tls D. Benjamin
Internet-Draft Google LLC
Intended status: Experimental July 29, 2019
Expires: January 30, 2020
Legacy RSASSA-PKCS1-v1_5 codepoints for TLS 1.3
draft-davidben-tls13-pkcs1-00
Abstract
This document allocates code points for the use of RSASSA-PKCS1-v1_5
with client certificates in TLS 1.3.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 2
3. PKCS#1 v1.5 SignatureScheme Types . . . . . . . . . . . . . . 2
4. Security Considerations . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. Normative References . . . . . . . . . . . . . . . . . . 4
6.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
TLS 1.3 [RFC8446] removed support for RSASSA-PKCS1-v1_5 [RFC8017] in
CertificateVerify messages in favor of RSASSA-PSS. While RSASSA-PSS
is a long-established signature algorithm, some legacy hardware
cryptographic devices lack support for it. Due to performance
requirements, such devices are uncommon in TLS servers, but are
sometimes used by TLS clients for client certificates. Moreover, TLS
negotiates the protocol version before client certificates, so this
limitation can further impact adjacent connections that do not use
affected keys.
This document allocates code points to use these legacy keys with
client certificates in TLS 1.3.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. PKCS#1 v1.5 SignatureScheme Types
The following SignatureScheme values are defined for use with TLS
1.3.
enum {
rsa_pkcs1_sha256_legacy(TBD1),
rsa_pkcs1_sha384_legacy(TBD2),
rsa_pkcs1_sha512_legacy(TBD3),
} SignatureScheme;
The above code points indicate a signature algorithm using RSASSA-
PKCS1-v1_5 [RFC8017] with the corresponding hash algorithm as defined
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in [SHS]. They are only defined for signatures in the client
CertificateVerify message and are not defined for use in other
contexts. In particular, servers intending to advertise support for
RSASSA-PKCS1-v1_5 signatures in the certificates themselves should
use the rsa_pkcs1_* constants defined in [RFC8446].
Clients MUST NOT advertise these values in the "signature_algorithms"
extension of the ClientHello. They MUST NOT accept these values in
the server CertificateVerify message.
Servers that wish to support clients authenticating with legacy
RSASSA-PKCS1-v1_5-only keys MAY send these values in the
"signature_algorithms" extension of the CertificateRequest message
and accept them in the client CertificateVerify message. Clients
with such legacy keys MAY negotiate the use of these signature
algorithms if offered by the server. Clients SHOULD NOT negotiate
them with keys that support RSASSA-PSS.
4. Security Considerations
Prior to this document, legacy RSA keys would prevent client
certificate deployments from adopting TLS 1.3. The new code points
allow such deployments to upgrade without replacing the keys. TLS
1.3 fixes a privacy flaw [PRIVACY] with client certificates, so
upgrading is a particular benefit to these deployments.
Additionally, TLS negotiates protocol versions before client
certificates. When sending a ClientHello, a TLS-1.3-capable client
cannot determine if the server will request a legacy key. It may
then offer TLS 1.3, to upgrade connections to other servers. A TLS-
1.3-capable server that requests client certificates cannot then
distinguish such a client from one with modern keys. It may then
negotiate TLS 1.3 and send a CertificateRequest. The connection
would then fail due to the legacy key, when it previously succeeded
at TLS 1.2.
To recover from this failure, one side must globally disable TLS 1.3
or the client must implement an external fallback. Disabling TLS 1.3
impacts connections that would otherwise be unaffected by this issue,
while external fallbacks break TLS's security analysis and may
introduce vulnerabilities [POODLE]. The new code points reduce the
pressure on implementations to select one of these mitigations.
However, the new code points also reduce the pressure on
implementations to migrate to RSASSA-PSS. The above considerations
do not apply to server keys, so these new code points are forbidden
for use with server certificates. RSASSA-PSS continues to be
required for TLS 1.3 servers using RSA keys.
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Finally, when implemented incorrectly, RSASSA-PKCS1-v1_5 admits
signature forgeries [MFSA201473]. Implementations producing or
verifying signatures with these algorithms MUST implement RSASSA-
PKCS1-v1_5 as specified in section 8.2 of [RFC8017]. In particular,
clients MUST include the mandatory NULL parameter in the DigestInfo
structure and produce a valid DER [X690] encoding. Servers MUST
reject signatures which do not meet these requirements.
5. IANA Considerations
IANA is requested to create the following entries in the TLS
SignatureScheme registry, defined in [RFC8446]. The "Recommended"
column should be set to "N", and the "Reference" column should be set
to this document.
+-------+-------------------------+
| Value | Description |
+-------+-------------------------+
| TBD1 | rsa_pkcs1_sha256_legacy |
| | |
| TBD2 | rsa_pkcs1_sha384_legacy |
| | |
| TBD3 | rsa_pkcs1_sha512_legacy |
+-------+-------------------------+
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<https://www.rfc-editor.org/info/rfc8017>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
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[SHS] Dang, Q., "Secure Hash Standard", National Institute of
Standards and Technology report,
DOI 10.6028/nist.fips.180-4, July 2015.
[X690] ITU-T, "Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ISO/IEC 8825-1:2002, 2002.
6.2. Informative References
[MFSA201473]
Delignat-Lavaud, A., "RSA Signature Forgery in NSS",
September 2014, <https://www.mozilla.org/en-
US/security/advisories/mfsa2014-73/>.
[POODLE] Moeller, B., "This POODLE bites: exploiting the SSL 3.0
fallback", October 2014,
<https://security.googleblog.com/2014/10/
this-poodle-bites-exploiting-ssl-30.html>.
[PRIVACY] Wachs, M., Scheitle, Q., and G. Carle, "Push away your
privacy: Precise user tracking based on TLS client
certificate authentication", 2017 Network Traffic
Measurement and Analysis Conference (TMA),
DOI 10.23919/tma.2017.8002897, June 2017.
Author's Address
David Benjamin
Google LLC
Email: davidben@google.com
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