PGP加密技术解析-优快云博客

http://en.wikipedia.org/wiki/Pretty_Good_Privacy

Pretty Good Privacy

From Wikipedia, the free encyclopedia

Pretty Good Privacy (PGP) is a computer program that provides cryptographic privacy and authentication. PGP is often used for signing, encrypting and decrypting e-mails to increase the security of e-mail communications. It was originally created by Philip Zimmermann in 1991.

PGP and other similar products follow the OpenPGP standard (RFC 4880) for encrypting and decrypting data.

[edit] How PGP encryption works

PGP encryption uses public-key cryptography and includes a system which binds the public keys to a user name and/or an e-mail address. The first version of this system was generally known as a web of trust to contrast with the X.509 system which uses a hierarchical approach based on certificate authority and which was added to PGP implementations later. Current versions of PGP encryption include both options through an automated key management server.

[edit] 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 (the message integrity property), and the former to determine whether it was actually sent by the person/entity claimed to be the sender (a digital signature). In PGP, these are used by default in conjunction with encryption, but can be applied to plaintext as well. The sender uses PGP to create a digital signature for the message with either the RSA or DSA signature algorithms. To do so, PGP computes a hash (also called a message digest) from the plaintext, and then creates the digital signature from that hash using the sender's private keys.

[edit] 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 overwhelming assurance of that association; deliberate (or accidental) spoofing is possible. PGP has, from its first versions, always included provisions for distributing a user's public keys in an 'identity certificate' which is so constructed cryptographically that any tampering (or accidental garble) is readily detectable. But merely making a certificate which is impossible to modify without being detected effectively is also insufficient. It 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/entity claiming it. From its first release, PGP products have included an internal certificate 'vetting scheme' to assist with this; a trust model which has been called a web of trust. 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 which 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 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 managed public key infrastructure scheme such as that used by S/MIME but has not been universally used. Users have been willing to accept certificates and check their validity manually or to simply accept them. The underlying problem has found no satisfactory solution.

[edit] Certificates

In the (more recent) OpenPGP specification, trust signatures can be used to support creation of certificate 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') identity 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 the certificate revocation lists of centralized 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, if in slightly different guise, and no fully satisfactory solution is known. PGP's original scheme, at least, leaves the decision 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 central certificate authority be accepted as correct.

[edit] 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 1996, cryptographer Bruce Schneier characterized an early version as being "the closest you're likely to get to military-grade encryption."^[1] Early versions of PGP have been found to have theoretical vulnerabilities and so current versions are recommended. In addition to protecting data in transit over a network, PGP encryption can also be used to protect data in long-term data storage such as disk files.

The cryptographic security of PGP encryption depends on the assumption that the algorithms used are unbreakable by direct cryptanalysis with current equipment and techniques. For instance, in the original version, the RSA algorithm was used to encrypt session keys; RSA's security depends upon the one-way function nature of mathematical integer factoring^[2]. Likewise, the secret key algorithm used in PGP version 2 was IDEA, which might, at some future time, be found to have a previously unsuspected cryptanalytic flaw. 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, the degree of their cryptographic vulnerability varies with the algorithm used. In practice, each of the algorithms in current use is not publicly known to have cryptanalytic weaknesses.

New versions of PGP are released periodically and vulnerabilities that developers are aware of are progressively fixed. Any agency wanting to read PGP messages would probably use easier means than standard cryptanalysis, e.g. rubber-hose cryptanalysis or black-bag cryptanalysis i.e. installing some form of trojan horse or keystroke logging software/hardware on the target computer to capture encrypted keyrings and their passwords. The FBI has already used this attack against PGP^[3]^[4] in its investigations. However, any such vulnerabilities apply not just to PGP, but to all encryption software.

In 2003, an incident involving seized Psion PDAs belonging to members of the Red Brigade indicated that neither the Italian police nor the FBI were able to decode PGP-encrypted files stored on them.^[5]

A more recent incident in December 2006 (see United States v. Boucher) involving US customs agents and a seized laptop PC which allegedly contained child pornography indicates that US Government agencies find it "nearly impossible" to access PGP-encrypted files. Additionally, a judge ruling on the same case in November 2007 has stated that forcing the suspect to reveal his PGP passphrase would violate his Fifth Amendment rights i.e. a suspect's constitutional right not to incriminate himself.^[6]^[7] The Fifth Amendment issue has been opened again as the case was appealed and the federal judge again ordered the defendant to provide the key. ^[8]

Evidence suggests that as of 2007, British police investigators are unable to break PGP^[9], so instead have resorted to using RIPA legislation to demand the passwords/keys.

[edit] History

[edit] Early history

Phil Zimmermann created the first version of PGP encryption in 1991. The name, "Pretty Good Privacy", is humorously ironic and was inspired by the name of a grocery store, "Ralph's Pretty Good Grocery," featured in radio host Garrison Keillor's fictional town, Lake Wobegon. This first version included a symmetric-key algorithm that Zimmermann had designed himself, named BassOmatic after a Saturday Night Live skit. Zimmermann had been a long-time anti-nuclear activist, and created PGP encryption so that similarly inclined people might securely use BBSs and securely store messages and files. No license was required for its non-commercial use. There was not even a nominal charge, and the complete source code was included with all copies. PGP found its way onto Usenet and from there onto the Internet, and it very rapidly acquired a considerable following around the world. Users and supporters included dissidents in totalitarian countries (some affecting letters to Zimmermann have been published, and some have been included in testimony before the US Congress), civil libertarians in other parts of the world (see Zimmermann's published testimony in various hearings), and the 'free communications' activists who call themselves cypherpunks (who provided both publicity and distribution).

[edit] Criminal investigation

Shortly after its release, PGP encryption found its way outside the United States, and in February 1993 Zimmermann became the formal target of a criminal investigation by the US Government for "munitions export without a license". Cryptosystems using keys larger than 40 bits were then considered munitions within the definition of the US 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 a curious way. He published the entire source code of PGP in a hardback book^[10], via MIT Press, which was distributed and sold widely. Anybody wishing to build their own copy of PGP could buy the $60 book, cut off the covers, separate the pages, and scan them using an OCR program, creating a set of source code text files. One could then build the application using the freely available GNU C Compiler. PGP would thus be available anywhere in the world. The claimed principle was simple: export of munitions—guns, bombs, planes, and software—was (and remains) restricted; but the export of books is protected by the First 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 (the Ninth Circuit Court of Appeals in the Bernstein case and the Sixth Circuit Court of Appeals in the Junger case).

US export regulations regarding 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 7 specific countries and a named list of groups and individuals.

[edit] PGP 3

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 which 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 use of the CAST-128 (a.k.a. CAST5) symmetric key algorithm, and the DSA and ElGamal asymmetric 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 had licensed RSA directly from RSADSI) 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 exclusively command line program, PGP 3 was designed from the start as a software library allowing users to work from a command line or inside a GUI environment. 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.

[edit] OpenPGP

Inside PGP Inc., there was still concern about patent issues. RSADSI was challenging the continuation of the Viacrypt RSA license to the newly merged firm. The company adopted an informal internal standard called "Unencumbered PGP": "use no algorithm with licensing difficulties". Because of PGP encryption's importance worldwide (it is thought to be the most widely chosen quality cryptographic system), 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 the IETF that 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 OpenPGP Working Group.

OpenPGP is on the Internet Standards Track and is under active development. The current specification is RFC 4880 (November 2007), the successor to RFC 2440. Many e-mail clients provide OpenPGP-compliant email security as described in RFC 3156.

The Free Software Foundation has developed its own OpenPGP-compliant program called GNU Privacy Guard (abbreviated GnuPG or GPG). GnuPG is freely available together with all source code under the GNU General Public License (GPL) and is maintained separately from several Graphical User Interfaces (GUIs) that interact with the GnuPG library for encryption, decryption and signing functions (see KGPG, Seahorse, MacGPG). Several other vendors have also developed OpenPGP-compliant software.

[edit] Network Associates acquisition

In December 1997, PGP Inc. was acquired by Network Associates, Inc. Zimmermann and the PGP team became NAI employees. NAI continued to pioneer export through software publishing, being the first company to have a legal export strategy by publishing source code. Under its aegis, the PGP team added disk encryption, desktop firewalls, intrusion detection, and IPsec VPNs to the PGP family. After the export regulation liberalizations of 2000 which no longer required publishing of source, NAI stopped releasing source code, over the PGP team's objection. There was consternation amongst PGP users worldwide at this and, inevitably, some conspiracy theories as well.

In early 2001, Zimmermann left NAI. He served as Chief Cryptographer for Hush 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 cancelled all support for PGP products, with the exception of the re-named commandline product. NAI (now McAfee) continues to sell and support the product under the name McAfee E-Business Server.

[edit] Current situation

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 newly formed company, PGP was funded by Rob Theis of Doll Capital Management (DCM) and Terry Garnett of Venrock Associates. PGP Corporation is supporting existing PGP users and honoring NAI support contracts. Zimmermann now serves as a special advisor and consultant to PGP Corporation, as well as continuing to run his own consulting company. In 2003 PGP Corporation created a new server-based product offering called PGP Universal. In mid-2004, PGP Corporation shipped its own command line version called PGP Command Line, which integrates with the other PGP Encryption Platform applications. In 2005 PGP Corporation made its first acquisition—the German software company Glueck and Kanja Technology AG, which is now PGP Deutschland AG. Since the 2002 purchase of NAI PGP assets, PGP Corporation has offered worldwide PGP technical support from their office in Draper, Utah and Offenbach, Germany and as well Tokyo, Japan.

[edit] PGP Corporation encryption applications

This section describes commercial programs available from PGP Corporation. For information on other programs compatible with the OpenPGP specification, see OpenPGP implementations below.

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 which can be managed by an optional central policy server. PGP encryption applications include e-mail and attachments, digital signatures, laptop 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 and/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 application, the products feature desktop e-mail, digital signatures, IM security, whole disk encryption, file and folder security, self decrypting archives, and secure shredding of deleted files. Capabilities are licensed in different ways depending on 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 local keyserver, 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 current shipping versions are PGP Desktop 9.10 and PGP Universal 2.10

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 the S/MIME, allowing communications with any user of a NIST specified standard.