Pretty Good Privacy (PGP) is a data encryption and decryption computer program that provides cryptographic privacy and authentication for data communication. PGP is often used for signing, encrypting and decrypting e-mails to increase the security of e-mail communications. It was created by Philip Zimmermann in 1991.
PGP encryption uses a serial combination of hashing, data compression, symmetric-key cryptography, and, finally, public-key cryptography; each step uses one of several supportedalgorithms. Each public key is bound 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]Compatibility
As PGP evolves, PGP systems that support newer features and algorithms are able to create encrypted messages that older PGP systems cannot decrypt, even with a valid private key. Thus, it is essential that partners in PGP communication understand each other's capabilities or at least agree on PGP settings.
[edit]Confidentiality
PGP can be used to send messages confidentially. For this, PGP combines symmetric-key encryption and public-key encryption. The message is encrypted using a symmetric encryption algorithm, which requires a symmetric key. Each symmetric key is used only once and is also called a session key. The session key is protected by encrypting it with the receiver's public key thus ensuring that only the receiver can decrypt the session key. The encrypted message along with the encrypted session key is sent to the recipient.
[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 the 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 key.
[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) impersonation is possible. PGP has, from its first versions, always included provisions for distributing a user's public keys in an 'identity certificate' which is also constructed cryptographically so 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. No satisfactory solution has been found for the underlying problem.
[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. 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 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 decrypt 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. In November 2009 a British citizen was convicted under RIPA legislation and jailed for 9 months for refusing to provide police investigators with encryption keys to PGP-encrypted files
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