Application Area Working Group W. Chuang
Internet-Draft Google, Inc.
Expires: April 7, 2016 October 5, 2015

S/MIME Proxy Forwarding


S/MIME provides the means to protect message body content but does not protect the sender and recipient identity nor other headers. We propose a means to do so by proxying the sending and receiving via a trusted SMTP server. This document describes how the proxyies are discovered and the wrapped messages created. This then describes how the proxy forwarding is done.

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Table of Contents

1. Motivation

MMaking private the email sender and recipient remains an open issue for email delivery despite several privacy protecting RFCs. S/MIME [RFC5751] provides for the means to wrap the message body but not the header in an effective way, Secure Headers Fields [RFC7508] provides a means to wrap most headers but not the envelope sender and recipient, and Secure SMTP over TLS [RFC2487] has limitations described as follows. Traditionally mail delivery through SMTP is sent via cleartext and many SMTP servers still only offer this. To enhance security, SMTP may offer TLS, but the STARTTLS option may be hidden by an active Man-In- the-middle (MitM) that prevents the initiation of the handshake that makes the message delivery private (EFF link). Further, a sophisticated MitM adversary may allow encryption but with their own TLS keys so they can eavesdrop all the while depending on certain clients not to check the validity of the server TLS certificates.

Thus mail delivery despite significant effort is still vulnerable to eavesdropping or man-in-in-middle. The recent Snowden leaks about the PRISM program have shown that relationship metadata about who is communicating with whom is often as important as the message contents.

2. Introduction

We propose that S/MIME wrapped, meaning encrypted messages, be sent through partly or fully trusted SMTP MSA and MDA servers via proxy accounts that act as forwarders that hide the true recipient and sender. While it is assumed that a (partly or fully) trusted network can be established between the sender and recipient to their respective proxy SMTP servers (MSA and MDA), mail delivery between the proxy SMTP servers is assumed to be untrusted i.e. vulnerable to passive monitoring and MitM. We propose a process described later by which only the non- identifying forwarding proxy accounts names are potentially seen by the eavesdropper and that the message body is wrapped in a way that does not leak the contents nor the true sender or recipient identity. The proxy accounts should sustain enough aggregate traffic and be unrelated to the original sender and recipient such that traffic analysis is very difficult. The following diagram illustrates the proxying process. This forwarding via proxies is analogous to a networking proxying and forwarding of Chaum-mixes wrapped messages in Onion routing. Our approach also uses proxies and Chaum message wrapping but differs in that the sender and receiver have trusted proxies that are statically defined and willing to support mail flow (which may be large). To differentiate we call this process S/MIME Grunion forwarding after the small fish that likes to bury itself in the sand when spawning.

+--------+  +--------+    +-----+    +--------+    +-----+
| True   |  | Sender |  +-+     +-+  | Proxy  |  +-+     +-+
| Sender +--> MSA    +-->Untrusted+--> Sender +-->Untrusted+-+
|        |  |        |  +-+     +-+  | MSA    |  +-+     +-+ |
+--------+  +--------+    +-----+    +--------+    +-----+   |
  |  +----------+    +-----+    +----------+  +----------+
  |  | Proxy    |  +-+     +-+  | Recipient|  | True     |
  +--> Recipient+-->Untrusted+--> MDA      +--> Recipient|
     | MDA      |  +-+     +-+  |          |  |          |
     +----------+    +-----+    +----------+  +----------+

Figure 1: SMTP Delivery with Partly Trusted Proxying

We differentiate the proxy as partly or fully trusted as an optional profile provides increased security at the cost of additional encryption and special handling for bounced messages. The partly trusted proxy is used when the sender and recipient do not trust the proxy MSA and MDA SMTP servers to see the body content, the full path, or message headers. The sender proxy is not trusted to know who the recipient is, and the recipient proxy is not trusted to see who the sender is. However the proxies are trusted to forward the message on behalf of the sender and to do cryptographic processing. It also similarly does not trust the network between the end-point to see the body content, the full path, or message headers. This approach uses S/MIME message/822 header [RFC5751] (RFC5751 sec 3.1) to wrap the whole message including the header and body. A new headers suitably modified with the proxy sender and recipients as described shortly is prepended. Special consideration must be made to protect the end-point privacy when specifying a return path in case of bounce messages, when handling message headers generated during the delivery process and when signing the message.

+-------+    +--------+    +------+    +-----------+  +-----------+
| True  |    | Proxy  |  +--      --+  | Proxy     |  | True      |
| Sender+----> Sender +--> Untrusted+--> Recipient +--> Recipient |
|       |    | + MSA  |  |          |  | + MDA     |  |           |
+-------+    +--------+  +--      --+  +-----------+  +-----------+

Figure 2: SMTP Delivery with Fully Trusted Proxying

The fully trusted proxy is used when the sender and recipient trust the proxy SMTP servers to be given the clear-text headers but wishes to use the proxied wrapping of the message when the message is delivered between the MSA and MDA. Here the network between end-point and the proxy SMTP servers is fully trusted meaning is capable of keeping messages private. This might be guaranteed by communication through a closed network behind a firewall, or it might be on the open internet but guaranteed to prevent an active MiTM by mandating use of SMTP TLS with certificate chain checks. It also assumed that bounce messages are not generated from the end-points to the proxying SMTP server as the bounce with the original headers might reveal content to the untrusted network. This restriction for fully trusted proxying precludes more elaborate send paths that the partially trusted proxying can support. Fully trusted proxying will also be capable of handling traditional S/MIME CMS [RFC5652] parts be it signed or encrypted if the sender wishes to keep the message content private from the proxy servers yet authenticated. The advantage of the partly trusted proxy approach is that by being able to read the headers at the proxies, it does not special handling for return-path or 'Received:' headers which greatly simplifies message setup at the client, and during forwarding which reduces runtime overhead. While both partly and fully trusted proxying modes should be supported, it is likely that fully trusted proxying will be much easier to deploy while maintaining most of the desired privacy properties. Because of the ease of deployment, we proposed that another use of the fully trusted proxy model could be used as a general replacement for opportunistic TLS even when the original message did not use S/MIME so long as both the sender and recipient SMTP support this model. This will be described later in the document.

2.1. Fully Trusted Proxy

The Fully Trusted Proxy diagram below illustrates that the proxy- sender wraps the original message once and sets a proxy header using the proxy-sender and proxy-recipient where the content may already be a RFC5751 S/MIME wrapped message. The Grunion S/MIME wrapped message and proxy head is what is seen by the untrusted network until it is received by the proxy-recipient which unwraps the original message and forwards it to the true recipient. The proxy sender is also capable of unwrapping the Grunion message in case of bounces. In the following table, the sender proxy address is assumed to be in the same domain as the sender, and similarly the recipient proxy address is assumed to be in the same domain as the recipient.

The later tables illustrates the differences between partly vs fully trusted proxy methods. The diagrams illustrate the MIME tree structure where the CMS corresponds to S/MIME content wrapping. Note that CMS may wrap the message body only or both header and message body. The tables describe the TO and FROM headers addresses and the SMTP TO and FROM addresses (envelope TO and FROM) at each step. The Message column indicates S/MIME message body wrapping where the public keys used are: ps- proxy sender, pr- proxy recipient, r- recipient. The domains are: S- sender, R- recipient and potentially PS- proxy sender, PR-proxy recipient. The headers are: h- original header, and h_p- proxy header.

Step             FROM      TO        SMTP FROM   SMTP TO   Message (after)
Sender(orig)     send@S    recip@R                         (h,b)
Sender           proxy@S   proxy@R   send@S      proxy@S   (h_p, Kpr(h, Kr(b)))
Proxy-Sender     proxy@S   proxy@R   proxy@S     proxy@R   (h_p, Kpr(h, Kr(b)))
  + MSA
Proxy-Recipient  proxy@S   proxy@R                         (h, Kr(b))
  + MDA
Recipient        send@S    recip@R                         (h, b)

Figure 3: Fully Trusted Message Header and Body

Note that the SMTP FROM (MAIL FROM) address is rewritten at the Proxy Sender MSA from sender@S to proxy@S as that server notices the Grunion message being forwarded.

2.2. Partly Trusted Proxy

The Partly Trusted Proxy diagram illustrates that the message is wrapped thrice with proxy headers by the true sender so as to hide the true path even from the proxies.

Step             FROM      TO        SMTP FROM   SMTP TO   Message (after)
Sender(orig)     send@S    recip@R                         (h, b)
Sender MSA       send@S    proxy@PS  send@S      proxy@PS  (h_ps, Kps(
                                                             (h_pr, Kpr(
                                                                (h_r, Kr(h, b))
Proxy-Sender MSA proxy@PS  proxy@PR  proxy@PS    proxy@PR  (h_pr, Kpr(
                                                             (h_r, Kr(h, b))))
Proxy-Recipient  proxy@PR  recip@R   proxy@PR    recip@R   (h_r, Kr(h, b))
Recipient MSA    proxy@PR  recip@R                         (h_r, Kr(h, b))
Recipient        send@S    recip@R                         (h, b)


Figure 4: Partly Trusted Message Header and Body

Proxy accounts define a new altered delivery path that hides the original sender and recipient by replacing them with a new sender and recipient email address i.e. proxy accounts. Discovery of the proxy accounts is through the X.509 certificates or CMS through a new certificate extension or by a CMS extension that points to the aforementioned extension. In the X.509 case, this is found by searching the certificate chain from the end-entity certificate through the intermediate Certificate Authority certificates. Both the sender and the recipient must each specify a certificate chain where one of the certificates has the extension for the S/MIME Grunion routing to work. The extensions are described shortly.

3. Proxying Metadata

Grunion routing requires several CMS and X.509 Certificate extensions for the purposes of discovering proxying certificates, for determining which forwarding profile to use, and to support message delivery during forwarding. One feature is duplicating the proxying certificate discovery specification in both CMS and X.509 certificate. The advantage of the X.509 extension is that it must be valid for duration of the certificate validity i.e. proxying support must be valid during that time whereas the CMS extension has no such guarantee. However there is a bootstrapping problem which the CMS extension resolves. The certificate issuer might want to prove that the proxying path does deliver mail via the proxy SMTP server to or from the recipient and server. The issuer could generate a test message via the CMS format. Once the path is proven a certificate with the X.509 extensions could be issued.

3.1. CMS Extension

TrustedProxySupported := SEQUENCE {
    certificatePaths SubjectInfoAccessSyntax OPTIONAL
    certificatesIdentifier ProxyCertificateIdentifier OPTIONAL
ProxyCertificateIdentifier := SEQUENCE {
    keyIdentifier  KeyIdentifier OPTIONAL,
    proxyCertIssuer GeneralNames OPTIONAL,
    proxyCertSerialNumber CertificateSerialNumber OPTIONAL
KeyIdentifier ::= OCTET STRING

Figure 5: TrustedProxySupported

Senders supporting this protocol should advertise this support via a CMS signed-attribute as described in RFC5652 [RFC5652] and RFC2634 [RFC2634] called TrustedProxySupported. The OID for this attribute is TBD. This attribute may take an repeated argument certificatePaths which is the same format as Subject Information Accesss in RFC5280 [RFC5280] section which describes a means to obtain certificates from a remote location e.g. URL. This follows the guidelines there for fetching certificates using a specified protocol and then how to interpret the fetched content. There is a second argument certificateIdentifier that specifies which certificates found at the remote location can be used for TrustedProxying. This is the same format as the Authority Key Identifier information in RFC5280 [RFC5652] section If the certificatePaths and/or certificateIdentifier argument are not present, then it is assumed that the proxying certificates for the Grunion forwarding profile is discovered from the certificates in the message signature i.e. certificates in the CMS SignedData part. Specifying more than one certificateIdentifier provides path diversity and when multiple are present the sender should randomly pick one to use. The ASN.1 syntax is:

Messages with the TrustedProxySupported attribute must specify a Grunion proxying certificate either referenced by the certificate specified by certificatesPaths/certificateIdentifier or from the signature certificate with a TrustedProxyCertificate extension (next section). This is intended to help the sender find information needed to set up S/MIME Grunion forwarding but does not necessarily indicate that the current message is using S/MIME Grunion forwarding.

TrustedProxyForwarding := SEQUENCE {
    bounceMessage EnvelopedData OPTIONAL
    profile TrustedProxyProfile
TrustedProxyProfile : = ENUMERATED {

Figure 6: TrustedProxyForwarding

There is a second signed attribute TrustedProxyForwarding that indicates a message is using the Grunion forwarding. The OID is TBD. This takes two arguments. The first bounceMessage is an optional pre- generated bouncy message. This is not set if using the fully trusted proxying or if the message is already a bounce. If the pre-generated bounce message is needed, then the sender creates the bounceMessage for each wrapped CMS part as otherwise the bounce message is lost during unwrapping. They use the same structure as partly trusted proxying to protect privacy but addressed in the reverse direction i.e. from current location back to the sender. The inner message contains an indication that the message has bounced, some identifying message-id header to correlate the bounce to the original message, and a description of the current proxy that will likely generate the bounce e.g. destination address. Then working from sender back to the current location, the message is recursively wrapped with that destination's public key. A new header with the destination address as TO is appended. The second argument is the Grunion forwarding profile used i.e. whether partly or fully trusted proxying is assumed.

3.2. Certificate Extension

TrustedProxyCertificate := SEQUENCE {
    trustedProxy IA5String
    proxyProfile TrustedProxyProfiles
TrustedProxyProfiles : = SEQUENCE {
    profile TrustedProxyProfile

Figure 7: TrustedProxyCertificate

We propose a new X.509 certificate [RFC5280] extension that designates that a specified email account can act as a trusted proxy (either partly or fully trusted) in the process of S/MIME grunion forwarding. The TrustedProxyCertificate extension implies a binding of the proxy account to that certificate's public key and associated private-key for the purposes of email encryption and signing, and that such a binding will exist as long as the certificate is valid. The extension also serves as another subject alternative name argument trustedProxy. Consequently the certificate containing the specified email address must have a valid issuance path and valid name in the presence of any path constraints using the process specified in RFC5280. Note that trusted proxy certificate may be an intermediate CA certificate. There is a second argument proxyProfile that specifies whether the trusted proxy supports partly trusted or fully trusted profiles or both. OID for the extension is TBD. If this certificate extension appears as part of the user's signature, it must not be placed on the end-entity certificate user certificate as it would otherwise share the keys with the end-entity. The ASN.1 syntax is:

We also propose that the TrustedProxySupported be an additional certificate extensions that also implies support for a trusted proxy with an external certificate chain distinct from the current one. Unlike TrustedProxyCertificate this extension doesn't imply that the proxy certificate is in the user's issuance path. TrustedProxySupported would generally be placed on the user end-entity certificate and are analogous to the CMS extensions in that describes how to fetch the proxy certificate and interpret the content. Keep in mind that unlike the CMS extension, these binds the user certificate to trusted proxy for the duration of the validity of the certificate. This means that use of the trusted proxy to reach the user will be available through the life of the certificate.

4. Message Sending And Receiving

The following describes the mechanisms for implementing Grunion S/MIME forwarding.

4.1. Outbound

The message is composed normally using the true sender and true recipient or recipients. In the fully trusted case, the sender may first process the message i.e. encrypt/sign using traditional S/MIME 3.2 CMS [RFC5751] and the recipient's public key, and encoded as a application/pkcs7-mime part. The message MIME tree is embedded message/rfc822 part as described in RFC5751 section 3.1. The new message is encrypted following the instructions in RFC5751 section 3.3 to create an application/pkcs7-mime MIME part using the proxy recipient's public key. A new proxy header is generated with the proxy- sender and proxy-recipient as the TO and FROM addresses and prepended. Only the original message or innermost wrapped message should be signed using the sender's private key to maintain the privacy of the end- points.

In the partly trusted case, message creation uses almost the same wrapping strategy as the partial proxying. Wrapping is done from the recipient back to the proxy sender recursively. The wrapping takes the message body in a message/rfc822 part and wrapped with that step's recipient public key. A new proxy header is generated using that step sender and recipient. To protect the sender and recipient identity there should be at least two SMTP proxying steps, but more are possible and could be encoded and decoded without modification. This document does not describe how to discover those intermediaries because so far there hasn't been a use case identified for them. Any message signing done should be on the original unwrapped message to protect the privacy of the endpoints and using the sender's private key.

4.2. Inbound

Each proxying SMTP server looks for TrustedProxyForwarding CMS extension. If found, it checks whether the profile is supported, otherwise bounce the message. It then checks to see if the FROM field is a supported user, and then if the CMS specified RecipientInfo matches an available key. If so unwraps the message including extracting the message/rfc822 if found, otherwise bounce the message. The proxying SMTP server observes the new header's TO address as the forwarding destination.

4.3. Handling Bounce Message

One issue with practical mail delivery is that some messages cannot be delivered and must be bounced. In the partially trusted case, the initial bounce will takes the TrustedProxyForwarding bounceMessage content as the body content and sends it along the return path. This will be forwarded and unwrapped in a privacy preserving way. If the bounce initiator understands Grunion forwarding, it should extract bounceMessage, and send that content as the bounce message. If it doesn't understand this formatting, it may bounce the entire message and the initial recipient of the bounce will have to extract the bounceMessage content in addition to unwrapping and forwarding. In the fully trusted proxy case, the proxy sender should add itself as a recipient in addition to the proxy recipient to enable decryption of the message it sent but was bounced. If there is a bounce it should rewrap the bounce message using the sender proxy public key using the fully trusted CMS structure and addressing. Note that the bounced message in both cases transfers in encrypted form between the proxies retracing the original sending path thus maintaining privacy of the contents and the headers. p

4.4. Header Processing

All user generated headers are wrapped in both proxying models. However there are dynamically generated headers appended to the message as it are propagated along the SMTP send path. Fully trusted proxying means that the proxies can see, and preserve headers which is very useful for debugging without breaking the privacy contract. However SMTP proxying servers should be careful not propagate information about the sender or recipient such as their IP or DNS address. For example the "Received" header is dynamically generated at SMTP receive time with debug information about the environment, timestamp proxy address and potentially the sender. Grunion forwarders should scrub the sender or recipient (on bounce) address. Similarly authentication and result headers such as DKIM, SPF and DMARC can similarly be propagated. Partly trusted headers is much more restrictive in order to protect the more restrictive privacy contract. In the current design, all dynamically generated headers are not propagated to the recipient at the proxy SMTP servers.

5. General Delivery Traffic with Grunion Delivery

A domain may advertise that all messages may be delivered via Grunion forwarding. As such it could replace opportunistic SMTP TLS to maintain message privacy so long as the sender and recipient SMTP servers can support Grunion forwarding and can act as the proxies. Unlike SMTP TLS which advertises TLS capability on a per session basis, this capability is advertised as being supported as long as the certificate is valid. As the TrustedProxyCertificate extension may be placed on the intermediate CA certificate, it can be present in other usage scenarios besides S/MIME email such as part of the SMTP server TLS certificates chain. A stateful SMTP server could track past intermediate certificates presented to it to determine whether a new delivery can be transmitted with the message wrapped in Grunion S/MIME even if the current session doesn't specify STARTTLS. If a message is already wrapped by the sender with the proxy recipient key, the message should not be rewrapped by the proxying sender SMTP server. To maintain endpoint privacy between the endpoint to the SMTP server only the fully trusted profile should be used.

6. References

[RFC2487] Hoffman, P., "SMTP Service Extension for Secure SMTP over TLS", RFC 2487, DOI 10.17487/RFC2487, January 1999.
[RFC2634] Hoffman, P., "Enhanced Security Services for S/MIME", RFC 2634, DOI 10.17487/RFC2634, June 1999.
[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, DOI 10.17487/RFC5280, May 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification", RFC 5751, DOI 10.17487/RFC5751, January 2010.
[RFC7508] Cailleux, L. and C. Bonatti, "Securing Header Fields with S/MIME", RFC 7508, DOI 10.17487/RFC7508, April 2015.

Author's Address

Weihaw Chuang Google, Inc. 1600 Amphitheatre Parkway Mountain View, CA 94043 US EMail: