Cryptography 101

Cryptograph Basics

• Cryptography - science or study of protecting information whether in transit or at rest
  • Renders the information unusable to anyone who can’t decrypt it
  • Takes plain text, applies cryptographic method, turn it into cipher text
• Cryptanalysis - study and methods used to crack cipher text
• Linear Cryptanalysis - works best on block ciphers
• Differential Cryptanalysis - applies to symmetric key algorithms
  • Compares differences in the inputs to how each one affects the outcome
• Integral cryptanalysis - input vs output comparison same as differential; however, runs multiple computations of the same block size input
• Plain text doesn’t necessarily mean ASCII format - it simply means unencrypted data
• Nonrepudiation - means by which a recipient can ensure the identity of the sender and neither party can deny sending

Encryption Algorithms and Techniques

• Algorithm - step-by-step method of solving a problem
• Two General Forms of Cryptography
  • Substitution - bits are replaced by other bits
  • Transposition - doesn’t replace; simply changes order
• Encryption Algorithms - mathematical formulas used to encrypt and decrypt data
• Steam Cipher - readable bits are encrypted one at a time in a continuous stream
  • Usually done by an XOR operation
  • Work at a high rate of speed
• Block Cipher - data bits are split up into blocks and fed into the cipher
  • Each block of data (usually 64 bits) encrypted with key and algorithm
  • Are simpler and slower than stream ciphers
• XOR - exclusive or; if inputs are the same (0,0 or 1,1), function returns 0; if inputs are not the same (0,1 or 1,0), function returns 1
• Key chosen for cipher must have a length larger than the data; if not, it is vulnerable to frequency attacks

Symmetric Encryption

• Symmetric Encryption - known as single key or shared key
  • One key is used to encrypt and decrypt the data
  • Problems include key distribution and management
  • Suitable for large amounts of data
  • Harder for groups of people because more keys are needed as group increases
  • Does nothing for nonrepudiation; only performs confidentiality
• Algorithms
  • DES - block cipher; 56 bit key; quickly outdated and now considered not very secure
  • 3DES - block cipher; 168 bit key; more effective than DES but much slower
  • AES (Advanced Encryption Standard) - block cipher; 128, 192 or 256 bit key; replaces DES; much faster than DES and 3DES
  • IDEA (International Data Encryption Algorithm) - block cipher; 128 bit key; originally used in PGP 2.0
  • Twofish - block cipher; up to 256 bit key
  • Blowfish - fast block cipher; replaced by AES; 64 bit block size; 32 to 448 bit key; considered public domain
  • RC (Rivest Cipher) - RC2 to RC6; block cipher; comparable key length up to 2040 bits; RC6 (latest version) uses 128 bit blocks and 4 bit working registers; RC5 uses variable block sizes and 2 bit working registers. RC4 is a stream cipher

Asymmetric Encryption

• Uses two types of keys for encryption and decryption
• Public Key - generally used for encryption; can be sent to anyone
• Private Key - kept secret; used for decryption
• Comes down to what one key encrypts, the other decrypts
• The private key is used to digitally sign a message
• Algorithms
  • Diffie-Hellman - developed as a key exchange protocol; used in SSL and IPSec; if digital signatures are waived, vulnerable to MITM attacks
  • Elliptic Curve Cryptosystem (ECC) - uses points on elliptical curve along with logarithmic problems; uses less processing power; good for mobile devices
  • El Gamal - not based on prime number factoring; uses solving of discrete logarithm problems
  • RSA - achieves strong encryption through the use of two large prime numbers; factoring these create key sizes up to 4096 bits; modern de facto standard
• Only downside is it’s slower than symmetric especially on bulk encryption and processing power

Hash Algorithms

• Hash - one-way mathematical function that produces a fix-length string (hash) based on the arrangement of data bits in the input
• Algorithms
  • MD5 (Message Digest algorithm) - produces 128 bit hash expressed as 32 digit hexadecimal number; has serious flaws; still used for file download verification
  • SHA-1 - developed by NSA; 160 bit value output
  • SHA-2 - four separate hash functions; produce outputs of 224, 256, 384 and 512 bits; not widely used
  • SHA-3 - uses sponge construction
  • RIPEMD-# - works through 80 stages, executing 5 blocks 16 times each; uses modulo 32 addition
• Collision - occurs when two or more files create the same output
  • Can happen and can be used an attack; rare, though
• DUHK Attack (Don’t Use Hard-Coded Keys) - allows attackers to access keys in certain VPN implementations; affects devices using ANSI X9.31 with a hard-coded seed key
• Rainbow Tables - contain precomputed hashes to try and find out passwords
• Salt - used with a hash to obscure the hash; collection of random bits
• Things to Remember
  • Hashes are used for integrity
  • Hashes are one-way functions
• Tools
  • HashCalc
  • MD5 Calculator
  • HashMyFiles

Steganography

• Steganography - practice of concealing a message inside another medium so that only the sender and recipient know of its existence
• Ways to Identify
  • Text - character positions are key - blank spaces, text patterns
  • Image - file larger in size; some may have color palette faults
  • Audio & Video - require statistical analysis
• Methods
  • Least significant bit insertion - changes least meaningful bit
  • Masking and filtering (grayscale images) - like watermarking
  • Algorithmic transformation - hides in mathematical functions used in image compression
• Tools
  • QuickStego
  • gifshuffle
  • SNOW
  • Steganography Studio
  • OpenStego

PKI System

• Public Key Infrastructure (PKI) - structure designed to verify and authenticate the identity of individuals
• Registration Authority - verifies user identity
• Certificate Authority - third party to the organization; creates and issues digital certificates
• Certificate Revocation List (CRL) - used to track which certificates have problems and which have been revoked
• Validation Authority - used to validate certificates via Online Certificate Status Protocol (OCSP)
• Trust Model - how entities within an enterprise deal with keys, signatures and certificates
• Cross-Certification - allows a CA to trust another CS in a completely different PKI; allows both CAs to validate certificates from either side
• Single-authority system - CA at the top
• Hierarchical trust system - CA at the top (root CA); makes use of one or more RAs (subordinate CAs) underneath it to issue and manage certificates

Digital Certificates

• Certificate - electronic file that is used to verify a user’s identity; provides nonrepudiation
• X.509 - standard used for digital certificates
• Contents of a Digital Certificate
  • Version - identifies certificate format
  • Serial Number - used to uniquely identify certificate
  • Subject - who or what is being identified
  • Algorithm ID (Signature Algorithm) - shows the algorithm that was used to create the certificate
  • Issuer - shows the entity that verifies authenticity
  • Valid From and Valid To - dates certificate is good for
  • Key Usage - what purpose the certificate serves
  • Subject’s Public Key - copy of the subject’s public key
  • Optional Fields - Issuer Unique Identifier, Subject Alternative Name, and Extensions
• Some root CAs are automatically added to OSes that they already trust; normally are reputable companies
• Self-Signed Certificates - certificates that are not signed by a CA; generally not used for public; used for development purposes
  • Signed by the same entity it certifies

Digital Signatures

• When signing a message, you sign it with your private key and the recipient decrypts the hash with your public key
• Digital Signature Algorithm (DSA) - used in generation and verification of digital signatures per FIPS 186-2

Full Disk Encryption

• Data at Rest (DAR) - data that is in a stored state and not currently accessible
  • Usually protected by full disk encryption (FDE) with pre-boot authentication
  • Example of FDE is Microsoft BitLocker and McAfee Endpoint Encryption
  • FDE also gives protection against boot-n-root

Encrypted Communication

• Often-Used Encrypted Communication Methods
  • Secure Shell (SSH) - secured version of telnet; uses port 22; relies on public key cryptography; SSH2 is successor and includes SFTP
  • Secure Sockets Layer (SSL) - encrypts data at transport layer and above; uses RSA encryption and digital certificates; has a six-step process; largely has been replaced by TLS
  • Transport Layer Security (TLS) - uses RSA 1024 and 2048 bits; successor to SSL; allows both client and server to authenticate to each other; TLS Record Protocol provides secured communication channel
  • Internet Protocol Security (IPSEC) - network layer tunneling protocol; used in tunnel and transport modes; ESP encrypts each packet
  • PGP - Pretty Good Privacy; used for signing, compress and encryption of emails, files and directories; known as hybrid cryptosystem - features conventional and public key cryptography
  • S/MIME - standard for public key encryption and signing of MIME data; only difference between this and PGP is PGP can encrypt files and drives unlike S/MIME
• Heartbleed - attack on OpenSSL heartbeat which verifies data was received correctly
  • Vulnerability is that a single byte of data gets 64kb from the server
  • This data is random; could include usernames, passwords, private keys, cookies; very easy to pull off
  • nmap -d –script ssl-heartbleed –script-args vulns.showall -sV [host]
  • Vulnerable versions include Open SSL 1.0.1 and 1.0.1f
  • CVE-2014-0160
• FREAK (Factoring Attack on RSA-EXPORT Keys) - man-in-the-middle attack that forces a downgrade of RSA key to a weaker length
• POODLE (Paddling Oracle On Downgraded Legacy Encryption) - downgrade attack that used the vulnerability that TLS downgrades to SSL if a connection cannot be made
  • SSl 3 uses RC4, which is easy to crack
  • CVE-2014-3566
  • Also called PoodleBleed
• DROWN (Decrypting RSA with Obsolete and Weakened eNcyption) - affects SSL and TLS services
  • Allows attackers to break the encryption and steal sensitive data
  • Uses flaws in SSL v2
  • Not only web servers; can be IMAP and POP servers as well

Cryptography Attacks

• Known plain-text attack - has both plain text and cipher-text; plain-text scanned for repeatable sequences which is compared to cipher text
• Chosen plain-text attack - attacker encrypts multiple plain-text copies in order to gain the key
• Adaptive chosen plain-text attack - attacker makes a series of interactive queries choosing subsequent plaintexts based on the information from the previous encryptions; idea is to glean more and more information about the full target cipher text and key
• Cipher-text-only attack - gains copies of several encrypted messages with the same algorithm; statistical analysis is then used to reveal eventually repeating code
• Replay attack
  • Usually performed within context of MITM attack
  • Hacker repeats a portion of cryptographic exchange in hopes of fooling the system to setup a communications channel
  • Doesn’t know the actual data - just has to get timing right
• Chosen Cipher Attack
  • Chooses a particular cipher-text message
  • Attempts to discern the key through comparative analysis
  • RSA is particularly vulnerable to this
• Side-Channel Attack
  • Monitors environmental factors such as power consumption, timing and delay
• Tools
  • Carnivore and Magic Lantern - used by law enforcement for cracking codes
  • L0phtcrack - used mainly against Windows SAM files
  • John the Ripper - UNIX/Linux tool for the same purpose
  • PGPcrack - designed to go after PGP-encrypted systems
  • CrypTool
  • Cryptobench
  • Jipher
• Keys should still change on a regular basis even though they may be “unhackable”
• Per U.S. government, an algorithm using at least a 256-bit key cannot be cracked