Cryptography

Reference:

Network Security

PRIVATE Communication in a PUBLIC World.

by Kaufman, Perlman & Speciner.

Secret Key Cryptography

• Single key used to encrypt and decrypt.
• Key must be known by both parties.
• Assuming we live in a hostile environment (otherwise - why the need for cryptography?), it may be hard to share a secret key.

Public Key Cryptography (a.k.a. asymmetric cryptography)

• Relatively new field - 1975 (as far as we know, the NSA is not talking).
• Each entity has 2 keys:
• private key (a secret)
• public key (well known).
• Public keys are used for encrypting.
• Private keys are used for decrypting.

Digital Signature

• Public key cryptography is also used to provide digital signatures.

Ed: The following doesn't make much sense without the pictures...

Transmitting over an insecure channel.

Alice wants to send Bob a private message.

Apublic is Alice’s public key.

Aprivate is Alice’s private key.

Bpublic is Bob’s public key.

Bprivate is Bob’s private key.

"Hello Bob, Wanna get together?"

encrypt using Bpublic

decrypt using Bprivate

OK Alice, Your place or mine?

decrypt using Aprivate

encrypt using Apublic

Bob’s Dilemma

• Nobody can read the message from Alice, but anyone could produce it.
• How does Bob know that the message was really sent from Alice?

• Bob may be comforted to know that only Alice can read his reply.

Alice can sign her message!

• Alice can create a digital signature and prove she sent the message (or someone with knowledge of her private key).
• The signature can be a message digest encrypted with Aprivate.

Message Digest

• Also known as “hash function” or “one-way transformation”.
• Transforms a message of any length and computes a fixed length string.
• We want it to be hard to guess what the message was given only the digest.
• Guessing is always possible.

Alice’s Signature

• Alice feeds her original message through a hash function and encrypts the message digest with Aprivate.
• Bob can decrypt the message digest using Apublic.
• Bob can compute the message digest himself.
• If the 2 message digests are identical Bob knows Alice sent the message.

Sign with Aprivate

check signature using Apublic

encrypt using Bpublic

decrypt using Bprivate

Why the digest?

• Alice could just encrypt her name, and then Bob could decrypt it with Apublic.
• Why wouldn’t this be sufficient?

Implications

• Suppose Alice denies she sent the message?

• Bob can prove that only someone with Alice’s key could have produced the message.

Another possible problem

• Suppose O.J. receives a message from Alice including a digital signature.
• O.J. sends the same message to Bob claiming to be Alice, and includes the digital signature from the message Alice sent to him.
• Bob is convinced Alice sent the message!

Solution?

• Always start your messages with:
• Dear O.J.
• Create a digest from the encrypted message and sign that digest.
• There are many other schemes as well.

Speed

• Secret key encryption/decryption algorithms are much faster than public key algorithms.
• Many times a combination is used:
• use public key cryptography to share a secret key.
• use the secret key to encrypt the bulk of the communication.

Secure Protocols

• There are a growing number of applications for secure protocols:
• email
• electronic commerce
• electronic voting
• homework submission

• Many application protocols include use of cryptography as part of the application level protocol.
• Secure Sockets Layer (SSL) is a different approach - a new layer is added that provides a secure channel (currently TCP only).
• Most secure WWW communication is SSL based.

Security

• There is a course on security offered in the spring - much more in depth.
• I have lots of books for anyone interested in secure protocols, encryption standards and algorithms, etc.
• Usually a popular Term Project Topic!