... a non-proprietary protocol for encrypting email communication using public key cryptography. It is based on the original PGP (Pretty Good Privacy) software. The OpenPGP protocol defines standard formats for encrypted messages, signatures, and certificates for exchanging public keys.
The primary purpose of OpenPGP relates to end-to-end encrypted email communication, but it is also utilized for encrypted messaging, password managers and other use cases. OpenPGP provides support for all major operating systems like Windows, macOS, GNU/Linux, Android and iOS. The openPGP standard was proposed to the Internet Engineering Task Force (IETF) and accepted as a standard in 1997. The standard has been continuously improved since that time and there is no suggestion intelligence agencies can break it. 
OpenPGP has an impressive resume and a host of applications, developer libraries and miscellaneous tools have implemented the OpenPGP standard either directly or with additional software. Applicable domains include:
- Email encryption [archive]: for various platforms, browser plugins, webmail providers with browser plugins, and webmail providers with in-browser cryptography
- Server software [archive]: webmail clients, keyservers, mailing list software and password managers
- Developer libraries [archive]: various OpenPGP libraries, libraries supporting OpenPGP smartcards, and developer tools
- Miscellaneous tools [archive]: web-based tools and other applications
Figure: How PGP Works 
To learn more about this topic, refer to the following resources:
- OpenPGP website [archive]
- OpenPGP on GitHub [archive]
- Pretty Good Privacy on Wikipedia [archive]
- Verifying Software Signatures
To search for keys by email address, Key ID or Fingerprint, visit the
keys.openpgpg.org server [archive] (.onion [archive]). This publicly available service ("keyserver") enables the uploading, discovery, distribution and management of OpenPGP-compatible keys.
Issues with PGP
PGP has very bad usability and cryptography. Common PGP clients such as GnuPG are also not very secure. Thus, it is recommended against by many cryptography experts.
- What's the matter with PGP? [archive]
- The PGP Problem [archive]
- I'm giving up on PGP [archive]
- GPG And Me [archive]
- signify: Securing OpenBSD From Us To You [archive]
In English Language
- OpenPGP For Beginners [archive]
- OpenPGP Getting Started [archive]
- OpenPGP Help Spread [archive]
- How to use GnuPG securely [archive]
- See also if there are any crypto parties [archive] in your vicinity.
- Try typing "crypto party" followed by the nearest city in a search engine of your choice.
In German Language
- Informationen über OpenPGP [archive]
- kostenlose Schulungsangebote [archive]
- Förderung von OpenPGP [archive]
- Wie man GnuPG sicher verwenden kann [archive]
Air Gapped OpenPGP Key
Clearsign with Multiple Keys
The OpenPGP Web of Trust
Bootstrapping OpenPGP Keys from the Internet
Some OpenPGP users are unsure how to stay totally anonymous or what steps to take if there is no trusted path to an OpenPGP key.
Some people just write an unencrypted mail to the recipient and ask them to send their public key. The recipient will most likely either send their public key or at least its fingerprint.
This method works against passive attacks since an observer will not know what has been discussed in the following encrypted mails. However, this method totally fails against active attacks. A man-in-the-middle could replace the recipient's key with their own malicious key. The sender would then use the wrong key, the man-in-the-middle would decrypt the message, read it, and re-encrypt it with the legitimate key and forward it to the recipient. Neither the sender nor recipient would ever discover that their messages were being read by an adversary. This is the whole reason why the trust model path and key signing is recommended in the first place.
As an alternative, some people also publish their OpenPGP fingerprint or their OpenPGP public key on their personal or other websites. This method is more secure if the website is accessible over TLS -- even more so when both server and client are using HSTS [archive] (and DNSSEC [archive]) -- and/or as an onion service with a
.onion domain. Of course, this presupposes the visitor is aware what kind of transportation mechanism is provided. In this case, the adversary would have to:
- break the TLS or onion encryption where someone wants to obtain the key or fingerprint; or
- compromise the server
Depending on the adversary this may take more resources. It should be noted that both the public CA system of TLS and Tor onion services have security issues; see TLS and Onion Services Security for further information.
Based on the preceding discussion, it is safest if the same website is accessible over both https and
.onion. This configuration allows the user to visit both sites and then check if the OpenPGP fingerprint/key results match.
To further improve the situation the key holder can spread its fingerprint and/or OpenPGP key to other websites. Some key holders attach their OpenPGP fingerprint to their email signature (a short attachment to any mail) and to public mailing lists they are participating in. This behavior makes it difficult for an adversary to successfully spoof all these information avenues. Spreading the fingerprint and/or key in various domains is only helpful if the person searching is either aware of that fact or has undertaken research on their own initiative.
GnuPG Key Encryption vs OpenPGP Hardware Protection
Table: GnuPG Key Encryption vs OpenPGP Hardware Protection Comparison
|Domain||GnuPG Encryption of OpenPGP Private Key||Hardware Protection |
|OpenPGP private key can be protected by software encryption||Yes ||No |
|OpenPGP private key can be stored encrypted on storage||Yes ||No  |
|Software or device implementation: 100% Freedom Software||Yes||No |
|Can be independently reproduced and audited||Yes||No |
|Number of people capable of auditing the implementation||Small ||Very small |
|OpenPGP private key is protected by hardware, making it very difficult to extract the key from the device's storage||No||Yes |
|Wipes OpenPGP private key once physical tampering is detected (FIPS 140-2 Level 3 [archive])||No||No (?) |
|OpenPGP private key access notification||No||Some, yes (?) |
|Secret  can be entered on a secure external device ||No||Some, yes |
|OpenPGP private key can no longer be abused once detached from malware infected machine||No ||Yes |
|User difficulty in remembering secret||Medium ||Easy |
|Usability||Medium ||Difficult |
|OpenPGP private key security once lost (without machine compromise) ||Protected by software encryption  ||Protected by hardware    |
|OpenPGP private key cannot be used by malware  when key attached  and passphrase stolen  or pin cached||No||No|
|OpenPGP private key cannot be extracted, deleted or revoked by malware||No ||Yes|
|Encryption of user's data is still effective once malware infected||No ||No |
|Signatures cannot be created by malware||No||No|
|Signature counter on hardware device||No||Yes |
|Signature counter on computer display trustworthy if attached on malware infected computer||Not applicable||No |
|Signature counter trustworthy if attached on malware-free machine ||Not applicable||Yes|
In conclusion, both options have unique advantages. Unfortunately, it is not yet possible to combine both options. 
Why aren't the SKS keyserver wiki steps always functional?
The SKS keyserver network has recently come under attack after a critical vulnerability was discovered which allows certificates to be spammed using a flaw in the OpenPGP protocol itself. Future releases of OpenPGP software will likely mitigate this flaw, but high profile contributors to the protocol suggest that data should not be retrieved form the network at present if possible. For more details, see here [archive].  
Is the OpenPGP keyserver part of the SKS pool?
No. The federation model of the SKS pool has various problems in terms of reliability, abuse-resistance, privacy, and usability. We might do something similar to it, but keys.openpgp.org will never be part of the SKS pool itself.
- https://www.openpgp.org/about/ [archive]
- https://www.openpgp.org/ [archive]
- https://en.wikipedia.org/wiki/Pretty_Good_Privacy#OpenPGP [archive]
- https://en.wikipedia.org/wiki/File:PGP_diagram.svg [archive]
- As defined by TUF: Attacks and Weaknesses:
- https://lists.gnupg.org/pipermail/gnupg-users/2015-January/052185.html [archive]
- Token or smartcard.
- If a secure password has been chosen to protect the OpenPGP private key.
- The OpenPGP private key is stored unencrypted on the storage of the smartcard or token.
- Stored on HDD, SSD, USB etc.
- Stored on the memory chip of the smartcard or token.
* gnupg-users mailing list: Possible to combine smartcard PIN with key password? [archive]
* Patrick Schleizer specifically asked Werner Koch about this at c3c1. This is neither possible nor a planned or likely future feature.
- The specification may be Freedom Software, but currently there are no blueprints for smartcards or tokens that meet this definition.
- Due to the absence of blueprints and copyright, no other company can reproduce/audit security.
- Written in C and using cryptography. This is a difficult endeavor and beyond most individuals.
- Hardware is more difficult than software and this particular type of hardware is even harder. Only the manufacturing engineers are capable of understanding it. In the case of closed hardware implementations, which is the case for most (if not all) tokens and smartcards , open and independent audits are impossible.
- It is very difficult to read the storage of smartcards and tokens. Professional data recovery companies usually decline requests for recovery from such storage.
- The author is unaware of any OpenPGP tokens or smartcards that claim to possess this security feature.
- It is understood some external smartcard readers blink a flashlight and/or tone when the key is being accessed.
- Password versus pin.
- On an external device that is usually not infected by malware.
- For example spr332 devices.
- A skilled adversary will take a copy of both the OpenPGP private key and the passphrase, just in case that the user manages to move to a malware-free machine.
- Since adversaries cannot extract the key to get a copy, once the compromise has been undone -- either by chance, by doing a clean re-installation which the malware did not survive or because the malware is noticed -- future files/mails encrypted to the private key can no longer be decrypted by the adversary. No new malicious signatures can be made anymore. Revoking previously compromised keys would still be advisable, because the adversary may have created a large number of signatures for all sorts of texts and/or files.
- A long passphrase.
- A medium-sized pin.
- Arguably, OpenPGP is not that simple. In comparison, OTR has a smaller feature set (for example, no signatures for publicly released files), but encryption is much more usable. OTR is younger and has more users.
- Initial setup is harder than GnuPG private key password encryption and generally difficult.
* no malware on user's computer
* either the storage (HDD, SSD, USB etc.) holding the OpenPGP private key vs token or smartcard has been lost or stolen.
- The adversary would require either:
* a backdoor in GnuPG itself; or
* a backdoor in the distribution that shipped GnuPG; or
* found a vulnerability in GnuPG; or
* found an effective weakness in the encryption algorithm used by GnuPG.
- No vulnerabilities in GnuPG's private OpenPGP key encryption has ever been reported.
- Quote Wikipedia [archive]:
Smart cards can be physically disassembled by using acid, abrasives, or some other technique to obtain unrestricted access to the on-board microprocessor. Although such techniques involve a fairly high risk of permanent damage to the chip, and irrecoverable loss of the secret keys therein, they permit much more detailed information (e.g. photomicrographs of encryption hardware) to be extracted.
- Other, non-OpenPGP smartcards have been cracked in the past using ultra-expensive electron-scanning microscopes. Source: theguardian, How codebreakers cracked the secrets of the smart card [archive]
- http://www.cl.cam.ac.uk/~sps32/mcu_lock.html [archive]
- More information is required on how realistic, difficult and expensive these attacks are. Additional reader contributions on this issue are most welcome.
- If the machine in use has been compromised by malware like a trojan horse.
- On storage (HDD, SSD, USB etc.) vs on token or smartcard.
- By keylogger or extracted from memory once cached.
- Therefore make sure to have a backup on storage that is never attached to that machine.
- An adversary who managed to compromise the user's machine can use a keylogger to sniff the OpenPGP private key password once it is entered next time or extract it from memory if it is still cached. Any of the user's encrypted email, files, etc. (that are read from the user's devices or that have been extracted from other sources, such as by man-in-the-middle attacks or obtained from the user's mail provider and so forth) can then be decrypted.
- An adversary who managed to compromise the user's machine can wait until the user caches its pin next time. Otherwise, same as above. Once a machine has been compromised, nothing it shows can be trusted anymore, even if the PIN is never cached. If the PIN is cached or not is up to the software on the user's machine which can be no longer trusted once compromised. The adversary could install their own key caching software (gnupg-agent). Instead of the user's request "decrypt mail X", malware can also intercept this and make it "decrypt mail Y and X".
- Users could attach their smartcard to other computers, perhaps non-compromised, perhaps offline machines and check the signature counter. For example output, see: Using an OpenPGP Smartcard with GnuPG [archive].
- Once malware is running on a machine, nothing the machine displays can be trusted. It could be manipulated by the malware.
- Assumption: The token or card reader has not been compromised by malware running on the user's machine.
- New, experimental keyservers have been established which afford protection against this attack.
- The author notes the potential downsides of this attack:
- If you fetch a poisoned certificate from the keyserver network, you will break your GnuPG installation.
- Poisoned certificates cannot be deleted from the keyserver network.
- The number of deliberately poisoned certificates, currently at only a few, will only rise over time.
- We do not know whether the attackers are intent on poisoning other certificates.
- We do not even know the scope of the damage.
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