1 Web security. 2 Applications –Email –electronic commerce –electronic voting –homework submission –E-bank –Paper submission.

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Web security

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Applications

– Email– electronic commerce– electronic voting– homework submission– E-bank– Paper submission

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Security Requirments

Authentication: identifying someone (or something) reliably. Proving you are who you say you are.

Authorization: permission to access a resource.

Integrity: data has not changed by someone else.

Confidentiality: data is only received by the target

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Terminology

Encryption: Scramble data so that only someone with a secret can make sense of the data.

Decryption: Descrambling encrypted data. DES: Data Encryption Standard: secret key

cryptographic function standardized by NBS (NIST).

AES: Advanced Encryption Standard: recent standard for symmetric cryptography

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Terminology (cont.)

Secret Key Cryptography: a cryptographic scheme where the same key is used to encrypt and decrypt data. Also called symmetric encryption.

Public Key Cryptography: a cryptographic scheme where different keys are used for encryption and decryption (asymmetric).

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Terminology (more!) Firewall: a network component that

separates two networks and (typically) operates in the upper layers of the OSI reference model (Application layer).

Screening Router: a discriminating router that filters packets based on network layer (and sometimes transport layer) protocols and addresses.

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Attack types

DoS (denial of service): attempting to disrupt or deny a useful service.

Eavesdropping: interception of a third-party (malicious) host

Modification: changing data Fabrication: producing data on behalf of

another person.

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Historical Ciphers

Caesar cipher

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Data Encryption Standard (DES)

Designed at IBM in 1975– Changes suggested by the NSA

Standardized by NIST in 1977– Official cipher for civilian cryptography– Reviewed by the NSA – Used by CIA

Combines substitutions and permutations– Operates on bits

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Feistel Network

Iterative structure Efficient hardware

implementation Non-linear element F

provides security Multiple rounds

provide mixing (diffusion) between the two halves

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The AES Cipher - Rijndael designed by Rijmen-Daemen in Belgium

has 128/192/256 bit keys, 128 bit data

an iterative cipher– treats data in 4 groups of 4 bytes

– expands 128-bit key to 4 groups of 4 bytes(larger keys are expanded to more groups)

– operates an entire block in every round

– round number dependent on key size

designed to be:– resistant against known attacks

– speed and code compactness on many CPUs

– design simplicity

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RSA by Rivest, Shamir & Adleman of MIT in 1977

best known & widely used public-key scheme

based on exponentiation in a finite field over integers modulo a prime – exponentiation takes O((log n)3) operations (easy)

uses large integers (eg. 1024 bits)

security due to cost of factoring large numbers

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RSA Key SetupGeneration of a public/private key pair:

• select two large primes at random: p, q

• compute the system modulus n = p·q

• select at random the encryption key e (n)=(p-1)(q-1)

– where 1<e<(n), gcd(e,(n))=1

• solve following equation to find decryption key d :e·d=1 mod (n) and 0≤d≤n

• public encryption key: KU={e,n}

• private decryption key: KR={d,n}

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RSA Example1. Select primes: p=17 & q=11

2. Compute n = pq =17×11=187

3. Compute (n)=(p–1)(q-1)=16×10=160

4. Select e : gcd(e,160)=1; choose e=7

5. Determine d: de=1 mod 160 and d < 160 Value is d=23 since 23×7=161= 10×160+1

6. Public key KU={7,187}

7. Private key KR={23,187}

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RSA Use to encrypt a message M the sender:

– obtains public key of recipient KU={e,n}

– computes: C=Me mod n, where 0≤M<n

to decrypt the ciphertext C the owner:– uses the private key KR={d,n}

– computes: M=Cd mod n

note that the message M must be smaller than the modulus n

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RSA Example cont’d sample RSA encryption/decryption is:

given message M = 88 (88<187)

encryption:

C = 887 mod 187 = 11

decryption:

M = 1123 mod 187 = 88

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

• Each entity has 2 keys:– private key (a secret)– public key (well known).

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• Private keys are used for decrypting.• Public keys are used for encrypting.

encryptionplaintext ciphertext

public key

decryptionciphertext plaintext

private key

Using Keys

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Digital Signature• Public key cryptography is also used to

provide digital signatures.

signingplaintext signed message

private key

verificationsigned message plaintext

public key

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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.

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Hello Bob,Wanna get together?

AliceAlice BobBob

encrypt using Bpublic decrypt using Bprivate

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OK Alice,Your place or mine?

AliceAlice BobBob

decrypt using Aprivate encrypt using Apublic

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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.

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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.

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AliceAlice BobBob

Sign with Aprivate check signature using Apublic

encrypt using Bpublic decrypt using Bprivate

Revised Scheme

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Another possible problem• Suppose Bill receives a message from Alice

including a digital signature.

“meet me at the library tonight”

• Bill sends the same message to Joe so that it looks like the message came from Alice.

• Bill includes the digital signature from the message Alice sent to him.

• Joe is convinced Alice sent the message!

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Solution?

• Always start your messages with:– Dear Bill,

• Create a digest from the encrypted message and sign that digest.

• There are many other schemes as well.

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Authentication

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Authentication

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Secure Protocols

• Many application protocols include the use of cryptography as part of the application level protocol.– The cryptographic scheme employed is

part of the protocol.– If stronger cryptographic tools become

available we need to change the protocol.

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SSL and TLS

• Secure Sockets Layer (SSL) is a different approach - a new layer is added that provides a secure channel over a TCP only link.

• TLS is Transport Layer Security (IETF standard based on SSL).

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SSL layer

Application

SSL

TCP

IP

Application

SSL

TCP

IP

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Advantages of SSL/TLS

• Independent of application layer

• Includes support for negotiated encryption techniques.– easy to add new techniques.

• Possible to switch encryption algorithms in the middle of a session.

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HTTPS Usage

• HTTPS is HTTP running over SSL.– used for most secure web transactions.– HTTPS server usually runs on port 443.– Include notion of verification of server via a

certificate.– Central trusted source of certificates.