Computer Networking Lecture Notes

2013 September 19 • Application Security

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Outline

• Application security
• Protection mechanisms.
• Cryptography
  • Breaking cryptography.
  • Symmetric- and asymmetric-key cryptography.
• Identity: authentication and verification.

Application Security

• What is application security?
  • Move data from sender to receiver securely.
• “Securely” means whatever you want.
  • Private — only the sender and receiver understand the data.
  • Inviolate — the data sent is the data received.
  • Certain — Each side knows the other side is involved.

Security Attacks

• The definition of security is the security policy.
  • Mix ‘n’ match to make your own policy.
• The attacks against security policies are legion.
  • Privacy — eavesdropping.
  • Inviolate — replay.
  • Certain — relay (man in the middle), spoofing.

Protection Mechanisms

• Security attacks outnumber protective mechanisms.
  • Privacy — encryption.
  • Inviolate — checksumming, message digests.
  • Certainty — digital signatures.
• Attacks are broad-based and diffuse.
• Protections are more powerful but have limited scope.

Cryptography

• Cryptography is a fundamental data transformation in security.
• A cryptographic algorithm (a cypher) obscures (encrypts) data relative to a secret (a key).
• The algorithm (or a related one) also unobscures (decrypts) the obscure data relative to the secret (or a related one).
• Encrypting or decrypting data without the secret should be impossible.

Cypher Example

• The Ceasar cypher shifts letters in an alphabet A by integer k.
  • Encryption: C_i → C_{(i + k) mod |A|}.
  • Decryption: C_i → C_{(i - k) mod |A|}.
  • Key: k
• Example:
  • if k = 5, cow encrypts to htb.
  • if k = 4, htb decrypts to dpx.

Cryptographic Security

• A secure cypher is impossible to decrypt without the key.
  • There’s one mathematically proven secure cypher: the one-time pad cypher.
• All other cyphers may be secure.
  • Security comes from the computational complexity of decrypting without a key.
  • This is fragile, technology-dependent security.
    • R.I.P. DES.

Breaking Cryptography

• A cryptographic algorithm is broken when cyphertext can be decrypted without a key.
  • Or the key can be recovered from the cypertext (WiFi security).
• Practically, quasi-security comes from the difficulty of breaking cyphers.
  • A message with a useful life of X years should be protected by a cypher that takes at least X years to break.
  • A common, but dangerous, interpretation.

Symmetric-Key Cryptography

• A symmetric-key cypher uses the same key for encryption and decryption.

  

  E:Encryption algorithm
  D:Decryption algorithm
  K:Key
  P:Plaintext (data)
  C:Cyphertext
  

Asymmetric-Key Cryptography

• An asymmetric-key cypher uses different but related keys for encryption and decryption.

  

  E:Encryption algorithm
  D:Decryption algorithm
  K_BP:Bob’s public key
  K_BS:Bob’s secret key
  P:Plaintext (data)
  C:Cyphertext
  

Cryptographic Techniques

• Cryptographic algorithms combine character substitution and transposition techniques.
• Substitution: plaintext characters are mapped to other plaintext characters.
  • The Caesar cypher.
• Transposition: permute a plaintext character sequence.

  Most modern cyphers: DES, AES, …

Problem Solved?

• Are we done? Does good cryptography provide security?
• Unfortunately, no.
  • Generating good keys is hard.
  • Good keys are inconvenient to handle.
  • Distributing keys is hard and expensive.
• Cryptography is ineffective for other common attack points.
  • Physical security, social engineering, …

Authentication

• The authentication problem: how does Alice know the message is from Bob?
• One possible solution is:
  1. Alice knows only Bob knows secret k.
  2. If X can prove to Alice it knows k, then
  3. Alice knows X is Bob.
• The cryptographic algorithm acts as a digital signature.
• Can the solution be implemented?

Asymmetric-Key Authentication

• Suppose Bob’s secret is K_BS, his secret key.
  • Alice challenges X with C = E(P, K_BP).
  • X responds with P\('\).
  • If P\('\) = P, then P\('\) = D(C, K_BS) and X is Bob.
  • If P\('\) \({}\ne\{}) P, then X is still unauthenticated.
• This is the basic idea; there are lots of details to fill in.
• Problem solved?

Attack!

• Oh no:
  • Alice is made to think that Bob knows K_ES, Eve’s secret key.
  • Alice authenticates Eve as Bob.
  • Poor Alice.
• Alice needs a way to reject the false claim that Bob knows a secret when he doesn’t.
• How can Alice do this?

Verifying Identity

• A verified digital signature only proves that X knows a secret.
  • It doesn’t prove (or establish) that X is Bob.
• Alice doesn’t know, and Eve’s going to lie.
• A third party needs to establish to Alice that

  if X knows k, then X is Bob.

• How does that go?

Identity Verification

• Introducing Vera, a trusted, independent third party.
  1. Bob verifies his identity to Vera, gives her a public key.
  2. Vera creates a digital certificate for Bob.
  3. Alice asks Vera about this Bob character.
  4. Vera sends Alice Bob’s digital certificate.

Digital Certificates

• A digital certificate can contain lots of information, but it must contain
  • Bob’s public key, and
  • Vera’s digital signature.
• Does this solve the problem?
  • Not really, but
  • It’s (presumably) harder for Eve to masquerade as Vera.
• X.509 is a ITU-T digital-certificate standard.

SSH Example

• Secure Shell (SSH) is a remote log-in service replacing telnet and rlogin.
• SSH provides authentication, integrity, and privacy.
• SSH v. 2 uses three protocols for security policy: SSH-TRANS, SSH-AUTH, and SSH-CON.

SSH Transport

• SSH-Trans is a transport (end-to-end) service providing an encrypted channel.
• The underlying connection is a byte-stream (TCP).
• The SSH-Trans protocol:
  • The client opens a byte-stream to the server.
  • The client authenticates the server.
  • They establish a session key to encrypt data over the stream.

Server Authentication

• What gives the client the server’s public key?
• The server:

  we ssh opus
  'opus (10.1.13.154)' authenticity can’t be established.
  RSA key fingerprint is d0:4e:70:f0:db:2a:dd:77:8b:67:a2:cd:0e:0f:3a:26.
  Do you want to continue (yes/no)? yes
  Added 'opus' (RSA) to known-hosts list.
  Last login: Sun Aug 19 13:06:26 2012 from rockhopper
  
  op

• ssh remembers the key if accepted.

  op exit
  Connection to opus.monmouth.edu closed.
  
  we ssh opus
  Last login: Thu Sep 20 18:20:16 2012 from westrunton
  
  op 

SSH Connections

• SSH security services are so useful they’ve been packaged for use by other applications.

  

• This technique is called tunneling.

Summary

• Security is a game; make sure you understand the rules.
• Cryptography in one form or another is a basic security tool.
  • But there are many security threats it doesn’t address.
• Claiming and establishing identity is an important operation in secure systems.
  • And the current identity infrastructure is rickety.

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This page last modified on 2012 September 20.