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Public-Key Certificates and Distributed Security

The basic tool that permits widespread use of PGP is the public-key certificate. The essential elements of a public-key certificate are:

  The public key itself.
  A user ID consisting of the name and E-mail address of the owner of the key.
  One or more digital signatures for the public key and user ID.

The signer testifies that the user ID associated with this public key is valid. The digital signature is formed using the private key of the signer. Anyone in possession of the corresponding public key can verify that the signature is valid. If any change is made, either to the public key or the user ID, the signature will no longer compute as valid.

Public-key certificates are used in several security applications that require public-key cryptography. In fact, it is the public-key certificate that makes distributed security applications using public keys practical.

One approach that might be taken to use public-key certificates is to create a central certifying authority. This is the approach recommended for use with the privacy-enhanced mail (PEM) scheme. Each user must register with the central authority and engage in a secure exchange that includes independent techniques for verifying user identity. Once the central authority is convinced of the identity of a key holder, it signs that key. If everyone who uses this scheme trusts the central authority, then a key signed by the authority is automatically accepted as valid.

There is nothing inherent in the PGP formats or protocols to prevent the use of a centralized certifying authority. However, PGP is intended as an E-mail security scheme for the masses. It can be used in a variety of informal and formal environments. Accordingly, PGP is designed to support a so-called web of trust, in which individuals sign each other’s keys and create an interconnected community of public-key users.

If user B has physically passed his public key to user A, then user A knows that this key belongs to user B and signs it. User A keeps a copy of the signed key and also returns a copy to user B. Later, user B wishes to communicate with user D and sends this person the public key, with user A’s signature attached. User D is in possession of user A’s public key and also trusts user A to certify the keys of others. User D verifies user A’s signature on user B’s key and accepts user B’s key as valid.

COMPUTING TRUST

Although PGP does not include any specification for establishing certifying authorities or for establishing trust, it does provide a convenient means of using trust, associating trust with public keys, and exploiting trust information.

Each user can collect a number of signed keys and store them in a PGP file known as a public-key ring. Associated with each entry is a key legitimacy field that indicates the extent to which PGP will trust that this is a valid public key for this user; the higher the level of trust, the stronger is the binding of this user ID to this key. This field is computed by PGP. Also associated with the entry are zero or more signatures that the key ring owner has collected that sign this certificate. In turn, each signature has associated with it a signature trust field that indicates the degree to which this PGP user trusts the signer to certify public keys. The key legitimacy field is derived from the collection of signature trust fields in the entry. Finally, each entry defines a public key associated with a particular owner, and an owner trust field is included that indicates the degree to which this public key is trusted to sign other public-key certificates; this level of trust is assigned by the user. The signature trust fields can be thought of as cached copies of the owner trust field from another entry.

Trust Processing

If user A inserts a new public key on the public-key ring, PGP must assign a value to the trust flag that is associated with the owner of this public key. If the owner is in fact A, and this public key also appears in the private-key ring, then a value of ultimate trust is automatically assigned to the trust field. Otherwise, PGP asks user A for an assessment of the trust to be assigned to the owner of this key, and user A must enter the desired level. The user can specify that this owner is unknown, untrusted, marginally trusted, or completely trusted.

When the new public key is entered, one or more signatures may be attached to it. More signatures may be added later on. When a signature is inserted into the entry, PGP searches the public-key ring to see if the author of this signature is among the known public-key owners. If so, the OWNERTRUST value for this owner is assigned to the SIGTRUST field for this signature. If not, an unknown user value is assigned.

The value of the key legitimacy field is calculated on the basis of the signature trust fields present in this entry. If at least one signature has a signature trust value of ultimate, then the key legitimacy value is set to complete. Otherwise, PGP computes a weighted sum of the trust values. A weight of 1/X is given to signatures that are always trusted and 1/Y to signatures that are usually trusted, where X and Y are user-configurable parameters. When the total of weights of the introducers of a key/user ID combination reaches 1, the binding is considered to be trustworthy, and the key legitimacy value is set to complete. Thus, in the absence of ultimate trust, at least X signatures that are always trusted or Y signatures that are usually trusted or some combination, is needed.


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