Internet DRAFT - draft-hallambaker-consent


                                                           P. Hallam-Baker
               Internet Draft                                VeriSign Inc. 
               Document: draft-hallambaker-consent-00.txt        July 2004 
               Expires: October 2005                                       
                               Proof of Consent Mechanism 
            Status of this Memo 
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               We propose a mechanism Proof of Consent that allows an 
               email recipient to provide verifiable proof of æopt-inÆ 
               consent to receive email. Proof of Consent may be used 
               to automatically whitelist email from mailing lists and 
               email forwarded from another email server. 
               Proof of Consent is designed to require minimal state 
               maintenance by both the email sender and the recipient 
               and to be deployable with minimal impact on existing 
               email infrastructure. 
            1. Requirements 
               The Proof of Consent mechanism provides a verifiable and 
               revocable proof that a recipient consented to receive 
               email from a specified source. It is designed to address 
               several of the existing problems of maintaining mailing 
               lists and other consent based bulk mail such as 
               newsletters and solicited advertisements. 
               We note that other protocols such as Really Simple 
               Syndication (RSS) and Network News Transport Protocol 
               (NNTP) offer facilities that are similar to mailing lists 
               without the need for a proof of consent mechanism. 
               Despite the existence of these mechanisms email remains 
               the most commonly used means of obtaining data of this 
               type and it is appropriate to address these requirements 
               in the context of email mailing lists despite the 
               existence of alternative protocols that already meet 
            1.1 Subscription Consent Problem 
               The SMTP protocol does not provide a means to allow a 
               mail server receiving a message alleged to have been sent 
               to a mailing list message to determine whether it was 
               solicited or not. 
            1.2 Reputation Attacks  
               An SMTP mail server may be falsely accused of having sent 
               messages to a non-subscriber.  
               As the situation currently stands there is no way to 
               determine the truth or falsehood of such allegations. A 
               party may even sign up for a newsletter with opposing 
               views so as to be able to complain about the messages 
               sent and hope to cause the mailing list to be put on a 
               Events of this type have occurred in connection with 
               mailing lists of every political persuasion. When 
               complaints are made about a blacklisting they are 
               typically met with further hearsay accusations that the 
               accused was ænotoriousÆ. 
            1.3 Unsubscribe Problem 
               Mailing list users often find it difficult to 
               unsubscribe. In many cases requests to be unsubscribed 
               are sent to the entire list. 
               The un-subscription problem can be a serious problem when 
               an intermediary such as a mailing list gateway is 
            1.4 Abandoned Subscription Problem 
               Users often fail to unsubscribe from mailing lists, 
               causing huge volumes of mail to accumulate unread in an 
               unused account or an unread mail folder. In some cases 
               the subscribed email account is also abandoned. 
               Often a mail user will subscribe to a mailing list on a 
               speculative basis and find that they do not read the list 
            1.5 Abandoned List Problem 
               Mailing lists are often created and then abandoned after 
               a period of time after falling into disuse. This can 
               create serious problems if a spammer after discovers an 
               abandoned list without an administrator and starts to 
               send messages to the list. 
            1.6 Maintenance Problem 
               The management of a high volume mailing list requires a 
               considerable amount of effort, largely because of the 
               need to manage the problems of subscribers who are unable 
               to unsubscribe, have abandoned subscriptions etc. Another 
               increasing concern for mailing list administrators is 
               that their lists may be blocked by blacklists, often 
               through no fault of their own due to reputation attacks. 
            2. Deployment Constraints 
               The deployed email infrastructure is the result of more 
               than twenty years of development, much of which has taken 
               place in an ad-hoc fashion. As such it is vital that any 
               proposal to change that infrastructure be compatible with 
               the infrastructure as deployed and not merely as it is 
               described in theoretical specifications. 
            2.1 SMTP Deployment Limitations 
               Many SMTP servers are poorly configured and perform 
               arbitrary and in many cases capricious modifications to 
               messages as they are transmitted. 
            2.2 SMTP Client Limitations 
               Adding features to SMTP clients is a slow process. The 
               features offered by mail readers have not changed 
               significantly in the past five years, basic principles of 
               mail delivery have been unchanged for almost ten years. 
               As a result few end users are likely to upgrade their 
               mail client in order to be able to take advantage of a 
               protocol meeting the requirements described. 
            2.3 Separate Servers for Incoming and Outgoing Mail 
               Enterprises are increasingly using separate mail servers 
               for incoming and outgoing mail. Even if the end user 
               interacts with a single server it is likely that incoming 
               mail will be pre-processed by some form of mail proxy, 
               particularly if anti-spam filtering is being performed. 
            2.4 Network Protocol Access Limitations 
               Many ISPs limit or block completely use of certain 
               Internet protocols. These blocks may be imposed for a 
               variety of reasons that include preventing spam, service 
               differentiation and caprice. 
            2.5 Existing Features Insufficiently Observed 
               Many of the problems with mailing lists could be 
               addressed by simply deploying the existing proposals in 
               [RFC2369]. These provide a mechanism that allows mailing 
               lists to use URIs to specify how users can perform 
               functions unsubscribe from a list. 
               A header of particular interest in this specification is 
               the Mailing-List header that uniquely identifies a 
               mailing list. 
            3. The Proof of Consent Protocol 
               The chief deficiency in the email protocols with respect 
               to the requirements is that an incoming mail server has 
               no means of determining whether a recipient did 
               legitimately consent to opt-in.  
               Most mailing list applications support the use of a 
               challenge/response protocol to verify subscription 
               requests. The mailing list sends the subscriber a 
               challenge token consisting of a string of characters. To 
               confirm subscription to the list the subscriber returns 
               the token to the mailing list. 
               The Proof of Consent mechanism proposes the use of a 
               second token that is created by the subscriberÆs incoming 
               mail server and forwarded together with the challenge 
               token to the mailing list. The mailing list then includes 
               a copy of the token in every email message sent to 
               provide the proof that it was solicited. 
               This mechanism minimizes the need for state maintenance 
               at each stage in the mail transfer protocol, avoiding the 
               need for additional per user records to be stored at 
               either the incoming or outgoing servers. 
            3.1 Initialization 
               The incoming mail server establishes a shared secret k. 
               In the case that there are multiple incoming servers the 
               shared secret may be established manually or by means of 
               some form of key agreement protocol using public key 
               based credentials. 
               Once established, the shared secret is maintained for an 
               extended time period such as a year. Incoming mail 
               servers must keep a record of all shared secrets that 
               have ever been used and the validity intervals in which 
               they were used. 
            3.2 Confirmation Code 
               During the subscription process we establish a 
               confirmation code that is cryptographically bound to the 
               mailing list name, the recipient email address and the 
               date of subscription by means of the shared secret: 
               Code = MAC (mailing-list, recipient, date) 
                     The string the mailing list will use in the 
                     Mailing-List header 
                     The email address of the recipient 
                     The day on which the subscription took place. 
               Each message sent from the mailing list contains a 
               Confirmation-Code header as follows: 
                  Confirmation-Code: <date> <confirmation> 
               The mailing list already needs to maintain a per-
               subscriber record of mailing addresses. The additional 
               state required to support the confirmation code protocol 
               is negligible. 
               We require the subscription date to be stored explicitly 
               for two reasons, first it requires the mailing list 
               administrator to take notice of the age of subscriptions, 
               secondly it allows the incoming mail server to reject 
               mail with a stale confirmation code that is many years 
               old. This allows a mail server to reject mail sent to a 
               mailing list that a previous holder of an account name 
               subscribed to. 
               The incoming mail server can authenticate a confirmation 
               code (but not the attached message) by means of the 
               shared secret. Note that it is not necessary for the MAC 
               algorithm to be standardized, it is sufficient for the 
               sender and receiver to use the same one. 
            3.3 Subscription Process 
               The confirmation code is generated during the 
               subscription process and communicated to the mailing list 
               We assume that any subscription process involves a 
               confirmation process by means of an email callback loop 
               challenge. The incoming mail server detects a mailing 
               list request for subscription confirmation and causes it 
               to be redirected so that the appropriate confirmation 
               code can be added. 
               The callback loop challenge contains a new header 
               constructed as follows: 
               Challenge-Code: <mailing-list> <recipient> <opaque> 
                     The string the mailing list will use in the 
                     Mailing-List header 
                     The email address of the recipient 
                     An opaque code defined by the mailing list 
                     confirmation server to be used for confirmation 
               If the user responds to the confirmation request the 
               appropriate challenge code is generated and forwarded to 
               the indicated address along with the opaque code to 
               establish that the user did intend to subscribe. 
            3.4 Legacy Subscriptions 
               The confirmation code protocol must support gradual 
               introduction. It must be possible for a mailing list to 
               deploy the protocol without having to re-subscribe 
               existing users. It would be advantageous if this can be 
               achieved in such a way that allows a mailing list 
               administrator to be able to deploy the protocol 
               incrementally and still be able to establish that 
               unsolicited messages are never sent. 
               When a mailing list server sends a message to a legacy 
               subscriber the confirmation-code header is still present 
               but only the date field is filled, the confirmation field 
               is absent. This alerts the incoming server to the fact 
               that the message is purported to be a legacy 
               If the incoming server supports the confirmation code 
               protocol it may query the user to determine whether the 
               subscription is actually authorized and if so provide the 
               relevant confirmation code to the mailing list server to 
               avoid the need for subsequent authorizations.  
               A similar procedure may be employed when the confirmation 
               code protocol is deployed on an existing incoming mail 
               server. In this case it is recommended that the service 
               provide some mechanism to allow the user to send 
               confirmation codes to a group of mailing lists at the 
               same time. 
               Mailing list administrators may defend themselves against 
               malicious allegations by having a copy of the mailing 
               list signed by a digital notary at the time the protocol 
               is deployed. The signature format may be chosen in such a 
               way that validity of each entry in the list may be 
               determined independently without revealing any 
               information about any other list member. Various schemes 
               suggest themselves including use of Merkle hash trees or 
               the XML Signature manifest object. It is recommended that 
               any mechanism chosen use a random mask value within each 
               entry to prevent attackers from finding out that a 
               specified party has subscribed to a list. 
               This information could be made available through some 
               form of protocol, however it is likely that requiring 
               existing users to re-subscribe will prove more 
            3.5 Revocation of Confirmation Codes 
               The mechanism described only provides for the 
               authenticity of subscription requests to be established. 
               No assurance is provided for un-subscription requests, 
               nor is the protocol easily modified to achieve this 
               The confirmation code is cryptographically bound to the 
               mailing list identifier. An incoming mail server or spam 
               filtering application can filter incoming mail on the 
               basis of the mailing list identifier. Although this 
               requires the service to maintain state the overhead is 
               minimal and saves considerable resources in the long run. 
               A mailing list may choose not to observe requests to 
               unsubscribe but there is no incentive to do so if the 
               messages are unlikely to be read. 
            4. Security Considerations 
            4.1 Replay Attack 
               The consent to receive mechanism provides proof that the 
               recipient of an email message has consented to receive 
               messages from a specified source. It does not provide 
               definitive proof that a particular message originates 
               from the source specified. 
               An attacker that obtains a consent token for a particular 
               recipient/sender combination can generate an arbitrary 
               volume of messages containing the token.  
               This vulnerability is unlikely to represent a significant 
               risk in the context of spam mitigation. It is highly 
               unlikely that a spammer could obtain a sufficiently large 
               number of consent tokens to make bulk distribution of 
               spam through this mechanism feasible.  
               The vulnerability may be eliminated through use of the 
               consent token mechanism in combination with an 
               authentication mechanism. 
            Copyright Statement 
            Copyright (C) The Internet Society (year).  This document 
            is subject to the rights, licenses and restrictions 
            contained in BCP 78, and except as set forth therein, the 
            authors retain all their rights." 
            "This document and the information contained herein are 
            provided on an "AS IS" basis and THE CONTRIBUTOR, THE 
            Author's Addresses 
            Phillip Hallam-Baker 
            VeriSign Inc.