PKI & SSL

Dr. Andy Wu

BCIS 4630 Fundamentals of IT Security

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Overview

• Digital signature• PKI• Public Key Applications

– Digital certificate– Secure Socket Layer

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Non-Repudiation• Non-repudiation means that someone cannot later

deny something that he/she agreed to earlier.• To establish non-repudiation, one must prove two

things:– Authentication: the other person was the person who

signed (Use the “Authentication” utilization of asymmetric encryption to prove it).

– Integrity of document: the document the other person signed has not been modified since it leaves him/her (Use hash to prove it).

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Non-Repudiation• Hash + Asymmetric Encryption = Digital Signature• Digital Signature

– The message to be digitally signed is first hashed and then the message digest is encrypted with the sender’s private key.

– It’s one step further than integrity by authenticating the identity of the sender.

• The sender cannot deny sending the file because she supposedly is the only person having access to her private key.

• The sender cannot deny the document in your possession wasn’t the one she signed if the hash value hasn’t changed.

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Digital Signature - Signing

• The sender digitally signs the message by– Calculating the hash value of the message.– Encrypting the hash with the sender’s private

key.• Optionally, the sender may encrypt the

message after the hash is run, but this is not a required part of the signing.

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Digital Signature - Signing

Source: Tom Austin, PKI, A Wiley Tech Brief.

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Digital Signature - Verification

• The recipient verifies the digital signature by:– Obtaining the sender’s public key and using it to

decrypt the encrypted hash value.– Using the same hash algorithm used by the

sender, calculating the hash value of the message.

– Comparing the locally computed hash value to the decrypted hash value.

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Digital Signature - Verification

Source: Tom Austin, PKI, A Wiley Tech Brief.

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Digital Signature - Verification

• The recipient verifies that:– The message has not been tempered with

since it was signed (integrity).– The person who supposedly sent the message

really did (authentication).

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Why PKI?

If anyone can upload a public key on behalf of someone else…

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Why PKI?

• John and Diane want to communicate securely. • John can generate his own public/private key

pair and send his public key to Diane or place it in a directory that is available to everyone (a key depository).

• Without a PKI, Katie also can generate her own key pair, and place her public under Diane’s name. This in effect allows Katie to steal Diane’s identity.

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Public Key Infrastructure• Uses symmetric and asymmetric cryptographic

technologies. These technologies can be used by:– E-mail clients– Virtual private network products– Web server components– Domain controllers– Etc.

• A Framework for different products and technologies to integrate with each other to supply various security services.

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Components of PKI• PKI is made up of:

– Hardware– Applications– Policies– Services– Programming interfaces– Cryptographic algorithms– Protocols– Users– Utilities

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What Does PKI Do?

• It provides third-party (institutional) trust.• It binds people’s public key to their identity.• Before a person’s public key is placed in a

key depository, his/her public key must be bound to his/her identity.

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What Does PKI Do?

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Who Stands Behind PKI?

• In PKI environments, entities called certificate authorities (Cas) provide a service similar to that of the Department of Motor Vehicles (DMV).

• A CA issues a digital certificate containing necessary identification information about an entity, combines the entity’s public key with the certificate, and digitally signed it with the CA’s private key.

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Certificate Authorities• The trusted authority for certifying an entity’s identity and

creating an electronic document indicating that entities are who they claim to be.– The electronic document is referred to as a digital certificate.– It establishes an association between the entity’s identity and the

entity’s public key.– The entity keeps the corresponding private key separately.

• The CA signs an entity’s public key with the CA’s private key.– If you can decrypt the certificate with the CA’s public key (most

popular ones stored on your computer as Windows was installed), you can trust the entity’s public key.

– If not, the entity’s public key (and identity too) is suspicious.

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Registration Authority

• The registration authority (RA) is the component that accepts a request for a digital certificate.

• They performs the necessary steps of registering and authenticating the entity requesting a certificate.

• RAs cannot generate certificates.– Once they finish processing the request and verifying

the individual’s identity, the request is sent to the CA.

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Fields in A Certificate• Version Number

– The version of the X.509 standard that was followed to create the certificate. • Serial number

– A unique number identifying a specific certificate issued by a particular CA. • Signature algorithm

– The hashing algorithm and the digital signature algorithm used to digitally sign the certificate.

• Issuer– The CA that generated and digitally signed the certificate.

• Validity– The period during which the certificate is valid.– Defined by the “Valid from” and “Valid to” dates.

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Fields in A Certificate• Subject

– Owner of the certificate.– Can be a network device, an application, a department, a company, or a

person.• Public key

– The public key being bound to the certified subject.– Also identifies the algorithm used to create the key pair.

• Extensions– Allow additional data to be encoded into the certificate to expand the

functionality of the certificate.• CA digital signature

– Hash (thumbprint) algorithm used for the signature.– The signature itself.

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Certificate Examples

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Certificate Revocation• A certificates is revoked when its validity needs to be

ended before its actual expiration date.• A certificate has to be revoked if:

– The private key that is related to the public key in the certificate has been or may have been compromised.

– The subscriber has breached or unable to exercise a material obligation stipulated in the contract.

• The frequency with which revocation information (in the form of a certificate revocation list, or CRL) is updated and distributed is an extremely important consideration.

Certificate Revocation• Examples of situations where a certificate needs to be

revoked: – A user loses a laptop or a smart card that stored a private

key. – An improper software implementation has been uncovered

that directly affected the security of a private key.– A user has fallen victim to a social engineering attack and

inadvertently given up a private key.– Data held within the certificate no longer apply to the

specified individual.– An employee has left a company.

Certificate Suspension

• Instead of being revoked, a certificate is sometimes suspended, meaning it is temporarily put on hold.

• The CRL would list this certificate and its serial number, and in the field that describes why a certificate was revoked, it would instead indicate a hold state.

• Suspension may be removed later.• It is not wise to trust a suspended certificate.

Reasons for Suspension• A certificate may be suspended due to some

temporary problems– The key might have been compromised, but the

investigation is still under way.– The subscriber has not paid annual service fee to the CA.– An individual is on vacation and wants to ensure that the

certificate will not be compromised or used during that time

• Once the temporary problems are resolved, a request can be made to the CA to remove the certificate from the CRL.

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Chain of Trust• A CA vouches for other entities’ identity. But who

vouches for the CA?• A CA can have its identity endorsed by another CA.

– That other CA can, in turn, be endorsed by yet another CA.• This creates a hierarchical chain of trust.• Ultimately, however, there will be CA(s) whose identity

we must accept by faith for the whole PKI system to work.

• Windows and other OSes trust a number of root CAs when the OS is installed.

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Chain of Trust

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Chain of Trust

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Web Communication Encryption• Secure Sockets Layer (SSL) is a general-purpose

protocol developed by Netscape for managing the encryption of information being transmitted over the Internet.

• SSL is the most common public key application.It provides:– Authentication

• Allows the client to make sure it is talking to the correct server.

– Confidentiality• Establishes a strong, cryptographically protected connection

between the client and the server.

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Secure Socket Layer• HTTP is used for the transfer of hyperlinked data over

the Internet, from web servers to browsers.• One of the primary drivers behind the development of

SSL/TLS was the desire to hide the complexities of cryptography from end users.

• When using an SSL/TLS-enabled browser, this can be done simply by requesting a secure connection from a web server instead of a nonsecure connection.– When a secure connection is needed, SSL/TLS is used and

appears in the address as https://.

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SSL - Authentication• SSL uses digital certificate for authenticating the server to the

client. This is mandatory.• Authenticating the client to the server can be desirable in high

security environments. But this is not the mandatory (default) setting of SSL.

• The certificate only tells the user that a server is what it claims to be, based on his/her trust placed in the CA that issues the certificate.

• It does not say anything about the contents of the communication, e.g., the certificate for a program’s installer only says that it was made by the entity shown on the certificate. It doesn’t guarantee that the program is safe.

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SSL - Confidentiality• For the client and server to communicate, both sides

must agree on a commonly held protocol (SSL v1, v2, v3, or TLS v1) and encryption algorithm.– Commonly available cryptographic algorithms include

Diffie-Hellman and RSA. • The channel is protected by encryption against

eavesdropping. – Each packet is encrypted using the symmetric key before

transfer across the network, and then decrypted by the receiver.

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SSL Handshake

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SSL Handshake• ClientHello: Ask the server to begin negotiating security services

by using SSL.– Contains a CipherSuite field that states the cryptographic services

the client can support.• ServerHello: Contains a CipherSuite field that states the exact

cryptographic parameters, algorithms, key sizes, etc. to be used for the session.– It is chosen from the options in the ClientHello message.

• ServerKeyExchange: Contains the server’s public key embedded in a certificate (server certificate).

• ServerHelloDone: Indicates that the server has finished its initial negotiation messages.

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SSL Handshake• ClientKeyExchange: Client tells the server its key

information – the symmetric keys encrypted using the server’s public key.– One key is used to encrypt data the client sends to the

server. The other for encrypting the opposite data flow.• ChangeCipherSpec: It explicitly indicates that the

security services should now be invoked.– Symmetric encryption algorithm, message integrity

algorithm, etc.– Client and server now take the cue and exchange data

securely.

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SSL Handshake• Finished: Both systems verify the success of negotiation.• Client may be required by the server to authenticate itself to the

server.– The server sends CertificateRequest after its own

ServerKeyExchange message.– The client responds with a Certificate message after

ServerHelloDone.– The client also sends a CertificateVerify message that contains

signed hashes of data that the client and the server both possess at this point.

– The server then can verify the client’s certificate.

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SSL with Client Authentication

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Common Errors in SSL

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Common Errors in SSL

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