CSCE 465 Lecture 24

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Authentication Protocols

Examples:

• Wi-Fi (WPA, WEP, tamulink-wpa uses WPA2-Enterprise suite of encryption/authentication protocols)
• Printing (CUPS, HPLIP, Samba, etc.)
• Kerberos

Roadmap

• Authentication Handshakes
• Login only (one-side authentication)
• mutual authentication (both sides authenticate)
• Integrity/encryption of data
• Mediated authentication (with KDC)

Authentication Handshakes

Secure communication almost always includes an ititial handshake

• Authenticate each other
• Establish session keys
• not trivial; flaws undermine security

Authentication with Shared Secret

1. Alice: "I'm Alice."
2. Bob: "Alice, I challenge you with this random number Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R} ."
3. Alice: "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(K_{\text{Alice-Bob}}, R)}

Multiple variations are possible: e.g. send encrypted random number and ask Alice to decrypt

Weaknesses:

• Authentication is not mutual: Trudy can convince Alice that she is Bob
• Trudy can hijack conversation after initial exchange
• If shared key is derived from password, Trudy can mount offline password-guessing attack
• Trudy may comprimise Bob's database and later impersonate Alice

Authentication with Public Key

1. Alice: "I'm Alice."
2. Bob: "Alice, I challenge you with this random number Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R} ."
3. Alice: "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \text{Sign}_{\text{Alice}} \{ R \}} ."

Variation: send Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R} encrypted with Alice's public key, ask for decrypted Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R} .

Bob's database is more secure; less information can be obtained if hacked, BUT

Weaknesses:

• Authentication is still not mutual: Trudy can still convince Alice that she is Bob
• Trudy can still hijack conversation after initial exchange.
• NEW: Trudy can trick Alice into signing something
  • Alice should use different private key for authentication than for other purposes.

Mutual Authentication

(both sides authenticate)

Shared Secret Key

1. Alice: "I'm Alice."
2. Bob: "Alice, I challenge you with this random number Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R_1} ."
3. Alice: "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(K_{\text{Alice-Bob}}, R_1)} "
4. Alice: "Bob, I challege you with this random number $R_{2}$."
5. Bob: "$f(K_{\text{Alice-Bob}},R_{2})$"

This can be optimized to just

1. Alice: "I'm Alice. Bob, I challenge you with this random number $R_{2}$."
2. Bob: "$f(K_{\text{Alice-Bob}},R_{2})$. Alice, I challenge you with this random number $R_{1}$."
3. Alice: "$f(K_{\text{Alice-Bob}},R_{1})$"

Reflection Attack

It's still possible for trudy to impersonate Alice

• 1. Open connection as usual, and get Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R_1} from Bob
  2. Open a new connection, send Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R_1} as the first "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R_2} " and receive the result back
  3. Send result of second connection over first connection

Lesson: don't have Alice and Bob do exactly the same thing.

• Use different keys: totally different or Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{\text{Alice-Bob}} = K_{\text{Bob-Alice}}+1}
• Refuse to compute challenge for old random number
• Initiator should be the first to prove identity (assumption: initiator is more likely to be bad guy)

Password Guessing

Since Bob sends back Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(K_{\text{Alice-Bob}}, R_2)} , it's possible to attempt password guessing attack

Countermeasure:

1. Alice: "I'm Alice."
2. Bob: "Alice, I challenge you with this random number Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R_1} ."
3. Alice: "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(K_{\text{Alice-Bob}}, R_1)} . Bob, I challenge you with this random number Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R_2} ."

Timestamp

Requires synchronized clocks

1. Alice: send encrypted value of current timestamp
2. Bob: Reply with encrypted value of timestamp + 1

Establishment of Session Keys

Suppose following authentication happened:

1. Alice: "I'm Alice"
2. Bob: "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R} "
3. Alice: "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{\text{Alice-Bob}}\{R\}} "

• Can't use Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{\text{Alice-Bob}}\{R\}} as session key (already transmitted over network)
• Can't use Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{\text{Alice-Bob}}\{R+1\}} as session key (Trudy could impersonate Bob and get Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R+1} from Alice
• In general, something should be done with key, not Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R} .

Two-Way Public Key Based Authentication

Better Approach:

• Alice chooses and encrypts Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R_1} with Bob's public key
• Bob chooses and encrypts Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R_2} with Alice's public key
• Session key is Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R_1 \oplus R_2}
• Trudy will have to compromise both Alice and Bob

Even better:

• Alice and Bob establish session key with Diffie-Hellman key exchange
• Alice and Bob sign quantity they send
• Trudy can't learn anything about session key even if she compromises both Alice and Bob (since key is negotiated through Diffie-Hellman)

One-Way Public Key Based Authentication

Often it's only necessary to authenticate the server (e.g. SSL)

1. Encrypt Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R} with Bob's public key
2. Diffie-Hellman key exchange (Bob signs D-H key)

Mediated authentication (with KDC)

Trusted, third-party Server controls authentication to other machines

In principle:

1. Alice to KDC: Alice wants Bob
2. KDC generates Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{AB}}
3. KDC to Alice: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{\text{Alice}}\{ K_{AB} \}}
4. KDC to Bob: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{\text{Bob}}\{ K_{AB} \}}

Concerns:

• Trudy may claim to be Alice and talk to KDC (may not get anything useful)
• Messages encrypted by Alice may get to Bob before KDC's message (Bob does not have key from KDC yet)
• May be difficult for KDC to connect to Bob

In practice:

1. Alice to KDC: Alice wants Bob
2. KDC generates Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{AB}}
3. KDC to Alice: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{\text{Alice}}\{ K_{AB} \}} and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{\text{Bob}}\{ K_{AB} \}}
4. Alice to Bob: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle K_{\text{Bob}}\{ K_{AB} \}} (this is called a ticket)

Solves second issue, but still not completely secure.