Cryptography(basics)

"Ancient Crypto": Substitution ciphers

Caesar cipher shifts letter with a constant of K

RSA Encryption/Decryption

0. Given (n,e) and (n,d) as computed as RSA scheme. 1. To encrypt bit pattern, m, compute c =m ^ e mod n 2. To decrypt received bit pattern, c , compute m = c ^ d mod n

RSA Scheme

1. Choose two large prime numbers p,q. 2. Compute n=pq, z=(p-1)(q-1) 3. Choose e (with e<n) that has no common factors with z. 4. Choose d such that ed-1 is exactly divisible by z. (ez mod z =1). 4. Public key is (n,e), Private key is (n,d).

S-box

4 bit substitution boxes that provide the encryption of the bits... they each contain lookup tables used by the algorithm as instructions on how the bits should be encrypted

Electronic Code Book (ECB)

A mode of operation for a block cipher, with the characteristic that each possible block of plaintext has a defined corresponding ciphertext value, and vice versa

Initialization Vector (IV)

A non-secret binary vector used as the initializing input algorithm for the encryption of a plaintext block sequence to increase security by introducing additional cryptographic variance and to synchronize cryptographic equipment.

Parity check

A process used to detect errors in memory or data communication.

Advanced Encryption Standard (AES)

A symmetric cipher that was approved by the NIST in late 2000 as a replacement for DES.

Elliptic Curve Cryptography (ECC)

A type of public key cryptosystem that requires a shorter key length than many other cryptography systems (including the de facto industry standard, RSA).

padding

Additional data that is added to a hash to make it the correct number of bytes.

PKI (Public Key Infrastructure)

An encryption system that is composed of a CA, certificates, software, services, and other cryptographic components, for the purpose of verifying authenticity and enabling validation of data and entities.

Block ciphers

Blocks ciphers perform encryption by breaking a message into fixed-length units, called blocks. Advantages of block ciphers include the following: -Implementation of block ciphers is easier than stream-based cipher implementation. -Block ciphers are generally less susceptible to security issues. -They are generally used more in software implementations. -Block ciphers employ both confusion and diffusion. Block ciphers often use different modes: ECB, CBC, CFB, and CTR.

Conditions of cryptanalysis

Cipher-text only Known-plaintext(known both plaintext and ciphertext) Chosen-plaintext(can select prepared plaintext and see ciphertext) Chosen-ciphertext(chosen-plaintext & select ciphertext and see plaintext)

How to construct random seed?

Coin Flips Human behavior Atomic decay Thermal noise Electromagnetic noise

Confusion and Diffusion

Confusion: algorithm to reduce the predictability of ciphertext when changing one character in plaintext. Diffusion: spread the information from the plaintext over the entire ciphertext.

ECB weakness

Data patterns may remain visible Susceptible to replay attacks, block insertion/deletion.

Pros of Substitution ciphers

Easy to remember and use

Encrypt-then-MAC

Encrypt cleartext, MAC ciphertext, append MAC to ciphertext Provides integrity of ciphertext and plaintext Recommended by most researchers.

Chaining

Encryption of each block depends on previous blocks. Problem: first block has no prior block? Plaintext of first block can be inferred Solution : initialization vector

Symmetric Key Encryption

Encryption system in which a single key is used for both encryption and decryption.

Asymmetric Key Encryption

Encryption system in which two keys are used: a public key used only to encrypt data, and a private key used only to decrypt it.

Cryptographic Checksum

Ensures data integrity. Its value can only be evaluated by computers that know the cryptographic key.

Modes of Operation

Five standard modes: ECB: Electronic Code Book CBC: Cipher Block Chaining CFB: Cipher Feedback OFB: Output Feedback CTR: Counter

HMAC

Hash-based Message Authentication Code. An HMAC is a fixed length string of bits similar to other hashing algorithms such as MD5 and SHA-1, but it also uses a secret key to add some randomness to the result.

MAC-then-Encrypt

MAC cleartext, append the MAC to cleartext, encrypt whole Similar properties as Encrypt-and-MAC

Encrypt-and-MAC

MAC cleartext, encrypt cleartext, append MAC to ciphertext Protect plaintext integrity but not ciphertext

cryptographic primitives

Modular mathematical functions that perform one task reliably. They form the basic building blocks of modern cryptography.

Cons of Substitution ciphers

Obvious patters in ciphertext

Pseudorandom Generator (PRG)

PRG takes one number and produces a sequence of bits.

OTP drawbacks

Perfectly secure but impractical

Stream ciphers using PRG

Problem: OTP key is as long as the message. Solution: Pseudo random key -- stream ciphers

RSA Encryption

RSA (Rivest-Shamir-Adleman) is the most common internet encryption and authentication system. The system used an algorithm that involves multiplying two large prime numbers to generate a public key, used to encrypt data and decrypt an authentication, and a private key, used to decrypt the data and encrypt an authentication.

Stream Cipher: One Time Pad(OTP)

Stream ciphers encrypt one bit or one byte at a time Key is only used to encrypt one message.

Substitution and transposition

Substitution: one set of bits is exchanged for another Transposition: rearranging cyphertext order to break any repeating patterns in the underlying plaintext.

Message Integrity

The ability to be certain that the message being sent arrives at the proper destination without being copied or changed

Cryptoanalysis

Trying to break an encrypted message.

Encryption Substitution ciphers

ci := (pi + k) mod 26

Decryption Substitution ciphers

pi := (ci-k) mod 26

stream vs block

transmit byte by byte, vs in groups

encryption algorithm

uses a secret key to encrypt messages

Modern cryptography

uses secure mathematical processes and cryptographic keys