Cryptography And Secure Systems

12/8/2015

AppCrypto Team

What does it mean a system to be secure?

It boils down to the trust you place on the system!

Guru

Trust on the system

How secure is the system

You decide to safeguard your valuable assets like hardly used gold jewelries in a bank instead of keeping them in your house.

Guru

You are placing more trust on the bank’s ability to safeguard your valuable asset

than your own house.

You learn this fact by experience – you

friends valuable assets kept at home

got robbed.

None of your friends have had any bad

experience with the bank safeguarding

their assets.

Secure systems are built to satisfy the following security goals.

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Integrity Availability

Confidentiality

Authentication

Non-repudiation

We will not discuss availability in this presentation as cryptography is not generally used to assure availability (i.e. the service is available whenever you need it) Guru

Denial of Service (DoS) attacks disrupts the available it of a service.

Can I trust the system not to show my sensitive data to any unauthorized parties?

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Can I trust the system not to modify my data by any unauthorized parties?

Can I trust the system not to allow to impersonate me to the system?

Can I trust the system to hold each individual accountable of their actions?

Confidentiality

Integrity

Authentication

Non-repudiation

Confidentiality

Integrity

Authentication

Non-repudiation

Cryptography

Can’t read!

Can’t modify!

Can’t impersonate!

Can’t deny doing!

Trust sounds intuitive, but how do I objectively measure trust?

What does it mean a system to be secure?

Guru

The measure of trust (how secure your system) depends on who you are trying to protect against. We call it the attack model.

An Attack model captures the capabilities, collisions and intentions of adversaries.

An adversary is a bad user who want to break

the security of your system.

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In order to demonstrate trust in the system under the given attack model

We need to prove the security of the cryptographic protocols under that attack model.

A cryptographic protocol secure under one attack model (e.g. attacker can see one message) may not be secure under more stronger attack model (e.g. attacker can see many messages).

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Usually, most of us will be using time tested and proven cryptographic primitives such as ciphers, message authentication codes, digital signatures.

We will be constructing new protocols using these primitives. However, we need to make sure that our protocols are secure under the chosen attack model.

A cryptographic protocol built using secure cryptographic primitives may not necessarily be secure if not built right.

Adversary

Capabilities

Intentions

In cryptography, we assume that attackers

have limited computational power. In other words, they are computationally

bounded.

There could be passive attackers or active attackers. Passive attackers simply listens to your

messages whereas much more powerful

active attacks modify your messages.

Collisions

Multiple adversaries may collude together to break a cryptographic

protocol.

Let’s look at the security goals that are directly tied to cryptography mentioned earlier.

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Confidentiality

Can’t read!

Ciphertext (C) Encrypt Plaintext (M)

Plaintext(M) Decrypt Ciphertext(C)

Use cipher.

The message to be encrypted is called

“plaintext”

The encrypted message is called

“ciphertext”

Ciphers

Symmetric ciphers

Asymmetric ciphers

Also called “Symmetric Key Cryptosystems”

Also called “Public Key Cryptosystems”

Alice Bob

Alice and Bob share the same key.

Alice Bob

Alice and Bob do not share the same key. Public key & Private

key pair

Roughly speaking, a cipher is secure if it is hard to obtain plaintext from the ciphertext without knowing the key used. Guru

In a cipher, only the key is secret; everything else (encryption algorithm, decryption algorithm) is public.

Alice Bob

Messages exchanged in a TLS protocol are encrypted using a symmetric cipher.

Integrity

Can’t modify!

Cryptographic Hashing Algorithm

Document

Key

Secure Hash

Alice Bob

Messages exchanged in a TLS protocol are hashed using a keyed hash algorithm.

Send the hash along with the document to the receiver.

Receiver can compute the hash and compare to verify.

Message Hash

Use hashing.

Authentication

Can’t impersonate!

Use hashing, Public Key Ciphers (certificates).

Three ways to authenticate

Use what you have (e.g. key, badge, certificates)

Use what you know (e.g. passwords, PINs)

Use what you are (e.g. fingerprints, iris)

Password authentication (Server: am I talking to Alice?)

Alice Server (www.example.com)

Hash pwd Salt Password

Certificate based authentication (Alice: Am I taking to example.com?)

Alice Server (www.example.com)

Example.com certificate

Example.com certificate

Check if the certificate is signed by a trusted root

authority.

Check if the new hash matches the

stored one.

Transfer $1M to Bob

Non-repudiation

Can’t deny doing!

Use Public Key Ciphers (digital signatures).

Alice

Public key Private key

Transfer $1M to Bob

Digitally signed using Alice’s private key

Alice cannot later deny that she did not order bank to transfer as it has her digital signature on it

(only she can produce that signature)

Verify signature using Alice’s public key

The security of a system boils down to the trust.

Cryptography is a tool that we can use to build trust in security services (confidentiality, integrity, authentication and non-repudiation) and in turn in systems build using these security services.

If the foundation of your house (cryptographic tools used) is not

strong, your house (software system) will collapse!

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