1. CDK: Chapter 7 TvS: Chapter 9
Lecture 18: Security
CDK: Chapter 7
TvS: Chapter 9

2. Security in 1 machine Within 1 machine, the OS is responsible for
Verifying users’ identities
Checking access rights to shared objects
Not too difficult! But a network brings new problems:
Different machines – identity mapping
Network weaknesses ….
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3. Security Problems of Network
Messages can be:
Read => loss of secrecy/privacy
Altered
Created containing forged information
Copied and replayed
Can also have denial of service attacks
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4. Cryptography All the problems with messages find solutions based on
The inclusion (and checking) of extra information (e.g. sequence nos.)
and
Encryption
Using some algorithm, plaintext is converted to encrypted text
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5. Secure Channels How to make client-server communication secure?
1. Authentication of the communicating parties. We may need to ensure message integrity, confidentiality, etc.
2. Authorization. Is the client authorized to have that request carried out? Relates to controlling access to the resources (granting access rights).
A secure channel (Voydock and Kent, 1983) protects senders and receivers against interception, modification and fabrication of messages.
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6. Properties of Secure Channels
A secure channel connects two processes and:
Ensures each process knows reliably the identity of the principal on whose behalf the other process is acting
Each message includes a physical or logical time stamp to prevent replay or reordering
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7. Digital Signatures
Need an electronic version of a signature to authenticate a message
Unlike paper signatures, we need to stop a signature being attached to other messages by cutting and pasting! So a signature will include the message or a secure digest derived from the signed message.
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8. Secure digests A digest is also called a secure hash function.
Need to be sure that, given the digest value, the receiver cannot invent another message which yields the same digest value!
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9. Uses of signatures There are two aspects:
Authentication – receiver knows that the message is from the signer, because it is assumed impossible to forge a signature
Non-repudiation – the sender can’t deny sending the message
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10. Keys in Cryptography Two kinds:
Shared secret keys – the major problem is how to establish a shared secret key between principals who never meet!
Public/private key pairs – every one has such a pair; everyone knows all the public keys, only the principal knows the private. Need other key to decrypt message.
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11. Public key encryption
Requires 100 – 1000 times as much processing power as secret-key algorithms
Rely on the “impossibility” of deriving the private key from the public one
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12. Pragmatics
Better not to rely on secrecy of encryption algorithms – in practice clients and servers need to know, or even negotiate, these – rely only on keys
Bigger keys take longer to crack – but it is often just a matter of time, and the timescale gets shorter as attackers get more powerful computers
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13. Sending a confidential message
A -> B: Message encrypted with B’s public key
Only B can decrypt
OR (more practically)
A -> B: 1-off session key, K, encrypted with B’s public key
A -> B: Message encrypted with K
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14. Digital Signature with Public Keys
A -> B: A’s identity, and the message encrypted with A’s private key
B can decrypt using A’s public key – and the fact that this works proves that A encrypted it – as only A knows that key.
To keep the above confidential, A could encrypt the whole with B’s public key!
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15. Key distribution Need a way to get the public keys
This needs to be secure – e.g. if an enemy intercepts the request for a key, it can reply with one it invented
On an intranet, we can design a simple protocol for an authentication server
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16. Digital Certificates
In the wider world, we need to be able to be sure that servers (and clients) are who they claim to be (and give their public keys).
A certificate is an unforgeable document issued by a more trustworthy source
Thus a certification chain is established
Revocation – hard, use expiry date
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17. Needham-Schroeder Authentication Protocol
Designed for use with an authentication server, S, which has secret keys for all principals
How to get two processes, A and B, communicating securely without revealing secret keys to each other
Used in Kerberos (within Intranets)
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18. Cryptography notationsKA
Alice’s secret key KB
Bob’s secret key
KAB
Secret key shared between Alice and Bob
KApriv
Alice’s private key (known only to Alice)
KApub
Alice’s public key (published by Alice for all to read) { M } K
Message
encrypted with key [ ]K
signed with key
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19. The Needham–Schroeder secret-key authentication protocol (7.14 in CDK)
Header
Message
Notes
1. A->S:
A requests S to supply a key for communication
A, B, NA
with B.
S returns a message encrypted in A’s secret key,
2. S->A:
{NA , B, KAB,
containing a newly generated key KAB and a
{KAB, A}KB}KA
‘ticket’ encrypted in B’s secret key. The nonce NA
demonstrates that the message was sent in response
to the preceding one. A believes that S sent the
message because only S knows A’s secret key.
3. A->B:
{KAB, A}KB
A sends the ‘ticket’ to B.
{NB}KAB
B decrypts the ticket and uses the new key KAB to
4. B->A:
encrypt another nonce NB.
A demonstrates to B that it was the sender of the
5. A->B:
{NB - 1}KAB
previous message by returning an agreed
transformation of NB.
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20. The same also from TvS (Fig 9.17)
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21. Summary Whole 3rd year course on using Cryptography in Networks ….
This lecture has only scratched the surface – read Chapter 7 of CDK or Chapter 9 of TvS if you are interested
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