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NETWORK SECURITY
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Contents
6.1 Digital Certificates 6.2 Pubic Key Infrastructure 6.3 Key Management 6.4 Cryptographic Transport Protocols
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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.
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4. 6.1 Digital Certificates 06 APPLYING CRYTOGRAPHY
Alice must retrieve Bob’s public key, how can Alice be sure that it is actually Bob’s key that she is retrieving? What if an imposter posted that public key under Bob’s name?
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5. 6.1 Digital Certificates
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6.1 Digital Certificates
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.
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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.
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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
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6.1 Digital Certificates
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).
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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).
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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.
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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.
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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.
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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).
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6.1 Digital Certificates
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.
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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 communication
Verify the identity of clients and servers on the Web
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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.
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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 transmissions.
Digital certificates can also be used to authenticate the authors of documents.
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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.
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6.1 Digital Certificates
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6.1 Digital Certificates
Most server digital certificates combine both server authentication and secure communication between clients and servers on the Web.
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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.
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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.
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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.
With single-sided certificates it is necessary to have a backup copy of the certificate with each message in order to ensure that the could be decrypted again later if necessary. With dual-sided certificates only the encryption certificate must be repeatedly backed up, while the signing certificate could be retained once on the system. This reduces the risk of having multiple copies of certificates that could be maliciously used by attackers.
Second, dual-sided certificates facilitate certificate handling in organizations. Copies of each employee’s encryption certificates can be kept in a central storage repository. This permits the organization, if necessary, to access any encrypted messages of any employees. Because it is not necessary to keep copies of individual employee signing certificates, this makes an employee’s digital certificate unavailable for another employee to use maliciously.
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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.
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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 X.509 V2 supported new issuer and subject identifier fields that were absent from Version 1.
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6.1 Digital Certificates
The current version, X.509 V3, was defined in 1996, and introduced the extension field.
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6.1 Digital Certificates
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29. 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.
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30. 6.2 Public Key Infrastructure
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.
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31. 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.
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32. 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.”
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33. 6.2 Public Key Infrastructure
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.
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34. 6.2 Public Key Infrastructure
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35. 6.2 Public Key Infrastructure
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36. 6.2 Public Key Infrastructure
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37. 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.
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38. 6.2 Public Key Infrastructure
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.
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39. 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.
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40. 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.
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41. 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.
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42. 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.
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43. 6.2 Public Key Infrastructure
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44. 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.
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45. 6.2 Public Key Infrastructure
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46. 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.
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47. 6.2 Public Key Infrastructure
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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.
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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 attackers.
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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 digitally sign the document.
Proper key management includes key storage, key usage, and key handling procedures.
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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.
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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.
The reason for distributing parts of the key to multiple users is that the absence of one member would not prevent the key from being recovered.
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53. 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
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54. 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
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55. 6.4 Cryptographic Transport Protocols
Secure Shell (SSH)
a UNIX-based command interface and protocol for securely accessing a remote computer.
SSH is actually a suite of three utilities—slogin, scp, and ssh— that are secure versions of the unsecure UNIX counterpart utilities rlogin, rcp, and rsh. These commands are summarized in Table Both the client and server ends of the connection 12 are authenticated using a digital certificate, and passwords are protected by being encrypted.
SSH can even be used as a tool for secure network backups.
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56. 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
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57. 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).
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58. 6.4 Cryptographic Transport Protocols
PPTP
most widely used (a part of Windows OS)
IP traffic in encrypted and then sent over public network
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59. 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
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60. 6.4 Cryptographic Transport Protocols
Transport Protocol
S/MIME uses digital certificates to protect the messages.
S/MIME functionality is built into the vast majority of modern software and interoperates between them.
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