and Security
Trust Security
A system is either secure or it is not. Whether a user considers a systems to be secure is a matter of trust.
Difficult to get right, impossible to get perfect!
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Security Topics
1. Providing a secure communications channel authentication, confidentiality and integrity. 2. Handling authorization who is entitled to use what in the system? 3. Providing effective Security Management cryptographic keys distribution, add/remove users, certificates
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Types of Threats
Interception unauthorized access to data (e.g., communication eavesdropping). Interruption a service becomes unavailable (e.g., lost/corrupted files, DoS attacks). Modification unauthorized changes to, and tampering of, data. Fabrication non-normal, additional activity.
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Security Mechanisms
Encryption fundamental technique: used to implement confidentiality and integrity. Authentication verifying claimed identity of users, client, server,(e.g., use passwords) Authorization verifying allowable operations. Auditing who did what to what and when/how did they do it? Logging activities.
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Design Issue: Simplicity
Designing a secure computer system is considered a difficult task, regardless of whether it is also a DS. The use of a few simple mechanisms that are easily understood and trusted to work would be the ideal situation. Unfortunately, the real world is not this clear cut, as introducing security mechanisms to an already complex system can often matters worse. However, this is still a design goal to aim for!
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Security Mechanisms
Fundamental technique within any distributed systems security environment:
Cryptography.
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Participants/Components
Intruders and eavesdroppers in communication.
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Example: Caesar Cipher
Used by Julius Caesar to communicate to his army (one of the first persons to have ever employed encryption for the sake of securing messages ) Shift each letter a certain number of spaces
For example, for a shift of 19
ABCDEFGHIJKLMNOPQRSTUVWXYZ TUVWXYZABCDEFGHIJKLMNOPQRS
Easily broken e.g. by frequency analysis.
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Much Better: One-Time Pads
Imagine two identical pads of paper
On each page there is a random number
(Or two CDs that have recordings of white noise)
To encrypt,
Take first/next character of plaintext message, rotate by number on top page of pad Tear off page of pad and burn it Repeat until entire message encrypted
To decrypt,
Take first/next character of encrypted message, rotate backward by number on top page of duplicate pad Tear off first page of pad and burn it Repeat
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Two Kinds of Keys
Message
Symmetric lock and key
Locking key
(Identical) unlocking key
Asymmetric lock and key
Locking key
(Different) unlocking key
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Symmetric case Ek(M)
Recipient opens using identical (symmetric) unlocking key
Dk(Ek(M)) = M
Put message in lockbox Close and lock using senders locking key Transport to recipient
Dk2(Ek(M)) = M Ek(M) Asymmetric case
Recipient opens using different (asymmetric) unlocking key
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Introducing Alice, Bob & Co.
Alice and Bob are good the guys. Chuck and Eve are usually the bad guys.
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Secret Communication with a Shared Secret Key
Alice and Bob share a secret key k. Alice uses k and an agreed encryption function Ek(M) to encrypt and send any number of messages M to Bob. Bob reads the encrypted messages using the corresponding decryption function Dk (M).
Alice and Bob can go on using k as long as it is safe to assume that k has not been compromised.
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Added Value by a Public Key
For Alice to send a confidential message to Bob, she uses Bobs public key. Only Bob can decrypt this message (with his private key). In addition, Bob also wants to be sure the message is actually from Alice, so Alice uses her private key to sign the message, and Bob uses Alices public key to decrypt it. If a correctly formatted message appears, Bob knows Alice sent it.
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Notation for Cryptography
Notation KA, B
Description Secret key shared by A and B Public key of A Private key of A
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Symmetric Encryption Algorithms
DES: The US Data Encryption Standard (1977). No longer strong in its original form. 56-bit key, 350 kbytes/sec. Triple-DES: applies DES three times with two different keys. 112-bit key, 120 Kbytes/sec IDEA: International Data Encryption Algorithm (1990). 128-bit key, 700 kbytes/sec AES: A proposed US Advanced Encryption Standard (1997). 128/256-bit key.
The above speeds are for a Pentium II processor at 330 MHZ
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Public-Key Cryptosystem
RSA: The first practical algorithm (Rivest, Shamir and Adelman 1978) and still the most frequently used. Key length is variable, 512-2048 bits. Speed 1-7 kbytes/sec. (350 MHz PII processor) Elliptic curve: A recently-developed method, shorter keys and faster. Asymmetric algorithms are ~1000 x slower and are therefore not practical for bulk encryption, but their other properties make them ideal for ...