Management of Public Keys and PKIs in .NET

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3.3 Management of Public Keys and PKIs
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As seen numerous times before, many encryption mechanisms and even some key exchange mechanisms, such as IKE rely on pre-shared symmetric secrets. Usage of symmetric keys for trust establishment introduces serious scalability problems, since it requires that each node pair in the system share a unique secret. A network consisting of n nodes, may have to potentially deal with O(n2) keys. In a large network, such a large number of keys can pose a serious key management problem. On the other hand, using public private key pairs solves many of the scalability problems related to manually distributed pairwise secrets: each node often needs to only hold one key pair, leading to n key pair for a network of n nodes. However, using public keys has its own set of issues:
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When Alice sends a message to Bob, Alice uses Bob s public key to encrypt the message, so that only Bob can, with his private key, decrypt the message and get hold of its content. However, this assumes that Alice knows Bob s public key beforehand. Either, Bob must have sent his public key to Alice, or, Alice, must have downloaded it from some place. At any rate, Alice must have a way of being able to trust the fact that the public key she has received does actually belong to Bob. When we say that Alice sends a message encrypted with Bob s public key, one important fact that is usually forgotten is that this by itself does not protect the integrity of the message. Anybody could have sent an encrypted message on behalf of Alice, since Bob s public key is accessible to anybody who has access to the public key database. This means that Alice needs to also add her signature to the message, signing some part of the message with her own private key. Bob can verify Alice s signature using Alice s public key, trusting that nobody else can forge Alice s signature, since Alice s private key is a well-kept secret. However, again, Bob needs to either acquire Alice s public key out-of-band or from Alice directly.
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We see that when using public key methods two main problems need to be solved: 1. The public key of each party must be made available to the other parties. People need to know Bob s public key to be able to send him encrypted messages or verify his signatures. Either Bob needs to send his public key to people in advance or there needs to be a trusted authority where people can go and download Bob s public key.
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2. There needs to be a way to tie the public key to the identity of its owner in a trusted fashion. When people get Bob s public key from some source, they need to make sure that it actually belongs to Bob. Even when they receive the key from Bob himself, they need to make sure that it is actually Bob who sent it. Both problems can be solved with introducing certificates for any entity that presents its public key to other entities in the network. A public key certificate includes the owner s identity and public key, so that the recipient of the certificate can determine who the public key belongs to. In order to assert the authenticity of the certificate, it must be signed by an authority that both the presenter and recipient of the certificate trust. This entity is called certificate authority. The CA is actually the entity that issues the certificate for all the clients within the network, regardless of where the private public key pair for that entity is generated. To ensure both identity and public key in the certificate belong to the same client, the CA authenticates the client thoroughly (often physical presentation of identity rather than electronic one). Unfortunately, managing certificates is not a simple task. Beside the authentication, many other issues, such as safeguarding the private keys, managing certificate lifetimes and verifying their validity, revoking certificates if needed, are involved. For this purpose, use of certificates for authentication and security purposes typically require an entire infrastructure, typically referred to as PKI. Since we are devoting an entire chapter ( 9) to PKIs, we will not continue with management issues for public keys and end this chapter at this point.
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