST be only one CN
component present. Additionally, if the certificate contains an RFC 822
address, this name shall appear in the end entity component of the
address - on the left-hand side of the '@' symbol.
In the case of a domain signature, an additional naming rule is
defined: the 'name mapping rule'. The name mapping rule states that
for a domain signing authority, the domain part of its name MUST be the
same as, or an ascendant of, the domain name of the message
originator(s) that it is representing. The domain part is defined
as follows:
* In the case of an X.500 distinguished subject name of an X.509
certificate, the domain part is the country, organisation,
organisational unit, state, and locality components of the
distinguished name.
* In the case of an RFC 2247 distinguished name, the domain part
is the domain components of the distinguished name.
* If the certificate contains an RFC 822 address, the domain
part is defined to be the RFC 822 address component on the right-
hand side of the '@' symbol.
For example, a domain signing authority acting on behalf of John Doe of
the Acme corporation, whose distinguished name is 'cn=John Doe,
ou=marketing,o=acme,c=us' and whose e-mail address is
John.Doe@marketing.acme.com, could have a certificate containing a
distinguished name of 'cn=domain-signing-authority,o=acme,c=us' and an
RFC 822 address of 'domain-signing-authority@acme.com'. If John Doe has
an RFC 2247 defined address of 'cn=John Doe,dc=marketing,dc=acme,dc=us'
then an address of 'cn=domain-signing-authority,dc=acme,dc=us' could be
used to represent the domain signing authority.
When the X.500 distinguished subject name has consecutive organisational
units and/or localities it is important to understand the ordering of
these values in order to determine if the domain part of the domain
signature is an ascendant. In this case, when parsing the distinguished
subject name from the most significant component (i.e. country, locality
or organisation) the parsed organisational unit or locality is deemed to
be the ascendant of consecutive (unparsed) organisational units or
localities.
When parsing an RFC 2247 subject name from the most significant
component (i.e. the 'dc' entry that represents the country, locality or
organisation) the parsed 'dc' entry is deemed to be the ascendant of
consecutive (unparsed) 'dc' entries.
For example, a domain signing authority acting on behalf of John Doe of
the Acme corporation, whose distinguished name is 'cn=John Doe,
ou=marketing,ou=defence,o=acme,c=us' and whose e-mail address is
John.Doe@marketing.defence.acme.com, could have a certificate containing
a distinguished name of 'cn=domain-signing-authority,ou=defence,o=acme,
c=us' and an RFC 822 address of
'domain-signing-authority@defence.acme.com'. If John Doe has an RFC 2247
defined address of 'cn=John Doe,dc=marketing,dc=defense,dc=acme,dc=us'
then the domain signing authority could have a distinguished name of
'cn=domain-signing-authority,dc=defence,dc=acme,dc=us'.
Any message received where the domain part of the domain signing agent's
name does not match, or is not an ascendant of, the originator's domain
name MUST be flagged.
This naming rule prevents agents from one organisation masquerading as
domain signing authorities on behalf of another. For the other types of
signature defined in this document, no such named mapping rule is
defined.
Implementations conforming to this standard MUST support this name
mapping convention as a minimum. Implementations MAY choose to
supplement this convention with other locally defined conventions.
However, these MUST be agreed between sender and recipient domains prior
to secure exchange of messages.
On verifying the signature, a receiving agent MUST ensure that the
naming convention has been adhered to. Any message that violates the
convention MUST be flagged.
3.1.2 Signature Type Attribute
An S/MIME signed attribute is used to indicate the type of signature.
This should be used in conjunction with the naming conventions specified
in the previous section. When an S/MIME signed message containing the
signature type attribute is received it triggers the software to verify
that the correct naming convention has been used.
The ASN.1 [4] notation of this attribute is: -
SignatureType ::= SEQUENCE OF OBJECT IDENTIFIER
id-sti OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) 9 }
-- signature type identifier
If present, the SignatureType attribute MUST be a signed attribute, as
defined in [5]. If the SignatureType attribute is absent and there are
no further encapsulated signatures the recipient SHOULD assume that the
signature is that of the message originator.
All of the signatures defined here are generated and processed as
described in [5]. They are distinguished by the presence of the
following values in the SignatureType signed attribute:
id-sti-domainSig OBJECT IDENTIFIER ::= { id-sti 2 }
-- domain signature.
id-sti-addAttribSig OBJECT IDENTIFIER ::= { id-sti 3 }
-- additional attributes signature.
id-sti-reviewSig OBJECT IDENTIFIER ::= { id-sti 4 }
-- review signature.
For completeness, an attribute type is also specified for an originator
signature. However, this signature type is optional. It is defined as
follows:
id-sti-originatorSig OBJECT IDENTIFIER ::= { id-sti 1 }
-- originator's signature.
All signature types, except the originator type, MUST encapsulate other
signatures. Note a DOMSEC defined signature could be encapsulating an
empty signature as defined in section 3.
A SignerInfo MUST NOT include multiple instances of SignatureType. A
signed attribute representing a SignatureType MAY include multiple
instances of different SignatureType values as an AttributeValue of
attrValues [5], as long as the SignatureType 'additional attributes' is
not present.
If there is more than one SignerInfo in a signerInfos (i.e. when
different algorithms are used) then the SignatureType attribute in all
the SignerInfos MUST contain the same content.
The following sections describe the conditions under which each of these
types of signature may be generated, and how they are processed.
3.2 Domain Signature Generation and Verification
A 'domain signature' is a proxy signature generated on a user's behalf
in the user's domain. The signature MUST adhere to the naming
conventions in 3.1.1, including the name mapping convention. A 'domain
signature' on a message authenticates the fact that the message has
been released from that domain. Before signing, a process generating a
'domain signature' MUST first satisfy itself of the authenticity of the
message originator. This is achieved by one of two methods. Either the
'originator's signature' is checked, if S/MIME signatures are used
inside a domain. Or if not, some mechanism external to S/MIME is used,
such as the physical address of the originating client or an
authenticated IP link.
If the originator's authenticity is successfully verified by one of the
above methods and all other signatures present are valid, including
those that have been encrypted, a 'domain signature' can be added to a
message.
If a 'domain signature' is added and the message is received by a Mail
List Agent (MLA) there is a possibility that the 'domain signature'
will be removed. To stop the 'domain signature' from being removed the
steps in section 5 MUST be followed.
An entity generating a domain signature MUST do so using a certificate
containing a subject name that follows the naming convention specified
in 3.1.1.
If the originator's authenticity is not successfully verified or all
the signatures present are not valid, a 'domain signature' MUST NOT be
generated.
On reception, the 'domain signature' SHOULD be used to verify the
authenticity of a message. A check MUST be made to ensure that both the
naming convention and the name mapping convention have been used as
specified in this standard.
A recipient can assume that successful verification of the domain
signature also authenticates the message originator.
If there is an originator signature present, the name in that
certificate SHOULD be used to identify the originator. This information
can then be displayed to the recipient.
If there is no originator signature present, the only assumption that
can be made is the domain the message originated from.
A domain signer can be assumed to have verified any signatures that it
encapsulates. Therefore, it is not necessary to verify these signatures
before treating the message as authentic. However, this standard does
not preclude a recipient from attempting to verify any other signatures
that are present.
The 'domain signature' is indicated by the presence of the value
id-sti-domainSig in a 'signature type' signed attribute.
There MAY be one or more 'domain signature' signatures in an S/MIME
encoding.
3.3 Additional Attributes Signature Generation and Verification
The 'additional attributes' signature type indicates that the
SignerInfo contains additional attributes that are associated with the
message.
All attributes in the applicable SignerInfo MUST be treated as
additional attributes. Successful verification of an 'additional
attributes' signature means only that the attributes are authentically
bound to the message. A recipient MUST NOT assume that its successful
verification also authenticates the message originator.
An entity generating an 'additional attributes' signature MUST do so
using a certificate containing a subject name that follows the naming
convention specified in 3.1.1. On reception, a check MUST be made to
ensure that the naming convention has been used.
A signer MAY include any of the attributes listed in [3] or in this
document when generating an 'additional attributes' signature. The
following attributes have a special meaning, when present in an
'additional attributes' signature:
1) Equivalent Label: label values in this attribute are to be treated as
equivalent to the security label contained in an encapsulated
SignerInfo, if present.
2) Security Label: the label value indicates the aggregate sensitivity
of the inner message content plus any encapsulated signedData and
envelopedData containers. The label on the original data is indicated
by the value in the originator's signature, if present.
An 'additional attributes' signature is indicated by the presence of the
value id-sti-addAttribSig in a 'signature type' signed
attribute. Other Object Identifiers MUST NOT be included in the sequence
of OIDs if this value is present.
There MAY be multiple 'additional attributes' signatures in an S/MIME
encoding.
3.4 Review Signature Generation and Verification
The review signature indicates that the signer has reviewed the message.
Successful verification of a review signature means only that the signer
has approved the message for onward transmission to the recipient(s).
When the recipient is in another domain, a device on a domain boundary
such as a Mail Guard or firewall may be configured to check review
signatures. A recipient MUST NOT assume that its successful verification
also authenticates the message originator.
An entity generating a signed review signature MUST do so using a
certificate containing a subject name that follows the naming convention
specified in 3.1.1. On reception, a check MUST be made to ensure that
the naming convention has been used.
A review signature is indicated by the presence of the value
id-sti-reviewSig in a 'signature type' signed attribute.
There MAY be multiple review signatures in an S/MIME encoding.
3.5 Originator Signature
The 'originator signature' is used to indicate that the signer is the
originator of the message and its contents. It is included in this
document for completeness only. An originator signature is indicated
either by the absence of the signature type attribute, or by the
presence of the value id-sti-originatorSig in a 'signature type'
signed attribute.
4. Encryption and Decryption
Message encryption may be performed by a third party on behalf of a set
of originators in a domain. This is referred to as domain encryption.
Message decryption may be performed by a third party on behalf of a set
of recipients in a domain. This is referred to as domain decryption.
The third party that performs these processes is referred to in this
section as a "Domain Confidentiality Authority" (DCA). Both of these
processes are described in this section.
Messages may be encrypted for decryption by the final recipient and/or
by a DCA in the recipient's domain. The message may also be encrypted
for decryption by a DCA in the originator's domain (e.g. for content
analysis, audit, key word scanning, etc.). The choice of which of these
is actually performed is a system specific issue that depends on system
security policy. It is therefore outside the scope of this document.
These processes of encryption and decryption processes are shown in the
following table.
--------------------------------------------------------------------
| | Recipient Decryption | Domain Decryption |
|------------------------|----------------------|--------------------|
| Originator Encryption | Case(a) | Case(b) |
| Domain Encryption | Case(c) | Case(d) |
--------------------------------------------------------------------
Case (a), encryption of messages by the originator for decryption by the
final recipient(s), is described in CMS [5]. In cases (c) and (d),
encryption is performed not by the originator but by the DCA in the
originator's domain. In Cases (b) and (d), decryption is performed not
by the recipient(s) but by the DCA in the recipient's domain.
A client implementation that conforms to this standard MUST support
case (b) for transmission, case (c) for reception and case (a) for
transmission and reception.
A DCA implementation that conforms to this standard MUST support cases
(c) and (d), for transmission, and cases (b) and (d) for reception. In
cases (c) and (d) the 'domain signature' SHOULD be applied before the
encryption. In cases (b) and (d) the message SHOULD be decrypted before
the originators 'domain signature' is obtained and verified.
The process of encryption and decryption is documented in CMS [5]. The
only additional requirement introduced by domain encryption and
decryption is for greater flexibility in the management of keys, as
described in the following subsections. As with signatures, a naming
convention and name mapping convention are used to locate the correct
public key.
The mechanisms described below are applicable both to key agreement and
key transport systems, as documented in CMS [5]. The phrase 'encryption
key' is used as a collective term to cover the key management keys used
by both techniques.
The mechanisms below are also applicable to individual roving users who
wish to encrypt messages that are sent back to base.
4.1 Domain Confidentiality Naming Conventions
A DCA MUST be named 'domain-confidentiality-authority'. This name MUST
appear in the 'common name(CN)' component of the subject field in the
X.509 certificate. Additionally, if the certificate contains an RFC 822
address, this name MUST appear in the end entity part of the address,
i.e. on the left-hand side of the '@' symbol.
Along with this naming convention, an additional naming rule is defined:
the 'name mapping rule'. The name mapping rule states that for a DCA,
the domain part of its name MUST be the same as, or an ascendant of (as
defined in section 3.1.1), the domain name of the set of entities that
it represents. The domain part is defined as follows:
* In the case of an X.500 distinguished name of an X.509 certificate,
the domain part is the country, organisation, organisational
unit, state, and locality components of the distinguished name.
* In the case of an RFC 2247 distinguished name, the domain part
is the domain components of the distinguished name.
* If the certificate contains an RFC 822 address, the domain part is
defined to be the RFC 822 address part on the right-hand side of the
'@' symbol.
For example, a DCA acting on behalf of John Doe of the Acme
corporation, whose distinguished name is 'cn=John Doe, ou=marketing,
o=acme,c=us' and whose e-mail address is John.Doe@marketing.acme.com,
could have a certificate containing a distinguished name of
'cn=domain-confidentiality-authority,o=acme,c=us' and an e-mail
address of 'domain-confidentiality-authority@acme.com'. If John Doe
has an RFC 2247 defined address of 'cn=John Doe,dc=marketing,
dc=defense,dc=acme,dc=us' then the domain signing authority could have
a distinguished name of 'cn=domain-signing-authority,dc=defence,dc=acme,
dc=us'. The key associated with this certificate would be used for
encrypting messages for John Doe.
Any message received where the domain part of the domain encrypting
agents name does not match, or is not an ascendant of, the domain name
of the entities it represents MUST be flagged.
This naming rule prevents messages being encrypted for the wrong domain
decryption agent.
Implementations conforming to this standard MUST support this name
mapping convention as a minimum. Implementations may choose to
supplement this convention with other locally defined conventions.
However, these MUST be agreed between sender and recipient domains
prior to sending any messages.
4.2 Key Management for DCA Encryption
At the sender's domain, DCA encryption is achieved using the recipient
DCA's certificate or the end recipient's certificate. For this, the
encrypting process must be able to correctly locate the certificate for
the corresponding DCA in the recipient's domain or the one corresponding
to the end recipient. Having located the correct certificate, the
encryption process is then performed and additional information required
for decryption is conveyed to the recipient in the recipientInfo field
as specified in CMS [5]. A DCA encryption agent MUST be named according
to the naming convention specified in section 4.1. This is so that the
corresponding certificate can be found.
No specific method for locating the certificate to the corresponding
DCA in the recipient's domain or the one corresponding to the end
recipient is mandated in this document. An implementation may choose
to access a local certificate store to locate the correct certificate.
Alternatively, a X.500 or LDAP directory may be used in one of the
following ways:
1. The directory may store the DCA certificate in the recipient's
directory entry. When the user certificate attribute is requested,
this certificate is returned.
2. The encrypting agent maps the recipient's name to the DCA name in the
manner specified in 4.1. The user certificate attribute associated
with this directory entry is then obtained.
This document does not mandate either of these processes. Whichever one
is used, the name mapping conventions must be adhered to, in order to
maintain confidentiality.
Having located the correct certificate, the encryption process is then
performed. A recipientInfo for the DCA or end recipient is then
generated, as described in CMS [5].
DCA encryption may be performed for decryption by the end recipient
and/or by a DCA. End recipient decryption is described in CMS [5]. DCA
decryption is described in section 4.3.
4.3 Key Management for DCA Decryption
DCA decryption uses a private-key belonging to the DCA and the necessary
information conveyed in the DCA's recipientInfo field.
It should be noted that domain decryption can be performed on messages
encrypted by the originator and/or by a DCA in the originator's domain.
In the first case, the encryption process is described in CMS [5]; in
the second case, the encryption process is described in 4.2.
5. Applying a Domain Signature when Mail List Agents are Present.
It is possible that a message leaving a DOMSEC domain may encounter a
Mail List Agent (MLA) before it reaches the final recipient. There is a
possibility that this would result in the 'domain signature' being
stripped off the message. We do not want a MLA to remove the 'domain
signature'. Therefore, the 'domain signature' must be applied to the
message in such a way that will prevent a MLA from removing it.
A MLA will search a message for the "outer" signedData layer, as defined
in ESS [3] section 4.2, and strip off all signedData layers that
encapsulate this "outer" signedData layer. Where this "outer" signedData
layer is found will depend on whether the message contains a
mlExpansionHistory attribute or an envelopedData layer.
There is a possibility that a message leaving a DOMSEC domain has
already been processed by a MLA, in which case a 'mlExpansionHistory'
attribute will be present within the message.
There is a possibility that the message will contain an envelopedData
layer. This will be the case when the message has been encrypted within
the domain for the domain's "Domain Confidentiality Authority", see
section 4.0, and, possibly, the final recipient.
How the 'domain signature' is applied will depend on what is already
present within the message. Before the 'domain signature' can be
applied the message MUST be searched for the "outer" signedData layer,
this search is complete when one of the following is found: -
- 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 a signedData layer containing a mlExpansionHistory attribute has
been found then: -
1) Strip off the signedData layer (after remembering the included
signedAttributes).
2) Search the rest of the message until an envelopedData layer or
the original content is found.
3) a) If an envelopedData layer has been found then: -
- Strip off all the signedData layers down to the envelopedData
layer.
- Locate the RecipientInfo for the local DCA and use the
information it contains to obtain the message key.
- Decrypt the encryptedContent using the message key.
- Encapsulate the decrypted message with a 'domain
signature'
- If local policy requires the message to be encrypted using
S/MIME encryption before leaving the domain then encapsulate
the 'domain signature' with an envelopedData layer containing
RecipientInfo structures for each of the recipients and an
originatorInfo value built from information describing this
DCA.
If local policy does not require the message to be encrypted
using S/MIME encryption but there is an envelopedData at a
lower level within the message then the 'domain signature'
MUST be encapsulated by an envelopedData as described above.
An example when it may not be local policy to require S/MIME
encryption is when there is a link crypto present.
b) If an envelopedData layer has not been found then: -
- Encapsulate the new message with a 'domain signature'.
4) Encapsulate the new message in a signedData layer, adding the
signedAttributes from the signedData layer that contained the
mlExpansionHistory attribute.
If no signedData layer containing a mlExpansionHistory attribute has
been found but an envelopedData has been found then: -
1) Strip off all the signedData layers down to the envelopedData
layer.
2) Locate the RecipientInfo for the local DCA and use the information
it contains to obtain the message key.
3) Decrypt the encryptedContent using the message key.
4) Encapsulate the decrypted message with a 'domain signature'
5) If local policy requires the message to be encrypted before
leaving the domain then encapsulate the 'domain signature' with an
envelopedData layer containing RecipientInfo structures for each
of the recipients and an originatorInfo value built from
information describing this DCA.
If local policy does not require the message to be encrypted using
S/MIME encryption but there is an envelopedData at a lower level
within the message then the 'domain signature' MUST be
encapsulated by an envelopedData as described above.
If no signedData layer containing a mlExpansionHistory attribute has
been found and no envelopedData has been found then: -
1) Encapsulate the message in a 'domain signature'.
5.1 Examples of Rule Processing
The following examples help explain the above rules. All of the signedData
objects are valid and none of them are a domain signature. If a signedData
object was a domain signature then it would not be necessary to validate any
further signedData objects.
1) A message (S1 (Original Content)) (where S = signedData) in which the
signedData does not include an mlExpansionHistory attribute is to
have a 'domain signature' applied. The signedData, S1, is verified.
No "outer" signedData is found, after searching for one as defined
above, since the original content is found, nor is an envelopedData
or a mlExpansionHistory attribute found. A new signedData layer, S2,
is created that contains a 'domain signature', resulting in the
following message sent out of the domain (S2 (S1 (Original Content))).
2) A message (S3 (S2 (S1 (Original Content))) in which none of the
signedData layers includes an mlExpansionHistory attribute is to have
a 'domain signature' applied. The signedData objects S1, S2 and S3
are verified. There is not an original, "outer" signedData layer
since the original content is found, nor is an envelopedData or a
mlExpansionHistory attribute found. A new signedData layer, S4, is
created that contains a 'domain signature', resulting in the
following message sent out of the domain (S4 (S3 (S2 (S1 (Original
Content))).
3) A message (E1 (S1 (Original Content))) (where E = envelopedData) in
which S1 does not include a mlExpansionHistory attribute is to have
a 'domain signature' applied. There is not an original, received
"outer" signedData layer since the envelopedData, E1, is found at the
outer layer. The encryptedContent is decrypted. The signedData, S1,
is verified. The decrypted content is wrapped in a new signedData
layer, S2, which contains a 'domain signature'. If local policy
requires the message to be encrypted, using S/MIME encryption, before
it leaves the domain then this new message is wrapped in an
envelopedData layer, E2, resulting in the following message sent out
of the domain (E2 (S2 (S1 (Original Content)))), else the message is
not wrapped in an envelopedData layer resulting in the following
message (S2 (S1 (Original Content))) being sent.
4) A message (S2 (E1 (S1 (Original Content)))) in which S2 includes a
mlExpansionHistory attribute is to have a 'domain signature'
applied. The signedData object S2 is verified. The mlExpansionHistory
attribute is found in S2, so S2 is the "outer" signedData. The signed
attributes in S2 are remembered for later inclusion in the new outer
signedData that is applied to the message. S2 is stripped off and the
message is decrypted. The signedData object S1 is verified. The
decrypted message is wrapped in a signedData layer, S3, which
contains a 'domain signature'. If local policy requires the message
to be encrypted, using S/MIME encryption, before it leaves the
domain then this new message is wrapped in an envelopedData layer,
E2. A new signedData layer, S4, is then wrapped around the
envelopedData, E2, resulting in the following message sent out of
the domain (S4 (E2 (S3 (S1 (Original Content))))). If local policy
does not require the message to be encrypted, using S/MIME
encryption, before it leaves the domain then the message is not
wrapped in an envelopedData layer but is wrapped in a new signedData
layer, S4, resulting in the following message sent out of the domain
(S4 (S3 (S1 (Original Content). The signedData S4, in both cases,
contains the signed attributes from S2.
5) A message (S3 (S2 (E1 (S1 (Original Content))))) in which none of
the signedData layers include a mlExpansionHistory attribute is to
have a 'domain signature' applied. The signedData objects S3 and S2
are verified. When the envelopedData E1 is found the signedData
objects S3 and S2 are stripped off. The encryptedContent is
decrypted. The signedData object S1 is verified. The decrypted
content is wrapped in a new signedData layer, S4, which contains a
'domain signature'. If local policy requires the message to be
encrypted, using S/MIME encryption, before it leaves the domain
then this new message is wrapped in an envelopedData layer, E2,
resulting in the following message sent out of the domain (E2 (S4
(S1 (Original Content)))), else the message is not wrapped in an
envelopedData layer resulting in the following message (S4 (S1
(Original Content))) being sent.
6) A message (S3 (S2 (E1 (S1 (Original Content))))) in which S3
includes a mlExpansionHistory attribute is to have a 'domain
signature' applied. The signedData objects S3 and S2 are verified.
The mlExpansionHistory attribute is found in S3, so S3 is the
"outer" signedData. The signed attributes in S3 are remembered
for later inclusion in the new outer signedData that is applied to
the message. The signedData object S3 is stripped off. When the
envelopedData layer, E1, is found the signedData object S2 is
stripped off. The encryptedContent is decrypted. The signedData
object S1 is verified. The decrypted content is wrapped in a new
signedData layer, S4, which contains a 'domain signature'. If local
policy requires the message to be encrypted, using S/MIME encryption,
before it leaves the domain then this new message is wrapped in an
envelopedData layer, E2. A new signedData layer, S5, is then wrapped
around the envelopedData, E2, resulting in the following message sent
out of the domain (S5 (E2 (S4 (S1 (Original Content))))). If local
policy does not require the message to be encrypted, using S/MIME
encryption, before it leaves the domain then the message is not
wrapped in an envelopedData layer but is wrapped in a new signedData
layer, S5, resulting in the following message sent out of the domain
(S5 (S4 (S1 (Original Content). The signedData S5, in both cases,
contains the signed attributes from S3.
7) A message (S3 (E2 (S2 (E1 (S1 (Original Contnent)))))) in which S3
does not include a mlExpansionHistory attribute is to have a
'domain signature' applied. The signedData object S3 is verified.
When the envelopedData E2 is found the signedData object S3 is
stripped off. The encryptedContent is decrypted. The signedData
object S2 is verified, the envelopedData E1 is decrytped and the
signedData object S1 is verified. The signedData object S2 is
wrapped in a new signedData layer S4, which contains a
'domain signature'. Since there is an envelopedData E1 lower down
in the message, the new message is wrapped in an envelopedData
layer, E3, resulting in the following message sent out of the domain
(E3 (S4 (S2 (E1 (S1 (Original Content)))))).
6. Security Considerations
This specification relies on the existence of several well known names,
such as domain-confidentiality-authority. Organisations must take care
with these names, even if they do not support DOMSEC, so that
certificates issued in these names are only issued to legitimate
entities. If this is not true then an individual could get a certificate
associated with domain-confidentiality-authority@acme.com and as a
result might be able to read messages the a DOMSEC client intended for
others.
Implementations MUST protect all private keys. Compromise of the
signer's private key permits masquerade.
Similarly, compromise of the content-encryption key may result in
disclosure of the encrypted content.
Compromise of key material is regarded as an even more serious issue for
domain security services than for an S/MIME client. This is because
compromise of the private key may in turn compromise the security of a
whole domain. Therefore, great care should be used when considering its
protection.
Domain encryption alone is not secure and should be used in conjunction
with a domain signature to avoid a masquerade attack, where an attacker
that has obtained a DCA certificate can fake a message to that domain
pretending to be another domain.
7. DOMSEC ASN.1 Module
DOMSECSyntax
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) domsec(10) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS All
-- The types and values defined in this module are exported for
-- use in the other ASN.1 modules. Other applications may use
-- them for their own purposes.
SignatureType ::= SEQUENCE OF OBJECT IDENTIFIER
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 16 }
id-sti OBJECT IDENTIFIER ::= { id-smime 9 } -- signature type
identifier
-- Signature Type Identifiers
id-sti-originatorSig OBJECT IDENTIFIER ::= { id-sti 1 }
id-sti-domainSig OBJECT IDENTIFIER ::= { id-sti 2 }
id-sti-addAttribSig OBJECT IDENTIFIER ::= { id-sti 3 }
id-sti-reviewSig OBJECT IDENTIFIER ::= { id-sti 4 }
END -- of DOMSECSyntax
8. References
[1] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC 2633,
June 1999.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Hoffman, P., "Enhanced Security Services for S/MIME", RFC 2634,
June 1999.
[4] International Telecommunications Union, Recommendation X.208, "Open
systems interconnection: specification of Abstract Syntax Notation
(ASN.1)", CCITT Blue Book, 1989.
[5] Housley, R., "Cryptographic Message Syntax", RFC 2630, June 1999.
9. Authors' Addresses RFC 3183
Tim Dean Title: Domain Security Services using S/MIME
DERA Malvern Author(s): T. Dean, W. Ottaway
St. Andrews Road Status: Experimental
Malvern Date: October 2001
Worcs Mailbox: tbdean@QinetiQ.com,
WR14 3PS wjottaway@QinetiQ.com
Pages: 24
Characters: 57129
SeeAlso/Updates/Obsoletes: None
Phone: +44 (0) 1684 894239 I-D Tag: draft-ietf-smime-domsec-09.txt
Fax: +44 (0) 1684 896660
Email: t.dean@eris.dera.gov.uk
William Ottaway URL: ftp://ftp.rfc-editor.org/in-notes/rfc3183.txt
DERA Malvern
St. Andrews Road
Malvern
Worcs
WR14 3PS
Phone: +44 (0) 1684 894079 This document describes how the S/MIME (Secure/Multipurpose Internet
Fax: +44 (0) 1684 896660 Mail Extensions) protocol can be processed and generated by a number
Email: w.ottaway@eris.dera.gov.uk of components of a communication system, such as message transfer
agents, guards and gateways to deliver security services. These
services are collectively referred to as 'Domain Security Services'.
10. Full Copyright Statement This memo defines an Experimental Protocol for the Internet community.
It does not specify an Internet standard of any kind. Discussion and
suggestions for improvement are requested. Distribution of this memo
is unlimited.
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