NETWORK
SECURITY
Contents
6.1 Digital Certificates
6.2 Pubic Key Infrastructure 6.3 Key Management
6.4 Cryptographic Transport Protocols
6.1 Digital Certificates
• Alice receives a package containing an
encrypted document from Bob. It is secure as it was encrypted.
• Yet how can she know that it came from Bob? Because Alice’s asymmetric public key is widely available, anyone could use it to encrypt the document.
• The answer is to use a digital signature.
6.1 Digital Certificates
6.1 Digital Certificates
6.1 Digital Certificates
6.1.1 Defining Digital Certificates
– Digital certificates can be used to associate or “bind” a user’s identity to a public key.
– A digital certificate is the user’s public key that has itself been “digitally signed” by a
reputable source entrusted to sign it.
6.1 Digital Certificates
– Digital certificates prevent a man-in-the-
middle attack that impersonates the owner of the public key.
– Digital certificates can also be used to identify objects other than users, such as servers and applications.
6.1 Digital Certificates
– A digital certificate typically contains the following information:
• Owner’s name or alias
• Owner’s public key
• Name of the issuer
• Digital signature of the issuer
• Serial number of the digital certificate
• Expiration date of the public key
6.1 Digital Certificates
6.1.2 Authorizing, Storing and Revoking
•Several entities and technologies are used for authorizing, storing, and revoking digital certificates.
•These include the Certificate Authority (CA) and Registration Authority (RA), a Certificate Repository (CR), and a Certificate
Revocation List (CRL).
6.1 Digital Certificates
Authority (CA) & Registration (RA)
•Instead of a user verifying his own identity, a third-party person or agency is used.
•An entity that issues digital certificates for others is known as a Certificate Authority (CA).
6.1 Digital Certificates
• A user provides information to a CA that verifies her identity.
• Also, the user generates public and
private keys and sends the public key to the CA (or in some instances the CA may create the keys).
• The CA inserts this public key into the certificate.
6.1 Digital Certificates
• A CA can be external to the organization, or it can be a CA internal to the
organization.
• Some organizations set up a subordinate entity, called a Registration Authority
(RA), to handle some CA tasks such as processing certificate requests and
authenticating users.
6.1 Digital Certificates
Certificate Revocation List (CRL)
•Digital certificates normally have an expiration date.
•Expired digital certificates should then be revoked.
•Revoked digital certificates are listed in a Certificate Revocation List (CRL), which can be accessed to check the certificate status of other users.
6.1 Digital Certificates
Certificate Repository (CR)
•It is important that the CA publishes the certificates and CRLs to a directory.
•This directory can be managed locally or in a publicly accessible directory, which is
called a Certificate Repository (CR).
6.1 Digital Certificates
6.1.3 Types of Digital Certificates
•There are different types of digital certificates.
•In addition, some digital certificates are
single-side while others can be dual-sided.
•Also, standards exist for digital certificates.
6.1 Digital Certificates
• In addition to being used to verify the sender’s identity, digital certificates can also be used to:
– Encrypt channels to provide secure
communication between clients and servers – Encrypt messages for secure Internet e-mail
communication
– Verify the identity of clients and servers on the Web
6.1 Digital Certificates
– Verify the source and integrity of signed executable code
• There are three basic categories of digital certificates:
– personal digital certificates, – Server digital certificates, and
– software publisher digital certificates.
6.1 Digital Certificates
Personal Digital Certificates
•Personal digital certificates are issued by a CA or RA directly to individuals.
•Personal digital certificates are typically used to secure e-mail transmissions.
•Digital certificates can also be used to authenticate the authors of documents.
6.1 Digital Certificates
Server Digital Certificates
•Server digital certificates are often issued from a Web server to a client.
•Typically perform two functions.
– First, they can ensure the authenticity of the Web server.
– Second, server certificates can ensure the
authenticity of the cryptographic connection to the Web server.
6.1 Digital Certificates
6.1 Digital Certificates
• Most server digital certificates combine both server authentication and secure communication between clients and servers on the Web.
6.1 Digital Certificates
• Software Publisher Digital Certificates
• Software publisher digital certificates are provided by software publishers.
• The purpose of these certificates is to
verify that their programs are secure and have not been tampered with.
6.1 Digital Certificates
Single Side and Dual Side
•Digital certificates can be either single- sided or dual-sided.
•When Bob sends one digital certificate to Alice along with his message, that is known as a single-sided certificate.
6.1 Digital Certificates
• Dual-sided certificates are certificates in which the functionality is split between two certificates.
– The signing certificate is used to sign a message to prove that that sender is
authentic.
– The encryption certificate is used for the actual encryption of the message.
6.1 Digital Certificates
• Dual-sided certificates have two advantages.
– First, dual-sided certificates reduce the need for storing multiple copies of the signing
certificate.
– Second, dual-sided certificates facilitate certificate handling in organizations.
6.1 Digital Certificates
X.509 Digital Certificates
•The most widely accepted format for digital certificates is defined by the International
Telecommunication Union (ITU) X.509 international standard.
•X.509 V1 first appeared in 1988. X.509 V2 supported new issuer and subject identifier fields that were absent from Version 1.
6.1 Digital Certificates
• The current version, X.509 V3, was defined in 1996, and introduced the extension field.
6.1 Digital Certificates
6.2 Public Key Infrastructure
• One of the important management tools for the use of digital certificates and
asymmetric cryptography is public key infrastructure.
• Public key infrastructure involves public- key cryptography standards, trust models, and key management.
6.2 Public Key Infrastructure
6.2.1 What is Public Key Infrastructure
•In an organization where multiple users have multiple digital certificates, it quickly can
become overwhelming to manage all of these entities.
•In short, there needs to be a consistent means to manage digital certificates.
•Public key infrastructure (PKI) is just that.
6.2 Public Key Infrastructure
• It is a framework for all of the entities
involved in digital certificates—including hardware, software, people, policies and procedures—to create, store, distribute, and revoke digital certificates.
• In short, PKI is digital certificate management.
6.2 Public Key Infrastructure
• PKI is often erroneously applied to a broader range of cryptography topics beyond managing digital certificates.
• It is sometimes defined as that which supports “other public key-enabled
security services” or “certifying users of a security application.”
6.2 Public Key Infrastructure
6.2.2 Public-Key Cryptographic Standards (PKCS)
•Public-key cryptography standards (PKCS) is a numbered set of PKI standards that
have been defined by the RSA Corporation.
•These standards are based on the RSA public-key algorithm.
6.2 Public Key Infrastructure
6.2 Public Key Infrastructure
6.2 Public Key Infrastructure
6.2 Public Key Infrastructure
• Applications and products that are
developed by vendors may choose to support the PKCS standards.
• For example, Microsoft Windows Vista provides native support for exporting
digital certificates based on PKCS #7 and
#12.
6.2 Public Key Infrastructure
6.2.3 Trust Model
•Trust may be defined as confidence in or reliance on another person or entity.
•A trust model refers to the type of trusting relationship that can exist between
individuals or entities.
6.2 Public Key Infrastructure
• In one type of trust model, direct trust, a relationship exists between two individuals because one person knows the other
person.
• Direct trust is not feasible when dealing with multiple users who each have digital certificates.
6.2 Public Key Infrastructure
• A third party trust refers to a situation in which two individuals trust each other
because each trusts a third party.
• This is the role that a CA plays: for
example, it verifies Mary, Amanda, and Javier to Alice.
6.2 Public Key Infrastructure
• There are essentially three PKI trust models that use a CA. These are
– the hierarchical trust model,
– the distributed trust model, and – the bridge trust model.
6.2 Public Key Infrastructure
Hierarchical Trust Model
•The hierarchical trust model assigns a single hierarchy with one master CA called the root.
•This root signs all digital certificate authorities with a single key.
•A hierarchical trust model can be used in an organization where one CA or RA is
responsible.
6.2 Public Key Infrastructure
6.2 Public Key Infrastructure
Distributed Trust Model
•Instead of having a single CA as in the hierarchical trust model, the distributed trust model has multiple CAs that sign digital certificates.
•The distributed trust model is the basis for digital certificates issued by Internet users.
6.2 Public Key Infrastructure
6.2 Public Key Infrastructure
Bridge Trust Model
•The bridge trust model is similar to the distributed trust model in that there is no single CA that signs digital certificates.
•However, with the bridge trust model there is one CA that acts as a “facilitator” to
interconnect all other CAs.
6.2 Public Key Infrastructure
6.3 Key Management
• Keys form the very foundation of PKI systems
– it is important to be carefully managed.
• A quick look at Certificate Life Cycle of PKI:
– Creation: the certificate is created and issued to the user.
– Suspension: if the certificate’s validity must be temporarily suspended.
– Revocation: the certificate is no longer valid.
– Expiration: the certificate can no longer be used.
6.3 Key Management
• Key Storage
– The means of storing keys in a PKI system is important.
– Public keys can be stored by embedding them within digital certificates.
– Private keys can be stored on the user’s local system.
– Drawback to software-based storage: may leave keys open to attacks:
• vulnerabilities in the client OS may expose keys to
6.3 Key Management
• Key Usage
– If more security is needed than a single set of public and private keys, multiple pairs of dual keys can be created.
– One pair of keys may be used to encrypt
information and the public key could be backed up to another location.
– Second pair would be used only for digital sig- natures and the public key in that pair would never be backed up.
– In the event that the public encryption key was stolen, the attacker would still not be able to
6.3 Key Management
• Key Handling
• Procedures in key handling:
– Escrow – refers to a situation in which keys are managed by a third party, such as a trusted CA.
– Expiration – Keys have expiration dates.
• Some systems set keys to expire after a set period of time by default.
– Renewal – an existing key can be renewed, no need to generate new keys.
– Revocation – all keys should expire after a set period of time, a key may need to be revoked prior to its
expiration date.
• revoked keys cannot be reinstated.
6.3 Key Management
• Key Handling
• Procedures in key handling (cont’d):
– Recovery – What happens if an employee is hospitalized for an extended period, yet the organization needs to
transact business using her keys?
• Different techniques may be used.
– Suspension – the revocation of a key is permanent; key suspension is for a set period of time.
• A suspended key can be later reinstated.
– Destruction – key destruction removes all private and
public keys along with the user’s identification information in the CA.
• When a key is revoked or expires, the user’s information remains on the CA for audit purposes.
6.4 Cryptographic Transport Protocols
• File Transfer Protocol
– does not use encryption
– usernames, passwords, and files are transferred are in clear-text.
– very vulnerable to MITM attacks
– one way to secure it is using SFTP
6.4 Cryptographic Transport Protocols
• Secure Socket Layer (SSL)/Transport Layer Security (TLS)
– SSL uses a public key to encrypt data that is transferred over the SSL connection.
– TLS a protocol that guarantees privacy and data integrity
– TLS is an extension of SSL – Often referred as SSL/TLS
• SSL/TLS
– TLS Handshake Protocol authentication – TLS Record Protocol encryption
6.4 Cryptographic Transport Protocols
• Secure Shell (SSH)
– a UNIX-based command interface and protocol for securely accessing a remote computer.
6.4 Cryptographic Transport Protocols
• Web Protocols
– originally insecure, port 80 – to secure it:
• HTTP over SSL/TLS HTTPS, port 443
• SHTTP not widely used
6.4 Cryptographic Transport Protocols
• VPN Protocols (Tunneling)
– securing the communication channel – there are many tunneling protocols:
• PPTP
• L2TP
• L3TP
• IPSec
• etc (combination L2TP/IPSec, L3TP/IPSec).
6.4 Cryptographic Transport Protocols
• PPTP
– most widely used (a part of Windows OS) – IP traffic in encrypted and then sent over
public network
6.4 Cryptographic Transport Protocols
• IPSec
– a set of protocols to securely exchange packets
• Applications – no need to modify to run under IPsec.
• Users – do not need to be trained on specific security procedures.
• Software – no changes needed at local client, as IPsec is implemented in a device
6.4 Cryptographic Transport Protocols
• E-mail Transport Protocol
– S/MIME uses digital certificates to protect the e-mail messages.
– S/MIME functionality is built into the vast majority of modern e-mail software and interoperates between them.
