Difference between revisions of "RFC6091"
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Internet Engineering Steering Group (IESG). Not all documents | Internet Engineering Steering Group (IESG). Not all documents | ||
approved by the IESG are a candidate for any level of Internet | approved by the IESG are a candidate for any level of Internet | ||
− | Standard; see Section 2 of RFC 5741. | + | Standard; see Section 2 of [[RFC5741|RFC 5741]]. |
Information about the current status of this document, any errata, | Information about the current status of this document, any errata, | ||
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document authors. All rights reserved. | document authors. All rights reserved. | ||
− | This document is subject to BCP 78 and the IETF Trust's Legal | + | This document is subject to [[BCP78|BCP 78]] and the IETF Trust's Legal |
Provisions Relating to IETF Documents | Provisions Relating to IETF Documents | ||
(http://trustee.ietf.org/license-info) in effect on the date of | (http://trustee.ietf.org/license-info) in effect on the date of | ||
Line 49: | Line 49: | ||
The IETF has two sets of standards for public key certificates: one | The IETF has two sets of standards for public key certificates: one | ||
− | set for the use of X.509 certificates | + | set for the use of X.509 certificates [[RFC5280]], and one for OpenPGP |
− | certificates | + | certificates [[RFC4880]]. At the time of this writing, TLS [[RFC5246]] |
standards are defined to use X.509 certificates. This document | standards are defined to use X.509 certificates. This document | ||
specifies a way to negotiate the use of OpenPGP certificates for a | specifies a way to negotiate the use of OpenPGP certificates for a | ||
Line 58: | Line 58: | ||
implementations that make use of X.509 certificates are not affected. | implementations that make use of X.509 certificates are not affected. | ||
− | These extensions are not backward-compatible with | + | These extensions are not backward-compatible with [[RFC5081]], and the |
major differences are summarized in Appendix A. Although the OpenPGP | major differences are summarized in Appendix A. Although the OpenPGP | ||
CertificateType value is being reused by this memo with the same | CertificateType value is being reused by this memo with the same | ||
− | number as that specified in | + | number as that specified in [[RFC5081]] but with different semantics, |
we believe that this causes no interoperability issues because the | we believe that this causes no interoperability issues because the | ||
latter was not widely deployed. | latter was not widely deployed. | ||
Line 68: | Line 68: | ||
The term "OpenPGP key" is used in this document as in the OpenPGP | The term "OpenPGP key" is used in this document as in the OpenPGP | ||
− | specification | + | specification [[RFC4880]]. We use the term "OpenPGP certificate" to |
refer to OpenPGP keys that are enabled for authentication. | refer to OpenPGP keys that are enabled for authentication. | ||
This document uses the same notation and terminology used in the TLS | This document uses the same notation and terminology used in the TLS | ||
− | Protocol specification | + | Protocol specification [[RFC5246]]. |
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", | The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", | ||
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this | "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this | ||
− | document are to be interpreted as described in | + | document are to be interpreted as described in [[RFC2119]]. |
== Changes to the Handshake Message Contents == | == Changes to the Handshake Message Contents == | ||
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as the extension number for the extensions in both the client hello | as the extension number for the extensions in both the client hello | ||
message and the server hello message. The hello extension mechanism | message and the server hello message. The hello extension mechanism | ||
− | is described in | + | is described in [[RFC5246]]. |
This extension carries a list of supported certificate types the | This extension carries a list of supported certificate types the | ||
Line 102: | Line 102: | ||
this document. Reusing a single specification for both client and | this document. Reusing a single specification for both client and | ||
server is common in other specifications, such as the TLS protocol | server is common in other specifications, such as the TLS protocol | ||
− | itself | + | itself [[RFC5246]]. |
enum { client, server } ClientOrServerExtension; | enum { client, server } ClientOrServerExtension; | ||
Line 161: | Line 161: | ||
An OpenPGP certificate appearing in the certificate message is sent | An OpenPGP certificate appearing in the certificate message is sent | ||
using the binary OpenPGP format. The certificate MUST contain all | using the binary OpenPGP format. The certificate MUST contain all | ||
− | the elements required by Section 11.1 of | + | the elements required by Section 11.1 of [[RFC4880]]. |
OpenPGP certificates to be transferred are placed in the Certificate | OpenPGP certificates to be transferred are placed in the Certificate | ||
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"certificate_unobtainable" fatal alert if the certificate with the | "certificate_unobtainable" fatal alert if the certificate with the | ||
given fingerprint cannot be found. The "certificate_unobtainable" | given fingerprint cannot be found. The "certificate_unobtainable" | ||
− | fatal alert is defined in Section 5 of | + | fatal alert is defined in Section 5 of [[RFC6066]]. |
Implementations of this protocol MUST ensure that the sizes of key | Implementations of this protocol MUST ensure that the sizes of key | ||
IDs and fingerprints in the OpenPGPSubKeyCert and | IDs and fingerprints in the OpenPGPSubKeyCert and | ||
− | OpenPGPSubKeyFingerprint structures comply with | + | OpenPGPSubKeyFingerprint structures comply with [[RFC4880]]. Moreover, |
it is RECOMMENDED that the keys to be used with this protocol have | it is RECOMMENDED that the keys to be used with this protocol have | ||
the authentication flag (0x20) set. | the authentication flag (0x20) set. | ||
The process of fingerprint generation is described in Section 12.2 of | The process of fingerprint generation is described in Section 12.2 of | ||
− | + | [[RFC4880]]. | |
The enumerated types "cert_fingerprint" and "cert" of | The enumerated types "cert_fingerprint" and "cert" of | ||
− | OpenPGPCertDescriptorType that were defined in | + | OpenPGPCertDescriptorType that were defined in [[RFC5081]] are not used |
and are marked as obsolete by this document. The "empty_cert" type | and are marked as obsolete by this document. The "empty_cert" type | ||
has replaced "cert" and is a backward-compatible way to specify an | has replaced "cert" and is a backward-compatible way to specify an | ||
Line 256: | Line 256: | ||
== Security Considerations == | == Security Considerations == | ||
− | All security considerations discussed in | + | All security considerations discussed in [[RFC5246]], [[RFC6066]], and |
− | + | [[RFC4880]] apply to this document. Considerations about the use of | |
the web of trust or identity and certificate verification procedures | the web of trust or identity and certificate verification procedures | ||
are outside the scope of this document. These are considered issues | are outside the scope of this document. These are considered issues | ||
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The protocol for certificate type negotiation is identical in | The protocol for certificate type negotiation is identical in | ||
operation to cipher suite negotiation as described in the TLS | operation to cipher suite negotiation as described in the TLS | ||
− | specification | + | specification [[RFC5246]], with the addition of default values when the |
extension is omitted. Since those omissions have a unique meaning | extension is omitted. Since those omissions have a unique meaning | ||
and the same protection is applied to the values as with cipher | and the same protection is applied to the values as with cipher | ||
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When using OpenPGP fingerprints instead of the full certificates, the | When using OpenPGP fingerprints instead of the full certificates, the | ||
− | discussion in Section 5 of | + | discussion in Section 5 of [[RFC6066]] for "Client Certificate URLs" |
applies, especially when external servers are used to retrieve keys. | applies, especially when external servers are used to retrieve keys. | ||
However, a major difference is that although the | However, a major difference is that although the | ||
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sent, are always identified by their fingerprint, which serves as a | sent, are always identified by their fingerprint, which serves as a | ||
cryptographic hash of the certificate (see Section 12.2 of | cryptographic hash of the certificate (see Section 12.2 of | ||
− | + | [[RFC4880]]). | |
The information that is available to participating parties and | The information that is available to participating parties and | ||
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This document uses a registry and the "cert_type" extension | This document uses a registry and the "cert_type" extension | ||
− | originally defined in | + | originally defined in [[RFC5081]]. Existing IANA references have been |
updated to point to this document. | updated to point to this document. | ||
In addition, the "TLS Certificate Types" registry established by | In addition, the "TLS Certificate Types" registry established by | ||
− | + | [[RFC5081]] has been updated in the following ways: | |
1. Values 0 (X.509) and 1 (OpenPGP) are defined in this document. | 1. Values 0 (X.509) and 1 (OpenPGP) are defined in this document. | ||
2. Values from 2 through 223 decimal inclusive are assigned via "RFC | 2. Values from 2 through 223 decimal inclusive are assigned via "RFC | ||
− | Required" | + | Required" [[RFC5226]]. |
3. Values from 224 decimal through 255 decimal inclusive are | 3. Values from 224 decimal through 255 decimal inclusive are | ||
− | reserved for Private Use | + | reserved for Private Use [[RFC5226]]. |
== Acknowledgements == | == Acknowledgements == | ||
Line 312: | Line 312: | ||
=== Normative References === | === Normative References === | ||
− | + | [[RFC2119]] Bradner, S., "Key words for use in RFCs to Indicate | |
− | Requirement Levels", BCP 14, RFC 2119, March 1997. | + | Requirement Levels", [[BCP14|BCP 14]], [[RFC2119|RFC 2119]], March 1997. |
− | + | [[RFC4880]] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. | |
− | Thayer, "OpenPGP Message Format", RFC 4880, | + | Thayer, "OpenPGP Message Format", [[RFC4880|RFC 4880]], |
November 2007. | November 2007. | ||
− | + | [[RFC5226]] Narten, T. and H. Alvestrand, "Guidelines for Writing an | |
− | IANA Considerations Section in RFCs", BCP 26, RFC 5226, | + | IANA Considerations Section in RFCs", [[BCP26|BCP 26]], [[RFC5226|RFC 5226]], |
May 2008. | May 2008. | ||
− | + | [[RFC5246]] Dierks, T. and E. Rescorla, "The Transport Layer Security | |
− | (TLS) Protocol Version 1.2", RFC 5246, August 2008. | + | (TLS) Protocol Version 1.2", [[RFC5246|RFC 5246]], August 2008. |
− | + | [[RFC6066]] Eastlake 3rd, D., "Transport Layer Security (TLS) | |
− | Extensions: Extension Definitions", RFC 6066, | + | Extensions: Extension Definitions", [[RFC6066|RFC 6066]], |
January 2011. | January 2011. | ||
=== Informative References === | === Informative References === | ||
− | + | [[RFC5081]] Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport | |
− | Layer Security (TLS) Authentication", RFC 5081, | + | Layer Security (TLS) Authentication", [[RFC5081|RFC 5081]], |
November 2007. | November 2007. | ||
− | + | [[RFC5280]] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., | |
Housley, R., and W. Polk, "Internet X.509 Public Key | Housley, R., and W. Polk, "Internet X.509 Public Key | ||
Infrastructure Certificate and Certificate Revocation | Infrastructure Certificate and Certificate Revocation | ||
− | List (CRL) Profile", RFC 5280, May 2008. | + | List (CRL) Profile", [[RFC5280|RFC 5280]], May 2008. |
− | Appendix A. Changes from RFC 5081 | + | Appendix A. Changes from [[RFC5081|RFC 5081]] |
This document incorporates a major change in the "Server Certificate" | This document incorporates a major change in the "Server Certificate" | ||
and "Client Certificate" TLS messages that will make implementations | and "Client Certificate" TLS messages that will make implementations | ||
− | following this protocol incompatible with those following | + | following this protocol incompatible with those following [[RFC5081]]. |
This change requires the subkey IDs used for TLS authentication to be | This change requires the subkey IDs used for TLS authentication to be | ||
marked explicitly in the handshake procedure. This was decided in | marked explicitly in the handshake procedure. This was decided in | ||
Line 351: | Line 351: | ||
that can be used with this protocol. | that can be used with this protocol. | ||
− | + | [[RFC5081]] required that an OpenPGP key or subkey be marked with the | |
authentication flag; thus, authentication would have failed if this | authentication flag; thus, authentication would have failed if this | ||
flag was not set or if this flag was set in more than one subkey. | flag was not set or if this flag was set in more than one subkey. |
Latest revision as of 03:08, 22 October 2020
Internet Engineering Task Force (IETF) N. Mavrogiannopoulos Request for Comments: 6091 KUL Obsoletes: 5081 D. Gillmor Category: Informational Independent ISSN: 2070-1721 February 2011
Using OpenPGP Keys for Transport Layer Security (TLS) Authentication
Abstract
This memo defines Transport Layer Security (TLS) extensions and associated semantics that allow clients and servers to negotiate the use of OpenPGP certificates for a TLS session, and specifies how to transport OpenPGP certificates via TLS. It also defines the registry for non-X.509 certificate types.
Status of This Memo
This document is not an Internet Standards Track specification; it is published for informational purposes.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6091.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Contents
Introduction
The IETF has two sets of standards for public key certificates: one set for the use of X.509 certificates RFC5280, and one for OpenPGP certificates RFC4880. At the time of this writing, TLS RFC5246 standards are defined to use X.509 certificates. This document specifies a way to negotiate the use of OpenPGP certificates for a TLS session, and specifies how to transport OpenPGP certificates via TLS. The proposed extensions are backward-compatible with the current TLS specification, so that existing client and server implementations that make use of X.509 certificates are not affected.
These extensions are not backward-compatible with RFC5081, and the major differences are summarized in Appendix A. Although the OpenPGP CertificateType value is being reused by this memo with the same number as that specified in RFC5081 but with different semantics, we believe that this causes no interoperability issues because the latter was not widely deployed.
Terminology
The term "OpenPGP key" is used in this document as in the OpenPGP specification RFC4880. We use the term "OpenPGP certificate" to refer to OpenPGP keys that are enabled for authentication.
This document uses the same notation and terminology used in the TLS Protocol specification RFC5246.
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.
Changes to the Handshake Message Contents
This section describes the changes to the TLS handshake message contents when OpenPGP certificates are to be used for authentication.
Client Hello
In order to indicate the support of multiple certificate types, clients MUST include an extension of type "cert_type" to the extended client hello message. The "cert_type" TLS extension is assigned the value of 9 from the TLS ExtensionType registry. This value is used as the extension number for the extensions in both the client hello message and the server hello message. The hello extension mechanism is described in RFC5246.
This extension carries a list of supported certificate types the client can use, sorted by client preference. This extension MUST be omitted if the client only supports X.509 certificates. The "extension_data" field of this extension contains a CertificateTypeExtension structure. Note that the CertificateTypeExtension structure is being used both by the client and the server, even though the structure is only specified once in this document. Reusing a single specification for both client and server is common in other specifications, such as the TLS protocol itself RFC5246.
enum { client, server } ClientOrServerExtension;
enum { X.509(0), OpenPGP(1), (255) } CertificateType;
struct { select(ClientOrServerExtension) { case client: CertificateType certificate_types<1..2^8-1>; case server: CertificateType certificate_type; } } CertificateTypeExtension;
No new cipher suites are required to use OpenPGP certificates. All existing cipher suites that support a key exchange method compatible with the key in the certificate can be used in combination with OpenPGP certificates.
Server Hello
If the server receives a client hello that contains the "cert_type" extension and chooses a cipher suite that requires a certificate, then two outcomes are possible. The server MUST either select a certificate type from the certificate_types field in the extended client hello or terminate the session with a fatal alert of type "unsupported_certificate".
The certificate type selected by the server is encoded in a CertificateTypeExtension structure, which is included in the extended server hello message using an extension of type "cert_type". Servers that only support X.509 certificates MAY omit including the "cert_type" extension in the extended server hello.
Server Certificate
The contents of the certificate message sent from server to client and vice versa are determined by the negotiated certificate type and the selected cipher suite's key exchange algorithm.
If the OpenPGP certificate type is negotiated, then it is required to present an OpenPGP certificate in the certificate message. The certificate must contain a public key that matches the selected key exchange algorithm, as shown below.
Key Exchange Algorithm OpenPGP Certificate Type
RSA RSA public key that can be used for encryption.
DHE_DSS DSA public key that can be used for authentication.
DHE_RSA RSA public key that can be used for authentication.
An OpenPGP certificate appearing in the certificate message is sent using the binary OpenPGP format. The certificate MUST contain all the elements required by Section 11.1 of RFC4880.
OpenPGP certificates to be transferred are placed in the Certificate structure and tagged with the OpenPGPCertDescriptorType "subkey_cert". Since those certificates might contain several subkeys, the subkey ID to be used for this session is explicitly
specified in the OpenPGPKeyID field. The key ID must be specified even if the certificate has only a primary key. The peer, upon receiving this type, has to either use the specified subkey or terminate the session with a fatal alert of "unsupported_certificate".
The option is also available to send an OpenPGP fingerprint, instead of sending the entire certificate, by using the "subkey_cert_fingerprint" tag. This tag uses the OpenPGPSubKeyFingerprint structure and requires the primary key fingerprint to be specified, as well as the subkey ID to be used for this session. The peer shall respond with a "certificate_unobtainable" fatal alert if the certificate with the given fingerprint cannot be found. The "certificate_unobtainable" fatal alert is defined in Section 5 of RFC6066.
Implementations of this protocol MUST ensure that the sizes of key IDs and fingerprints in the OpenPGPSubKeyCert and OpenPGPSubKeyFingerprint structures comply with RFC4880. Moreover, it is RECOMMENDED that the keys to be used with this protocol have the authentication flag (0x20) set.
The process of fingerprint generation is described in Section 12.2 of RFC4880.
The enumerated types "cert_fingerprint" and "cert" of OpenPGPCertDescriptorType that were defined in RFC5081 are not used and are marked as obsolete by this document. The "empty_cert" type has replaced "cert" and is a backward-compatible way to specify an empty certificate; "cert_fingerprint" MUST NOT be used with this updated specification, and hence that old alternative has been removed from the Certificate struct description.
enum { empty_cert(1), subkey_cert(2), subkey_cert_fingerprint(3), (255) } OpenPGPCertDescriptorType;
uint24 OpenPGPEmptyCert = 0;
struct { opaque OpenPGPKeyID<8..255>; opaque OpenPGPCert<0..2^24-1>; } OpenPGPSubKeyCert;
struct { opaque OpenPGPKeyID<8..255>; opaque OpenPGPCertFingerprint<20..255>; } OpenPGPSubKeyFingerprint;
struct { OpenPGPCertDescriptorType descriptorType; select (descriptorType) { case empty_cert: OpenPGPEmptyCert; case subkey_cert: OpenPGPSubKeyCert; case subkey_cert_fingerprint: OpenPGPSubKeyCertFingerprint; } } Certificate;
Certificate Request
The semantics of this message remain the same as in the TLS specification. However, if this message is sent, and the negotiated certificate type is OpenPGP, the "certificate_authorities" list MUST be empty.
Client Certificate
This message is only sent in response to the certificate request message. The client certificate message is sent using the same formatting as the server certificate message, and it is also required to present a certificate that matches the negotiated certificate type. If OpenPGP certificates have been selected and no certificate is available from the client, then a certificate structure of type "empty_cert" that contains an OpenPGPEmptyCert value MUST be sent. The server SHOULD respond with a "handshake_failure" fatal alert if client authentication is required.
Other Handshake Messages
All the other handshake messages are identical to the TLS specification.
Security Considerations
All security considerations discussed in RFC5246, RFC6066, and RFC4880 apply to this document. Considerations about the use of the web of trust or identity and certificate verification procedures are outside the scope of this document. These are considered issues to be handled by the application layer protocols.
The protocol for certificate type negotiation is identical in operation to cipher suite negotiation as described in the TLS specification RFC5246, with the addition of default values when the extension is omitted. Since those omissions have a unique meaning and the same protection is applied to the values as with cipher suites, it is believed that the security properties of this negotiation are the same as with cipher suite negotiation.
When using OpenPGP fingerprints instead of the full certificates, the discussion in Section 5 of RFC6066 for "Client Certificate URLs" applies, especially when external servers are used to retrieve keys. However, a major difference is that although the "client_certificate_url" extension allows identifying certificates without including the certificate hashes, this is not possible in the protocol proposed here. In this protocol, the certificates, when not sent, are always identified by their fingerprint, which serves as a cryptographic hash of the certificate (see Section 12.2 of RFC4880).
The information that is available to participating parties and eavesdroppers (when confidentiality is not available through a previous handshake) is the number and the types of certificates they hold, plus the contents of the certificates.
IANA Considerations
This document uses a registry and the "cert_type" extension originally defined in RFC5081. Existing IANA references have been updated to point to this document.
In addition, the "TLS Certificate Types" registry established by RFC5081 has been updated in the following ways:
1. Values 0 (X.509) and 1 (OpenPGP) are defined in this document.
2. Values from 2 through 223 decimal inclusive are assigned via "RFC
Required" RFC5226.
3. Values from 224 decimal through 255 decimal inclusive are
reserved for Private Use RFC5226.
Acknowledgements
The authors wish to thank Alfred Hoenes and Ted Hardie for their suggestions on improving this document.
References
Normative References
RFC2119 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
RFC4880 Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
RFC5226 Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
RFC5246 Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
RFC6066 Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, January 2011.
Informative References
RFC5081 Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
Layer Security (TLS) Authentication", RFC 5081, November 2007.
RFC5280 Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008.
Appendix A. Changes from RFC 5081
This document incorporates a major change in the "Server Certificate" and "Client Certificate" TLS messages that will make implementations following this protocol incompatible with those following RFC5081. This change requires the subkey IDs used for TLS authentication to be marked explicitly in the handshake procedure. This was decided in order to place no limitation on the OpenPGP certificates' contents that can be used with this protocol.
RFC5081 required that an OpenPGP key or subkey be marked with the authentication flag; thus, authentication would have failed if this flag was not set or if this flag was set in more than one subkey. The protocol in this memo has no such limitation.
Authors' Addresses
Nikos Mavrogiannopoulos ESAT/COSIC Katholieke Universiteit Leuven Kasteelpark Arenberg 10, bus 2446 Leuven-Heverlee, B-3001 Belgium
EMail: [email protected]
Daniel Kahn Gillmor Independent 119 Herkimer St. Brooklyn, NY 11216-2801 US
EMail: [email protected]