SMIME Working Group Sean Turner, IECA
Internet Draft Jim Schaad, Soaring Hawk
Expires June 4, 2007
December 4, 2006
Multiple Signatures in S/MIME
draft-ietf-smime-multisig-00.txt
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Copyright (C) The Internet Society (2006).
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 'stronger' algorithm(s). This
document defines a signed 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.
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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.
1.1 Requirements Terminology
Though this document is not an Internet Draft, we use the convention
that the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [MUSTSHOULD].
1.2 Discussion
This draft is being discussed on the 'ietf-smime' mailing list. To
subscribe, send a message to ietf-smime-request@imc.org with the
single word subscribe in the body of the message. There is a Web
site for the mailing list at <http://www.imc.org/ietf-smime/>.
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2. Rationale
2.1 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.
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 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.
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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 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.
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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.
A 3rd type of attack is a generic downgrade attack. The premise is
to remove the 'better' signature to leave easier to attack
signature.
2.2 Attribute Design
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.
3. Multiple Signature Indication
The MultipleSignatures attribute type specifies a pointer to a
signer’s other MultipleSignatures attribute(s). For example, if a
signer applies three signatures there must be two attribute values
for MultipleSignatures 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 MultipleSignatures 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
MultipleSignatures attribute:
id-aa-multipleSignatures OBJECT IDENTIFIER ::= { iso(1) member-
body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) TBD }
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multipleSignatures attribute values have the ASN.1 type
MultipleSignature:
MultipleSignature ::= SEQUENCE {
bodyHashAlg DigestAlgorithIdentifier,
signAlg SignatureAlgorithmIdentifier,
signAttrsHash SignAttrsHash,
cert ESSCertIDv2 OPTIONAL}
SignAttrsHash ::= SEQUENCE {
algID AlgorithmIdentifier,
hash OCTET STRING }
bodyHashAlg includes the digest algorithmIdentifier for the
referenced MultipleSignatures attribute.
signAlg includes the signature algorithmIdentifier for the refrenced
MultipleSignatures attribute.
signAttrsHash has two fields:
- aldId MUST match the digest algorithm for the SignerInfo in
which this MultipleSignatures attribute value is placed.
- hash is the hash value of the signedAttrs (see section 4.3).
cert is optional. It identities the certificate used to sign the
SignerInfo that contains the other MultipleSignatures attribute(s).
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
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4. Message Generation and Processing
The following are the additional procedures for Message Generation
when using the MultipleSignatures 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.
- 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 MultipleSignatures attribute in
signedAttrs.
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4.4 Message Digest Calculation Process
4.4.1 MultipleSignatures Signed Attribute Generation
The procedure for generating the MultipleSignatures 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 MultipleSignature 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.
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 MultipleSignatures
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
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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 MultipleSignature 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 MultipleSignature
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 MultipleSignature attribute that points to
the 3rd SignerInfo present in the 1st and 2nd SignerInfos.
5.0 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.
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 is the mathematics of
the signature itself (i.e., can one actually successfully do the
computations and get the correct answer). This piece ends up with a
binary answer, either it succeeds or it fails there is no middle
ground. Not necessaryily true, what about an unevaluatable
algorithm - this is nether success nor failure. From the verifiers
perspective the answer is still fail since they can’t actuall do the
math - so I think it’s still binary.
The second is the validation of the source of the public key. For
CMS, this is generally determined by extracting the public key from
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a certificate. The certificate needs to be evaluated. This is done
by the procedures outlined in [PKIX-CERT]. 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 [PKIX-CERT], 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 part 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.
-- state if you cannot do the math?
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. Segregate SignerInfo objects according to who signed.
2. Take the best result from the items in the grouping; this is the
result for the grouping.
3. Take the worst result from all of the groups; 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 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
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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.
In Step 3:
- Very application dependent processing other options are:
o strip bad sig from the outside in - signed mail.
o strip bad sigs from the inside out - work flow.
- Modifications of the algorithm due to the presence of other types
of layers. I.e. EncryptedData/EnvelopedData or AuthenticatedData.
Implications/differences for AuthenticatedData.
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6. Security Considerations
If another entity is providing hash to be signed, then ensure it is
a “trustworthy” source.
//** Needs more work
7 References
7.1 Normative References
[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.
[ESSCertID] Schaad, J., "ESS Update: Adding CertID Algorithm
Agility", draft-ietf-smime-esscertid-01.txt, April 2006.
7.2 Non-Normative 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) }
;
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-- Section 3.0
id-multipleSignatures OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) TBD }
MultipleSignature ::= SEQUENCE {
bodyHashAlg DigestAlgorithIdentifier,
signAlg SignatureAlgorithmIdentifier,
signAttrsHash SignAttrsHash,
cert ESSCertIDv2 OPTIONAL }
SignAttrsHash ::= SEQUENCE {
algID AlgorithmIdentifier,
hash OCTET STRING }
END – of MultipleSignatures
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Editor’s Address
Sean Turner
IECA, Inc.
Email: turners (at) ieca.com
Jim Schaad
Soaring Hawk Consulting
Email: jimsch (at) exmsft.com
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