Learning Practical Cryptography

Cryptosmith Video Series #1

Rick Smith December, 2016

Fiction, not Fact.Reality was worse

“Get Smart”© NBC Television

Genuine 1960s phone security,

not including the radio,or the power.

Installed in trucks, ships.

The cell phone: asmall, practical

device

Modern Mobile Crypto Apps✤ Strong crypto, easy to install✤ But do they really protect your data?

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???

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A crypto failure on the web.But what does it mean?

Do we need to fix it?

from “IOS Security” April 2016© Apple, Inc.

Learn the words and conceptsand it makes sense

From the Cryptography 1 course offered on Courseraby Dan Boneh, Stanford University

Makekey pair

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Shared Secret

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Shared Secret

From Authentication © 2002. Used by permission

Video = automated diagrams

Start/Stop the Video

} Like, Save, Sharing, etc.

Full Screen 1080p

Controls

} Notes on how to watch this video

} Link to the next video, or to

related videos

Video Notes: a link to additional notes about

the video’s contents

Grade School Crypto

Ciphers hide the

meaning of text

Zfmebop efab qeb jbxkfkd lc

qbuq

encryption, decryption, ciphers, keys, algorithms, code cracking, cryptanalysis, letter frequency analysis

Eavesdropping on a Mobile Purchase

Cryptosmith Video Series #2

Rick Smith, December, 2016

Buying a Coffee Pot

Buying a Coffee Pot

DANGERZONE

Safe Zone

Safe Zone

Protecting Private Data:Encrypt itKeep it in a safe zone

AES

b4cd11bcf084c19d112879d0

The Key

b4cd11bcf084c19d112879d0

The Key

The datato encrypt

AES

The decrypted

data

Encrypt

Decrypt

b4cd11bcf084c19d112879d0

The Key

b4cd11bcf084c19d112879d0

The Key

Secret Sharing with Public Key Crypto

Cryptosmith Video Series #3

Rick Smith, April, 2017

b4cd11bcf084c19d112879d0

Secret Key

The datato encrypt

The decrypted

dataCipher

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Secret Key

To protect our sensitive data…

… we must protect the secret key

Cipher

Encrypt

Decrypt

How do we share the secret key?

The key is easy to intercept

b4cd11bcf084c19d112879d0

Secret Key

b4cd11bcf084c19d112879d0

Secret Key

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Secret Key

The Phone’s Special InfoThe Server’s Special Info

Share a Secret Keywithout earlier preparations

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Secret Key

b4cd11bcf084c19d112879d0

Secret Key

Eavesdropping does not leak the shared secret

?

Special Info = Public Key

✤ A public key hides a private key inside

Generatea public-private

key pairRandom input

Public Key

Private Key

Special Info = Public Key

✤ A public key hides a private key inside✤ We can share the public key in public✤ A public key hides a private key inside

Public Key

✤ A public key hides a private key inside✤ We can share the public key in public✤ The private key must remain secret

Special Info = Public Key

✤ A public key hides a private key inside✤ We can share the public key in public

Public Key

Private Key

The Phone’s Special InfoThe Server’s Special Info

The Server’s Public Key

The Phone’s Public Key

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Secret Key

Use the phone’s private keyand the server’s public keyto create their shared secret key

For the Phone:

Building the Shared Secret

Building the Shared Secret

The Server’s Public Key

The Phone’s Public Key

Use the server’s private keyand the phone’s public keyto create their shared secret key

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Secret Key

For the Server:

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Secret Key

Private Key Private Key

Building the Shared SecretChoose a library function:✤ Elliptic Curve ✤ Diffie-Hellman

Secret Sharing

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Shared Secret

Private Key

Public Key

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Secret Key

b4cd11bcf084c19d112879d0

Secret Key

A reliable, secure data link requires

✤ Public key exchange toconstruct a shared secret

✤ A strong cipher whose keysare known to both devices

Encrypt

Decrypt

Transport LayerSecurity (TLS - SSL)

Transport Layer Security (TLS - SSL)

Cryptosmith Video Series #4

Rick Smith, March, 2017

What’s in a name?

TLSTransport Layer Security

SSLSecure Sockets Layer

TLSTransport Layer Security

SSLSecure Sockets Layer

protocol:

got it!

1. set of rules that explain correct conduct and proceduresfor 2 or more parties

2. set of rules for handling andformatting computer data

TLS evolved from the SSL Protocol✤ Introduced in 1994✤ https://

The TLS Protocolfor safe communication between 2 devices

1. Create a secret-sharing public key pair

2. Use public-key crypto to establish a shared secret

3. Use the shared secret to establisha secure link using a secret-key cipher.

4. Use the secure link to exchange encrypted messages.

4 Steps

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Shared Secret

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Shared Secret

Step 1: Create a key pair

Makekey pair Phone creates a new pair when

it opens a new connection

Step 2: Establish a shared secret

Makekey pair

1. Phone sends public key and asks for a TLS connection

2. Server creates a key pair3. Server sends its public key4. Devices compute the shared secret

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Shared Secret

b4cd11bcf084c19d112879d0

Shared Secret

Step 3: Establish encrypted link

Hash

b4cd11bcf084c19d112879d0

Shared Secret

b4cd11bcf084c19d112879d0

Shared Secret

Hash

812d0b4ccd1f019d4c181b79

Phone Key

812d0b4ccd1f019d4c181b79

Phone Key

12d0bf4c5c1b819d61c4c179

Server Key

12d0bf4c5c1b819d61c4c179

Server Key

1. Devices create crypto keysusing a “hash” function

✤ Separate keys for separate roles

Step 3: Establish encrypted link

Encrypt

Decrypt

hello?hello?

812d0b4ccd1f019d4c181b79

Phone Key

1. Devices create crypto keysusing a “hash” function

✤ Separate keys for separate roles2. Phone sends message to test

the encrypted link

812d0b4ccd1f019d4c181b79

Phone Key

1. Devices create crypto keysusing a “hash” function

✤ Separate keys for separate roles

Step 3: Establish encrypted link

okay!

1. Devices create crypto keysusing a “hash” function

✤ Separate keys for separate roles2. Phone sends message to test

the encrypted link3. Server sends reply to confirm

the test message arrived

12d0bf4c5c1b819d61c4c179

Server Key

12d0bf4c5c1b819d61c4c179

Server Key

1. Devices create crypto keysusing a “hash” function

✤ Separate keys for separate roles2. Phone sends message to test

the encrypted link

okay!

Cipher

Cipher

Decrypt

Encrypt

Step 4: Exchange messages safely

Cipher

Cipher

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The Key

Decrypt

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Encrypt

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The Key

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NFF %8C UIF BSL

NFF %7G UIF CSL

Message IntegrityThe text must not be altered

How do we detect an altered message?

Hash functions!

Protecting Message Integrity

Cryptosmith Video Series #5

Rick Smith, January, 2017

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The Key

Decrypt

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Encrypt

b4cd11bcf084c19d112879d0

The Key

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NFF %8G UIF BSL

NFF %7G UIF CSL

Message IntegrityHow do we detect altered text?

We add extra data to detect changes.

NFF %8G UIF BSL K(check value)

Use the numeric values of the data in the message to calculate a check value.

Sending: N+F+F+%+8+G+U+I+F+B+S+L = K

Received: N+F+F+%+7+G+U+I+F+C+S+L = W

NFF %8G UIF BSL K(check value)

NFF %7G UIF CSL K(check value)

Recalculate the check value for the received message.Damage yields a different check value.

One Way Hash functions

✤ More than a simple check digit calculation✤ Input: 1 or more bytes, and can take a huge input✤ Output: a fixed size (example: 128 bits for “SHA 128”)

A GOOD GLASS IN THE BISHOP'S HOSTEL IN THE … … FROM THE TREE THROUGH THE SHOT FIFTY FEET OUT

43 84 9f 75 61 4a 49 8b 00 d5 28 a4 59 fc 9d de

Hash

One Way Hash functions

✤ Not encryption: no procedure to simply invert it✤ Hashing a second time mixes the data even more

A GOOD GLASS IN THE BISHOP'S HOSTEL IN THE … … FROM THE TREE THROUGH THE SHOT FIFTY FEET OUT

43 84 9f 75 61 4a 49 8b 00 d5 28 a4 59 fc 9d de

UnHashHash

b5 3c e2 fa 7c 4f a7 c8 8e 33 b4 2e 57 da 13 30

Hash

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Hash

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UK

Message IntegrityInclude the message hash (“UK”)

Check thehash values

Hash

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Hash

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UK

Message IntegrityAny change causes a mismatch

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UK

Check thehash values

Hash

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Hash

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UK

Message IntegrityWhat about intentional changes?

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GV

Check thehash values

The hash calculation shouldinclude secret information

Keyed Hashing✤ Include secret information - a key - to prevent

attackers from making undetected changes

A GOOD GLASS IN THE BISHOP'S … THROUGH THE SHOT FIFTY FEET OUT

43 84 9f 75 61 4a 49 8b

Hash

b4cd11bc f084c19d 112879d0

The KeyThe Message

Keyed Hashing✤ Recipient uses the shared secret key to check for

alterations in the message

A GOOD GLASS IN THE BISHOP'S … THROUGH THE SHOT FIFTY FEET OUT 43 84 9f 75 61 4a 49 8b

b4cd11bc f084c19d 112879d0

The KeyThe Message

The Wrapped Message The transmitted hash

Keyed Hashing✤ Recipient uses the shared secret key to check for

alterations in the message

A GOOD GLASS IN THE BISHOP'S … THROUGH THE SHOT FIFTY FEET OUT

43 84 9f 75 61 4a 49 8b

b4cd11bc f084c19d 112879d0

The KeyThe Message

The transmitted hash

43 84 9f 75 61 4a 49 8b

Hash

=

Crypto mechanisms can fail

WEPWhat makes crypto weak