cryptographic techniques instructor: jerry gao ph.d. san jose state university email:...

Cryptographic Techniques

Instructor: Jerry Gao Ph.D.

San Jose State Universityemail: [email protected]

URL: http://www.engr.sjsu.edu/gaojerry

May, 2000

Topic: Crytographic Techniques

- Encryption and decryption

- Symmetric encryption

- Asymmetric or public-key encryption

- Message digesting or hashing

- Digital signatures and enveloping- digital signatures, dual signatures, blind signatures

- Public key management- certificates and certification authorities

Jerry Gao Ph.D. 5/20000

Outline

All Rights Reserved


Introduction to Crytographic Techniques



In cryptographics, a message in human readable form is referred to plaintext or cleartext.

Encryption:

Encryption refers to a process which disguises a cleartext message to hide its substance and generates a message, known as ciphertext.

Plaintext is denoted by P, whereas ciphertext is denoted by C.The encryption function E operates on P to produce C based on a key value K:

E k (P) = C

Encryption and Decryption

Encryption

Key

Plaintext Ciphtext



Encryption and Decryption

Decryption:

The reverse process of encryption is known as a decryptiondecryption, in which a cihhertext is converted back to its original cleartext form.

In the reverse process, the decryption function D operates on C to produce Pbased on a key value K:

D k (C) = P

Encryption

Key

Ciphertext Plaintext



Symmetric Encryption


Symmetric encryption (secret-key encryption, or single-key encryption):

Secret-key encryption refers to the use of a shared key for both encryption bythe transmitter and decryption by the receiver.

Work step:- Sender sends a message after encryption with a secret-key.- Receiver decrypt the message with the same secret key after receiving it.

Internet

Anne sends a message

BobreceivesAnne’smessage

Encryptwith a secret key

Decryptwith a secret key

EncryptedMessage

EncryptedMessage


Symmetric Encryption


Special features of symmetric encryption:- Two communication parties use and share the same key.- Encryption and decryption is carried out based on the same key.- Both parties must agree on the secret key before communications.

Advantages:- Useful in many cases which only involves two-party communications.

Problems and limitations:

- Complex in key distribution (key generation, transmission, and storage).- Not scalable.- Impractical to support large group of communications in networks.- Impossible to support exchanging messages with a large group of previouslyunknown parties over a public open network.

Example: DES (the Data Encryption Standard)


Asymmetric Encryption


Asymmetric encryption (public-key encryption):

Public-key encryption (asymmetric encryption) use a pair of keys for each party.- One key is known as a “public key”, which is known to other parties.- One key is known as a “private key” or a “secret key”, which must be confidential, and is known only to its owner.

- The public key encrypts the message.- The private key decrypts the message.

Internet

Anne sends a message

BobreceivesAnne’smessage

Encryptwith a public keyPK(Bob)

Decryptwith Bob’ private keySK(Bob)

ScrambledMessage

ScrambledMessage


Asymmetric Encryption


Special features of asymmetric encryption (public-key encryption):

SK(PK(M)) = MAdvantages:

- Easy to key distribution (key generation, maintain and storage)- Scalable to support large group of users in an public network- Easy to support unknown users since no previous consent is needed.- Better protection than symmetric encryption.

Problems and limitations:

- Slow performance

Applications: E-commerce, snoop-proof email system.

Standards: the best known public-key encryption algorithm --> RSA


, Sh

RSA Algorithm


The RSA algorithm was developed by Rivest, Shamir, and Adleman at MIT in 1978.

The basic algorithm is outlined below:

1: Choose two large distinct primes, p and q.2: Compute the product (modulus) n = pq. 3: Randomly choose encryption key e, such that e and (p-1)(q-1) are relatively prime.4: Finally use Euclid’s algorithm to compute the decryption key, d such that

e . d = 1 (mod (p-1) . (q -1)).

Where, d and n are relatively prime. e and n are the public key. d is the secret key.

To encrypt a message M, eC = M mod n

To decrypt C, dM = C mod n


Comparing Symmetric Encryption and Asymmetric Encryption


Features Secret-Key Encryption Public-Key Encryption

No. of keys Single Key Pair of Keys

Types of keys Secret Key only One Public Key andone Private Key

Key Management Simple but difficult to Need digital certificatesmanage and trusted third parties

Performance Very fast Slower

Usage Used for bulk data Used for less demandingencryption, such as applications such as a phone system, encryption small doc. or

to sign messages

Standards Data Encryption the RSA algorithmStandard (DES)


Message Digesting or Hashing


Message digesting is a way to provide integrity without confidentiality.

The basic idea is to apply a digesting or hash algorithm to the (long) message to produce a (short) message digest.

Since the encryption is only applied to a very small quantity, and message digesting is very much faster than encryption. Checking==>When the message arrives, the receiver computes a hash of the message using the same algorithm. If this matches the decrypted MIC that came with the message, then the message has not been tampered with.

A good Hash function has two properties:- It must be difficult to invert.- It must be resistant to collision.

Two well-known Hash functions in payment protocols are MD5 and SHA.


Computing a message integrity check (MIC)


Message

MIC MessageHash

DigestAlgorithm

BlockCipher

MessageIntegrity Check

Secret Key


Digital Signatures


What is a digital signature?

- Digital signatures are implemented using public-key encryption.

- A digital signature is a cryptographic mechanism that performs a similar function to a written signature.

- Created using PPK cryptograph and message digests.

Encryption allows a messagesender the ability to digitally sign messages, thus creating a digital signature for themessage. When a message digest is computed and then encrypted using the sender’s private key, and later append to the message, the result is called the digital signatureof the message.


Digital Signatures


Purposes: Digital signatures are used to ensure the integrity and authentication.-- To verify the origin and contents of a message.-- Digital signatures are used for sender authentication.

Why digital signatures?

- Public-key algorithms are computation-intensive. With large messages they may be too expensive or too slow for the application.

- Digital signatures based on digested message provide alternative solutions.

Application:Digital signatures can be used to endorse an electronic document in a way that can be later validated for authenticity.


Digital Enveloping


What is a digital envelop?

- When a digitally signed message is further encrypted using the receiver’s publickey, and the message is said to be contained in a digital envelope.

Purposes: Digital signatures are used to ensure the integrity and authentication.-- To verify the origin and contents of a message.-- Digital signatures are used for sender authentication.-- To ensure the integrity of communication messages.

Why digital signatures?

- Public-key algorithms are computation-intensive. With large messages they may be too expensive or too slow for the application.

- Digital signatures based on digested message provide alternative solutions.????


Generating A Digital Signature


Message

MessageSignature

Message Digest

Encrypt

HashingAlgorithm

Sender’sPrivate Key(SK sender)

To Receiver


Receiving and Checking A Digital Signature

Topic: Fundamental Crytographic Concepts

Signature

Original Message Digest Message Digest

Decrypt HashingAlgorithm

Sender’sPublic Key(PK sender)

Message

Compare two values


Enveloping a message for a recipient

Topic: Fundamental Cryptographic Concepts

Message

Encrypted MessageEncrypted Content

Encryption Key

Random ContentEncryption Key

Encryption

Symmetric KeyEncryption

Recipient’sPublic KeyPK recipient

Creating a Secure Digital Envelope

John’s letter

John’s letter

10100011

Encrypted Message

MessageDigest

Symmetricrandom key

Symmetricrandom key

DigestAlgorithm

Steve’s Public key

10100011

EncryptedDigest

John’s Public key

10100011

Encrypted Message

From: John:

To Steve:



Digital Certificates


Why digital certificate?

- To ensure all participants in a communication or an e-commerce transaction are authenticated.

What is a digital certificate?

- A digital message which is digitally signed by a trusted certificate authorityto a party in a communication or an e-commerce transaction to ensure its authenticity with a public key.

- A digital certificate includes: a) a party’s ID informationb) its public key issued by a CA

CA --> a certificate authority (a trusted third-party)

Applications:Through the use of a common third party, digital certificates provide an easy and convenient way to ensure that the participants in an electronic commercetransaction can trust each other.


WebSite’ssever information

MessageDigest

CA’sprivate key

Certificate Authority (CA)

DigestAlgorithm


X.509 Certificate

Creating a Server Certificate






X.509 CertificateMessageDigest

MessageDigest

Digest Algorithm

CA’s Public Key

Decryption

Compare?

Server Authentication by Client


cryptographic techniques instructor: jerry gao ph.d. san jose state university email:...

Documents

singlekey encryption

key value

private key

shared key

asymmetric encryption

secret key decrypt

reverse process of encryption

encryption function