1. Cryptography: Digital Signatures Message Digests Authentication
Security in Computing
Cryptography:
Digital Signatures
Message Digests
Authentication

2. Digital Signatures Non-repudiation (non-denial)
How do you legally prove who actually send you the message?
Receiver can verify the claimed identity of sender
Sender cannot later repudiate contents of the message
Receiver cannot possibly have concocted message by himself/herself

3. Digital Signatures Purpose
Authenticity of legal / financial documents is determined by authorised handwritten signatures
Photocopies may be forged and don’t count!
Growing need for computing systems to replace physical transport of paper documents
The problem is devising a method of “digital signatures” to be used as legal signatures

4. Digital Signatures Symmetric Scheme A, KA (B, RA, t, M)
Trusted Central Authority (CA) is needed
CA shares secret key K with sender (Alice) and receiver (Bob): KA and Kb
Alice (A)sends message (M)encrypted with her key (KA) to her banker Bob (B) via CA
A, KA (B, RA, t, M)
A = Alice’s identity;
B = Bob’s identity; RA = Random number chosen by Alice; t = timestamp to ensure freshness
KA(...) means the message is encrypted using the secret key Alice shares with CA

5. Digital Signatures
CA sees message is from Alice and decrypts it using Alice’s secret key, KA
CA then uses Bob’s secret, KB, to encrypt an extended message containing:
Alice’s original plaintext message to Bob
the “signed message” encrypted with CA’s secret key, KCA(A, t, M)
Extended message is then sent to Bob
KB (A, RA, t, M, KCA(A, t, M))
KA(...) means the message is encrypted using the secret key Alice shares with CA

6. Digital Signatures Verification: A case in Court
Alice later denies sending message to Bob
Bob tells judge that CA will only accept message form Alice if encrypted with her key, KA
Bob produces Exhibit 1: KCA(A, t, M))
Judge asks trustworthy CA to decrypt Exhibit I who also testifies that Bob is telling the truth
Judge rules in favour of Bob, case dismissed!

7. Digital Signatures Problems Digital Signature Public key Scheme?
Replay attacks: Timestamp; random number
Digital Signature Public key Scheme?
Using timestamps throughout minimises the problem
Based on timestamps old messages are rejected
To guard against instant replay attacks Bob checks RA of every incoming message
RA((Random number in Alice’s message) is checked to see if any other incoming message contains the same RA. If not then the assumption is that this is new message / request.

8. Message Digests Integrity One –way Hash function
Digital signatures couple together two distinct functions (criticism): “authentication” (i.e. verification) & secrecy
“Authentication” is often needed but secrecy is not
Message digests provide a more efficient approach to digital signatures

9. Message Digests Comparing Digital Signature and Message Digests
Digital Signature: A, KA(B, RA, t, M) (from Alice to Bob via CA)
KB (A, RA, t, P, KCA(A, t, P)) (from CA to BOB)
Extended message to Bob

10. Message Digests Message Digest Public Key Scheme? Message Digest:
Instead, consider CA computing a message digest MD(P) by applying MD to P
KB ( A, RA, t, P KCA( A, t, MD (P) ) )
Thus, message digest MD(P) is signed and NOT P (computationally faster to do so)
Message Digest Public Key Scheme?

11. Message Digests Applications
Message Digest 5 (MD5) / & MD6 are prominent by Ronald Rivest
Secure Hash Algorithm (SHA-1) + newer versions of SHA-1
Others (Investigate)

12. Authentication Authentication Vs Digital Signatures
Digital Signatures guarantee non-repudiation at some point in the future
Authentication – verifying someone’s identity during communication
Authentication – how do you know it’s Alice and not Trudy (imposter) you are ‘talking’ to?

13. Authentication Authentication Vs Authorisation
Authorisation – what a process is permitted to do
Server: Is Bob’s process allowed to delete a file ?
Authentication – are you actually communicating with the process you think you are?
Server: Needs to determine if this is Bob’s process

14. Authentication Based on shared secret key (symmetric)
Commonly known as challenge-response protocols
Alice and Bob share secret key, KAB
Alice or Bob sends a random number to the other
Alice or Bob transforms it in some way and returns the result back to the other

15. Two way Authentication: challenge –response
Alice
Bob A RB
KAB (RB) RA
This protocol contains five messages. Can it be shortened?
Alice sends her identity to Bob
Bob chooses a challenge to be sure this came from Alice and not Trudy
The challenge Bob sends is a large random number RB as plaintext to Alice
Alice encrypts message using shared secret key and sends ciphertext KAB(RB) to Bob
Bob decrypts ciphertext and checks RB
Bob is now sure he is talking to Alice and not Trudy (Trudy does not know KAB)
*But Alice does not know she is talking to Bob (Trudy could have sent plaintext RB to Alice!)
Therefore, Alice sends Bob her own RA and when Bob returns KAB(RA), Alice knows it’s from him
To establish a session key (optional), Alice picks a session kye KS and sends Bob KAB (KS)
*As far as Alice is concerned Trudy could have intercepted the first message (see diagram) and sent the random number to her. So Alice does same as what Bob did.
KAB (RA)
Source: A S Tanenbaum