Trusted third party
Encyclopedia
In cryptography
, a trusted third party (TTP) is an entity which facilitates interactions between two parties who both trust the third party; The Third Party reviews all critical transaction communications between the parties, based on the ease of creating fraudulent digital content. In TTP models, the relying parties use this trust to secure their own interactions. TTPs are common in any number of commercial transactions and in cryptographic digital transactions as well as cryptographic protocols, for example, a certificate authority
(CA) would issue a digital identity ceritificate to one of the two parties in the next example. The CA then becomes the Trusted-Third-Party to that certificates issuance. Likewise transactions that need a third party recordation would also need a third-party repository service of some kind or another.
. Without ever having met Bob, Alice may need to obtain a key to use to encrypt messages to him. In this case, a TTP is a third party who may have previously seen Bob (in person), or is otherwise willing to vouch that this key (typically in an identity certificate) belongs to the person indicated in that certificate, in this case, Bob. In discussions, this third person is often called Trent. Trent gives it to Alice, who then uses it to send secure messages to Bob. Alice can trust this key to be Bob's if she trusts Trent. In such discussions, it is simply assumed that she has valid reasons to do so (of course there is the issue of Alice and Bob being able to properly identify Trent as Trent and not someone impersonating Trent).
as a part of a public key infrastructure
) changes little. As in many environments, the strength of trust is as weak as its weakest link. When the infrastructure of a trusted CA is breached the whole chain of trust is broken. The recent incident at CA
DigiNotar
broke the trust of the Dutch governments PKI
is a text-book example of the weaknesses of the system and the effects of it.
The PGP
cryptosystem includes a variant of the TTP in the form of the web of trust
. PGP users digitally sign each others' identity certificates and are instructed to do so only if they are confident the person and the public key belong together. A key signing party
is one way of combining a get-together with some certificate signing. Nonetheless, doubt and caution remain sensible as some users have been careless in signing others' certificates.
Trusting humans, or their organizational creations, can be risky. For example, in financial matters, bonding companies have yet to find a way to avoid losses in the real world.
acts as a trusted third party for authenticating or acknowledging signatures on documents. A TTP's role in cryptography is much the same, at least in principle. A certificate authority
partially fills such a notary function, attesting to the identity of a key's owner, but not to whether the party was mentally aware or was apparent free from duress (nor does the certificate authority attest to the date of the signature).
Cryptography
Cryptography is the practice and study of techniques for secure communication in the presence of third parties...
, a trusted third party (TTP) is an entity which facilitates interactions between two parties who both trust the third party; The Third Party reviews all critical transaction communications between the parties, based on the ease of creating fraudulent digital content. In TTP models, the relying parties use this trust to secure their own interactions. TTPs are common in any number of commercial transactions and in cryptographic digital transactions as well as cryptographic protocols, for example, a certificate authority
Certificate authority
In cryptography, a certificate authority, or certification authority, is an entity that issues digital certificates. The digital certificate certifies the ownership of a public key by the named subject of the certificate...
(CA) would issue a digital identity ceritificate to one of the two parties in the next example. The CA then becomes the Trusted-Third-Party to that certificates issuance. Likewise transactions that need a third party recordation would also need a third-party repository service of some kind or another.
An example
Suppose Alice and Bob wish to communicate securely — they may choose to use cryptographyCryptography
Cryptography is the practice and study of techniques for secure communication in the presence of third parties...
. Without ever having met Bob, Alice may need to obtain a key to use to encrypt messages to him. In this case, a TTP is a third party who may have previously seen Bob (in person), or is otherwise willing to vouch that this key (typically in an identity certificate) belongs to the person indicated in that certificate, in this case, Bob. In discussions, this third person is often called Trent. Trent gives it to Alice, who then uses it to send secure messages to Bob. Alice can trust this key to be Bob's if she trusts Trent. In such discussions, it is simply assumed that she has valid reasons to do so (of course there is the issue of Alice and Bob being able to properly identify Trent as Trent and not someone impersonating Trent).
Actual practice
How to arrange for (trustable) third parties of this type is an unsolved problem. So long as there are motives of greed, politics, revenge, etc., those who perform (or supervise) work done by such an entity will provide potential loopholes through which the necessary trust may leak. The problem, perhaps an unsolvable one, is ancient and notorious. That large impersonal corporations make promises of accuracy in their attestations of the correctness of a claimed public-key-to-user correspondence (e.g., by a certificate authorityCertificate authority
In cryptography, a certificate authority, or certification authority, is an entity that issues digital certificates. The digital certificate certifies the ownership of a public key by the named subject of the certificate...
as a part of a public key infrastructure
Public key infrastructure
Public Key Infrastructure is a set of hardware, software, people, policies, and procedures needed to create, manage, distribute, use, store, and revoke digital certificates. In cryptography, a PKI is an arrangement that binds public keys with respective user identities by means of a certificate...
) changes little. As in many environments, the strength of trust is as weak as its weakest link. When the infrastructure of a trusted CA is breached the whole chain of trust is broken. The recent incident at CA
Certificate authority
In cryptography, a certificate authority, or certification authority, is an entity that issues digital certificates. The digital certificate certifies the ownership of a public key by the named subject of the certificate...
DigiNotar
DigiNotar
DigiNotar was a Dutch certificate authority owned by VASCO Data Security International. On September 3, 2011, after it had become clear that a security breach had resulted in the fraudulent issuing of certificates, the Dutch government took over operational management of DigiNotar's systems...
broke the trust of the Dutch governments PKI
Public key infrastructure
Public Key Infrastructure is a set of hardware, software, people, policies, and procedures needed to create, manage, distribute, use, store, and revoke digital certificates. In cryptography, a PKI is an arrangement that binds public keys with respective user identities by means of a certificate...
is a text-book example of the weaknesses of the system and the effects of it.
The PGP
Pretty Good Privacy
Pretty Good Privacy 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 texts, E-mails, files, directories and whole disk partitions to increase the security...
cryptosystem includes a variant of the TTP in the form of the web of trust
Web of trust
In cryptography, a web of trust is a concept used in PGP, GnuPG, and other OpenPGP-compatible systems to establish the authenticity of the binding between a public key and its owner. Its decentralized trust model is an alternative to the centralized trust model of a public key infrastructure ,...
. PGP users digitally sign each others' identity certificates and are instructed to do so only if they are confident the person and the public key belong together. A key signing party
Key signing party
In cryptography, a key signing party is an event at which people present their PGP-compatible keys to others in person, who, if they are confident the key actually belongs to the person who claims it, digitally sign the PGP certificate containing that public key and the person's name, etc...
is one way of combining a get-together with some certificate signing. Nonetheless, doubt and caution remain sensible as some users have been careless in signing others' certificates.
Trusting humans, or their organizational creations, can be risky. For example, in financial matters, bonding companies have yet to find a way to avoid losses in the real world.
Parallels outside cryptography
Outside cryptography, the law in many places makes provision for trusted third parties upon whose claims one may rely. For instance, a notary publicNotary public
A notary public in the common law world is a public officer constituted by law to serve the public in non-contentious matters usually concerned with estates, deeds, powers-of-attorney, and foreign and international business...
acts as a trusted third party for authenticating or acknowledging signatures on documents. A TTP's role in cryptography is much the same, at least in principle. A certificate authority
Certificate authority
In cryptography, a certificate authority, or certification authority, is an entity that issues digital certificates. The digital certificate certifies the ownership of a public key by the named subject of the certificate...
partially fills such a notary function, attesting to the identity of a key's owner, but not to whether the party was mentally aware or was apparent free from duress (nor does the certificate authority attest to the date of the signature).
See also
- Trusted computing baseTrusted computing baseThe trusted computing base of a computer system is the set of all hardware, firmware, and/or software components that are critical to its security, in the sense that bugs or vulnerabilities occurring inside the TCB might jeopardize the security properties of the entire system...
- Double-spendingDouble-spendingDouble-spending is a failure mode of digital cash schemes, when it is possible to spend a single digital token twice. Since, unlike physical token money such as coins, electronic files can be duplicated, and hence the act of spending a digital coin does not remove its data from the ownership of the...