Pretty Good Privacy

Not to be confused withGNU Privacy Guard (GPG).

Pretty Good Privacy

Original author(s)	Phil Zimmermann PGP Inc. Network Associates PGP Corp.
Developer(s)	Broadcom Inc.
Initial release	1991(1991)
Stable release	11.4.0 Maintenance Pack 2 / May 23, 2023(2023-05-23)
Written in	C
Operating system	macOS,Windows
Standard(s)	OpenPGP: RFC 4880, 5581, 6637 PGP/MIME: RFC 2015, 3156
Type	Encryption software
License	Commercialproprietary software
Website	www.broadcom/products/advanced-threat-protection/encryption

Pretty Good Privacy(PGP) is anencryption programthat providescryptographicprivacyandauthenticationfordata communication.PGP is used forsigning,encrypting, and decrypting texts,e-mails,files, directories, and whole disk partitions and to increase thesecurityof e-mail communications.Phil Zimmermanndeveloped PGP in 1991.

PGP and similar software follow the OpenPGP standard (RFC 4880), an open standard forencryptingand decryptingdata.Modern versions of PGP areinteroperablewithGnuPGand other OpenPGP-compliant systems.

Design

How PGP encryption works visually

PGP encryption uses a serial combination ofhashing,data compression,symmetric-key cryptography,and finallypublic-key cryptography;each step uses one of several supportedalgorithms.Each public key is bound to a username or an e-mail address. The first version of this system was generally known as aweb of trustto contrast with theX.509system, which uses a hierarchical approach based oncertificate authorityand which was added to PGP implementations later. Current versions of PGP encryption include options through an automated key management server.

PGP fingerprint

Apublic key fingerprintis a shorter version of a public key. From a fingerprint, someone can validate the correct corresponding public key. A fingerprint like C3A6 5E46 7B54 77DF 3C4C 9790 4D22 B3CA 5B32 FF66 can be printed on a business card.

Compatibility

As PGP evolves, versions that support newer features andalgorithmscan create encrypted messages that older PGP systems cannot decrypt, even with a valid private key. Therefore, it is essential that partners in PGP communication understand each other's capabilities or at least agree on PGP settings.

Confidentiality

PGP can be used to send messages confidentially.For this, PGP uses ahybrid cryptosystemby combiningsymmetric-key encryptionand public-key encryption. The message is encrypted using a symmetric encryption algorithm, which requires asymmetric keygenerated by the sender. The symmetric key is used only once and is also called asession key.The message and its session key are sent to the receiver. The session key must be sent to the receiver so they know how to decrypt the message, but to protect it during transmission it is encrypted with the receiver's public key. Only the private key belonging to the receiver can decrypt the session key, and use it to symmetrically decrypt the message.

Digital signatures

PGP supports message authentication and integrity checking. The latter is used to detect whether a message has been altered since it was completed (themessage integrityproperty) and the former, to determine whether it was actually sent by the person or entity claimed to be the sender (adigital signature). Because the content is encrypted, any changes in the message will fail the decryption with the appropriate key. The sender uses PGP to create a digital signature for the message with either theRSAorDSAalgorithms. To do so, PGP computes a hash (also called amessage digest) from the plaintext and then creates the digital signature from that hash using the sender's private key.

Web of trust

Main article:Web of trust

Both when encrypting messages and when verifying signatures, it is critical that the public key used to send messages to someone or some entity actually does 'belong' to the intended recipient. Simply downloading a public key from somewhere is not a reliable assurance of that association; deliberate (or accidental) impersonation is possible. From its first version, PGP has always included provisions for distributing user's public keys in an 'identity certification', which is also constructed cryptographically so that any tampering (or accidental garble) is readily detectable. However, merely making a certificate that is impossible to modify without being detected is insufficient; this can prevent corruption only after the certificate has been created, not before. Users must also ensure by some means that the public key in a certificate actually does belong to the person or entity claiming it. A given public key (or more specifically, information binding a user name to a key) may be digitally signed by a third-party user to attest to the association between someone (actually a user name) and the key. There are several levels of confidence that can be included in such signatures. Although many programs read and write this information, few (if any) include this level of certification when calculating whether to trust a key.

The web of trust protocol was first described by Phil Zimmermann in 1992, in the manual for PGP version 2.0:

As time goes on, you will accumulate keys from other people that you may want to designate as trusted introducers. Everyone else will each choose their own trusted introducers. And everyone will gradually accumulate and distribute with their key a collection of certifying signatures from other people, with the expectation that anyone receiving it will trust at least one or two of the signatures. This will cause the emergence of a decentralized fault-tolerant web of confidence for all public keys.

The web of trust mechanism has advantages over a centrally managedpublic key infrastructurescheme such as that used byS/MIMEbut has not been universally used. Users have to be willing to accept certificates and check their validity manually or have to simply accept them. No satisfactory solution has been found for the underlying problem.

Certificates

Main article:Public key certificate

In the (more recent) OpenPGP specification,trust signaturescan be used to support creation ofcertificate authorities.A trust signature indicates both that the key belongs to its claimed owner and that the owner of the key is trustworthy to sign other keys at one level below their own. A level 0 signature is comparable to a web of trust signature since only the validity of the key is certified. A level 1 signature is similar to the trust one has in a certificate authority because a key signed to level 1 is able to issue an unlimited number of level 0 signatures. A level 2 signature is highly analogous to the trust assumption users must rely on whenever they use the default certificate authority list (like those included in web browsers); it allows the owner of the key to make other keys certificate authorities.

PGP versions have always included a way to cancel ('revoke') public key certificates. A lost or compromised private key will require this if communication security is to be retained by that user. This is, more or less, equivalent to thecertificate revocation listsof centralised PKI schemes. Recent PGP versions have also supported certificate expiration dates.

The problem of correctly identifying a public key as belonging to a particular user is not unique to PGP. All public key/private key cryptosystems have the same problem, even if in slightly different guises, and no fully satisfactory solution is known. PGP's original scheme at least leaves the decision as to whether or not to use its endorsement/vetting system to the user, while most other PKI schemes do not, requiring instead that every certificate attested to by a centralcertificate authoritybe accepted as correct.

Security quality

To the best of publicly available information, there is no known method which will allow a person or group to break PGP encryption by cryptographic, or computational means. Indeed, in 1995,cryptographerBruce Schneiercharacterized an early version as being "the closest you're likely to get to military-grade encryption."Early versions of PGP have been found to have theoretical vulnerabilities and so current versions are recommended.In addition to protectingdata in transitover a network, PGP encryption can also be used to protect data in long-term data storage such as disk files. These long-term storage options are also known as data at rest, i.e. data stored, not in transit.

The cryptographic security of PGP encryption depends on the assumption that the algorithms used are unbreakable by directcryptanalysiswith current equipment and techniques.

In the original version, theRSAalgorithm was used to encrypt session keys. RSA's security depends upon theone-way functionnature of mathematicalinteger factoring.Similarly, the symmetric key algorithm used in PGP version 2 wasIDEA,which might at some point in the future be found to have previously undetected cryptanalytic flaws. Specific instances of current PGP or IDEA insecurities (if they exist) are not publicly known. As current versions of PGP have added additional encryption algorithms, their cryptographic vulnerability varies with the algorithm used. However, none of the algorithms in current use are publicly known to have cryptanalytic weaknesses.

New versions of PGP are released periodically and vulnerabilities fixed by developers as they come to light. Any agency wanting to read PGP messages would probably use easier means than standard cryptanalysis, e.g.rubber-hose cryptanalysisorblack-bag cryptanalysis(e.g. installing some form oftrojan horseorkeystroke loggingsoftware/hardware on the target computer to capture encryptedkeyringsand their passwords). TheFBIhas already used this attack against PGPin its investigations. However, any such vulnerabilities apply not just to PGP but to any conventional encryption software.

In 2003, an incident involving seizedPsionPDAsbelonging to members of theRed Brigadeindicated that neither theItalian policenor the FBI were able to decrypt PGP-encrypted files stored on them.

A second incident in December 2006, (seeIn re Boucher), involvingUS customs agentswho seized alaptop PCthat allegedly containedchild pornography,indicates that US government agencies find it "nearly impossible" to access PGP-encrypted files. Additionally, a magistrate judge ruling on the case in November 2007 has stated that forcing the suspect to reveal his PGP passphrase would violate hisFifth Amendmentrights i.e. a suspect's constitutional right not to incriminate himself.The Fifth Amendment issue was opened again as the government appealed the case, after which a federal district judge ordered the defendant to provide the key.

Evidence suggests that as of 2007,British policeinvestigators are unable to break PGP,so instead have resorted to usingRIPAlegislation to demand the passwords/keys. In November 2009 a British citizen was convicted under RIPA legislation and jailed for nine months for refusing to provide police investigators with encryption keys to PGP-encrypted files.

PGP as acryptosystemhas been criticized for complexity of the standard, implementation and very low usability of the user interfaceincluding by recognized figures in cryptography research.It uses an ineffective serialization format for storage of both keys and encrypted data, which resulted in signature-spamming attacks on public keys of prominent developers ofGNU Privacy Guard.Backwards compatibility of the OpenPGP standard results in usage of relatively weak default choices of cryptographic primitives (CAST5cipher,CFBmode, S2K password hashing).The standard has been also criticized for leaking metadata, usage of long-term keys and lack offorward secrecy.Popular end-user implementations have suffered from various signature-striping, cipher downgrade and metadata leakage vulnerabilities which have been attributed to the complexity of the standard.

History

Early history

Phil Zimmermanncreated the first version of PGP encryption in 1991. The name, "Pretty Good Privacy" was inspired by the name of agrocerystore, "Ralph's Pretty Good Grocery", featured in radio hostGarrison Keillor's fictional town,Lake Wobegon.This first version included asymmetric-key algorithmthat Zimmermann had designed himself, namedBassOmaticafter aSaturday Night Livesketch. Zimmermann had been a long-timeanti-nuclear activist,and created PGP encryption so that similarly inclined people might securely useBBSsand securely store messages and files. No license fee was required for its non-commercial use, and the completesource codewas included with all copies.

In a posting of June 5, 2001, entitled "PGP Marks 10th Anniversary",Zimmermann describes the circumstances surrounding his release of PGP:

It was on this day in 1991 that I sent the first release of PGP to a couple of my friends for uploading to the Internet. First, I sent it to Allan Hoeltje, who posted it to Peacenet, an ISP that specialized in grassroots political organizations, mainly in the peace movement. Peacenet was accessible to political activists all over the world. Then, I uploaded it to Kelly Goen, who proceeded to upload it to a Usenet newsgroup that specialized in distributing source code. At my request, he marked the Usenet posting as "US only". Kelly also uploaded it to many BBS systems around the country. I don't recall if the postings to the Internet began on June 5th or 6th. It may be surprising to some that back in 1991, I did not yet know enough about Usenet newsgroups to realize that a "US only" tag was merely an advisory tag that had little real effect on how Usenet propagated newsgroup postings. I thought it actually controlled how Usenet routed the posting. But back then, I had no clue how to post anything on a newsgroup, and didn't even have a clear idea what a newsgroup was.

PGP found its way onto theInternetand rapidly acquired a considerable following around the world. Users and supporters included dissidents in totalitarian countries (some affecting letters to Zimmermann have been published, some of which have been included in testimony before the US Congress),civil libertariansin other parts of the world (see Zimmermann's published testimony in various hearings), and the 'free communications' activists who called themselvescypherpunks(who provided both publicity and distribution); decades later,CryptoPartyactivists did much the same viaTwitter.

Criminal investigation

Shortly after its release, PGP encryption found its way outside theUnited States,and in February 1993 Zimmermann became the formal target of a criminal investigation by the US Government for "munitionsexport without a license ". At the time, cryptosystems using keys larger than40 bitswere considered munitions within the definition of theUS export regulations;PGP has never used keys smaller than 128 bits, so it qualified at that time. Penalties for violation, if found guilty, were substantial. After several years, the investigation of Zimmermann was closed without filing criminal charges against him or anyone else.

Zimmermann challenged these regulations in an imaginative way. In 1995, he published the entiresource codeof PGP in a hardback book,viaMIT Press,which was distributed and sold widely. Anybody wishing to build their own copy of PGP could cut off the covers, separate the pages, and scan them using anOCRprogram (or conceivably enter it as atype-in programif OCR software was not available), creating a set of source code text files. One could then build the application using the freely availableGNU Compiler Collection.PGP would thus be available anywhere in the world. The claimed principle was simple: export ofmunitions—guns, bombs, planes, and software—was (and remains) restricted; but the export ofbooksis protected by theFirst Amendment.The question was never tested in court with respect to PGP. In cases addressing other encryption software, however, two federal appeals courts have established the rule that cryptographic software source code is speech protected by the First Amendment (theNinth Circuit Court of Appealsin theBernstein caseand theSixth Circuit Court of Appealsin theJunger case).

US export regulationsregarding cryptography remain in force, but were liberalized substantially throughout the late 1990s. Since 2000, compliance with the regulations is also much easier. PGP encryption no longer meets the definition of a non-exportable weapon, and can be exported internationally except to seven specific countries and a list of named groups and individuals(with whom substantially all US trade is prohibited under various US export controls).

The criminal investigation was dropped in 1996.

PGP 3 and founding of PGP Inc.

During this turmoil, Zimmermann's team worked on a new version of PGP encryption called PGP 3. This new version was to have considerable security improvements, including a new certificate structure that fixed small security flaws in the PGP 2.x certificates as well as permitting a certificate to include separate keys for signing and encryption. Furthermore, the experience with patent and export problems led them to eschew patents entirely. PGP 3 introduced the use of theCAST-128(a.k.a. CAST5) symmetric key algorithm, and theDSAandElGamalasymmetric key algorithms, all of which were unencumbered by patents.

After the Federal criminal investigation ended in 1996, Zimmermann and his team started a company to produce new versions of PGP encryption. They merged with Viacrypt (to whom Zimmermann had sold commercial rights and who hadlicensedRSA directly fromRSADSI), which then changed its name to PGP Incorporated. The newly combined Viacrypt/PGP team started work on new versions of PGP encryption based on the PGP 3 system. Unlike PGP 2, which was an exclusivelycommand lineprogram, PGP 3 was designed from the start as asoftware libraryallowing users to work from a command line or inside aGUIenvironment. The original agreement between Viacrypt and the Zimmermann team had been that Viacrypt would have even-numbered versions and Zimmermann odd-numbered versions. Viacrypt, thus, created a new version (based on PGP 2) that they called PGP 4. To remove confusion about how it could be that PGP 3 was the successor to PGP 4, PGP 3 was renamed and released as PGP 5 in May 1997.

Network Associates acquisition

In December 1997, PGP Inc. was acquired byNetwork Associates, Inc.( "NAI" ). Zimmermann and the PGP team became NAI employees. NAI was the first company to have a legal export strategy by publishing source code. Under NAI, the PGP team added disk encryption, desktop firewalls, intrusion detection, andIPsecVPNsto the PGP family. After the export regulation liberalizations of 2000 which no longer required publishing of source, NAI stopped releasing source code.

Asset split

In early 2001, Zimmermann left NAI. He served as Chief Cryptographer forHush Communications,who provide an OpenPGP-based e-mail service,Hushmail.He has also worked with Veridis and other companies. In October 2001, NAI announced that its PGP assets were for sale and that it was suspending further development of PGP encryption. The only remaining asset kept was the PGP E-Business Server (the original PGP Commandline version). In February 2002, NAI canceled all support for PGP products, with the exception of the renamed commandline product.

McAfee

NAI, now known asMcAfee,continued to sell and support the commandline product under the name McAfee E-Business Server until 2013.In 2010,Intel CorporationacquiredMcAfee.In 2013, the McAfee E-Business Server was transferred to Software Diversified Services (SDS), which now sells, supports, and develops it under the name SDS E-Business Server.

For the enterprise, Townsend Security currentlyoffers a commercial version of PGP for theIBM iandIBM zmainframe platforms. Townsend Security partnered with Network Associates in 2000 to create a compatible version of PGP for the IBM i platform. Townsend Security again ported PGP in 2008, this time to the IBM z mainframe. This version of PGP relies on a free z/OS encryption facility, which utilizes hardware acceleration. SDS also offers a commercial version of PGP (SDS E-Business Server) for theIBM zmainframe.

PGP Corporation

In August 2002, several ex-PGP team members formed a new company,PGP Corporation,and bought the PGP assets (except for the command line version) from NAI. The new company was funded by Rob Theis of Doll Capital Management (DCM) and Terry Garnett of Venrock Associates. PGP Corporation supported existing PGP users and honored NAI's support contracts. Zimmermann served as a special advisor and consultant to PGP Corporation while continuing to run his own consulting company. In 2003, PGP Corporation created a new server-based product called PGP Universal. In mid-2004, PGP Corporation shipped its own command line version called PGP Command Line, which integrated with the other PGP Encryption Platform applications. In 2005, PGP Corporation made its first acquisition: theGermansoftware company Glück & Kanja Technology AG,which became PGP Deutschland AG.In 2010, PGP Corporation acquired Hamburg-based certificate authority TC TrustCenter and its parent company,ChosenSecurity,to form its PGP TrustCenterdivision.

After the 2002 purchase of NAI's PGP assets, PGP Corporation offered worldwide PGP technical support from its offices inDraper, Utah;Offenbach,Germany;andTokyo,Japan.

Symantec

On April 29, 2010,Symantec Corp.announced that it would acquire PGP Corporation for $300 million with the intent of integrating it into its Enterprise Security Group.This acquisition was finalized and announced to the public on June 7, 2010. The source code of PGP Desktop 10 is available for peer review.

In May 2018, a bug namedEFAILwas discovered in certain implementations of PGP which from 2003 could reveal the plaintext contents of emails encrypted with it.The chosen mitigation for this vulnerability in PGP Desktop is to mandate the useSEIPprotected packets in the ciphertext, which can lead to old emails or other encrypted objects to be no longer decryptable after upgrading to the software version that has the mitigation.

Broadcom

On August 9, 2019,Broadcom Inc.announced they would be acquiring the Enterprise Security software division of Symantec, which includes PGP Corporation.

PGP Corporation encryption applications

This section describes commercial programs available fromPGP Corporation.For information on other programs compatible with theOpenPGPspecification, seeExternal linksbelow.

While originally used primarily for encrypting the contents of e-mail messages and attachments from a desktop client, PGP products have been diversified since 2002 into a set of encryption applications that can be managed by an optional central policy server. PGP encryption applications include e-mails and attachments, digital signatures, full disk encryption, file and folder security, protection for IM sessions, batch file transfer encryption, and protection for files and folders stored on network servers and, more recently, encrypted or signed HTTP request/responses by means of a client-side (Enigform) and a server-side (mod openpgp) module. There is also a WordPress plugin available, called wp-enigform-authentication, that takes advantage of the session management features of Enigform with mod_openpgp.

The PGP Desktop 9.x family includes PGP Desktop Email, PGP Whole Disk Encryption, and PGP NetShare. Additionally, a number of Desktop bundles are also available. Depending on the application, the products feature desktop e-mail, digital signatures, IM security, whole disk encryption, file, and folder security, encryptedself-extracting archives,andsecure shreddingof deleted files. Capabilities are licensed in different ways depending on the features required.

The PGP Universal Server 2.x management console handles centralized deployment, security policy, policy enforcement, key management, and reporting. It is used for automated e-mail encryption in the gateway and manages PGP Desktop 9.x clients. In addition to its localkeyserver,PGP Universal Server works with the PGP public keyserver—called the PGP Global Directory—to find recipient keys. It has the capability of delivering e-mail securely when no recipient key is found via a secure HTTPS browser session.

With PGP Desktop 9.x managed by PGP Universal Server 2.x, first released in 2005, all PGP encryption applications are based on a new proxy-based architecture. These newer versions of PGP software eliminate the use of e-mail plug-ins and insulate the user from changes to other desktop applications. All desktop and server operations are now based on security policies and operate in an automated fashion. The PGP Universal server automates the creation, management, and expiration of keys, sharing these keys among all PGP encryption applications.

The Symantec PGP platform has now undergone a rename. PGP Desktop is now known as Symantec Encryption Desktop (SED), and the PGP Universal Server is now known as Symantec Encryption Management Server (SEMS). The current shipping versions are Symantec Encryption Desktop 10.3.0 (Windows and macOS platforms) and Symantec Encryption Server 3.3.2.

Also available are PGP Command-Line, which enables command line-based encryption and signing of information for storage, transfer, and backup, as well as the PGP Support Package for BlackBerry which enables RIM BlackBerry devices to enjoy sender-to-recipient messaging encryption.

New versions of PGP applications use both OpenPGP and theS/MIME,allowing communications with any user of aNISTspecified standard.

OpenPGP

Within PGP Inc., there was still concern surrounding patent issues. RSADSI was challenging the continuation of the Viacrypt RSA license to the newly merged firm. The company adopted an informal internal standard that they called "Unencumbered PGP" which would "use no algorithm with licensing difficulties". Because of PGP encryption's importance worldwide, many wanted to write their own software that would interoperate with PGP 5. Zimmermann became convinced that an open standard for PGP encryption was critical for them and for the cryptographic community as a whole. In July 1997, PGP Inc. proposed to theIETFthat there be a standard called OpenPGP. They gave the IETF permission to use the name OpenPGP to describe this new standard as well as any program that supported the standard. The IETF accepted the proposal and started the OpenPGPWorking Group.

OpenPGP is on theInternet Standards Trackand is under active development. Many e-mail clients provide OpenPGP-compliant email security as described in RFC 3156. The current specification is RFC 4880 (November 2007), the successor to RFC 2440. RFC 4880 specifies a suite of required algorithms consisting ofElGamal encryption,DSA,Triple DESandSHA-1.In addition to these algorithms, the standard recommendsRSAas described inPKCS #1v1.5 for encryption and signing, as well asAES-128,CAST-128andIDEA.Beyond these, many other algorithms are supported. The standard was extended to supportCamelliacipher by RFC 5581 in 2009, and signing and key exchange based onElliptic Curve Cryptography (ECC)(i.e.ECDSAandECDH) by RFC 6637 in 2012. Support forECCencryption was added by the proposedRFC 4880bisin 2014.

• PGP
  • RFC1991PGP Message Exchange Formats (obsolete)
• OpenPGP
  • RFC2440OpenPGP Message Format (obsolete)
  • RFC4880OpenPGP Message Format (obsolete)
  • RFC5581The Camellia Cipher in OpenPGP (obsolete)
  • RFC6637Elliptic Curve Cryptography (ECC) in OpenPGP (obsolete)
  • RFC9580OpenPGP
• PGP/MIME
  • RFC2015MIME Security with Pretty Good Privacy (PGP)
  • RFC3156MIME Security with OpenPGP

OpenPGP's encryption can ensure the secure delivery of files and messages, as well as provide verification of who created or sent the message using a process called digital signing. Theopen sourceoffice suiteLibreOfficeimplemented document signing with OpenPGP as of version 5.4.0 on Linux.Using OpenPGP for communication requires participation by both the sender and recipient. OpenPGP can also be used to secure sensitive files when they are stored in vulnerable places like mobile devices or in the cloud.

In late 2023, a schism occurred in the OpenPGP world: IETF's OpenPGP working group decided to choose a "crypto-refresh" update strategy for the RFC 4880 specification, rather than a more gradual "4880bis" path preferred by Werner Koch, author of GnuPG. As a result, Koch took his draft, now abandoned by the workgroup, and forked it into a "LibrePGP" specification.

Implementations

TheFree Software Foundationhas developed its own OpenPGP-compliant software suite calledGNU Privacy Guard,freely available together with all source code under theGNU General Public Licenseand is maintained separately from severalgraphical user interfacesthat interact with the GnuPG library for encryption, decryption, and signing functions (seeKGPG,Seahorse,MacGPG).Several other vendorshave also developed OpenPGP-compliant software.

The development of anopen sourceOpenPGP-compliant library, OpenPGP.js, written inJavaScriptand supported by theHorizon 2020 Framework Programmeof theEuropean Union,has allowed web-based applications to use PGP encryption in the web browser.

Limitations

With the advancement of cryptography, parts of PGP and OpenPGP have been criticized for being dated:

• The long length of PGP public keys, caused by the use of RSA and additional data other than the actual cryptographic key
• Lack offorward secrecy
• Use of outdated algorithms by default in several implementations
• Difficulty for the users to comprehend and poor usability
• Lack of ubiquity

In October 2017, theROCA vulnerabilitywas announced, which affects RSA keys generated by buggy Infineon firmware used onYubikey4 tokens, often used with OpenPGP. Many published PGP keys were found to be susceptible.Yubico offers free replacement of affected tokens.

See also

• Bernstein v. United States
• Electronic envelope
• Email encryption
• Email privacy
• GNU Privacy Guard
• Gpg4win
• Key server (cryptographic)
• PGP word list
• PGPDisk
• Pretty Easy privacy
• Privacy software
• Public-key cryptography
• S/MIME
• X.509
• ZRTP

Further reading

• Garfinkel, Simson(1995).PGP: Pretty Good Privacy.O'Reilly & Associates.ISBN1-56592-098-8.
• Levy, Steven(January 8, 2001).Crypto: How the Code Rebels Beat the Government—Saving Privacy in the Digital Age.Penguin Books.ISBN0140244328.
• Lucas, Michael W. (April 1, 2006).PGP & GPG Email for the Practical Paranoid.No Starch Press.ISBN978-1-59327-071-1.
• Zimmermann, Phil(June 1991)."Why I Wrote PGP".RetrievedMarch 3,2008.

External links

• OpenPGP::SDK
• MIT Public Key Directory for Registration and Search
• List of public keyservers
• IETF OpenPGP working group
• OpenPGP Alliance