Hi,
The draft has a number of nits and errors. Among others:
The related work section mentions KEYLESS and subcert being complementary that
is KEYLESS can perform the operations associated to the DC and/or those
associated to the cert key. I do not think that is correct. KEYLESS does not
support TLS 1.3 while DC only works with TLS 1.3. The LURK extension for TLS
1.3 [draft-mglt-lurk-tls13] should be mentioned instead. As LURK was mentioned
during the adoption period and until version 05 that should not cause any
issues.
Technologies only available for TLS 1.2 may be mentioned in the related work
section. [draft-mglt-lurk-tls12] should be mentioned similarly to KEYLESS as
it addresses the security concerns of KEYLESS.
There are other places where the extensions is mentioned together with TLS 1.2
that needs to be updated.
I also think that test vectors would be good as well as a link to a formal
verification publication (if available).
Please see all my comments inline, I hope they help.
Yours,
Daniel
----
Delegated Credentials for TLS
draft-ietf-tls-subcerts-09
[...]
1. Introduction
Typically, a TLS server uses a certificate provided by some entity
other than the operator of the server (a "Certification Authority" or
CA) [RFC8446] [RFC5280]. This organizational separation makes the
TLS server operator dependent on the CA for some aspects of its
operations, for example:
* Whenever the server operator wants to deploy a new certificate, it
has to interact with the CA.
* The server operator can only use TLS signature schemes for which
the CA will issue credentials.
These dependencies cause problems in practice. Server operators
often deploy TLS termination services in locations such as remote
data centers or Content Delivery Networks (CDNs) where it may be
difficult to detect key compromises. Short-lived certificates may be
used to limit the exposure of keys in these cases.
<mglt>
I believe it would be clearer to
summarize the problem and link it to the
use case. I would propose something
around:
These dependencies cause problems in
practice, the management of key exposure
necessarily requires an interaction with
the CA.
Typically server operators....
</mglt>
However, short-lived certificates need to be renewed more frequently
than long-lived certificates. If an external CA is unable to issue a
certificate in time to replace a deployed certificate, the server
would no longer be able to present a valid certificate to clients.
With short-lived certificates, there is a smaller window of time to
renew a certificates and therefore a higher risk that an outage at a
CA will negatively affect the uptime of the service.
To reduce the dependency on external CAs, this document proposes a
limited delegation mechanism that allows a TLS peer to issue its own
credentials within the scope of a certificate issued by an external
CA. These credentials only enable the recipient of the delegation to
speak for names that the CA has authorized. For clarity, we will
refer to the certificate issued by the CA as a "certificate", or
"delegation certificate", and the one issued by the operator as a
"delegated credential" or "DC".
<mglt>
>From the text it is unclear why the
signature scheme appears to be a
constraint as well how it does not opens
to some sort of downgrade attacks if
left to the operator.
</mglt>
3. Solution Overview
[...]
3.1. Rationale
Delegated credentials present a better alternative than other
delegation mechanisms like proxy certificates [RFC3820] for several
reasons:
* There is no change needed to certificate validation at the PKI
layer.
* X.509 semantics are very rich. This can cause unintended
consequences if a service owner creates a proxy certificate where
the properties differ from the leaf certificate. For this reason,
delegated credentials have very restricted semantics that should
not conflict with X.509 semantics.
* Proxy certificates rely on the certificate path building process
to establish a binding between the proxy certificate and the
server certificate. Since the certificate path building process
is not cryptographically protected, it is possible that a proxy
certificate could be bound to another certificate with the same
public key, with different X.509 parameters. Delegated
credentials, which rely on a cryptographic binding between the
entire certificate and the delegated credential, cannot.
* Each delegated credential is bound to a specific signature
algorithm that may be used to sign the TLS handshake ([RFC8446]
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Internet-Draft Delegated Credentials for TLS June 2020
section 4.2.3). This prevents them from being used with other,
perhaps unintended signature algorithms.
<mglt>
It is not clear to me why there is a
"may be used". I suppose it concerns the
use of the DC not the algorithm but that
was confusing.
I also believe that the specific
signature algorithm to sign the
delegated credential could be part of
the rational.
</mglt>
3.2. Related Work
Many of the use cases for delegated credentials can also be addressed
using purely server-side mechanisms that do not require changes to
client behavior (e.g., a PKCS#11 interface or a remote signing
mechanism [KEYLESS]). These mechanisms, however, incur per-
transaction latency, since the front-end server has to interact with
a back-end server that holds a private key. The mechanism proposed
in this document allows the delegation to be done off-line, with no
per-transaction latency. The figure below compares the message flows
for these two mechanisms with TLS 1.3 [RFC8446].
Remote key signing:
Client Front-End Back-End
|----ClientHello--->| |
|<---ServerHello----| |
|<---Certificate----| |
| |<---remote sign---->|
|<---CertVerify-----| |
| ... | |
Delegated credentials:
Client Front-End Back-End
| |<--DC distribution->|
|----ClientHello--->| |
|<---ServerHello----| |
|<---Certificate----| |
|<---CertVerify-----| |
| ... | |
These two mechanisms can be complementary. A server could use
credentials for clients that support them, while using [KEYLESS] to
support legacy clients.
<mglt>
I believe that this sentence does not
show any complementary as subcert and
KEYLESS are targeting different version
of TLS, so they can hardly be
complementary. However (and luckily)
LURK provides an extension for TLS 1.3
[draft-mglt-lurk-tls13] which enable a
complementary use of these mechanisms
these mechanisms. I believe that would
be good to indicate the reason they
complement each other which is that LURK
protects the credentials for its
operations. These operations could be
performed in the scope of subcert or TLS
1.3.
Note also that in a related section it
also worth mentioning that credentials
may be managed in different ways and
KEYLESS represents an valuable way to
protect and manage these credentials in
TLS 1.2. However, KEYLESS is known to
presents some vulnerabilities (PFS,
signing oracle) so the protection
remains limited while the LURK extension
for TLS 1.2 [draft-mglt-lurk-tls12]
addressed these issues and as a result
should provide a better protection.
</mglt>
The private key for a delegated credential
can be used in place of a certificate private key, so it is important
that the Front-End and Back-End are parties that have a trusted
relationship.
Use of short-lived certificates with automated certificate issuance,
e.g., with Automated Certificate Managment Environment (ACME)
<mglt>
Management
</mglt>
[RFC8555], reduces the risk of key compromise, but has several
limitations. Specifically, it introduces an operationally-critical
dependency on an external party. It also limits the types of
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Internet-Draft Delegated Credentials for TLS June 2020
algorithms supported for TLS authentication to those the CA is
willing to issue a certificate for. Nonetheless, existing automated
issuance APIs like ACME may be useful for provisioning delegated
credentials.
4. Delegated Credentials
While X.509 forbids end-entity certificates from being used as
issuers for other certificates, it is valid to use them to issue
other signed objects as long as the certificate contains the
digitalSignature KeyUsage ([RFC5280] section 4.2.1.3). We define a
new signed object format that would encode only the semantics that
are needed for this application. The credential has the following
structure:
struct {
uint32 valid_time;
SignatureScheme expected_cert_verify_algorithm;
opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
} Credential;
valid_time: Time in seconds relative to the beginning of the
delegation certificate's notBefore value after which the delegated
credential is no longer valid. This MUST NOT exceed 7 days.
<mglt>
I believe the behavior of the "verifying
peer" should also be specified maybe
with a reference.
</mglt>
expected_cert_verify_algorithm: The signature algorithm of the
credential key pair, where the type SignatureScheme is as defined
in [RFC8446]. This is expected to be the same as
CertificateVerify.algorithm sent by the server. Only signature
algorithms allowed for use in CertificateVerify messages are
allowed. When using RSA, the public key MUST NOT use the
rsaEncryption OID, as a result, the following algorithms are not
allowed for use with delegated credentials: rsa_pss_rsae_sha256,
rsa_pss_rsae_sha384, rsa_pss_rsae_sha512.
<mglt>
It is unclear whether the
expected_cert_verify_algorithm and
CertificateVerify.algorithm needs to be
checked and what needs to be done in
case of mismatch (with the RSA caveat).
I believe that should be clarified.
</mglt>
ASN1_subjectPublicKeyInfo: The credential's public key, a DER-
encoded [X.690] SubjectPublicKeyInfo as defined in [RFC5280].
The delegated credential has the following structure:
struct {
Credential cred;
SignatureScheme algorithm;
opaque signature<0..2^16-1>;
} DelegatedCredential;
algorithm: The signature algorithm used to verify
DelegatedCredential.signature.
<mglt>
I am wondering if any checks should be
performed with the
CertificateVerify.algorithm or if any
algorithm would be acceptable. Unless I
am missing something it seems a weak
algorithm can be used - assuming the
registry contains such weak algorithms.
</mglt>
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Internet-Draft Delegated Credentials for TLS June 2020
signature: The delegation, a signature that binds the credential to
the end-entity certificate's public key as specified below. The
signature scheme is specified by DelegatedCredential.algorithm.
[...]
4.1.1. Server Authentication
A client which supports this specification SHALL send a
"delegated_credential" extension in its ClientHello. The body of the
extension consists of a SignatureSchemeList:
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struct {
SignatureScheme supported_signature_algorithm<2..2^16-2>;
} SignatureSchemeList;
If the client receives a delegated credential without indicating
support, then the client MUST abort with an "unexpected_message"
alert.
If the extension is present, the server MAY send a delegated
credential; if the extension is not present, the server MUST NOT send
a delegated credential. The server MUST ignore the extension unless
TLS 1.3 or a later version is negotiated.
The server MUST send the delegated credential as an extension in the
CertificateEntry of its end-entity certificate; the client SHOULD
ignore delegated credentials sent as extensions to any other
certificate.
The expected_cert_verify_algorithm field MUST be of a type advertised
by the client in the SignatureSchemeList and is considered invalid
otherwise. Clients that receive invalid delegated credentials MUST
terminate the connection with an "illegal_parameter" alert.
<mglt>
I am wondering what would prevent any
downgrade attacks if the
SignatureSchemeList and
signature_algorithms have two different
sets of lists. My current understanding
is that these extensions are handled
independently, but I might be missing
something.
I am wondering if that would be
appropriated to specify the signature of
the CertificateVerify depending on the
presence of the delegated credential - I
mean the key used to generate it.
</mglt>
[...]
4.1.3. Validating a Delegated Credential
On receiving a delegated credential and a certificate chain, the peer
validates the certificate chain and matches the end-entity
certificate to the peer's expected identity. It also takes the
following steps:
1. Verify that the current time is within the validity interval of
the credential. This is done by asserting that the current time
is no more than the delegation certificate's notBefore value plus
DelegatedCredential.cred.valid_time.
2. Verify that the credential's remaining validity time is no more
than the maximum validity period. This is done by asserting that
the current time is no more than the delegation certificate's
notBefore value plus DelegatedCredential.cred.valid_time plus the
maximum validity period.
3. Verify that expected_cert_verify_algorithm matches the scheme
indicated in the peer's CertificateVerify message and that the
algorithm is allowed for use with delegated credentials.
<mglt>
I am wondering if a reference to specify
what allowed would not be needed unless
it means advertised in the extension.
</mglt>
4. Verify that the end-entity certificate satisfies the conditions
in Section 4.2.
5. Use the public key in the peer's end-entity certificate to verify
the signature of the credential using the algorithm indicated by
DelegatedCredential.algorithm.
If one or more of these checks fail, then the delegated credential is
deemed invalid. Clients and servers that receive invalid delegated
credentials MUST terminate the connection with an "illegal_parameter"
alert. If successful, the participant receiving the Certificate
message uses the public key in the credential to verify the signature
in the peer's CertificateVerify message.
[...]
7. Security Considerations
[...]
7.6. The Impact of Signature Forgery Attacks
When TLS 1.2 servers support RSA key exchange, they may be vulnerable
to attacks that allow forging an RSA signature over an arbitrary
message [BLEI]. TLS 1.2 [RFC5246] (Section 7.4.7.1.) describes a
mitigation strategy requiring careful implementation of timing
resistant countermeasures for preventing these attacks. Experience
shows that in practice, server implementations may fail to fully stop
these attacks due to the complexity of this mitigation [ROBOT]. For
TLS 1.2 servers that support RSA key exchange using a DC-enabled end-
entity certificate, a hypothetical signature forgery attack would
allow forging a signature over a delegated credential. The forged
credential could then be used by the attacker as the equivalent of a
man-in-the-middle certificate, valid for 7 days.
<mglt>
I do not see the relevance to TLS 1.3.
</mglt>
Server operators should therefore minimize the risk of using DC-
enabled end-entity certificates where a signature forgery oracle may
be present. If possible, server operators may choose to use DC-
enabled certificates only for signing credentials, and not for
serving non-DC TLS traffic. Furthermore, server operators may use
elliptic curve certificates for DC-enabled traffic, while using RSA
certificates without the DelegationUsage certificate extension for
non-DC traffic; this completely prevents such attacks.
Note that if a signature can be forged over an arbitrary credential,
the attacker can choose any value for the valid_time field. Repeated
signature forgeries therefore allow the attacker to create multiple
delegated credentials that can cover the entire validity period of
the certificate. Temporary exposure of the key or a signing oracle
may allow the attacker to impersonate a server for the lifetime of
the certificate.
________________________________
From: TLS <[email protected]> on behalf of Salz, Rich
<[email protected]>
Sent: Monday, June 29, 2020 12:00 PM
To: Joseph Salowey <[email protected]>; <[email protected]> <[email protected]>
Subject: Re: [TLS] 2nd WGLC for Delegated Credentials for TLS
I’d still like to see test vectors.
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