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Learning Objectives
Discuss Cryptography Terminology Discuss Symmetric Key Encryption Discuss Asymmetric Key Encryption Distinguish between Hashing and Encryption
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Cryptography?
Traditionally, cryptography refers to The practice and the study of encryption Transforming information in order to prevent
unauthorized people to read it.
Today, cryptography goes beyond encryption/decryption to include Techniques for making sure that encrypted
messages are not modified en route Techniques for secure
identification/authentication of communication partners.
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Your knowledge about Cryptography
1) Which of the following do cryptographic systems protect?
a) Data stored on local storage media (like hard drives) from access by unauthorized users.
b) Data being transmitted from point A to point B in a network
c) Both a and b
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Your knowledge about Cryptography
2) Which of the following security issues is addressed by cryptographic systems?
a) Confidentiality; i.e. protection against eavesdroppingb) Authentication; i.e. assurance parties involved in a
communication are who they claim to bec) Message integrity; i.e. assurance that messages are not
altered en routed) Availability; i.e. making sure that communication systems
are not shut down by intruders.e) All of the above
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Basic Terminology 1
Network
Plaintext“Hello”
Ciphertext “11011101”
Ciphertext “11011101”Plaintext“Hello”
DecryptionAlgorithm
Interceptor
Party A
Party B
Plaintext: original message to be sent. Could be text, audio, image, etc. Encryption/Decryption Algorithm: mathematical tool (software) used to
encrypt or decrypt Key: A string of bits used by to encrypt the plaintext or decrypt the ciphertext Ciphertext: encrypted message. Looks like a random stream of bits
+ Decryption key
EncryptionAlgorithm
+ Encryption key
HelloHello
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Basic Terminology 2 Encryption:
Converting plaintext into ciphertext using algorithms and keys The size of the ciphertext is proportional to the size of the plaintext Ciphertext is reversible to plaintext
Symmetric Key Encryption: Same key is used both for encryption and decryption Keys are usually identical or trivially identical*
Asymmetric Key Encryption: Also called Public/Private Key Encryption Two different keys are used: one for encryption, one for decryption
Party A Party B
Party A Party B
* Trivially identical means simple transformation could lead from one key to the another.
Flexcrypt: http://www.flexcrypt.com/flexcryptfree.html | Tools4noobs.com
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Your knowledge about Cryptography
3) Based on how symmetric encryption systems work, which of the following is the worst thing to happen?
a) An attacker gets a copy of the encryption and decryption algorithms
b) An attacker gets the decryption key
c) a and b are equally damaging
4) Which of the following presents more challenge for exchanging keys between partners?a) Asymmetric encryption
b) Symmetric encryption
c) A and b are equally challenging
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Exhaustive search and Key length
Key Length in bits Number of possible keys (2key length in bits)
1 2
2 4
4 16
8 256
16 65536
56 72057594037927900
112 5192296858534830000000000000000000 or 5.1923E+33
168 3.74144E+50
256 1.15792E+77
512 1.3408E+154
Attacker could use the right algorithm and do an exhaustive search (i.e. try all possible keys) in order to decrypt the ciphertext
Most attacks require the capture of large amount of ciphertext Every additional bit in the length of the key doubles the search time Every additional bit in the length of the key doubles the requirements in terms of
minimum processor’s speed to crack the key.
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Your knowledge about Cryptography
4) If you increase the key length from 56 bits to 66 bits. How much more key combinations an attacker who captures enough ciphertext will have to try in order to decipher the captured ciphertext using the appropriate algorithm?
_______________________________________
5) Assuming that it takes 7 days to try all possible combinations of a 56 bit key, how much time it would take to try all possible combinations when the key length is increased to 58 bits?
________________
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Weak vs. Strong Keys
Symmetric Key Encryption Usually for private of customer e-business Keys < 100-bit long are considered weak today. Keys 100-bit long or more are considered strong today.
Asymmetric Key Encryption Usually used for B2B e-commerce Key pairs must be much longer (512 bit and more) because of
the disastrous consequences of breaking the decryption keyKey Length in bits
Number of possible keys (2key length in bits) Type of communication
1 21 = 2
2 22 = 4
16 216 = 65536
56 256 = 72057594037927900 Private, symmetric, weak asymmetric (e.g. DES)
100 2100 = Private, symmetric
112 2112 = 5192296858534830000000000000000000 or 5.1923E+33 Business, asymmetric (e.g. 112-bit DES)
168 3.74144E+50 Business, asymmetric (e.g. 3DES)
256 1.15792E+77 Business, asymmetric (e.g. AES)
512 1.3408E+154 Business, asymmetric (e.g. RSA)
1024 to 4096 21024 to 24096 Business, asymmetric (e.g. RSA)
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Your knowledge about Cryptography
6) Most attacks require the capture of large amount of ciphertext, which can take a certain amount of time. Beside using strong keys what else can be done to make it harder to crack the key?
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Symmetric Key Encryption methods Two categories of methods
Stream cipher: algorithm operates on individual bits (or bytes); one at a time Block cipher: operates on fixed-length groups of bits called blocks
Only a few symmetric methods are used today
Methods Year approved Comments
Data Encryption Standard - DES 1977 1998: Electronic Frontier Foundation’s Deep Crack breaks a DES key in 56 hours
DES-Cipher Block Chaining
Triple DES – TDES or 3DES 1999
Advanced Encryption Standard – AES 2001 Its versions among the most used today
Other symmetric encryption methods
IDEA (International Data Encryption Algorithm), RC5 (Rivest Cipher 5), CAST (Carlisle Adams Stafford Tavares), Blowfish
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Data Encryption Standard (DES)
DES EncryptionProcess
64-Bit CiphertextBlock
64-Bit DES Symmetric Key(56 bits + 8 redundant bits)64-Bit Plaintext
Block
DES is a block encryption method, i.e. uses block cipher DES uses a 64 bit key; actually 56 bits + 8 bits computable
from the other 56 bits Problem: same input plaintext gives same output ciphertext
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DES-Cipher Block Chaining
First64-Bit Plaintext Block
DES EncryptionProcess
Second64-Bit Plaintext Block
First64-Bit Ciphertext Block
InitializationVector (IV)
DES EncryptionProcess
Second64-Bit Ciphertext Block
DES Key
DES Key
DES-CBC uses ciphertext from previous block as input making decryption by attackers even harder
An 64-bit initialization vector is used for first block
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Triple DES (3DES)
Sender Receiver
Encrypts original plaintext with the1st key
Decrypts ciphertext withthe 3d key
Decrypts output of firststep with the 2nd key
Encrypts output of thefirst step with the 2nd key
Encrypts output of secondstep with the 3d key; givesthe ciphertext to be sent
Decrypts output of secondstep with the 1st key; givesthe original plaintext
168-Bit Encryption with Three 56-Bit Keys
1st
2nd
3rd
3rd
2nd
1st
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Triple DES (3DES)
Sender Receiver
Encrypts plaintext with the1st key
Decrypts ciphertext withthe 1st key
Decrypts output with the 2nd key
Encrypts output with the2nd key
Encrypts output with the1st key
Decrypts output with the1st key
112-Bit Encryption With Two 56-Bit Keys
1st
2nd
1st
1st
2nd
1st
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Your knowledge about Cryptography
7) Based on the way DES and 3DES work, which of the following is true?a) 3DES requires more processing time than DES
b) Compared 3DES, DES requires more RAM
c) Both a and b
8) Given the increasing use of hand-held devices, 3DES will be more practical than DES.
a) True
b) False
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Advanced Encryption Standard - AES
Developed by two Belgian cryptographers, Joan Daemen and Vincent Rijmen, and submitted to the AES selection process under the name "Rijndael", a portmanteau of the names of the inventors
Offers key lengths of 128 bit, 192 bit, and 256 bit Efficient in terms of processing power and RAM
requirements compared to 3DES Can be used on a wide variety of devices including
Cellular phones PDAs Etc.
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DES, 3DES, and AES
DES
56
Weak
Moderate
Moderate
3DES
112 or 168
Strong
High
High
AES
128, 192, 256
Strong
Modest
Modest
Key Length (bits)
Key Strength
ProcessingRequirements
RAM Requirements
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Encryption Algorithms Used by MS Operating Systems
Operating SystemDefault Algorithm Other
Algorithms
Windows 2000 DESX (none)
Windows XP RTM DESX 3DES
Windows XP SP1 AES 3DES, DESX
Windows Server 2003 AES 3DES, DESX
Windows Vista AES 3DES, DESX
Windows Server 2008 AES 3DES, DESX (?)
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Public Key Encryption For confidentiality
Party A Party BDecrypt with
Party A’s Private KeyEncrypt with
Party A’s Public Key
Encrypt withParty B’s Public Key
Decrypt withParty B’s Private Key
EncryptedMessage
EncryptedMessage
Each Party uses other party’s public key for encryption Each Party uses own private key for decryption No need to exchange private key, but key needs to be very strong (512+
bit)
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Public Key Encryption methods
Asymmetric encryption methods are used both for Encryption in order to provide confidentiality Digital signature in order to provide partners’ authentication
Methods Year proposed Comments
RSA by Ron Rivest, Adi Shamir, and Leonard Adleman
1977 1995: First attack in lab conditions was reported
Elliptic Curve Cryptosystem - ECC 1985 Becoming widely used
Other symmetric encryption methods:
Dieffe-Hellman, El-Gamal
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Basic Terminology 3 Hashing:
Mathematical process for converting inputs into fixed-length outputs Hash function:
Algorithm that does the hashing. Uses an input + a shared secret or password. Example: MD5, Secure Hash Algorithm.
Hash: Fixed-length output of the hashing
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Encryption Versus Hashing
Encryption
Uses a key as aninput to an encryption method
Output is similar inlength to input
Reversible; ciphertextcan be decryptedback to plaintext
Use of Key
Length of Result
Reversibility
Hashing
Password is usually addedto text; the two arecombined, and thecombination is hashed
Output is of a fixedshort length, regardless of input
One-way function; hashcannot be “de-hashed” back to the original string
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Hashing & Public Key for authentication
Asymmetric Key Encryption is also used for authentication Usually used along with hashing
Confidentiality Authentication
Public KeyEncryption
Sender encrypts with receiver’s public key. Receiver decrypts with the receiver’s own private key.
Sender (supplicant) encrypts with own private key. Receiver (verifier) decrypts with the public key of the true party, usually obtained from a Certificate Authority.
Hashing Used in MS-CHAP for initial authentication and in HMACs for message-by-message authentication
Hashing and Public Key for authentication very used in cryptographic systems like SSL/TLS or IPSec
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Cryptographic Systems
Packaged set of cryptographic countermeasures used for protecting dialogues
Example: Secure Socket Layer/Transport Layer Security –SSL/TLS used in secured webservice
Each cryptographic system includes different security standards (algorithms, hashing methods, security parameters) that comm. partners needs to “agree” on.
Typical Process:
Handshaking stages
Ongoing communication stage: Message-by-Message authentication
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Cryptographic Systems (cont.)
Packaged set of cryptographic countermeasures used for protecting dialogues
Handshaking Stage 1:Initial Negotiation of Security Parameters
Handshaking Stage 2:Initial Authentication
(Usually mutual)
Handshaking Stage 3:Keying
(Secure exchange of keys and other secrets)
Ongoing Communication Stagewith Message-by-Message
Confidentiality, Authentication,and Message Integrity
ServerClient PC
Time
Encrypted for Confidentiality
PlaintextElectronic Signature(Authentication, Integrity)
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MS-CHAP* Hashing for Authentication
* Microsoft’s version of Challenge Handshake Authentication Protocol
CHAP is an authentication scheme used by Point to Point Protocol (PPP) servers to validate the identity of remote clients
1) After the completion of the link establishment phase, the server sends a "challenge" message to the client.
2) The client responds with a value calculated using a one-way hash function, such as an MD5 or SHA (Secure Hash Algorithm).
3) The server checks the response against its own calculation of the expected hash value. If the values match, the server acknowledges the authentication; otherwise it should terminate the connection.
4) At random intervals the server sends a new challenge to the peer and repeats steps 1 through 3.
Shared secret
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Message-by-Message Authentication using Hashing and Public Key
To Create the Digital Signature:
1. Hash the plaintext to create abrief Message Digest; this is NOT the Digital Signature.
2. Sign (encrypt) the messagedigest with the sender’s privatekey to create the Digital Signature.
3. Transmit the plaintext + digitalsignature, encrypted withsymmetric key encryption.
Plaintext
MD
DS
Hash
Sign (Encrypt)with Sender’sPrivate Key
4. Encrypted withSession Key
DS Plaintext
Sender Receiver
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Message-by-Message Authentication (cont.)
To Test the Digital Signature
5. Hash the received plaintextwith the same hashing algorithm
the sender used. This givesthe message digest.
6. Decrypt the digital signaturewith the sender’s public key.
This also should give themessage digest.
7. If the two match, themessage is authenticated.
MD
Received Plaintext
MD
DS
5. 6.
HashDecrypt withTrue Party’sPublic Key
7.Are they equal?
Plaintext
MD
DS
Hash
Sign (Encrypt)with Sender’sPrivate Key