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public Key Infrastructures

public Key Infrastructures 



Asymmetrical  encryption depends on public key integrity 

Tuvo a pproaches discourage rogue public keys 

Publishing Key fingerprints

Public Key Infrastructure (PKI)

: Distributed web of trust
: Hierarchical certiricate Authorities

Digitel Certincates


The distribution of public keys is a fundamental weakness in asymmetric encryption schemes. In addition to
when Alice wants to send a message to Bob, she looks up Bob's key from a public key directory. However



Fingerprints

signatures and on his business cards). When Alice obtains Bob's Key from a public key server she can
then compute her own fingerprint and compare it against the well known value.



Public Key Infrastructures

A more secure approach would be for Bob to have a third party that Alice trusts sign his public key. when
signed public key is referred to as a Certificate, and the trusted third party is referred to as a certificate



Digital Certificate

Owner : Public Key and Identity
Issuer : Detached signature and Identity
Period of Validity

Types
Certificate Authority Certificates
Server (or Self signed) Certificates

Certificate Authorities play the role of trusted third parties for the purpose of associating identities with their
legrtimate public keys. We now add the fictional character Trent to take the role of the trusted third party.
Assume that Trent has generated his own public private key pair, and signed his own public key. The self
signed Public key is referred to as a Certificate Signer's Certificate or"Certificate Authority Certificate".
Trent is now ready to act as a Certificate Authority (CA)

Bob is a retailer who would like (previously unknown) customers to be able to exchange sensitive
information with him in a secure manner. He first generates a public private key pair. He then convinces
Trent of his identity, and provides Trent with his public key. Trent will combine his own identity, Bob's
identity, Bob's public key, and a period of validity. He will then create a one way hash of the information,
and then encrypt this hash with his own private key, thus creating a detached digital signature. The
combined information and detached signature compose a digital certificate, which Trent gives to Bob. Trent
is referred to as the"issuer"of the certificate, and Bob as the"owner"of the certificate.

Alice is a Fotential customer who would like secure communications with retailers sucH as Bob. Knowing
that the is going to want Trent to authenticate sucH retailers for her, she has already obtained Trent's
certificates built in.
"certificate Authority Certificate". Standard web browsers, sucH as Firefox or Mozilla, come with sucH CA

Alice now connects to Bob's web site. Bob will give Alice his certificate. Alice will verify Bob's certificate
by extracting the issuer's detached signature, and verifying it with the CA certificate that was already in
her possession. Effectively, Alice is trusting the CA's word that this is in fact Bob that she is talking to.
with Bob's identity established, Alice and Bob can now use Public key encryption techniques to exchange
inf ormation secu re Iy.

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