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T101 Networks

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T101 Networks

13 – Message Integrity

- practical demonstrations
- part 3 is based on Cryptool
- Part 2 is based on nslookup
- review Lab 10
- find the “real” TTL of an A record
- set type=ns, then enter the domain to get a nameserver
- server nameserver
- set debug
- set type=a, then do the query

- find the first mail exchanger for an email address

- find the “real” TTL of an A record

- describe message integrity checking
- describe cryptographic hashing
- explain digital signatures
- explain how digital signatures can help to solve the identity problem

- Cryptography is…
- protecting privacy
- authentication of identities
- preservation of integrity

- …in an environment of mistrust

- the Man-in-the-Middle attack (MITM)
- intercepts the request for the bank’s public key, and sends his own public key to Alice

- Alice uses the MITM’s public key to encrypt the symmetric key
- the MITM can then decrypt the message and then send it to the bank
- Alice needs to be able to validate that the public key belongs to the bank
- asymmetric cryptography can solve this problem, but first, we need a way to check the integrity of a message

- in the early days of communication, parity and CRC checksums were added to messages
- exercise on handout, page 1
- they help to identify errors in transmission
- they do not guard against deliberate mistkaes

Key = Messagei

- Message is broken in blocks the same size as the key
- H0 has to be defined and published
- actually a bit trickier than this
- final hash Hn is the hash of the message

Plaintext = Hi-1

Symmetric

Encryption

Hi

- hash values are typically 128-256 bits long
- there must be multiple messages that produce the same hash
- for 128 bits, we only have 2128 different hash values
- surely we can find two messages with the same hash???

- two different messages with the same hash are called hash collisions
- there must be plenty of them
- tricky to find
- try brute force?
- for a 128 bit hash, you need to calculate the hash of 2128 different messages

- 2128 different messages
- if you can try 1,000,000,000 messages per second, then it will take:

2128

seconds

1,000,000,000

to find a collision

- 10,782,897,524,556,318,080,696 years
- so brute force doesn’t work
- but there could be a shortcut if there is a problem is found with the hashing algorithm
- more of this in the lab…

- two things that make a signature a signature
- only one person can create the signature
- everybody else can verify the signature

- take a document to be signed and produce the hash for it

Hash function

Document

Hash

Document

now if the document changes, we will notice, because the hash will be different

obvious flaw in this scheme!

- asymmetric encryption can use the private key to encrypt, instead of the public key

Private key

Plaintext

Asymmetric

Encryption

Ciphertext

- an example similar to this was seen in last week’s lab

- if you encrypt with the private key, then you need the public key to decrypt

Public key

Asymmetric

Decryption

Plaintext

Ciphertext

- this means that
- only the owner of the private key can encrypt (or sign)
- anybody can decrypt (or verify the signature)

- but what to encrypt (or sign)?
- you don’t need to encrypt (sign) the entire message, only the hash of the message
- The encryption of the hash value, using the private key, is called a digital signature.

- take a document to be signed and produce the hash for it

Hash function

Hash

Private key

Document

asymm

encrypt

Document

Signature

- take the document and calculate the hash for it, compare it against the decrypted signature

Public key

asymm

decrypt

Hash function

=

?

Signature

Document

Hash

Hash

- see page 5 of the notes

- a digital signature is the encryption of the hash value, using the private key
- the public key is used to check the signature is correct
- if the signature is correct, then the document is valid
- how does this help with the MITM problem of symmetric key exchange?

- Alice needs to check the validity of the public key, so maybe the bank should sign it’s own public key?
- in order for Alice to check the bank’s signature, she needs the bank’s public key
- Catch 22!

- maybe we can get a third party (Trusty Bob) to sign the bank’s public key
- then to check the signature of the bank’s public key, Alice needs Trusty Bob’s public key
- how does Alice get Trusty Bob’s public key?
- the MITM can intercept Trusty Bob’s public key if it sent over the network
- back to square one…?

- who do we trust to sign the bank’s public key?
- who is Trusty Bob?
- how do we get Trusty Bob’s public key, whilst being secure against the MITM?
- next week, the final piece to the jigsaw

- message integrity using hashing
- key exchange is susceptible to the MITM
- digital signature is the asymmetric encryption of the hash, using the private key
- who do we trust to sign public keys?
- how do we get their public key to check their signature?