In this research, we will focus on cryptosystems in which the management of the security is shared between many atomic entities, we call such systems Multi-Authority Cryptosystems. The first attempt to solve the problem of multi-authorities was the design of the X.509 norm which rules the management of trust in certificate delivery. The three following systems are based or fully compatible with X.509. These systems mainly deal with the problem of authenticating public keys and related identities. They are exclusively dedicated to the delivery of certificates.
Integrity is ensured via signatures.
To encipher short messages, they use public key cryptosystems. Long messages are enciphered using secret key cryptosystems which raises the problem of key distribution in which signature facilities are needed to certify the origin. However, there are cases in which third parties must intervene in the choice, the storage, the access or the recovery of the keys. The system is said to be fair because it establishes a right balance between security concerns and governmental needs of control but it may not avoid some of the Big Brother threats. This implements the key escrow concept in which a user's secret key is split into shares held by trustees to ensure key recovery if all trustees authorize it.