S/MIME WG Sean Turner, IECA
Internet Draft Jim Schaad, Soaring Hawk
Intended Status: Standard Track November 16, 2007
Expires: May 16, 2008
Multiple Signatures in S/MIME
draft-ietf-smime-multisig-03.txt
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Copyright (C) The IETF Trust (2007).
Abstract
CMS SignedData includes the SignerInfo structure to convey per-signer
information. SignedData supports multiple signers and multiple
signature algorithms per-signer with multiple SignerInfo structures.
If a signer attaches more than one SignerInfo, there are concerns
that an attacker could perform a downgrade attack by removing the
SignerInfo(s) with the 'strong' algorithm(s). This document defines
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the multiple-signatures attribute, its generation rules, and its
processing rules to allow signers to convey multiple SignerInfo while
protecting against downgrade attacks. Additionally, this attribute
may assist during periods of algorithm migration.
Conventions used in this document
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 [RFC2119].
Discussion
This draft is being discussed on the 'ietf-smime' mailing list. To
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Table of Contents
1. Introduction...................................................3
2. Rationale......................................................3
2.1. Attribute Design Requirements.............................4
3. Multiple Signature Indication..................................5
4. Message Generation and Processing..............................6
4.1. SignedData Type...........................................6
4.2. EncapsulatedContentInfo Type..............................7
4.3. SignerInfo Type...........................................7
4.4. Message Digest Calculation Process........................7
4.4.1. multiple-signatures Signed Attribute Generation......7
4.4.2. Message Digest calculation Process...................8
4.5. Signature Generation Process..............................8
4.6. Signature Verification Process............................8
5. Signature Evaluation Processing................................8
5.1. Evaluation of a SignerInfo object.........................9
5.2. Evaluation of a SignerInfo Set...........................10
5.3. Evaluation of a SignedData Set...........................11
6. Security Considerations.......................................11
7. IANA Considerations...........................................12
8. References....................................................12
8.1. Normative References.....................................12
8.2. Informative References...................................12
Appendix A. ASN.1 Module.........................................13
Appendix B. Background...........................................15
B.1. Attacks..................................................15
B.2. Hashes in CMS............................................15
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1. Introduction
The Cryptographic Message Syntax (CMS), see [CMS], defined SignerInfo
to provide data necessary for relying parties to verify the signer's
digital signature, which is also include in the SignerInfo structure.
Signers include more than one SignerInfo in a SignedData if they use
different digest or signature algorithms. Each SignerInfo exists
independently and new SignerInfo structures can be added or an
existing one(s) removed without perturbing the remaining
signature(s).
The concern is that if an attacker successfully attacked a hash or
signature algorithm; the attacker could remove all SignerInfo
structures except the SignerInfo with the successfully attacked hash
or signature algorithm; the relying party is then left with the
attacked SignerInfo even though the relying party supported more than
just the attacked hash or signature algorithm.
A solution is to have signers include a pointer to all the signer's
SignerInfo structures. If an attacker removes any SignerInfo, then
relying parties will be aware that an attacker has removed one or
more SignerInfo.
Note this attribute ought not be confused with the countersignature
attribute, see 11.4 of [CMS], as this is not intended to sign over an
existing signature rather it is to provide a pointer to additional
signer's signatures that are all at the same level. That is
countersignature provides a serial signature while the attribute
defined herein provides pointers to parallel signature by the same
signer.
2. Rationale
The rationale for this specification is to protect against downgrade
attacks that remove the 'strong' signature to leave the 'weak'
signature, which has presumably been successfully attacked. If a CMS
object has multiple SignerInfos, then the attacker, whether it be
Alice, Bob, or Mallory, can remove SignerInfos without the relying
party being aware that more than one was generated.
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Removal of a SignerInfo does not render the signature invalid nor
does it constitute an error. In the following scenario: a signer
generates a SignedData with two SignerInfo objects one with a 'weak'
algorithm and one with a 'strong' algorithm; there are three types of
relying parties:
1) Those that support only a 'weak' algorithm. If both SignerInfo
objects are present, the relying party processes the algorithm it
supports. If both SignerInfo objects are not present, the relying
party can easily determine that another SignerInfo has been
removed, but not changed. In both cases, if the 'weak' signature
verifies the relying party MAY consider the signature valid.
2) Those that support only a 'strong' algorithm. If both
SignerInfo objects are present, the relying party processes the
algorithm it supports. If both SignerInfo objects are not present,
the relying party can easily determine that another SignerInfo has
been removed, but the relying party doesn't care. In both cases,
if the 'strong' signature verifies the relying party MAY consider
the signature valid.
3) Those that support both a 'weak' algorithm and a 'strong'
algorithm. If both SignerInfo objects are present, the relying
party processes both algorithms. If both SignerInfo objects are
not present, the relying party can easily determine that another
SignerInfo has been removed.
Local policy MAY dictate that the removal of the 'strong' algorithm
results in an invalid signature. See section 5 for further
processing.
2.1. Attribute Design Requirements
The attribute will have the following characteristics:
1) Use CMS attribute structure;
2) Be computable before any signatures are applied;
3) Contain enough information to identify individual signatures
(i.e., a particular SignerInfo); and,
4) Contain enough information to resist collision, preimage, and
second premiage attacks.
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3. Multiple Signature Indication
The multiple-signatures attribute type specifies a pointer to a
signer's other multiple-signatures attribute(s). For example, if a
signer applies three signatures there must be two attribute values
for multiple-signatures in each SignerInfo. The 1st SignerInfo
points to the 2nd and 3rd SignerInfos. The 2nd SignerInfo points to
the 1st and 3rd SignerInfos. The 3rd SignerInfo points to the 1st and
2nd SignerInfos.
The multiple-signatures attribute MUST be a signed attribute. The
number of attributes included in a SignerInfo is the number of
signatures applied by a signer less one. This attribute is multi-
valued and there MAY be more than one AttributeValue present.
The following object identifier identifies the multiple-signatures
attribute:
id-aa-multipleSignatures OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) TBD }
multiple-signatures attribute values have the ASN.1 type
MultipleSignatures:
MultipleSignatures ::= SEQUENCE {
bodyHashAlg DigestAlgorithIdentifier,
signAlg SignatureAlgorithmIdentifier,
signAttrsHash SignAttrsHash,
cert ESSCertIDv2 OPTIONAL}
SignAttrsHash ::= SEQUENCE {
algID AlgorithmIdentifier,
hash OCTET STRING }
The fields in MultipleSignatures have the following meaning:
- bodyHashAlg includes the digest algorithmIdentifier for the
referenced multiple-signatures attribute.
- signAlg includes the signature algorithmIdentifier for the
referenced multiple-signatures attribute.
- signAttrsHash has two fields:
-- aldId MUST match the digest algorithm for the SignerInfo in
which this multiple-signatures attribute value is placed.
-- hash is the hash value of the signedAttrs (see section 4.3).
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- cert is optional. It identities the certificate used to sign the
SignerInfo that contains the other multiple-signatures
attribute(s). It MUST be present if the fields in the other
multiple-signatures attribute(s) are the same.
The following is an example:
SignedData
DigestAlg=sha1,sha256
SignerInfo1 SignerInfo2
digestAlg=sha1 digestAlg=sha256
signatureAlg=dsawithsha1 signatureAlg=ecdsawithsha256
signedAttrs= signedAttrs=
signingTime1 signingTime1
messageDigest1 messageDigest2
multiSig1= multiSig2=
bodyHash=sha256 bodyHash=sha1
signAlg=ecdsawithsha256 signAlg=dsawithsha1
signAttrsHash= signAttrsHash=
algID=sha1 algID=sha256
hash=value1 hash=value2
4. Message Generation and Processing
The following are the additional procedures for Message Generation
when using the multiple-signatures attribute. These paragraphs track
with section 5.1-5.6 in [CMS].
4.1. SignedData Type
The following steps MUST be followed by a signer when generating
SignedData:
- The signer MUST indicate the CMS version.
- The signer SHOULD include the digest algorithm used in
SignedData.digestAlgorithms, if the digest algorithm's identifier
is not already present.
- The signer MUST include the encapContentInfo. Note the
encapContentInfo is the same for all signers in this SignedData.
- The signer SHOULD add certificates sufficient to contain
certificate paths from a recognized "root" or "top-level
certification authority" to the signer, if the signer's
certificates are not already present.
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- The signer MAY include the Certificate Revocation Lists (CRLs)
necessary to validate the digital signature, if the CRLs are not
already present.
- The signer MUST:
-- Retain the existing signerInfo(s).
-- Include their signerInfo.
4.2. EncapsulatedContentInfo Type
The procedures for generating EncapsulatedContentInfo are as
specified in section 5.2 of [CMS].
4.3. SignerInfo Type
The procedures for generating a SignerInfo are as specified in
section 5.3 of [CMS] with the following addition:
The signer MUST include the multiple-signatures attribute in
signedAttrs.
4.4. Message Digest Calculation Process
4.4.1. multiple-signatures Signed Attribute Generation
The procedure for generating the multiple-signatures signed attribute
are as follows:
1) All other signed attributes are placed in the respective
SignerInfo structures but the signatures are not yet computed for
the SignerInfo.
2) The multiple-signatures attributes are added to each of the
SignerInfo structures with the SignAttrsHash.hash field containing
a zero length octet string.
3) The correct SignAttrsHash.hash value is computed for each of the
SignerInfo structures.
4) After all hash values have been computed, the correct hash
values are placed into their respective SignAttrsHash.hash fields.
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4.4.2. Message Digest calculation Process
The remaining procedures for generating the message-digest attribute
are as specified in section 5.4 of [CMS].
4.5. Signature Generation Process
The procedures for signature generation are as specified in section
5.5 of [CMS].
4.6. Signature Verification Process
The procedures for signature verification are as specified in section
5.6 of [CMS] with the following addition:
If the SignedData signerInfo includes the multiple-signatures
attribute, the attribute's values must be calculated as described in
section 4.4.1.
For every SignerInfo to be considered present for a given signer, the
number of MultipleSignatures AttributeValue(s) present in a given
SignerInfo MUST equal the number of SignerInfos for that signer less
one and the hash value present in each of the MultipleSignatures
AttributeValue(s) MUST match the output of the message digest
calculation from section 4.4.1 for each SignerInfo.
The hash corresponding to the 1st SignerInfo must match the value in
the multiple-signatures attribute that points to the 1st SignerInfo
present in the 2nd and 3rd SignerInfos. The hash corresponding to
the 2nd SignerInfo must match the value in the multiple-signatures
attribute that points to the 2nd SignerInfo present in the 1st and
3rd SignerInfos. The hash corresponding to the 3rd SignerInfo must
match the value in the multiple-signatures attribute that points to
the 3rd SignerInfo present in the 1st and 2nd SignerInfos.
5. Signature Evaluation Processing
This section describes recommended processing of signatures when
there are more than one SignerInfo present in a message. This may be
due to either multiple SignerInfos being present in a singled
SignedData object, or there are multiple SignerData objects embedded
in each other.
The text in this section is non-normative. The processing described
is highly recommended, but is not forced. Changes in the processing
which have the same results with somewhat different orders of
processing is sufficient.
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Order of operations:
1) Evaluate each SignerInfo object independently.
2) Combine the results of all SignerInfo objects at the same level
(i.e. attached to the same SignerData object)
3) Combine the results of the nested SignerData objects. Note that
this should ignore the presence of other CMS objects between the
SignedData objects.
5.1. Evaluation of a SignerInfo object
When evaluating a SignerInfo object, there are three different pieces
that need to be examined.
The first piece is the mathematics of the signature itself (i.e., can
one actually successfully do the computations and get the correct
answer). This result is one of three results. The mathematics
succeeds, the mathematics fails, or the mathematics cannot be
evaluated. The type of things that lead to the last state are non-
implementation of an algorithm or required inputs, such as the public
key, are missing.
The second piece is the validation of the source of the public key.
For CMS, this is generally determined by extracting the public key
from a certificate. The certificate needs to be evaluated. This is
done by the procedures outlined in [PROFILE]. In addition to the
processing described in that document, there may be additional
requirements on certification path processing that are required by
the application in question. One such set of addition processing is
described in [SMIME-CERT]. One piece of information that is part of
this additional processing is local and application policy. The
output of this processing can actually be one of four different
states: Success, Failure, Indeterminate and Warning. The first
three states are described in [PROFILE], Warning would be generated
when it is possible that some information is currently acceptable,
but may not be acceptable either in the near future or under some
circumstances.
The third piece of the validation is local and application policy as
applied to the contents of the SignerInfo object. This would cover
such issues as the requirements on mandatory signed attributes or
requirements on signature algorithms.
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5.2. Evaluation of a SignerInfo Set
Combining the results of the individual SignerInfos into a result for
a SignedData object requires knowledge of the results for the
individual SignerInfo objects, the require application policy and any
local policies. The default processing if no other rules are applied
should be:
1) Group the SignerInfo objects by the signer.
2) Take the best result from each signer.
3) Take the worst result from all of the different signers; this is
the result for the SignedData object.
Application and local policy can affect each of the steps outlined
above.
In Step 1:
- If the subject name or subject alternative name(s) cannot be used
to determine if two SignerInfo objects were created by the same
identity, then applications need to specify how such matching is to
be done. As an example, the S/MIME message specification [SMIME-
MSG] could say that as long as the same RFC 822 name exists in
either the subject name or the subject alt name they are the same
identity. This would be true even if other information that did
not match existed in these fields.
- Some applications may specify that this step should be skipped;
this has the effect of making each SignerInfo object independent of
all other SignerInfo objects even if the signing identity is the
same. Applications that specify this need to be aware that
algorithm rollover will not work correctly in this case.
In Step 2:
- The major policy implication at this step is the treatment of and
order for the indeterminate states. In most cases this state would
be placed between the failure and warning states. Part of the
issue is the question of having a multi-state or a binary answer as
to success or failure of an evaluation. Not every application can
deal with the statement "try again later". It may also be
dependent on what the reason for the indeterminate state is. It
makes more sense to try again later if the problem is that a CRL
cannot be found than if you are not able to evaluate the algorithm
for the signature.
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In Step 3:
- The same policy implications from Step 2 apply here.
5.3. Evaluation of a SignedData Set
For simple applications, the requirement can be made that the result
of evaluating a set of SignedData objects is the worst outcome of the
items. (I.e. one failure means the entire item fails). However not
all applications will want to have this behavior.
A work flow application could work as follows:
The second signer will modify the original content, keep the
original signature and then sign the message. This means that only
the outermost signature is of significance during evaluation. The
second signer is asserting that they successfully validated the inner
signature as part of its processing.
A Signed Mail application could work as follows:
If signatures are added for the support of [ESS] features, then the
fact that an outer layer signature can be treated as a non-
significant failure. The only thing that matters is that the
originator signature is valid. This means that all outer layer
signatures which fail can be stripped from the message prior to
display leaving only the inner-most valid signature to be displayed.
6. Security Considerations
Security considerations from the hash and signature algorithms used
to produce the SignerInfo apply.
If the hashing and signing operations are performed by different
entities, the entity performing the signature must ensure the hash
comes from a "trustworthy" source. This can be partially mitigated by
requiring that multiple hashes using different algorithms are
provided.
This attribute provides no protection if all of the algorithms used
in the signer attribute are 'cracked'.
Local policy and applications greatly affects signature processing.
The application of local policy and the requirements specific to an
application can both affect signature processing. This means that a
signature valid in one context or location can fail validation in a
different context or location.
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7. IANA Considerations
None: All identifiers are already registered. Please remove this
section prior to publication as an RFC.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[PROFILE] Housley, R., Polk, W., Ford, W., and D. Solo,
"Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC
3280, April 2002.
[SMIME-CERT] Ramsdell, B., and S. Turner, "Secure Multipurpose
Internet Mail Extensions (S/MIME) Version 3.2
Certificate Handling", work in progress.
[SMIME-MSG] Ramsdell, B., and S. Turner, "Secure Multipurpose
Internet Mail Extensions (S/MIME) Version 3.2 Message
Specification", work in progress.
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.
[ESSCertID] Schaad, J., "ESS Update: Adding CertID Algorithm
Agility", RFC 5035, August 2007.
8.2. Informative References
[ATTACK] Hoffman, P., Schneier, B., "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270, November
2005.
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Appendix A. ASN.1 Module
MultipleSignatures
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) multisig(TBD) }
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.
IMPORTS
-- Imports from RFC 3280 [PROFILE], Appendix A.1
AlgorithmIdentifier
FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
-- Imports from RFC 3852 [CMS], 12.1
DigestAlgorithmIdentifier, SignatureAlgorithmIdentifier
FROM CryptographicMessageSyntax2004
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) }
-- Imports from RFC XXX [ESSCertID], Appendix A
ESSCertIDv2
FROM ExtendedSecurityServices-2006
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) ess-2006(TBD) }
;
-- Section 3.0
id-multipleSignatures OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) TBD }
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MultipleSignatures ::= SEQUENCE {
bodyHashAlg DigestAlgorithIdentifier,
signAlg SignatureAlgorithmIdentifier,
signAttrsHash SignAttrsHash,
cert ESSCertIDv2 OPTIONAL }
SignAttrsHash ::= SEQUENCE {
algID AlgorithmIdentifier,
hash OCTET STRING }
END - of MultipleSignatures
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Appendix B. Background
This is an informative appendix that looks at the locations of hashes
CMS and possible attacks against them.
B.1. Attacks
The following types of resistance against known attacks, see
[ATTACK], is needed:
1) Collision Resistance: Find x and y where x != y and H(x) = H(y)
2) Preimage Resistance: Given y, find x where H(x) = y
3) Second Preimage Resistance: Given y, find x where H(x) = H(y)
Note: It is known that a collision resistance attack is simpler than
a second preimage resistance attack, and it is presumed that a second
preimage resistance attack is simplier than a preimage attack.
B.2. Hashes in CMS
Within a SignedInfo there are two places where hashes are applied and
hence can be attacked: the Body and the SignedAttributes. The
following outlines the entity that creates the hash, the entity that
attacks the hash, and the type of resistance required:
1) Hash of the Body (i.e., the octets comprising the value of the
encapContentInfo.eContent OCTET STRING omitting the tag and length
octets - as per 5.4 of [CMS]).
a) Alice creates the Body to be hashed:
i) Alice attacks the hash: This would require a successful
Collision Resistance attack.
ii) Mallory attacks the hash: This would require a successful
Second Preimage Reistance attack.
b) Alice hashes a body provided by Bob:
i) Alice attacks the hash: This would require a successful
Second Preimage Attack.
ii) Bob attacks the hash: This would require a successful
Collision Resistance attack. This can be upgraded to requiring
a successful Second Preimage Attack if Alice hash the ability
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to "change" the content of the body in some fashion. (One
example would be to use a keyed hash function.)
iii) Mallory attacks the hash: This would require a successful
Second Preimage Attack.
c) Alice signs using a hash value provided by Bob. (In this case
Alice is presumed to never see the body in question.)
i) Alice attacks hash: This would require a successful Preimage
Attack.
ii) Bob attacks hash: This would require a successful Collision
Resistance attack. Unlike case (b), there is nothing that
Alice can do to upgrade the attack required.
iii) Mallory attacks the hash: This would require a success
Preimage attack if the content is unavailable to Mallory and a
successful Second Preimage attack if the content is available
to Mallory.
2) Hash of SignedAttributes (i.e., the complete DER encoding of the
SignedAttrs value contained in the signedAttrs field - as per 5.4
of [CMS]).
There is a difference between hashing the body and hashing the
SignedAttrs value in that one SHOULD NOT accept a sequence of
attributes to be signed from a third party. In fact one SHOULD NOT
accept attributes to be included in the signed attributes list from a
third party. The attributes are about the signature you are applying
and not about the body. If there is meta-information that needs to
be attached to the body by a third party then they need to provide
their own signature and you need to be doing a countersignature.
(Note: the fact that the signature is to be used as a
countersignature is a piece of information that should be accepted,
but it does not directly provide an attribute that is inserted in the
attribute list.)
a) Alice attacks the hash: This requires a successful Collision
Resistance Attack.
b) Mallory attacks the hash: This requires a successful Second
Preimage Resistance attack.
c) Bob attacks the hash and provides the body hash used: This case
is analogous to the current attacks [ATTACK]. Based on prediction
of the signed attributes Alice will be using and the provided hash
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value and function. (It is expected that if Alice uses a keyed
hashing function as part of the signature this attack will be more
difficult.)
It should be noted that both of these attacks are considered to be
more difficult that the attack on the body since more structure is
designed into the data to be hashed than is frequently found in the
body and the data is shorter in length than that of the body.
The successful prediction of the Signing-Time attribute is expected
to more difficult than with certificates as the time would not
generally be rounded. Time stamp services can make this more
unpredictable by using a random delay before issuing the signature.
Allowing a third party to provide a hash value could potentially make
attack simpler when keyed hash functions are used since there is more
data than can be modified without changing the overall structure of
the Signed Attribute structure.
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Author's Addresses
Sean Turner
IECA, Inc.
Email: turners (at) ieca (dot) com
Jim Schaad
Soaring Hawk Consulting
Email: jimsch (at) exmsft (dot) com
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Turner & Schaad Expires May 16, 2008 [Page 19]
