| Internet-Draft | otp-token | September 2026 |
| Goto & West | Expires 25 March 2027 | [Page] |
This document defines the OTP-Token email header field, which can be used to deliver One-Time
Passcodes (OTP) in a machine-readable and origin-bound manner alongside the human-readable message
carrying that content today. Recipient Message User Agents (rMUA) can collaborate with other
entities in the ecosystem to assist in the delivery of these codes to the context which wishes to
verify their successful delivery.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://mikewest.github.io/otp-token/draft-goto-otp-token.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-goto-otp-token/.¶
Source for this draft and an issue tracker can be found at https://github.com/mikewest/otp-token.¶
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One-time passcodes delivered via email are widely used as part of flows which require verification of a user's contact information. Sign-in/-up flows, reauth for high-risk transactions, account recovery, and so on all might reasonably rely on verifying a user's access to a particular email address by sending a secret code to that address, and waiting for the user to prove that they know what code was sent by typing it into some other context.¶
Today, actions which require verification through email will begin in one context (say, a sign-in form on a website), but require users to hop to another context (their rMUA) to track down the relevant email after waiting for delivery, memorize a short code, and then hop back to the verifying context to type it in. This flow is frustrating, as context-switching leads to confusion and failure. It's also phishable, as users can be tricked into typing a code meant for a trusted context into an attacker-controlled site.¶
[SMS-ONE-TIME-CODES] showed that a machine-readable, origin-bound format for SMS-based OTPs can reduce both frustration and phishing by making it possible for software to easily extract OTPs and feed them into systems like [WEBOTP] or a platform's autofill mechanism. This approach can dramatically reduce the friction users experience, only in those cases where the context into which the code is delivered can be verified to be the destination to which the code has asserted a binding. This doesn't prevent phishing as users can still be tricked into typing the code manually, but it's a substantial improvement in the system's general security posture.¶
Leaning on that experience, this document proposes extending the core concept of a standardized delivery format from SMS to email, taking advantage of email's distinction between headers and user-visible content to do so.¶
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.¶
This document relies on [RFC5598] to define Recpient Mail User Agents (rMUA) and other aspects of email infrastructure.¶
This document further relies on Section 3 of [STRUCTURED-FIELDS] to define a number of concepts
around parsing and syntax, including: String, Byte Sequence, Parameters, and the sf-item ABNF
rule.¶
This document relies on [RFC6454] for the definition of an origin, and on Section 6.2 of that
document for its ASCII serialization. The serialized-origin grammar is defined in Section 7.1.¶
OTP-Token Header Field
The OTP-Token header field delivers an origin-bound OTP. For example:¶
OTP-Token: "123456"; origin="https://example.com"¶
This header is a Structured Field containing a String (Section 3.3.3 of [STRUCTURED-FIELDS]) representing the OTP, along with Parameters (Section 3.1.2 of [STRUCTURED-FIELDS]) representing the origin to which the OTP is bound.¶
OTP-Token MUST appear at most once in a message's header section. Instances appearing in the
header sections of MIME body parts MUST be ignored.¶
Note that headers might exceed the 998/78 character limitations specified in Section 2.2.3 of [RFC5322], and will be folded accordingly. Section 3.2 specifies how that case is to be handled.¶
The OTP-Token header MUST contain an origin Parameter:¶
origin: This parameter represents the origin to which the OTP is bound. Its value is a String
containing the ASCII serialization of an origin conforming to Section 6.2 of [RFC6454]. This
value MUST not be the serialization of an opaque origin ("null"), MUST be a potentially
trustworthy origin as defined in [SECURE-CONTEXTS], and MUST be in canonical form (lowercase,
default ports omitted, ASCII-only).¶
Note: [RFC6454] and [URL]/[HTML] do not entirely agree on origins or their serialization. The constraints above keep them more or less aligned for the subset of origins this document permits; reconciliation beyond that is well outside this document's scope.¶
Unknown Parameters are ignored in order to make future expansion possible if necessary (such as the supplemental verification tokens discussed in Section 6).¶
The following algorithm describes the process of parsing and validating the OTP-Token header into
an One-Time Passcode object, given a message and receipt-timestamp. Parsing fails if no valid
OTP is available:¶
If message's header section contains zero or more than one OTP-Token header, fail.¶
Let value be the result of unfolding message's header section's OTP-Token header as
specified in Section 2.2.3 of [RFC5322].¶
Let parsed be the result of parsing value as specified in Section 4.2 of
[STRUCTURED-FIELDS], with a field_type of "item". If parsing value as a Structured Field
fails, fail.¶
If parsed's bare_item is not a String, fail.¶
If parsed's parameters contains no item whose key is "origin", fail.¶
Let origin be the value of parsed's parameters' item whose key is "origin".¶
Fail if any of the following conditions are true:¶
Return a new One-Time Passcode whose:¶
Note: [STRUCTURED-FIELDS] has no concept of header folding. The ordering of steps 2 and 3 above is therefore necessary to enable proper parsing of message headers.¶
TODO. Probably something about DKIM?¶
Authentication mechanisms like federation and [WEBAUTHN] are clearly the directions in which the ecosystem should move. Improvements to OTP delivery are not arguments in the other direction. Still, it's important to recognize that email verification often serves as a last-resort fallback mechanism for account recovery. This document's proposal aims only to create low-cost opportunities to mitigate some of that validation path's inherent risks (and see Section 6 for discussion of potential mechanisms to further mitigate manual phishing risks).¶
TODO: Say something here about identifying the initiating context's origin and how it'll all be
platform-specific. Also note that these proposals address only one piece of a larger system by
allowing automated extraction of OTPs, but leave important, platform-specific details of their
integration with the rest of the system out of scope. Those mechanisms will be defined elsewhere
(e.g. HTML defines <input autocomplete="one-time-code">, iOS defines NSTextInput with
.oneTimeCode, and so on), so the most we can say here is that matching origins at the boundary
points is a cricial part of any security improvement this proposal offers.¶
IANA is asked to update the Provisional Message Header Field Names registry [RFC3864] with the following entry:¶
This document defines the simplest possible thing, matching the capabilities that the standardized SMS format offers. Given that we have a header which isn't human-visible (at least, not without effort), there are likely improvements we could explore that could increase the system's robustness.¶
Origin-bound OTPs as described here are an incomplete solution to phishing only insofar as they support systems which make it easy to hand the OTP to its asserted origin. In a perfect world, users would become accustomed to the low-friction automation of such a system, and would see the requiement to manually type an OTP into a page whose origin didn't match the OTPs assertion as an indication that something was off.¶
While we're waiting for the ubiquity of user expectations around such systems, it might be possible to help sites distinguish between manually-typed OTPs and those delivered via an automated system. This distinction seems potentially useful as an additional signal for risk analysis, as the latter can be trusted to perform security checks that the former might mistakenly bypass.¶
The simplest version of this distinction could add a supplemental validation token to the
OTP-Token header that would be difficult for a human to access:¶
OTP-Token: "123456"; origin="https://example.com"; token=:NjU0MzIx:¶
In this model, the token parameter would carry a high-entropy Byte Sequence (Section 3.3.5 of
[STRUCTURED-FIELDS]); think of it as a second, longer OTP. Because this value resides only in
email headers, it won't be visible to users in the message body, and will therefore be difficult
to ask users to copy/paste into a form.¶
Automated systems could deliver this token alongside (or instead of?) the human-visible OTP after matching the asserted origin, and refuse to hand it over in the absence of such a match. Sites receiving OTPs could take the presence of this token as a strong signal that the entity submitting the form had access to the entire email, not just to a portion of it that a user might have typed elsewhere.¶
Because manual entry carries an inherently higher risk of phishing or relay attacks, relying parties could treat OTP-only submissions as higher risk, potentially requiring (another) step-up authentication, restricting sensitive actions, or tightening session lifetimes.¶
Is specifying a single origin enough? Do we need scoping rules to tie OTPs to multiple
origins? Sites rather than origins?¶
Rather than relying on the relying party to examine the received timestamp, should we offer a
ttl/expires Parameter after which the automated system would refuse to offer the OTP?¶
Should we try to create tighter checks for the OTP's specified origin? That is, should we only accept origin bindings that can be authenticated via DKIM/DMARC?¶
And, of course, we should bikeshed the header's name: OTP-Token, OTP, One-Time-Passcode,
Super-Secret-Thing-That-Humans-Should-Not-Read, etc.¶
This can be considered an email-based implementation of [SMS-ONE-TIME-CODES], which paved the way to formalizing a machine-readable format for these short-lived verification codes.¶