Digital Signatures, Message Digest and Authentication Week-9

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

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

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

Digital Signatures CA sees message is from Alice and decrypts it using Alice’s secret key, K A CA then uses Bob’s secret, K B, to encrypt an extended message containing: Alice’s original plaintext message to Bob the “signed message” encrypted with CA’s secret key, K CA (A, t, M) Extended message is then sent to Bob K B (A, R A, t, M, K CA (A, t, M))

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, K A Bob produces Exhibit 1:K CA (A, t, M) Judge asks trustworthy CA to decrypt Exhibit 1 who also testifies that Bob is telling the truth Judge rules in favour of Bob, case dismissed!

Digital Signatures Problems Replay attacks: Suppose in the communication of two parties A and B; A is sharing his key to B to prove his identity but in the meanwhile Attacker C eavesdrop the conversation between them and keeps the information which are needed to prove his identity to B. Later C contacts to B and prove its authenticity.

Digital Signatures Solutions Solutions: Timestamp and Random Number Using timestamps throughout minimises the problem Based on timestamps old messages are rejected To guard against instant replay attacks Bob checks R A of every incoming message R A (Random number in Alice’s message) is checked to see if any other incoming message contains the same R A. If not then the assumption is that this is new message / request.

Message Digest (MD) 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

Message Digest (MD) Comparing Digital Signature and Message Digests A message digest is a fixed size numeric representation of the contents of a message, computed by a hash function. The sender can also generate a message digest and then encrypt the digest using the private key of an asymmetric key pair, forming a digital signature. The signature must then be decrypted by the receiver, before comparing it with a locally generated digest.

Digital Signatures Verification: A case in Court

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

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?

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

Authentication Based on shared secret key (symmetric) Commonly known as challenge-response protocols Alice and Bob share secret key, K AB 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

Two way Authentication: challenge –response Alice A K AB (R B ) Bob RARA K AB (R A ) RBRB

Two way Authentication: challenge –response - 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 R B as plaintext to Alice - Alice encrypts message using shared secret key and sends ciphertext K AB (R B ) to Bob - Bob decrypts ciphertext and checks R B

Two way Authentication: challenge –response - Bob is now sure he is talking to Alice and not Trudy (Trudy does not know K AB ) - But Alice does not know she is talking to Bob (Trudy could have sent plaintext R B to Alice!) - Therefore, Alice sends Bob her own R A and when Bob returns K AB (R A ), Alice knows it’s from him - To establish a session key (optional), Alice picks a session key K S and sends Bob K AB (K S )

Thank You !