Re: postfix filter to encrypt incoming emails with public gpg key
Dnia 27.10.2019 o godz. 17:52:03 Fourhundred Thecat pisze: > > I am using Postfix with Dovecot. I believe it is Dovecot who saves > messages to maildir. Not necessarily. I'm using Postfix with Dovecot too and I'm using Postfix's default local(8) to store mail. I didn't enable Dovecot's LDA. That needs to be specially set up in main.cf and/or master.cf. -- Regards, Jaroslaw Rafa r...@rafa.eu.org -- "In a million years, when kids go to school, they're gonna know: once there was a Hushpuppy, and she lived with her daddy in the Bathtub."
Re: postfix filter to encrypt incoming emails with public gpg key
Fourhundred Thecat: > On 27/10/2019 17.10, Wietse Venema wrote: > > Use the local(8) delivery agent. In your $HOME/.forward file, pipe > > the mail into a program that encrypts it with your public key, then > > writes the result to maildir. > > I am using Postfix with Dovecot. I believe it is Dovecot who saves > messages to maildir. How would that fit in ? > > > Postfix content filter is not suitable, as it is designed to > > handle multi-recipient email, whereas $HOME/.forward is guaranteed > > to handle only one recipient. > > I see. But, on the other hand, if I used a SMTMD-milter, it would be > processed before the queue, and would not interfere with the normal flow > through the pipeline. Again, it does not handle the multi-recipient case. Now, there may be an implicit assumption that email has only one, but that is not true in the general case (where I make recommendations). Wietse
Re: postfix filter to encrypt incoming emails with public gpg key
On 27/10/2019 18.20, Stephen Satchell wrote: >> are you perhaps confusing decryption with verifying the senders signature ? > > No. Signature verification and decrypting are two separate operations. > You will have to investigate how your mail client handles mail that has > been encrypted with one key, and signed by another. email is always encrypted with different key, than the one it is signed with. For signing, senders private key is used. For encryption, my my public key is used.
Re: postfix filter to encrypt incoming emails with public gpg key
On 10/27/19 7:38 AM, Fourhundred Thecat wrote: >> Further, the client would need to support the decryption of >> superencrypted mail, > there will be no "superencrypted" emails. As I explained in the first > sentence of my original description, I want to process only emails which > are not already encrypted. OK, that's sensible. >> once with your private key, then again with the >> public key of the sender. > are you perhaps confusing decryption with verifying the senders signature ? No. Signature verification and decrypting are two separate operations. You will have to investigate how your mail client handles mail that has been encrypted with one key, and signed by another.
Re: postfix filter to encrypt incoming emails with public gpg key
On 27 Oct 2019, at 10:52, Fourhundred Thecat <400the...@gmx.ch> wrote: > On 27/10/2019 17.10, Wietse Venema wrote: >> Use the local(8) delivery agent. In your $HOME/.forward file, pipe >> the mail into a program that encrypts it with your public key, then >> writes the result to maildir. > > I am using Postfix with Dovecot. I believe it is Dovecot who saves > messages to maildir. How would that fit in ? Then look at dovecot, it has options to store encrypted mail IIRC. (But that is OT for this list). -- I is for IDA who drowned in a lake J is for JAMES who took lye by mistake
Re: postfix filter to encrypt incoming emails with public gpg key
On 27/10/2019 17.10, Wietse Venema wrote: > Use the local(8) delivery agent. In your $HOME/.forward file, pipe > the mail into a program that encrypts it with your public key, then > writes the result to maildir. I am using Postfix with Dovecot. I believe it is Dovecot who saves messages to maildir. How would that fit in ? > Postfix content filter is not suitable, as it is designed to > handle multi-recipient email, whereas $HOME/.forward is guaranteed > to handle only one recipient. I see. But, on the other hand, if I used a SMTMD-milter, it would be processed before the queue, and would not interfere with the normal flow through the pipeline.
Re: postfix filter to encrypt incoming emails with public gpg key
Fourhundred Thecat: [encryption at rest, but not whole-disk encryption] > With my scheme, all emails would be stored encrypted on my server, and > decryption key does not exist on the server (emails are decrypted on my > local client) > > What would be the best way to implement this ? Use the local(8) delivery agent. In your $HOME/.forward file, pipe the mail into a program that encrypts it with your public key, then writes the result to maildir. Postfix content filter is not suitable, as it is designed to handle multi-recipient email, whereas $HOME/.forward is guaranteed to handle only one recipient. > Can such filter work, without ever storing plaintext email on disk ? Postfix (like other MTAs) persists email in the queue before it is delivered. If that is a problem, don't use an MTA. Instead, use a proxy. > Any other comments ? > > thanks, >
Re: postfix filter to encrypt incoming emails with public gpg key
On 27/10/2019 15.23, Stephen Satchell wrote: > OP, let me ask this: your proposal appears to be to modify the delivery > agent so that, instead of storing e-mail in cleartext, it insteads use > the public part of a public/private keypair to encrypt the payload of > incoming email. I did more research in the meantime, and I think the only reasonable way to implement my idea would be via a SMTP-Only Milter. > That way, your mail client would need to decrypt the > payload when reading the mail, using the private part of the keypair. yes. As it does now with messages that have been encrypted by the sender. > Further, the client would need to support the decryption of > superencrypted mail, there will be no "superencrypted" emails. As I explained in the first sentence of my original description, I want to process only emails which are not already encrypted. > once with your private key, then again with the > public key of the sender. are you perhaps confusing decryption with verifying the senders signature ?
Re: postfix filter to encrypt incoming emails with public gpg key
On 10/27/19 6:48 AM, Fourhundred Thecat wrote: > On 27/10/2019 13.29, Ansgar Wiechers wrote: >> Several years ago I wrote something like that [1]. However, if your mail >> server is untrusted I don't think there's a point in bothering. > > no server is 100% trusted. By this logic, should I therefore give up? It depends on what attack you are trying to protect against. You need to think outside the box, though. Have you considered not storing mail for any significant period on your server, and instead unloading new mail into an "inside" system? MTAs are "push" in that outsiders initiate connections to it to send or receive mail. So you have a workstation that connects to the pop3s port to pull down all mail, and do it aggressively, to minimize each mail message's exposure to ne'er-do-wells. > fetchmail is a mail-retrieval and forwarding utility; it fetches mail > from remote mailservers and forwards it to your local (client) > machine's delivery system. You can then handle the retrieved mail > using normal mail user agents such as mutt(1), elm(1) or Mail(1). The > fetchmail utility can be run in a daemon mode to repeatedly poll one or > more systems at a specified interval. This won't stop man-in-the-middle attacks, particularly the techniques used by sysadmins to satisfy LEO search warrants. But it will stop non-persistant wholesale harvesting of your mailstore. Of course, IMAP would not be available because fetchmail with POP3 would remove the mail when it's forwarded to your protected, Internet-isloated mailstore. OP, let me ask this: your proposal appears to be to modify the delivery agent so that, instead of storing e-mail in cleartext, it insteads use the public part of a public/private keypair to encrypt the payload of incoming email. That way, your mail client would need to decrypt the payload when reading the mail, using the private part of the keypair. Further, the client would need to support the decryption of superencrypted mail, once with your private key, then again with the public key of the sender. That dual-decryption capability is not available off-the-shelf to the best of my knowledge.
Re: postfix filter to encrypt incoming emails with public gpg key
On 27/10/2019 13.29, Ansgar Wiechers wrote: > Several years ago I wrote something like that [1]. However, if your mail > server is untrusted I don't think there's a point in bothering. no server is 100% trusted. By this logic, should I therefore give up? > Even if > you pass the mail through an encrypted transport channel and never store > it on disk, an attacker who has gained control of the server can still > intercept the message. an attacker listening on the server could intercept new message before it is encrypted. But he could not read messages from the past, now stored encrypted on the disk. > [1]: https://www.planetcobalt.net/sdb/crypter.shtml shouldn't this be implemented as a SMTP-Only Milter ?
Re: postfix filter to encrypt incoming emails with public gpg key
On 2019-10-27 Fourhundred Thecat wrote: > when new email arrives, and it is not already encrypted, I would like to > run it through a filter, which would encrypt the message with my public > gpg key, as if the original sender has sent the email encrypted. > > Why do I want to do this ? Why not ask the sender to send encrypted > messages to start with ? > > Lets say my bank sends me emails. I cannot forcer my bank to use gpg > encryption. I am happy they use email at all, instead of paper mail. > > My email server is untrusted. It can be hacked into and emails stolen. > Full disk encryption will not help, because the disk must be decrypted > during runtime. > > With my scheme, all emails would be stored encrypted on my server, and > decryption key does not exist on the server (emails are decrypted on my > local client) > > What would be the best way to implement this ? > > Can such filter work, without ever storing plaintext email on disk ? Several years ago I wrote something like that [1]. However, if your mail server is untrusted I don't think there's a point in bothering. Even if you pass the mail through an encrypted transport channel and never store it on disk, an attacker who has gained control of the server can still intercept the message. [1]: https://www.planetcobalt.net/sdb/crypter.shtml Regards Ansgar Wiechers -- "Abstractions save us time working, but they don't save us time learning." --Joel Spolsky
Re: postfix filter to encrypt incoming emails with public gpg key
Some while ago, I had a Perl script around Mail::GPG as mailbox_command, or inside a procmailrc, I'm not sure. I had it trigger only for a certain address extension, e.g. mailbox+...@domain.tld. It worked quite alright. > Can such filter work, without ever storing plaintext email on disk ? > > Any other comments ?
Re: postfix filter to encrypt incoming emails with public gpg key
> On Oct 27, 2019, at 12:20 PM, Matus UHLAR - fantomas > wrote: > > Encrypting mail at postfix level could create false sense of security. > How do you know that nobody can read it on the server bore it becomes > encrypted? > > And what's the poing of encrypting mail to you, when it came through the > world non-encrypted? Data at rest needs to be protected for a long time, it makes sense in some cases to encrypt stored messages. Retaining usability of an an encrypted mailstore is challenging, and I'd personally opt for an encrypted file-system so that the messages can be indexed and searched, others may have different priorities. -- Viktor.
Re: postfix filter to encrypt incoming emails with public gpg key
On 27/10/2019 10.25, Sam Tuke wrote: As well as fetching the public key, it'd need access to a private key too. I think the private key is considered the bigger problem, for various reasons. On 27.10.19 10:40, Fourhundred Thecat wrote: The scheme that I am describing needs only public key on the server. Not sure why you would think otherwise. encrypting mail at postfix level could create false sense of security. How do you know that nobody can read it on the server bore it becomes encrypted? And what's the poing of encrypting mail to you, when it came through the world non-encrypted? -- Matus UHLAR - fantomas, uh...@fantomas.sk ; http://www.fantomas.sk/ Warning: I wish NOT to receive e-mail advertising to this address. Varovanie: na tuto adresu chcem NEDOSTAVAT akukolvek reklamnu postu. I feel like I'm diagonally parked in a parallel universe.
Re: postfix filter to encrypt incoming emails with public gpg key
On 27/10/2019 10.25, Sam Tuke wrote: > As well as fetching the public key, it'd need access to a private key too. I > think the private key is considered the bigger problem, for various reasons. The scheme that I am describing needs only public key on the server. Not sure why you would think otherwise. > There have been a few attempts addressing the needs of this complex use case. > AFAICS none have been successful, but I'm out of date. > See the (abandoned?) STEED project and their whitepaper: > https://g10code.com/steed.html. That is by g10code - the creator of GPG. > Disclaimer: I once worked for them. quick look at the link suggests that this is not the problem I am trying to solve. but thanks anyway for your input.
Re: postfix filter to encrypt incoming emails with public gpg key
As well as fetching the public key, it'd need access to a private key too. I think the private key is considered the bigger problem, for various reasons. There have been a few attempts addressing the needs of this complex use case. AFAICS none have been successful, but I'm out of date. See the (abandoned?) STEED project and their whitepaper: https://g10code.com/steed.html. That is by g10code - the creator of GPG. Disclaimer: I once worked for them. Sam. On 27 October 2019 07:27:53 CET, lists wrote: >Let me try again. So the email comes in. Some programs gets your public >key and then encrypts the email on the server. Then when you retrieve >your email, it sends it out in what it believes is plain text or for >that matter can to TLS on the file, but you get a GPG message that you >then decrypt. > >So the reason this isn't normally done is a general purpose email >server would have to do this on per client basis, somehow getting the >proper public key for each client. > >Am I right? Close? > >If not I will shut up and wait for a guru to reply. > > > > > > > Original Message > > > >From: 400the...@gmx.ch >Sent: October 26, 2019 10:46 PM >To: postfix-users@postfix.org >Subject: Re: postfix filter to encrypt incoming emails with public gpg >key > > >On 27/10/2019 06.26, lists wrote: >> My bank insists I use their website for anything secure. I don't get >anything in my email that would be a security problem. > >I used bank just as an example. Feel free to substitute another >scenario, if you find mine hard to imagine. > >> Wouldn't a private key have to be held on your server to do what you >want? If so, that hacker can get the key. > >No. Definitely not. >Only public key is needed for asymmetric encryption. > >> Personally I would harden the server. It sounds like this is a >private server. You can use the firewall to vastly limit the countries >where your email can be retrieved. That is filter the hell out of all >email ports except 25. Besides filtering countries, I have a file of >about 30k of ipv4 cidrs from data centers that I block from all email >ports except 25 and all the web ports. No eyeballs in datacenters. > >Sure, I want to have both: >A secure server, AND encrypted emails. What is wrong with that ? -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
Re: postfix filter to encrypt incoming emails with public gpg key
TLS makes no difference, but you might as run the server as close to normal as possible. Original Message From: 400the...@gmx.ch Sent: October 26, 2019 11:52 PM To: postfix-users@postfix.org Subject: Re: postfix filter to encrypt incoming emails with public gpg key On 27/10/2019 07.27, lists wrote: > Let me try again. So the email comes in. Some programs gets your public key > and then encrypts the email on the server. I imagine, in theory it should work like this: New email comes in, and as it moves through the Postfix mail delivery pipeline, at some stage there is a simple filter, which performs an action. There should be some possibility to define simple rules, such as if recipeint = us...@mydomain.com perform action else continue Such process would need to have the users public key, obviously. But that is the least of an issue. I don't understand Postfix enough, to see how this can be implemented in practice. > Then when you retrieve your email, it sends it out in what it believes is > plain text or for that matter can to TLS on the file, but you get a GPG > message that you then decrypt. When I retrieve my message over IMAP, it will be retrieved as any other message, regardless whether it is encrypted or not. Also, TLS is irrelevant here. > So the reason this isn't normally done is a general purpose email server > would have to do this on per client basis, somehow getting the proper public > key for each client. I think the reason why this is not normally done, is that my request is quite exotic. I understand that. I think average mail user does not need this.
Re: postfix filter to encrypt incoming emails with public gpg key
On 27/10/2019 07.27, lists wrote: > Let me try again. So the email comes in. Some programs gets your public key > and then encrypts the email on the server. I imagine, in theory it should work like this: New email comes in, and as it moves through the Postfix mail delivery pipeline, at some stage there is a simple filter, which performs an action. There should be some possibility to define simple rules, such as if recipeint = us...@mydomain.com perform action else continue Such process would need to have the users public key, obviously. But that is the least of an issue. I don't understand Postfix enough, to see how this can be implemented in practice. > Then when you retrieve your email, it sends it out in what it believes is > plain text or for that matter can to TLS on the file, but you get a GPG > message that you then decrypt. When I retrieve my message over IMAP, it will be retrieved as any other message, regardless whether it is encrypted or not. Also, TLS is irrelevant here. > So the reason this isn't normally done is a general purpose email server > would have to do this on per client basis, somehow getting the proper public > key for each client. I think the reason why this is not normally done, is that my request is quite exotic. I understand that. I think average mail user does not need this.
Re: postfix filter to encrypt incoming emails with public gpg key
Let me try again. So the email comes in. Some programs gets your public key and then encrypts the email on the server. Then when you retrieve your email, it sends it out in what it believes is plain text or for that matter can to TLS on the file, but you get a GPG message that you then decrypt. So the reason this isn't normally done is a general purpose email server would have to do this on per client basis, somehow getting the proper public key for each client. Am I right? Close? If not I will shut up and wait for a guru to reply. Original Message From: 400the...@gmx.ch Sent: October 26, 2019 10:46 PM To: postfix-users@postfix.org Subject: Re: postfix filter to encrypt incoming emails with public gpg key On 27/10/2019 06.26, lists wrote: > My bank insists I use their website for anything secure. I don't get anything > in my email that would be a security problem. I used bank just as an example. Feel free to substitute another scenario, if you find mine hard to imagine. > Wouldn't a private key have to be held on your server to do what you want? If > so, that hacker can get the key. No. Definitely not. Only public key is needed for asymmetric encryption. > Personally I would harden the server. It sounds like this is a private > server. You can use the firewall to vastly limit the countries where your > email can be retrieved. That is filter the hell out of all email ports except > 25. Besides filtering countries, I have a file of about 30k of ipv4 cidrs > from data centers that I block from all email ports except 25 and all the web > ports. No eyeballs in datacenters. Sure, I want to have both: A secure server, AND encrypted emails. What is wrong with that ?
Re: postfix filter to encrypt incoming emails with public gpg key
On 27/10/2019 06.26, lists wrote: > My bank insists I use their website for anything secure. I don't get anything > in my email that would be a security problem. I used bank just as an example. Feel free to substitute another scenario, if you find mine hard to imagine. > Wouldn't a private key have to be held on your server to do what you want? If > so, that hacker can get the key. No. Definitely not. Only public key is needed for asymmetric encryption. > Personally I would harden the server. It sounds like this is a private > server. You can use the firewall to vastly limit the countries where your > email can be retrieved. That is filter the hell out of all email ports except > 25. Besides filtering countries, I have a file of about 30k of ipv4 cidrs > from data centers that I block from all email ports except 25 and all the web > ports. No eyeballs in datacenters. Sure, I want to have both: A secure server, AND encrypted emails. What is wrong with that ?
Re: postfix filter to encrypt incoming emails with public gpg key
My bank insists I use their website for anything secure. I don't get anything in my email that would be a security problem. That said, have you inquired if your bank will use pgp? I know that sounds like crazy talk, but some banks have PGP. (OT but note Amazon can do PGP too.) Wouldn't a private key have to be held on your server to do what you want? If so, that hacker can get the key. Personally I would harden the server. It sounds like this is a private server. You can use the firewall to vastly limit the countries where your email can be retrieved. That is filter the hell out of all email ports except 25. Besides filtering countries, I have a file of about 30k of ipv4 cidrs from data centers that I block from all email ports except 25 and all the web ports. No eyeballs in datacenters. Don't use roundcube or squirrelmail. Use email clients. Don't use cpanel or similar. Again keep the attack surface to a minimum. You can maintain a server strictly from command line. Use SSHGuard or fail2ban. Original Message From: 400the...@gmx.ch Sent: October 26, 2019 8:30 PM To: postfix-users@postfix.org Subject: postfix filter to encrypt incoming emails with public gpg key Hello, when new email arrives, and it is not already encrypted, I would like to run it through a filter, which would encrypt the message with my public gpg key, as if the original sender has sent the email encrypted. Why do I want to do this ? Why not ask the sender to send encrypted messages to start with ? Lets say my bank sends me emails. I cannot forcer my bank to use gpg encryption. I am happy they use email at all, instead of paper mail. My email server is untrusted. It can be hacked into and emails stolen. Full disk encryption will not help, because the disk must be decrypted during runtime. With my scheme, all emails would be stored encrypted on my server, and decryption key does not exist on the server (emails are decrypted on my local client) What would be the best way to implement this ? Can such filter work, without ever storing plaintext email on disk ? Any other comments ? thanks,
