FALLSEM2024-25_CSI3002_ETH_VL2024250101927_2024-09-30_Reference-Material-I.ppt

Applied Cryptography and Network Security MODULE 5  Key Management Cryptographic key management is the process of administering or managing cryptographic keys for a Cryptographic system. It involves the generation, creation, protection, storage, exchange, replacement, and use of ke...

Applied Cryptography and Network Security MODULE 5  Key Management Cryptographic key management is the process of administering or managing cryptographic keys for a Cryptographic system. It involves the generation, creation, protection, storage, exchange, replacement, and use of keys and enables selective restriction for certain keys. In addition to access restriction, key management also involves the monitoring and recording of each key’s access, use, and context. A key management system will also include key servers, user procedures, and protocols, including cryptographic protocol design. The security of the cryptosystem is dependent upon successful key management.  SYMMETRIC KEY DISTRIBUTION USING SYMMETRIC ENCRYPTION  Key Distribution  symmetric schemes require both parties to share a common secret key  issue is how to securely distribute this key  whilst protecting it from others  frequent key changes can be desirable  often secure system failure due to a break in the key distribution scheme  Key Distribution  given parties A and B have various key distribution alternatives: 1. A can select key and physically deliver to B 2. third party can select & deliver key to A & B 3. if A & B have communicated previously can use previous key to encrypt a new key 4. if A & B have secure communications with a third party C, C can relay key between A & B  In a distributed system, any given user or server may need to engage in exchanges with many other users and servers over time. Thus, each endpoint needs a number of keys supplied dynamically. The problem is especially difficult in a wide area distributed system. Thus, if there are n hosts, the number of required keys is n(n - 1) / 2 A network using node-level encryption with 1000 nodes would conceivably need to distribute as many as half a million keys. If that same network supported 10,000 applications, then as many as 50 million keys may be required for application-level encryption  Key Hierarchy  typically have a hierarchy of keys  session key  temporary key  used for encryption of data between users  for one logical session then discarded  master key  used to encrypt session keys  shared by user & key distribution center Key Hierarchy Key Distribution Scenario  Key Distribution Issues  hierarchies of KDC’s required for large networks, but must trust each other  session key lifetimes should be limited for greater security  use of automatic key distribution on behalf of users, but must trust system  use of decentralized key distribution (suitable for smaller network)  SYMMETRIC KEY DISTRIBUTION USING ASYMMETRIC ENCRYPTION  Symmetric Key Distribution Using Public Keys  public key cryptosystems are inefficient  so almost never use for direct data encryption  rather use to encrypt secret keys for distribution Simple Secret Key Distribution  Merkle proposed this very simple scheme  allows secure communications  no keys before/after exist Man-in-the-Middle-Attack Secret Key Distribution with Confidentiality and Authentication  Hybrid Key Distribution  retain use of private-key KDC  shares secret master key with each user  distributes session key using master key  public-key used to distribute master keys  especially useful with widely distributed users  rationale  performance  backward compatibility  Distribution of Public Keys  can be considered as using one of:  public announcement  publicly available directory  public-key authority  public-key certificates  Public Announcement  users distribute public keys to recipients or broadcast to community at large  eg. append PGP keys to email messages or post to news groups or email list  major weakness is forgery  anyone can create a key claiming to be someone else and broadcast it  until forgery is discovered can masquerade as claimed user  Publicly Available Directory  can obtain greater security by registering keys with a public directory  directory must be trusted with properties:  contains {name,public-key} entries  participants register securely with directory  participants can replace key at any time  directory is periodically published  directory can be accessed electronically  still vulnerable to tampering or forgery  Public-Key Authority  improve security by tightening control over distribution of keys from directory  has properties of directory  and requires users to know public key for the directory  then users interact with directory to obtain any desired public key securely  does require real-time access to directory when keys are needed  may be vulnerable to tampering Public-Key Authority  Public-Key Certificates  certificates allow key exchange without real-time access to public-key authority  a certificate binds identity to public key  usually with other info such as period of validity, rights of use etc  with all contentssigned by a trusted Public-Key or Certificate Authority (CA)  can be verified by anyone who knows the public-key authorities public-key Public-Key Certificates