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Domain 6: Cryptography & PKI

  1. Compare and contrast basic concepts of cryptography.
  2. Explain cryptography algorithms and their basic characteristics.
  3. Given a scenario, install and configure wireless security settings.
  4. Given a scenario, implement public key infrastructure.

1. Cryptography Concepts​

  • Encoding vs Encrypting
  • Cryptography
  • Symmetric encryption
    • Ciphers (Block & Stream)
    • Key Strength
    • Key Exchange
    • Session & Ephemeral Keys
  • Asymmetric encryption
  • Digital signatures / Nonrepudation
  • PKI: Public Key Infrastructure
  • Hashing
    • Rules
    • Issues
      • Salting
      • Collision Attacks
  • Elliptical Curve & Quantum Cryptography
  • Crypto best practices
  • Obfuscation & Steganography
  • Crypto Use Cases

Encoding vs Encrpyting​

  • Encrypting data is the method by which data is converted from a readable form to an encoded version that can only be decoded by another entity if they have access to a decryption key.

  • Encoding data is the process of transforming data from one form into another form using a known algorithm so that it can be reversed easily.

    • No key required
    • Used to change human text into machine text (i.e. ASCII)

Cryptography​

Cryptograpy is the process of converting ordinary plain text into unintelligeble text and vice-verse. It is a method of storing and transmitting data in a particular form so that only those for whom it is intended can read and process it.

Kerckhoffs's principle states: "Only secrecy of the key provides security".

Kerkhoff's principle: A cryptosystem should be secure even if everything about the system, except the key, is public knowledge.

Shanonon's maxim: "One ought to design systems under the assumption that the enemy will immediately gain full familiarity with them".

Symmetric Encryption​

  • Same key for both encryption & decryption
  • Secret key or private key
  • Advantages:
    • Easier to implement
    • Faster
  • Disadvantage: key distribution

Ciphers​

  • Block: chunks of data / fixed length group of bits
    • Requires padding if not enough data for a block
    • More complex, not as fast
    • May require initialization vector
  • Stream: bits encrypted one at a time
    • Faster / higher performance
    • Susceptible to malicious insertions

Key Strength​

  • Length and complexity of the key
  • Key entropy or randomness
  • Initialization Vector (IV): fixed size input of a random or pseudo-random value that ensures each message encrypts differently.
  • Nonce: random or pseudo-random number that is used only once and associated with a time stamp to increase key strength. A nonce can be used as an IV.

Key Exchange​

  • Process and method for sharing encryption keys.
  • In-band: key shared in the communications channel as the message.
  • Out-of-band: using another transmission media agreed upon in advance.
  • Forward secrey: if one key is compromised, subsequent keys will not also be compromised (Perfect Forward Secrey).

Session & Ephemeral Keys​

  • Session keys are randomly generated to perform both encryption and decryption during the communication of a session between two parties, and they are valid only during that one communication session, then deleted.

  • Ephemeral keys: also only used for one session, common to ephemeral key agreement protocols.

Asymmetric Encryption (Public/Private key encryption)​

  • Uses two keys, one to encrypt and other to decrypt
  • Extra computational overhead
  • Keys are mathematically related
  • Solves the issue of key exchange with symmetric encryption
  • Only private key needs to be kept secret
    • Private key can decrypt whatever is encrypted with public key, and vice-versa.
    • Public key can validate data ownership because it can only decrypt private key encrypted messages.
  • Used primarily for:
    • Secure exchange of shared keys used for symmetric encryption
    • Digital signatures

Digital signatures / Nonrepudation​

  • Also uses Public/Private Key pairs.
  • Message is "signed" using the sender's private key.
  • Anyone can verify signature by using the senders public key.
  • Used for repudation (proof of origin).
  • Used for message integrity (proof message hasn't been altered).
  • Doesn't protect message confidentiality.

PKI: Public Key Infrastructure​

  • Set of roles, policies, and procedures, needed to manage public-key (asymmetric) encryption.
  • Process of creating, managing, distributing, storing, using and revoking keys and digital certificates.

Hashing​

  • "Digital Fingerprint"
  • Work by taking a string of any length and producing a fixed-length string for output.

Hashing Rules​

  • Same speed no matter the data size
  • Impossible to regenerate the original message from it's hash value
  • Avoid hash collisions, each message has its own hash
  • Changing the original changes the hash value

Hashing Issues​

  • Precomputing hash values of common words (rainbow tables).
    • Solved with a Salt.
    • Salting adds a prefix consisting of a random string of characters before words are hashed.
  • Collision attacks: tries to find two input strings of a hash function that have the same output.

Elliptical Curve & Quantum Cryptography​

  • Elliptical Curve Cryptography (ECC)
    • Asymmetric public-key cryptosystem based on complex mathematical structures.
    • Uses smaller key sizes.
    • Very fast.
  • Quantum Cryptography
    • Relies on physics rather than mathematics.
    • Based on the quantum state of photons.
    • More secure.

Cypto best practices​

  • Use well-known and approved cryptographic algorithms.
    • NIST maintains publications and guidance for the use of approved cyptographic and hashing algorithms.
  • Adhere to require minimum key guidance for the chosen algorithm.
  • Use approved cryptographic modes.
  • Use strong random number generators.

Obfuscation & Steganography​

  • Obfuscation: act of making something difficult to understand.

    • Should rely on something not known or widely discovered.
    • Does not provide strong security.

  • Steganography: means "hidden writting".

    • Hiding messages, often in other media, so that unintended recipients are not even aware of any message.

Crypto Use Cases​

  • State of data
    • Data at rest (stored)
    • Data in transit (across a network)
    • Data in use (being processed)
  • Confidentiality
  • Integrity
  • Availability
  • Non-repudation
  • Obfuscation
  • Authentication
  • Resource & Security constraints

2. Cryptography Algorithms​

  • Symmetric Algorithms
  • Cipher modes
  • Asymmetric Encryption
  • Hashing algorithms
  • Rainbow Tables
  • Salts
  • Key Stretching

Symmetric Algorithms​

Act of making something difficult to understand.

  • Substitution cipher
    • ROT13 (rotates 13 places)
    • XOR with a secret key
  • Symmetric Algorithms
    • DES: Data Encryption Standard
      • Block cipher, using 64-bit blocks, 56-bit key + 8 bits of parity
      • Short key vulnerable to brute-force attacks
    • 3DES: Triple DES
      • DES computed three times
      • Uses a "key bundle" that comprises three different DES keys, each of 56 bits (total of 168 bits)
    • AES: Advanced Encryption Standard (Rijndael)
      • Free for any use public or private, commercial or non-commercial
      • Block cipher with 128 bit block size
      • Three key lengths: 128, 192 and 256 bits
      • Uses multiple encryption rouds:
        • 10 rounds for 128-bit keys
        • 12 rounds for 192-bit keys
        • 14 rounds for 256-bit keys
    • RC4/RC5/RC6: Rivest Cipher
      • RC4 is a stream cipher
      • RC5/RC6 are block ciphers
      • Works with key sizes between 40 and 2048 bits
    • Blowfish / Twofish
      • Symmetric block cipher that can use variable-length keys (32 to 448 bits)
      • Twofish uses 128-bit blocks
    • IDEA: Internal Data Encryption Algorithm
      • 128-bit key
      • Similar to DES, but more secure
      • Used in Pretty Good Privacy (PGP)
    • OTP: One-Time Pad
      • Most secure crypto implementation
      • Use of a key as long as the plain-text message
      • Only used once and then destroyed
    • Skipjack
      • NSA developed block cipher used in clipper chip
      • Uses an 80-bit key to encrypt 64-bit clocks of data

Cipher modes​

  • CTR: Counter Mode

    • Turns a block cipher into a stream cipher.
    • Used to generate a keystream.
    • Each block combines a nonce or IV with a sequentially assigned number to produce a unique counter block that is then encrypted.

  • CBC: Cipher-Block Chaining

    • Uses an IV with the first block.
    • Thereafter, each block of plain text is obfuscated with the cipher text from the previous block before it is encrypted.
    • Introduces more diffusion & reduces effects of plain-text attacks.

  • GCM: Galois/Counter Mode

    • Provides both integrity and confidentiality.
    • GCM uses CTR with 128-bit blocks.
    • Each 128-bit block is given an encrypted number. That result is then obfuscated with the plain text, producing cipher text.
    • GMAC (Galois Message Authentication Code) is an authentication-only variant of the GCM which can be used as an incremental message authentication code.

  • ECB: Electronic Codebook

    • Divides the message into blocks and the encrypts each block.
    • No longer recommended, same plain-text block is encrypted into same cipher-text block each time.

Asymmetric Algorithms​

  • RSA: Rivest, Shamir, and Adleman
    • Used for key exchange and digital signatures.
    • Keys can any length.
    • Algorithm works by multiplying two large primer numbers.
    • Derives two different numers: one public key and one private key.
  • D-H: Diffie-Hellman Key Exchange
    • Two parties, without prior arrangement can agree on a secret key that is known only to them.
    • Only used to generate a shared key (not encryption).
    • Key can be safely / secretly shared on a public network.
  • DHE: Diffie-Hellman Ephemeral / El Gamal
    • Uses a different key for every conversation.
    • Supports perfect forward secrecy.
  • ECC: Elliptic Curve Cryptography
    • Technique using elliptic curves to calcuate simple but difficult-to-break encryption keys.
    • Uses smaller key sizes to obtain the same level of security (160-bit ECC = 1024-bit RSA).
  • ECDHE: Elliptic Curve Diffe-Hellman Ephemeral
    • Variant of DHE using ECC for perfect forward secrecy.
  • PGP: Pretty Good Privacy / GPG: GNU Privacy Guard
    • Developed by Zimmerman in 1991, used to encrypt and sign email messages.

Hashing Algorithms​

  • SHA: Secure Hash Algorithm
    • Developed by US NSA
    • SHA-1: 160-bit hash from any variable-length string of data.
    • SHA-2: SHA-224, SHA-256, SHA-384, SHA-512 (based on their digest lengths).
    • SHA-3: published in 2012, not widely used yet.
  • MD2/MD4/MD5: Message Digest Algorithm
    • Most widely known hashing function.
    • Produces a 16-byte hash value, usually expressed as a 32 digit hexadecimal number.
    • Considered compromised. Rainbow tables have been published which allow people to reverse MD5 hashes made without good salts.
  • MAC: Message Authentication Code
    • Authentication of messages using a secret key.
    • Used in electronic fund transfers to protect against fraud.
  • HMAC: Hash-based Message Authentication Code
    • Combines a cryptographic hash function and a secret cryptographic key.
    • Does not encrypt the message, only the key.
  • RIPEMD: RACE Integrity Primitives Evaluation Message Digest
    • Design based on MD4
    • 160-bit version of the algorithm (RIPEMD-160) performs comparably to SHA1.

Rainbow Tables​

Rainbow Tables are precomputed tables for reversing cryptographic hash functions, where hashes are precomputed in advance.

Salt​

Salt is random data that is used as an additional input to hashes.

Key Stretching​

Procesess used to take a weak key and make it stronger, usually by kaing it longer.

  • Bcrypt
    • Based on the Blowfish algorithm.
    • Provides an adaptative hash function based on key factor.
  • PBKDF2: Password-Based Key Derivation Function 2
    • Algortihm applies a pseudo-random function to the password, combined with a salt of at least 64 bits, and then repeats the process at least 1000 times.

3. Wireless Security​

  • Wireless Access Methods
  • WPS: Wi-Fi Protected Setup
  • Wireless Cryptographic Protocols
  • Wi-FI Protected Access
    • WPA
    • WPA2
  • Authentication Protocols
  • Certificate Revocation

Wireless Access Methods​

  • Open Authentication: only need to know network name / SSID.
    • Captive Portal: web page that is launched first when connecting through a network.
  • Shared Authentication: client and wireless AP must negotiate and share a key prior to initiating communications.
    • PSK (Pre-shared key): each user uses same key to connect to the Wi-Fi network.
  • Enterprise: server handles distribution of cryptographic keys and/or digital certificates.
    • EAP (Extensible Authentication Protocol)

WPS: Wi-Fi Protected Setup​

Standard to simplify Wireless Access Point (AP) set-up for home users.

WPS Modes​

  • PIN entry
  • PBC: Push-button configuration
  • NFC: Near Field Communication

Wireless Cryptographic Protocols​

  • WEP (Wired Equivalent Privcay): original wireless encryption standard that should not be used anymore.
  • WPA (Wi-Fi Protected Access): developed in response to security concerns over WEP.
  • WPA2: required for Wi-Fi certified devices.
    • Uses AES for encryption.
    • Based on IEE 802.11i standard.

WPA (Wi-FI Protected Access)​

  • WPA-PSK (Personal): Uses a pre-shared key to authenticate and validate users on a wireless LAN/WLAN or Wi-Fi connection.
  • WPA-802.1X (Enterprise): Increased security for larger organization, required RADIUS authentication server.
  • TKIP (Temporal Key Integrity Protocol): Based on RC4, uses a unique key with each packet. (deprecated)

WPA2​

  • Counter Mode with CCMP (Cipher Block Chaining Message Authentication Code Protocol)
    • Replaced TKIP
    • Based on AES encryption cipher
    • CCM combines CTR for confidentiality and CBC-MAC for authentication and integrity
  • Fully implements the IEEE 802.11i-2004 Wi-Fi security standards.

Authentication Protocols​

  • EAP
    • Requires authentication server
    • Allows authentication methods beyond username/password
    • Provides support for public certifiactions
    • Four modes
      • PEAP - Protected EAP
      • EAP-TLS
      • EAP-TTLS (Tunneled TLS)
      • EAP-FAST (Flexible Authentication via Secure Tunneling)
  • IEEE 802.1x: Standard for port-based network access control.
  • RADIUS Federation
    • Using RADIUS to authenticate between entities.
    • As part of PEAP negotiation, client establishes A TLS session with a RADIUS server.

4. PKI: Public Key Infrastructure​

  • Public and Private Keys
  • Digital Certificate
    • X.509 Certificate Types
  • Certificate Authority (CA)
  • Registration Authorities (RA)
  • Certificate Trust Models
  • Key Escrow
  • Pinning

Public and Private keys​

Encrypt a document with the recipient's public key. Only their private key needs to be kept secret and only it can decrypt the message.

Digital Certificate​

A digitally signed block of data used to prove the ownership of a public key issued issued by a Certificate Authority.

  • Information about the key
  • Information about the identity of its owner (subject)
  • Information about digital signature of an entity that has verified the certificate's content (issuer)

The X.509 v3 standard defines the certificate formats and fields for public keys.

Digital Certificate Components​

  • Version: X.509 version
  • Serial Number: uniquely identifies certificate
  • Signature Algorithm ID: names of specific Public Key Algorithms that CA has used to sign scertificate (i.e., RSA with SHA-1)
  • Issuer (CA) x.500 Name: identity of CA Server who issued the certificate
  • Validity Period: period of time which certificate is valid with start date and expiration date
  • Subject X.500 Name: owner's identity with X.500 directory format
  • Subject Public Key Info
    • Algorithm ID
    • Public Key Value
  • Issuer Unique ID: information used to identify the issuer of the certificate
  • Subject Unique ID: information used to identify the owner of the certificate
  • Extension: additional information like Alternate name, CRL Distribution Point (CDP)
  • CA Digital Signature: atual digital signature of the CA

X.509 Certificate Types​

  • Root certificates: for root authorities, usually self-signed by that authority and often kept off-line.
  • Domain Validation (DV): includes only the domain name.
  • Organizational Validation (OV): organizations are vetted against official government sources (common four public-fancing websites).
  • Extended Validation (EV): highest level of trust, requires a comprehensive validation of the business.
  • Wildcard Certificates: allows subdomains for a single registered domain (*.example.com).
  • Subject Alternate Name (SAN): special X.509 that allows additional items (IP addresses, domain names, and so on).
  • Code signing certificates: used to sign computer code.
  • Machine/computer certificates: X.509 certificates assigned to a specific computer.
  • Email certificates: securing email (S/MIME).
  • User certificates: for individual users.

Certificate Authority (CA)​

  • Trusted entities
  • Internal (aka, self signed)
  • External / Third party (Symantec, GoDaddy, etc)
  • Duties:
    • Issues certificates
    • Verifies holder of a digital certificate
    • Ensures that holders of certificates are who they claim to be
  • Certificate-Signing Request (CSR)
    • Request from applicant to CA to apply for a digital certificate
    • Applicant's public key
    • Fully qualified domain name
    • Legally incorporated name of the company
    • Address

Registration Authorities (RA)​

  • Offload work from the CA
  • Validate user's or end-point's identities
  • Distribute keys
  • DOES NOT issue certificates

Certificate Revocation​

  • Certificate revocation is the process of invalidating a certificate before it's expiration date, often due to private key loss or compromise.
  • Certificate Revocation List (CRL)
    • Method for distributing certificate revocation information list. Must be often updated/maintained.
    • Certificate compared against CRL.
  • Online Certificate Status Protocol (OCSP)
    • Checks certificate status in real time.
    • OSCP Stapling
      • Reduces load on CA.
      • Allows web server to "staple" a time-stamped OCSP response as part of the TLS handshake with the client.
      • The web server is now responsible for handling OCSP requests instead of the CA.

Certificate Trust Models​

  • Single CA: simplest, no redundancy, self-signed certificate.
  • Hierarchical model:
    • Root CA, toop of the hierarchy, may be offline.
    • Intermediate CA, subordinate CAs provide redundancy and load balancing.
  • Certificate chaining.
  • Web of Trust: cross-certification model, a peer-to-pee trust relationship with other CAs.
  • Bridge CA: cross-certification model using a central point of trust.

Key Escrow​

  • Trusted third party maintains keys.
  • Addresses the possibility that a cryptographic key may be lost: if key is lost, then data is lost.
  • Key Recovery Agent is an entity that has the ability to recover a key, key components, or plain-text messages as needed.

Pinning​

  • Hashes of public keys for popular web servers are included with applications such as web browsers.
  • Mitigates the use of fraudulent certificates.
  • HTTP Public Key Pinning (HPKP): uses public key pins, which are essentially hashed values of the public key communicated to the browser client from the server in the HTTP header.