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Cryptography

Basic concepts and terms Public/Private Key Algorithms Key Distribution/Management Digital Signatures Miscellaneous Crypto

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Crytpo Intro

Why Encrypt? Protect stored information Protect

Encryption - process by which plaintext is converted to ciphertext using a key

Decryption - process by which ciphertext is converted to plaintext (with the appropriate key)

plaintext (cleartext)- intelligible data

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Crypto Terms

Cryptography - art/science relating to encrypting, decrypting information

cryptanalysis - art/science relating to converting ciphertext to plaintext without the (secret) key

Link encryption - the individual application of encryption to data on each link of a network (nodes are a weak link)

end-to-end encryption - the encryption of data from source system to end system (https)

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CISSP and Cryptology Limited Keys, There Use

Need to encrypt the key as well Took new programmer 15 minutes to break

Triple – DES, He put the compiled DLL into notepad and found a phrase

Symmetry Types

Nor Which Data To Encrypt Passwords? Nor What data not to encrypt

Different keys / approaches for different data Not the Underlying Mathematics

Nor Theoretical Probabilities Definition and Use

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Crypto Terms

Repudiation - In communication between parties, Denial by one party of having participated in communication (commonly heard as non-repudiation)

Traffic analysis - inference of information from analysis of traffic (presence, absence, frequency, etc.)

Traffic padding - generation of spurious data units Work Factor - effort/time needed to overcome a

protective measure

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One - Time Pad

Two identical pads (keys), one with sender, one with recipient

The random pads (keys) are the same length as the message

Unbreakable by exhaustive search Relies on physical security of the pads Pads can only be used once

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Cryptanalysis Terms

Ciphertext-only attack - attacker attempts to decrypt ciphertext

Known-plaintext attack - attacker attempts to decrypt ciphertext given knowledge of some plaintext (think “Login: “)

Chosen-plaintext attack - attacker obtains ciphertext corresponding to selected plaintext

Chosen-ciphertext attack - attacker obtains plaintext corresponding to selected ciphertext (in a public key system, when trying to deduce private key)

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Crypto System Properties

Encryption/decryption transformations must be efficient for all keys

System must be easy to use The security of the system should depend

ONLY on the secrecy of the keys and not on the secrecy of the encryption/decryption transformations

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Secrecy Requirements

If ciphertext and plaintext are known, it should be computationally infeasible to determine the deciphering algorithm

It should be computationally infeasible to systematically determine plaintext from intercepted ciphertext (Even if you decrypt ciphertext once, it should require the same amount of work to do it again.)

Note: “systematically” allows for a lucky guess Note: “Computationally infeasible” means great effort, doesn’t

account for advances in computing, mathematics

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Authenticity Requirements

If ciphertext and plaintext are known, it should be computationally infeasible to determine the enciphering algorithm

It should be computationally infeasible to find valid ciphertext (Even if you encrypt plaintext so that it can be decrypted once, it should require the same amount of work to do it again.)

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Public vs. Private Key Encryption

Private key encryption uses the same key for both encryption and decryption (faster) (DES)

Private key encryption known as symmetric Public key encryption uses two different keys, one to

decrypt and one to encrypt (RSA) Thus, one key can be “public”, because the other key is still

necessary for decryption More complex, key management is an issue

Public key encryption known as asymmetric

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Public Key

– Message encryption using a secret key (S) to encode the message and a public key (P) to encode the secret key

PS

S S

S

P

M essageM essage

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Encipherment Modes

Block Mode ciphers - Message broken into blocks, each block encrypted separately blocks of identical plaintext have identical ciphertext replay and substitution attacks easier (insertion or deletion

of blocks

Block chaining - parts of the previous block are inserted into the current block makes replay and substitution attacks much harder

DES is a block cipher

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Encipherment Modes

Stream Ciphers - Message broken into characters or bits and enciphered with a “key stream” key stream - should be

random and generated independently of the message stream

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Digital Encryption Standard (DES)

Developed by IBM in 1972 Never approved for national security applications 64-bit plain & cipher text block size 56-bit true key plus 8 parity bits Single chip (hardware) implementation

Most implementations now software

16 rounds of transpositions & substitutions Standard for unclassified government data symmetric, private key

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Applications of DES

Double DES Effective key length of 112 bits Work factor about the same as single DES

Triple DES Encrypt with first key Decrypt with second key Encrypt with first key Very secure

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Public key

RSA - Rivest, Shamir, and Adleman Introduced in 1976 Relies on factoring of large numbers

Elliptic Curve Cryptosystem (ECC) Highest strength/bit of public key stream Big savings over other public key systems

Computation Bandwidth Storage

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Strength Comparison

EC Key Size RSA Key Size MIPs Year160 1,024 1012

320 5,120 1036

600 21,000 1079

1,200 120,000 10168

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Key Selection

Similar to password selection, users are the weak link

Random number generators may not be random Key clustering - two different keys produce

equivalent ciphertext from equivalent plaintext

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Key Management

Using a public key system, A wants to talk to B

C is the Key Distribution Center, has A and B’s public key

A calls B, and the calling protocol contacts C C encrypts a session key, “k”, with the public

keys and sends the encrypted “k” to A and B A and B can then communicate

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Key Management Issues

Key storage, recovery Key revocation (lost, compromised keys) Must be fully automated No key in clear outside crypto system Choose key randomly from entire key space Key-encrypting key must be separate from

data keys Infrequently use keys with long life

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Key Escrow

Separate agencies maintain components of private key, which, when combined, can be used to decrypt ciphertext

Stated reason is to decrypt drug related communications

Clipper chip is an example secret algorithm Unpopular, unused

Issues include key storage, Big Brother

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Digital Signature

Used like a written signature, binds a message to an individual (Provides non-repudiation):

S is sender, R is recipient, and M is the Message R must be able to validate S’s signature on M No one can forge S’s signature If S denies signing M, a 3rd party must be able to resolve

the dispute between S and R

Easy with public key encryption - S encrypts with private key, R can decrypt with public (so can 3rd party)

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Digital Signature

Previous method results in M in ciphertext, anyone without S’s public key cannot view

An alternative is to compute a digest of the message using a public hash function, then encrypt the digest using private key. Thus, the only ciphertext is the hash.

MD2, MD4, and MD5 are public hash functions that create message digests. MD5 is strongest, a Rivest upgrade of MD4

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Hash Function

Hash function produces a message digest message digest also known as fingerprint,

imprint 2 messages with the same digest are

extremely unlikely Signer can’t claim a different message was

signed Recipient can’t claim a different message was

signed

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Digital Signature Standard (DSS)

Uses secure hash algorithm Condenses message to 160 bits Key size 512 - 1024 bits Proposed by NIST in 1991 Adopted???

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Message Authentication Code

General term used to describe digital signatures

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Basic public-key cryptography message formats

S P

S

P

M essage

P

M essage

PS

S

M essage

EncryptedM essage

SignedM essage

Signed and EncryptedM essage

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Attacks

Brute force attack Attempt to use all keys

expensive time consuming

But, processing speed doubles every 18 months, Clustered workstations, etc.

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Attacks

Analytic Use algorithm and algebraic manipulation to

reduce complexity Statistical

Use statistical weaknesses in design (more 1’s than 0s in keystream on average, for example)

Implementation Microsoft - PPTP, LANMAN passwords poor randomization on Netscape key

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Kerberos

Authentication and encryption system designed at MIT

Assigns a unique key, called a “ticket”, to each user on the network

“Ticket” embedded in user’s messages to identify sender

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Certificate Authority

Trusted, 3rd party organization CA (Certificate Authority) guarantees that the

individual granted a certificate is who he/she claims to be

CA usually has arrangement with financial institution to confirm identity

Critical to data security and electronic commerce

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Public Key Infrastructure

Known as PKI Necessary for widespread electronic

commerceNo absolute definition or standard! A system of digital certificates, Certificate

Authorities, and other registration authorities that verify and authenticate the validity of parties in Internet transactions

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Steganography

Art of hiding communications Why? Deny message exists Hide data transmissions

(remember the microdot?)

Common implementations hide information in graphic files, sound files, or slack space

Can be combined with encryption!

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Attacks

Ciphertext-only attack: The attacker has the ciphertext of several messages. Each of the messages has been encrypted using the same encryption algorithm.

Known-plaintext only:The attacker has the plaintext and ciphertext of one or more messages.

Chosen-plaintext attack: The attacker has the plaintext and ciphertext and can choose the plaintext that gets encrypted.

Chosen-ciphertext attack: The attacker can choose the ciphertext to be decrypted and has access to the resulting decrypted plaintext

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More Attacks

Man-in-the-middle attack: Eavesdropping on different conversations. Using digital signatures during the session-key exchange can circumvent the attack.

Dictionary attacks: Takes a password file with on-way function values and then takes the most commonly used passwords and run them through the same one-way function. These files are then compared.

Replay attack: An attacker copies a ticket and breaks the encryption and then tries to impersonate the client and resubmit the ticket at a later time to gain unauthorized access to a resource.