Internet Engineering Task Force (IETF) P. Wouters Request for Comments: 7929 Red Hat Category: Experimental August 2016 ISSN: 2070-1721
DNS-Based Authentication of Named Entities (DANE) Bindings for OpenPGP
OpenPGP is a message format for email (and file) encryption that lacks a standardized lookup mechanism to securely obtain OpenPGP public keys. DNS-Based Authentication of Named Entities (DANE) is a method for publishing public keys in DNS. This document specifies a DANE method for publishing and locating OpenPGP public keys in DNS for a specific email address using a new OPENPGPKEY DNS resource record. Security is provided via Secure DNS, however the OPENPGPKEY record is not a replacement for verification of authenticity via the "web of trust" or manual verification. The OPENPGPKEY record can be used to encrypt an email that would otherwise have to be sent unencrypted.
Status of This Memo
This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.
This document defines an Experimental Protocol for the Internet community. 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 7841.
Copyright (c) 2016 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.
OpenPGP [RFC4880] public keys are used to encrypt or sign email messages and files. To encrypt an email message, or verify a sender's OpenPGP signature, the email client Mail User Agent (MUA) or the email server Mail Transfer Agent (MTA) needs to locate the recipient's OpenPGP public key.
OpenPGP clients have relied on centralized "well-known" key servers that are accessed using the HTTP Keyserver Protocol [HKP]. Alternatively, users need to manually browse a variety of different front-end websites. These key servers do not require a confirmation of the email address used in the User ID (UID) of the uploaded OpenPGP public key. Attackers can -- and have -- uploaded rogue public keys with other people's email addresses to these key servers.
Once uploaded, public keys cannot be deleted. People who did not pre-sign a key revocation can never remove their OpenPGP public key from these key servers once they have lost access to their private key. This results in receiving encrypted email that cannot be decrypted.
Therefore, these key servers are not well suited to support MUAs and MTAs to automatically encrypt email -- especially in the absence of an interactive user.
This document describes a mechanism to associate a user's OpenPGP public key with their email address, using the OPENPGPKEY DNS RRtype. These records are published in the DNS zone of the user's email address. If the user loses their private key, the OPENPGPKEY DNS record can simply be updated or removed from the zone.
The OPENPGPKEY data is secured using Secure DNS [RFC4035].
The main goal of the OPENPGPKEY resource record is to stop passive attacks against plaintext emails. While it can also thwart some active attacks (such as people uploading rogue keys to key servers in the hopes that others will encrypt to these rogue keys), this resource record is not a replacement for verifying OpenPGP public keys via the "web of trust" signatures, or manually via a fingerprint verification.
This specification is one experiment in improving access to public keys for end-to-end email security. There are a range of ways in which this can reasonably be done for OpenPGP or S/MIME, for example, using the DNS, or SMTP, or HTTP. Proposals for each of these have
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been made with various levels of support in terms of implementation and deployment. For each such experiment, specifications such as this will enable experiments to be carried out that may succeed or that may uncover technical or other impediments to large- or small- scale deployments. The IETF encourages those implementing and deploying such experiments to publicly document their experiences so that future specifications in this space can benefit.
This document defines an RRtype whose use is Experimental. The goal of the experiment is to see whether encrypted email usage will increase if an automated discovery method is available to MTAs and MUAs to help the end user with email encryption key management.
It is unclear if this RRtype will scale to some of the larger email service deployments. Concerns have been raised about the size of the OPENPGPKEY record and the size of the resulting DNS zone files. This experiment hopefully will give the working group some insight into whether or not this is a problem.
If the experiment is successful, it is expected that the findings of the experiment will result in an updated document for standards track approval.
The OPENPGPKEY RRtype somewhat resembles the generic CERT record defined in [RFC4398]. However, the CERT record uses sub-typing with many different types of keys and certificates. It is suspected that its general application of very different protocols (PKIX versus OpenPGP) has been the cause for lack of implementation and deployment. Furthermore, the CERT record uses sub-typing, which is now considered to be a bad idea for DNS.
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 RFC 2119 [RFC2119].
This document also makes use of standard DNSSEC and DANE terminology. See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for these terms.
The OPENPGPKEY DNS resource record (RR) is used to associate an end entity OpenPGP Transferable Public Key (see Section 11.1 of [RFC4880]) with an email address, thus forming an "OpenPGP public key association". A user that wishes to specify more than one OpenPGP key, for example, because they are transitioning to a newer stronger
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key, can do so by adding multiple OPENPGPKEY records. A single OPENPGPKEY DNS record MUST only contain one OpenPGP key.
The type value allocated for the OPENPGPKEY RR type is 61. The OPENPGPKEY RR is class independent.
An OpenPGP Transferable Public Key can be arbitrarily large. DNS records are limited in size. When creating OPENPGPKEY DNS records, the OpenPGP Transferable Public Key should be filtered to only contain appropriate and useful data. At a minimum, an OPENPGPKEY Transferable Public Key for the user firstname.lastname@example.org should contain:
o The primary key X o One User ID Y, which SHOULD match 'email@example.com' o Self-signature from X, binding X to Y
If the primary key is not encryption-capable, at least one relevant subkey should be included, resulting in an OPENPGPKEY Transferable Public Key containing:
o The primary key X o One User ID Y, which SHOULD match 'firstname.lastname@example.org' o Self-signature from X, binding X to Y o Encryption-capable subkey Z o Self-signature from X, binding Z to X o (Other subkeys, if relevant)
The user can also elect to add a few third-party certifications, which they believe would be helpful for validation in the traditional "web of trust". The resulting OPENPGPKEY Transferable Public Key would then look like:
o The primary key X o One User ID Y, which SHOULD match 'email@example.com' o Self-signature from X, binding X to Y o Third-party certification from V, binding Y to X o (Other third-party certifications, if relevant) o Encryption-capable subkey Z o Self-signature from X, binding Z to X o (Other subkeys, if relevant)
When preparing a Transferable Public Key for a specific OPENPGPKEY RDATA format with the goal of minimizing certificate size, a user would typically want to:
o Where one User ID from the certifications matches the looked-up address, strip away non-matching User IDs and any associated certifications (self-signatures or third-party certifications).
o Strip away all User Attribute packets and associated certifications.
o Strip away all expired subkeys. The user may want to keep revoked subkeys if these were revoked prior to their preferred expiration time to ensure that correspondents know about these earlier than expected revocations.
o Strip away all but the most recent self-signature for the remaining User IDs and subkeys.
o Optionally strip away any uninteresting or unimportant third-party User ID certifications. This is a value judgment by the user that is difficult to automate. At the very least, expired and superseded third-party certifications should be stripped out. The user should attempt to keep the most recent and most well- connected certifications in the "web of trust" in their Transferable Public Key.
The RDATA Presentation Format, as visible in master files [RFC1035], consists of a single OpenPGP Transferable Public Key as defined in Section 11.1 of [RFC4880] encoded in base64 as defined in Section 4 of [RFC4648].
The DNS does not allow the use of all characters that are supported in the "local-part" of email addresses as defined in [RFC5322] and [RFC6530]. Therefore, email addresses are mapped into DNS using the following method:
1. The "left-hand side" of the email address, called the "local- part" in both the mail message format definition [RFC5322] and in the specification for internationalized email [RFC6530]) is encoded in UTF-8 (or its subset ASCII). If the local-part is written in another charset, it MUST be converted to UTF-8.
2. The local-part is first canonicalized using the following rules. If the local-part is unquoted, any comments and/or folding whitespace (CFWS) around dots (".") is removed. Any enclosing double quotes are removed. Any literal quoting is removed.
3. If the local-part contains any non-ASCII characters, it SHOULD be normalized using the Unicode Normalization Form C from [Unicode90]. Recommended normalization rules can be found in Section 10.1 of [RFC6530].
4. The local-part is hashed using the SHA2-256 [RFC5754] algorithm, with the hash truncated to 28 octets and represented in its hexadecimal representation, to become the left-most label in the prepared domain name.
5. The string "_openpgpkey" becomes the second left-most label in the prepared domain name.
6. The domain name (the "right-hand side" of the email address, called the "domain" in [RFC5322]) is appended to the result of step 2 to complete the prepared domain name.
For example, to request an OPENPGPKEY resource record for a user whose email address is "firstname.lastname@example.org", an OPENPGPKEY query would be placed for the following QNAME: "c93f1e400f26708f98cb19d936620da35 eec8f72e57f9eec01c1afd6._openpgpkey.example.com". The corresponding RR in the example.com zone might look like (key shortened for formatting):
c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY <base64 public key>
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4. Email Address Variants and Internationalization Considerations
Mail systems usually handle variant forms of local-parts. The most common variants are upper- and lowercase, often automatically corrected when a name is recognized as such. Other variants include systems that ignore "noise" characters such as dots, so that local- parts 'johnsmith' and 'John.Smith' would be equivalent. Many systems allow "extensions" such as 'john-ext' or 'mary+ext' where 'john' or 'mary' is treated as the effective local-part, and 'ext' is passed to the recipient for further handling. This can complicate finding the OPENPGPKEY record associated with the dynamically created email address.
[RFC5321] and its predecessors have always made it clear that only the recipient MTA is allowed to interpret the local-part of an address. Therefore, sending MUAs and MTAs supporting OPENPGPKEY MUST NOT perform any kind of mapping rules based on the email address. In order to improve chances of finding OPENPGP RRs for a particular local-part, domains that allow variant forms (such as treating local- parts as case-insensitive) might publish OPENPGP RRs for all variants of local-parts, might publish variants on first use (for example, a webmail provider that also controls DNS for a domain can publish variants as used by owner of a particular local-part) or just publish OPENPGP RRs for the most common variants.
Section 3 above defines how the local-part is used to determine the location where one looks for an OPENPGPKEY record. Given the variety of local-parts seen in email, designing a good experiment for this is difficult, as: a) some current implementations are known to lowercase at least US-ASCII local-parts, b) we know from (many) other situations that any strategy based on guessing and making multiple DNS queries is not going to achieve consensus for good reasons, and c) the underlying issues are just hard -- see Section 10.1 of [RFC6530] for discussion of just some of the issues that would need to be tackled to fully address this problem.
However, while this specification is not the place to try to address these issues with local-parts, doing so is also not required to determine the outcome of this experiment. If this experiment succeeds, then further work on email addresses with non-ASCII local- parts will be needed and, based on the findings from this experiment, that would be better than doing nothing or starting this experiment based on a speculative approach to what is a very complex topic.
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5. Application Use of OPENPGPKEY
The OPENPGPKEY record allows an application or service to obtain an OpenPGP public key and use it for verifying a digital signature or encrypting a message to the public key. The DNS answer MUST pass DNSSEC validation; if DNSSEC validation reaches any state other than "Secure" (as specified in [RFC4035]), the DNSSEC validation MUST be treated as a failure.
5.1. Obtaining an OpenPGP Key for a Specific Email Address
If no OpenPGP public keys are known for an email address, an OPENPGPKEY DNS lookup MAY be performed to seek the OpenPGP public key that corresponds to that email address. This public key can then be used to verify a received signed message or can be used to send out an encrypted email message. An application whose attempt fails to retrieve a DNSSEC-verified OPENPGPKEY RR from the DNS should remember that failure for some time to avoid sending out a DNS request for each email message the application is sending out; such DNS requests constitute a privacy leak.
Locally stored OpenPGP public keys are not automatically refreshed. If the owner of that key creates a new OpenPGP public key, that owner is unable to securely notify all users and applications that have its old OpenPGP public key. Applications and users can perform an OPENPGPKEY lookup to confirm that the locally stored OpenPGP public key is still the correct key to use. If the locally stored OpenPGP public key is different from the DNSSEC-validated OpenPGP public key currently published in DNS, the confirmation MUST be treated as a failure unless the locally stored OpenPGP key signed the newly published OpenPGP public key found in DNS. An application that can interact with the user MAY ask the user for guidance; otherwise, the application will have to apply local policy. For privacy reasons, an application MUST NOT attempt to look up an OpenPGP key from DNSSEC at every use of that key.
An OpenPGP public key can be associated with multiple email addresses by specifying multiple key UIDs. The OpenPGP public key obtained from an OPENPGPKEY RR can be used as long as the query and resulting data form a proper email to the UID identity association.
CNAMEs (see [RFC2181]) and DNAMEs (see [RFC6672]) can be followed to obtain an OPENPGPKEY RR, as long as the original recipient's email address appears as one of the OpenPGP public key UIDs. For example,
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if the OPENPGPKEY RR query for email@example.com (8d57[...]b7._openpgpkey.example.com) yields a CNAME to 8d57[...]b7._openpgpkey.example.net, and an OPENPGPKEY RR for 8d57[...]b7._openpgpkey.example.net exists, then this OpenPGP public key can be used, provided one of the key UIDs contains "firstname.lastname@example.org". This public key cannot be used if it would only contain the key UID "email@example.com".
If one of the OpenPGP key UIDs contains only a single wildcard as the left-hand side of the email address, such as "*@example.com", the OpenPGP public key may be used for any email address within that domain. Wildcards at other locations (e.g., "hugh@*.com") or regular expressions in key UIDs are not allowed, and any OPENPGPKEY RR containing these MUST be ignored.
Due to the expected size of the OPENPGPKEY record, applications SHOULD use TCP -- not UDP -- to perform queries for the OPENPGPKEY resource record.
Although the reliability of the transport of large DNS resource records has improved in the last years, it is still recommended to keep the DNS records as small as possible without sacrificing the security properties of the public key. The algorithm type and key size of OpenPGP keys should not be modified to accommodate this section.
OpenPGP supports various attributes that do not contribute to the security of a key, such as an embedded image file. It is recommended that these properties not be exported to OpenPGP public keyrings that are used to create OPENPGPKEY resource records. Some OpenPGP software (for example, GnuPG) supports a "minimal key export" that is well suited to use as OPENPGPKEY RDATA. See Appendix A.
DNSSEC is not an alternative for the "web of trust" or for manual fingerprint verification by users. DANE for OpenPGP, as specified in this document, is a solution aimed to ease obtaining someone's public key. Without manual verification of the OpenPGP key obtained via DANE, this retrieved key should only be used for encryption if the only other alternative is sending the message in plaintext. While this thwarts all passive attacks that simply capture and log all plaintext email content, it is not a security measure against active attacks. A user who publishes an OPENPGPKEY record in DNS still
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expects senders to perform their due diligence by additional (non- DNSSEC) verification of their public key via other out-of-band methods before sending any confidential or sensitive information.
In other words, the OPENPGPKEY record MUST NOT be used to send sensitive information without additional verification or confirmation that the OpenPGP key actually belongs to the target recipient.
DNSSEC does not protect the queries from Pervasive Monitoring as defined in [RFC7258]. Since DNS queries are currently mostly unencrypted, a query to look up a target OPENPGPKEY record could reveal that a user using the (monitored) recursive DNS server is attempting to send encrypted email to a target. This information is normally protected by the MUAs and MTAs by using Transport Layer Security (TLS) encryption using STARTTLS. The DNS itself can mitigate some privacy concerns, but the user needs to select a trusted DNS server that supports these privacy-enhancing features. Recursive DNS servers can support DNS Query Name Minimalisation [RFC7816], which limits leaking the QNAME to only the recursive DNS server and the nameservers of the actual zone being queried for. Recursive DNS servers can also support TLS [RFC7858] to ensure that the path between the end user and the recursive DNS server is encrypted.
Various components could be responsible for encrypting an email message to a target recipient. It could be done by the sender's MUA or a MUA plug-in or the sender's MTA. Each of these have their own characteristics. A MUA can ask the user to make a decision before continuing. The MUA can either accept or refuse a message. The MTA must deliver the message as-is, or encrypt the message before delivering. Each of these components should attempt to encrypt an unencrypted outgoing message whenever possible.
In theory, two different local-parts could hash to the same value. This document assumes that such a hash collision has a negligible chance of happening.
Organizations that are required to be able to read everyone's encrypted email should publish the escrow key as the OPENPGPKEY record. Mail servers of such organizations MAY optionally re-encrypt the message to the individual's OpenPGP key.
An MTA could be operating in a stand-alone mode, without access to the sender's OpenPGP public keyring, or in a way where it can access the user's OpenPGP public keyring. Regardless, the MTA MUST NOT modify the user's OpenPGP keyring.
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An MTA sending an email MUST NOT add the public key obtained from an OPENPGPKEY resource record to a permanent public keyring for future use beyond the TTL.
If the obtained public key is revoked, the MTA MUST NOT use the key for encryption, even if that would result in sending the message in plaintext.
If a message is already encrypted, the MTA SHOULD NOT re-encrypt the message, even if different encryption schemes or different encryption keys would be used.
If the DNS request for an OPENPGPKEY record returned an Indeterminate or Bogus answer as specified in [RFC4035], the MTA MUST NOT send the message and queue the plaintext message for encrypted delivery at a later time. If the problem persists, the email should be returned via the regular bounce methods.
If multiple non-revoked OPENPGPKEY resource records are found, the MTA SHOULD pick the most secure RR based on its local policy.
If the public key for a recipient obtained from the locally stored sender's public keyring differs from the recipient's OPENPGPKEY RR, the MUA SHOULD halt processing the message and interact with the user to resolve the conflict before continuing to process the message.
If the public key for a recipient obtained from the locally stored sender's public keyring contains contradicting properties for the same key obtained from an OPENPGPKEY RR, the MUA SHOULD NOT accept the message for delivery.
If multiple non-revoked OPENPGPKEY resource records are found, the MUA SHOULD pick the most secure OpenPGP public key based on its local policy.
The MUA MAY interact with the user to resolve any conflicts between locally stored keyrings and OPENPGPKEY RRdata.
A MUA that is encrypting a message SHOULD clearly indicate to the user the difference between encrypting to a locally stored and previously user-verified public key and encrypting to a public key obtained via an OPENPGPKEY resource record that was not manually verified by the user in the past.
To prevent amplification attacks, an Authoritative DNS server MAY wish to prevent returning OPENPGPKEY records over UDP unless the source IP address has been confirmed with [RFC7873]. Such servers MUST NOT return REFUSED, but answer the query with an empty answer section and the truncation flag set ("TC=1").
The hashing of the local-part in this document is not a security feature. Publishing OPENPGPKEY records will create a list of hashes of valid email addresses, which could simplify obtaining a list of valid email addresses for a particular domain. It is desirable to not ease the harvesting of email addresses where possible.
The domain name part of the email address is not used as part of the hash so that hashes can be used in multiple zones deployed using DNAME [RFC6672]. This does makes it slightly easier and cheaper to brute-force the SHA2-256 hashes into common and short local-parts, as single rainbow tables can be re-used across domains. This can be somewhat countered by using NextSECure version 3 (NSEC3).
DNS zones that are signed with DNSSEC using NSEC for denial of existence are susceptible to zone walking, a mechanism that allows someone to enumerate all the OPENPGPKEY hashes in a zone. This can be used in combination with previously hashed common or short local- parts (in rainbow tables) to deduce valid email addresses. DNSSEC- signed zones using NSEC3 for denial of existence instead of NSEC are significantly harder to brute-force after performing a zone walk.
Users may have a local key store with OpenPGP public keys. An application supporting the use of OPENPGPKEY DNS records MUST NOT modify the local key store without explicit confirmation of the user, as the application is unaware of the user's personal policy for adding, removing, or updating their local key store. An application MAY warn the user if an OPENPGPKEY record does not match the OpenPGP public key in the local key store.
Applications that cannot interact with users, such as daemon processes, SHOULD store OpenPGP public keys obtained via OPENPGPKEY up to their DNS TTL value. This avoids repeated DNS lookups that third parties could monitor to determine when an email is being sent to a particular user.
Anyone who can obtain a DNSSEC private key of a domain name via coercion, theft, or brute-force calculations, can replace any OPENPGPKEY record in that zone and all of the delegated child zones. Any future messages encrypted with the malicious OpenPGP key could then be read.
Therefore, an OpenPGP key obtained via a DNSSEC-validated OPENPGPKEY record can only be trusted as much as the DNS domain can be trusted, and is no substitute for in-person OpenPGP key verification or additional OpenPGP verification via "web of trust" signatures present on the OpenPGP in question.
The --armor or -a option of the gpg command should not be used, as it adds additional markers around the armored key.
When DNS software reading or signing of the zone file does not yet support the OPENPGPKEY RRtype, the Generic Record Syntax of [RFC3597] can be used to generate the RDATA. One needs to calculate the number of octets and the actual data in hexadecimal:
This is a copy of the original registration template submitted to IANA; the text (including the references) has not been updated.
A. Submission Date: 23-07-2014
B.1 Submission Type: [x] New RRTYPE [ ] Modification to RRTYPE B.2 Kind of RR: [x] Data RR [ ] Meta-RR
C. Contact Information for submitter (will be publicly posted): Name: Paul Wouters Email Address: firstname.lastname@example.org International telephone number: +1-647-896-3464 Other contact handles: email@example.com
F. What existing RRTYPE or RRTYPEs come closest to filling that need and why are they unsatisfactory?
The CERT RRtype is the closest match. It unfortunately depends on subtyping, and its use in general is no longer recommended. It also has no human usable presentation format. Some usage types of CERT require external URI's which complicates the security model. This was discussed in the dane working group.
G. What mnemonic is requested for the new RRTYPE (optional)?
H. Does the requested RRTYPE make use of any existing IANA registry or require the creation of a new IANA subregistry in DNS Parameters? If so, please indicate which registry is to be used or created. If a new subregistry is needed, specify the allocation policy for it and its initial contents. Also include what the modification procedures will be.
Currently, three software implementations of draft-ietf-dane-openpgpkey are using a private number.
This document is based on [RFC4255] and [SMIME] whose authors are Paul Hoffman, Jakob Schlyter, and W. Griffin. Olafur Gudmundsson provided feedback and suggested various improvements. Willem Toorop contributed the gpg and hexdump command options. Daniel Kahn Gillmor provided the text describing the OpenPGP packet formats and filtering options. Edwin Taylor contributed language improvements for various iterations of this document. Text regarding email mappings was taken from [MAILBOX] whose author is John Levine.