- Automated Encryption
- Trust Levels
- Conflict handling
- Key discovery and Opportunistic Encryption (Mail only)
- Screenshots from GpgOL
The idea behind automated encryption is simple: enable encryption without (much) user support. The goal is to not just protect users from passive attackers (e.g., someone listening), but also protect users from man-in-the-middle attacks and forgeries. At least, as much as possible without requiring much user support. In particular, we should only require help from the user if there is a good chance that she is being attacked.
This page is intended as a discussion base for validity display and opportunistic mail encryption and how to use the trust-model tofu+pgp for automated encryption.
Things to do after discussion and feedback not integrated in this draft:
- The yellow checker does not work as a symbol.
- Level 5 is not part of the screenshots.
- Write more about threats, mitigations, and attack scenarios.
What are our goals and how do we archive them?
- Prevent mass surveillance:
It's not difficult to imagine that all clear text emails are saved by many governments and immediately analyzed. By encrypting mail by default whenever possible, we dramatically increase the cost of this type of surveillance. For instance, the government would have to interfere with key discovery (similar to how [[https://www.eff.org/de/deeplinks/2014/11/starttls-downgrade-attacks|Verizon inhibited transport level security over SMTP]]) to prevent users from learning about their communication partners' keys.
Solution: way to find a reasonable key without user's help.
- Spam phishing:
Phishing is a common type of fraud. A simple example is an email that is apparently from your bank prompting you to take some action that requires you to log in. The link to the log-in site is actually to a site controlled by the attacker, which steals your credentials. Another example of such an attack is a mail containing malware in an attachment.
This type of attack can be prevented by using signatures to verify the sender's address. Since we don't require users to actively authenticate their communication partners, preventing this type of attack requires recognizing that the sender is attempting an impersonation.
Solution: there are two ways to detect this type of attack.
First, phishing attacks are successful, because the mail, etc. looks authentic. Thus, the attacker will try to imitate the real identity to avoid detection. A common technique is use an email address that is a homograph of the real email address, e.g., using a cyrillic a in place of a latin a. Google detects these types of phishing attacks by checking that email addresses fall into unicode's "highly restricted" restriction level" designation level. We could do something similar and show a warning if an email address doesn't pass the test.
Second, if we assume that the user will regularly receive signed emails from her bank, then we can exploit the communication history to show that signed messages from previously unseen / rarely seen email addresses shouldn't be trusted. This requires vigilance on the part of the user to realize that the message didn't verify, but should have. It also requires that the user be educated. Further, if a spamer uses the same email address & key many times, the email address may eventually appear to be trustworthy using this metric.
- Targeted (spear) phishing or CEO-Fraud:
This attack is similar to the spam phishing described above, but the stakes are higher. An example of this type of attack is when an assistant receives an email allegedly from the CEO requesting that the assistant immediately transfer some funds to a particular account. Unlike the above attack, in this case, the victim is targeted, and the potential monetary damage much higher.
Solution: again, automated techniques or the use of history cannot mitigate this attack; the employee must be trained to recognize certain signals. A possible mitigation is to have a list of fully trusted keys, and show messages that are signed with these keys differently. Note: this doesn't mean that the employee must necessarily curate this list; this can be done by the IT department.
- Man in the Middle attacks:
A Man-in-the-Middle (MitM) attack is when an adversary is actively decrypting and re-encrypting email. For this to work, the MitM must 1) get Alice and Bob to use keys that he controls and 2) re-encrypt every communication to avoid detection.
To get Alice and Bob to use keys that he controls, the MitM must intervene during the initial key discovery. In this case, we can detect the MitM attack when a valid message eventually gets through. This could occur if Alice receives a message via a channel that the attacker doesn't control.
If a good message gets through and is encrypted, Alice will be unable to decrypt it and she will probably tell Bob that something went wrong. Most likely, Alice and Bob will not be sufficiently technically savvy to diagnose the actual problem. Since everything will work when they use their usual communication channel, they will ignore the issue. To actually detect a conflict in this situation, Alice's MUA needs to fetch all of the keys specified in the PK-ESK packets. This will allow Alice's gpg to detect a conflict. Note: this scenario can occur due to a misconfiguration, e.g., the message is not encrypted to Alice.
If the message is only signed, then Alice will see that the message is signed with the wrong key. In this case, we can prompt Alice to contact Bob to figure out what the right key is, which gives us a chance of defeating the man-in-the-middle.
Note: if Bob proactively sends a message to Alice, then he will (hopefully) access Alice's key via an authenticated key stored, such as WKD, in which case, the attack would have to break the store's protection (e.g., TLS) to make sure Bob gets the attacker's key. On the other hand, if the attacker sends a forged message to Bob, and Bob just downloads the specified key from the key server, then the attacker has successfully intervened. This suggests that we should always check WKD for the right key, if possible.
If the MitM attempts to intervene after Alice and Bob have already successfully communicated, e.g., by sending Bob a forged message, then we can detect the MitM due to the conflict and we can prompt the users to exchange fingerprints to figure out the right key.
- Forensic detection of attacks: If an attack happened it will be detectable after the fact by the state of the attacked gnupg data directories. (See limitations below)
Limitations of the automation
The automated system can't protect against an attacker that controls the initial key exchange and persistently re-encrypts all messages with keys controlled by the attacker. This would be detectable if one message gets through and using a WKD would make this attack more expensive but until a User has manually exchanged / checked the fingerprint with his intended communication partner the User can't be sure if the communication is really secure.
This system still caters to users who need to authenticate some of their communication partners (see Level 3 / 4 below). It can be argued that the costs and detectability of such an attack would likely be higher then other attacks on the user's system such an attacker may be capable of.
Definitions of wording:
userid: The userid on a key that matches the smtp address of the sender. userid.validity is set as follows. By default, it is marginal. If the key is fully trusted via the web of trust, then it is Fully. If the key is explicitly marked as bad or unknown, then it is Never or Unknown, respectively.
tofu: Information we have about the communication history and reflects a bit the API of gpgme_tofu_info_t. As tofu info describes key + userid pairs, this is also sometimes called "binding".
key source: The source where the key was imported from, e.g. if it was automatically imported over https, or if it comes from public keyservers.
Key with enough history for basic trust:
(userid.validity <= Marginal AND tofu.signcount == 0 AND tofu.enccount == 0 AND key.source NOT in [cert, pka, danke, wkd])
With trust-model tofu+pgp this level is used only for Keys that were never used before to verify a signature and not obtained by a source that gives some indication that this is an actual key for this address.
The Key should not be used for opportunistic encryption to avoid the problem that the recipient might not be able to decrypt the message because it is a wrong or outdated key.
- Do not automatically encrypt to Level 0.
((userid.validity == Marginal AND tofu.validity < "Key with enough history for basic trust") AND key.source NOT in [cert, pka, dane, wkd]):
This level means that there is some confidence that the recipient actually can use the key as we have seen at least one signature or we have a weak trust path over the web of trust.
So it is unlikely that the recipient can't decrypt and thus its ok to use that Key for opportunistic encryption encryption but when receiving a signed message it should only show in details that a message was signed.
There may be some indication that the sender demonstrated a willingness to use crypto mails, especially if opportunistic encryption is disabled.
- Automatically encrypt to Level 1
- Don't show verified messages as much more trusted then an unverified message.
(userid.validity == Marginal AND ((tofu.validity >= "Key with enough history for basic trust" AND tofu.signfirst >= 3 Days ago) OR key.source IN [cert, pka, dane, wkd])):
The "automated user" that never uses any Certificate Manager or GnuPG will only see Level one and Level two as this is the highest level reachable through full automation.
We have basic confidence that the Sender is who he claims because we either have an established communication history or some "good enough" source of the key (e.g. the mail provider + https) provided the key for this sender.
Conflicts in this level are discussed more details below in the part about Conflict handling.
- Automatically encrypt to Level 2
- Show verified messages as more trusted then an unverified message.
userid.validity == Full AND "no key with ownertrust ultimate signed this userid."
Level 2 is good enough for most use cases, but some organizations or individuals or policies may require an assurance of confidentiality that may never reached through automation.
The idea is that Level 3-4 provide flexibility for Organizational Measures like: You may only send restricted documents to Level 4 keys.
This level is reached either through Web of Trust or if a user explicitly set the tofu Policy to "Good" for this key.
Automatically this Level can only be also be reached through WoT if a user trusted at least one other key.
This is also the level for S/MIME Mails.
- Automatically encrypt to Level 3
- You can assert in the ui that according to gnupg the sender is who he claims to be.
userid.validity == ultimate OR (userid.validity == full AND "any key with ownertrust ultimate signed this userid.")
Same reason as for Level 3. But even more restricted to direct trust, meaning that:
Either yourself or someone that is allowed to make that e.g. your central "CA" style person you may have in an organization has signed that key.
For a user this could also mean something like "with this communication partner I want to be absolutely sure that I'm always using the right key. So I manually verify the fingerprint. And mark that with a local or public signature on that key.
- Automatically encrypt to level 4
- Show verified messages as "the best". Stars and sprinkle level ;-)
Time delay for level 2
A time delay is supposed to make it more expensive for an attacker to reach level 2 as we then start to make claims about the attacker.
The assumption is that an attack that is kept up over some time is more difficult. Especially if it involves some external factors, like a phishing website etc. which might be turned off.
A time delay gives others the chance to intervene if they detect an attack e.g. if it is against a whole organization.
It also mitigates User Experience problems arising from the use of the encryption count in GnuPG's calculation for basic history because if you encrypt 20 drafts or 20 mails / files quickly to the same key it should not become level 2 before you have seen a signature.
A concern is that using the time of the first signature verification before reaching level 2 make lead to bad user experience. E.g.: You look at the same message after three days. Now it's level 2, last time you looked it was level 1.
HTTPS Trust as shortcut to level 2
Using HTTPS for key discovery will automatically bring a key to level 2 because in that case we have a claim by some authenticated source that this key really belongs to the according mail address.
If an attacker controls your HTTPS there are very likely cheaper and less detectable attacks on your communication then intercepting pgp encryption. e.g. compromising your system.
It's also harder to break HTTPS compared to SMTPS/IMAPS because every MUA offers to ignore certificate errors (which dirmngr does not) and a compromised router could claim that your MSP only offers SMTP / IMAP without encryption.
There should only be prominent information when reading a signed mail if:
- There is additional information that the sender is really is the intended communication partner. (Level >= 2)
This could be displayed as a checker or a seal ribbon or something. It should be prominent and next to the signed content. There should be a distinction between Levels two, three and four but it may be slight.
Don't treat signed mails worse then an unsigned mail
A MUA should not treat any signed mail worse then an unsigned mail. If a sender is not verified it should be displayed similar to an unsigned mail because in both cases you have no information that the Sender is actually your intended Communication partner. You may want to show a tofu Conflict more prominent as user interaction is required at this point.
Especially: ignore GPGME's Red suggestion An attacker would have removed the signature instead of invalidating it. It should be treated like an unsigned mail and only additional info in details should be shown for diagnostic purposes. Similarly when Red is set because a key is expired or so. It's not more negative then a unsigned mail so only if your MUA shows unsigned mails as "Red" may you treat signed mails this way, too ;-)
A TOFU conflict occurs when there are multiple keys with the same mailbox and it is not possible to automatically determine which ones are good. For instance, if two keys are cross-signed, then they are not considered to conflict; this is just a case of the user rotating her primary key.
Conflicts occur in two situations:
- A MitM controlled the initial key exchange. If a good message gets through, there will be a conflict. The "new" key is the correct key.
- An attacker attempts a MitM attack, but the user already has the right key. The "old" key is the correct key.
- An attacker sends a forged message. The "old" key is the correct key, or both are bad (the first key was also due to a forgery).
- There is a Troll trying to hurt usability so much that automated encryption is no longer used (i.e., many forgeries resulting in gratuitous conflicts)
- A user generated two keys e.g. on two devices and did not cross sign them and uses both.
- A user lost control of his old key, and did not have a revocation certificate.
Both misuse cases should be handled on the senders side because he controls or lost control of the involved keys and can take steps / inform himself what went wrong.
The second misuse case (inaccessible key) is likely more common than the first misuse case (multiple, valid keys). The first misuse case already leads to problems: communication partners need to choose which key to use.
Losing keys can also be assisted by software that provides a bad user experience or does not follow common practices.
Resolving conflicts on the senders side
When an application makes a signature, it should check that there are no other keys with the same user id. If there are and the private key material is available, then the application should prompt the user to make a cross signature
Similarly, if a signature is verified by a MUA, there are secret keys available with the same userid, and they are not cross signed, the user should be prompted to make a cross signature.
A tofu aware GUI should check when signing or from time to time if a different key is uploaded to a WKD for this userid and warn in that case. This will also make a permanent man in the middle attack by a mail service provider more expensive as it would mean providing a different key to the attacked user then to others.
Automated conflict resolution by recipients
Let us assume that there are two keys, K1 and K2, with the email address email@example.com, and that they are in conflict. In this case, we cannot with certainty resolve the conflict (otherwise, we would have resolved it automatically!). Consider:
- If we discover K2 because a good message finally got through a MitM attack, then K2 is the correct key.
- If we discover K2 due to a forgery, then K1 is the correct key.
Well, almost. In fact, the "correct key" could also be bad! For instance, if the user never interacted with Alice and got two different forgeries, then neither key is the correct one.
In all cases, we can resolve the conflict confidently if we fetch the key using WKS (assuming that HTTPS is not also being attacked, and the mail provider is acting in the interests of the user). (Note: whenever we encounter a signature by a new key, we shouldn't just get the key via the key servers, but also look it up by name using WKS. This immediately detects forgeries.)
Except, we don't want to do this, because we would have two good keys and something is wrong. If a user has lost control / lost his old key and is unable to revoke it we want to create problems for the sender, so that we can get notified by the sender that the new key should be used and we can mark the old key explicitly as bad.
If WKS is not available and we assume that at least one of the keys is probably good, then in all of the situations outlined above except for one (a good message gets through a MitM attack), then the old key is the correct key.
If we consider the good message from Bob getting through a MitM attack, we find that there are two situations: the good message is signed, and the good message is signed and encrypted.
If the message is signed and encrypted, then Alice will be unable to decrypt it (because the MitM didn't reencrypt it), and we won't actually see a conflict, because we never see the signature and consequently we never see a conflict. The failing-to-decrypt message, however, will likely cause Alice to talk to Bob. But, it is unlikely that they will correctly diagnose the problem, in particular, as once the MitM is back in control, everything will continue to work. Thus, they will likely conclude that there was a misconfiguration or a bug.
If the message is only signed, then Alice fetches the key used to sign the message (since she hasn't see it before) and we detect a conflict.
Thus, if we are feeling particularly bold, we might conclude that the above scenario (MitM + good signed message getting through) is sufficiently rare, that we'll automatically decide for old key if there is a conflict. This doesn't preclude us from showing the verification status as being "not trusted" (i.e., level 0) which might (in some ideal world) spur the user to try and figure out why mails from that particular user are no longer marked as verified.
Does this model protect against our threats?
- Man in the middle with full control of communication history: If Alice and Bob always encrypt & sign but Mallory had full control of their communication history and re-encrypts / resigns the messages regularly this attack could be detected:
a) When a Mail gets trough once e.g. if Mallory controls Alice's router and Alice uses an Internet Cafe once. In that case Bob would not be able to decrypt the message. That communication failure could lead to more investigation. A MUA may assist that by showing to which keys a mail was encrypted in case decryption fails.
b) If Alice publishes her key in a Web key directory Bob's MUA can detect that the key used for Alice does not match the one he always used and can signal this by not showing Alice's signatures as "Good" indicating that the communication is not protected.
The Attack is more problematic if Alice and Bob don't encrypt but just sign. In case a) this would mean that the mails that get through from the real Alice would not be shown as valid. But once a mail gets through from the real Alice the messages from both Alice and Mallory will not be shown as valid anymore, indicating that the communication is not secure.
- Man in the Middle attack with established communication:
This attack will be prevented by keeping the established key in use so messages won't get encrypted to Mallory. By not showing the validity indication anymore this attack is also detectable.
- Impostor attack: would still be prevented because the new key is never shown as valid / verified unless user interaction is done.
- The Troll Attack will be prevented as conflicts are not shown so prominent. Using a Web key directory can also mitigate this attack because it will prevent marking many keys as bad trough spam.
Key discovery and Opportunistic Encryption (Mail only)
A MUA should offer automated key discovery and opportunistic encryption. The WKD / WKS helps with automated key discovery and should be used (by using --locate-key)
To determine if a mail can be sent automatically encrypted one of the following rules must match for each recipient:
- There is a Fully / Ultimately valid key for the recipient.
- This is a marginally valid key for the recipient and the first UserID that matches the recipients mailbox has a signcount of at least one. Or the recipients key was obtained through a slightly authenticated source (e.g. WKD).
Auto Key Retrieve
Additionally to WKD lookup, if you receive a message from an Unknown Key a MUA should automatically retrieve it from a public keyserver or a Web Key directory. This Key can then be used for opportunistic encryption because you have seen a signature it is very likely that the recipient can decrypt. (auto-key-retrieve in gnupg)
Screenshots from GpgOL
No key was found. Level 0.
The first signed message form someone else. Level 1.
The second message. Level 1.
Basic trust! Level 2.
Full trust! Level 3.
Invalid sig. Level 0.
Tofu conflict. User attention required.