cryptography basics - dc206cryptography basics eugenia potseluevskaya (jennie) @epotseluevskaya...

Cryptography Basics

Eugenia Potseluevskaya(Jennie)

@epotseluevskaya

whoami

● Math education with the focus on cryptography and cybernetics

● 5 years in defensive security (mostly PKI, SIEM, and risk assessments)

● 8+ years in offensive security (vulnerability analysis, pentesting, security research, threat intelligence, ICS, IoT, etc.)

● PHDays forum organizer

/* buzzwords */ CISA, CISSP, OSCP, OSCE, Russian haxxor, Kaspersky Lab O_O

Cryptography is huge

In my diploma:

530 hours of various cryptography topics

including a year-long course of elliptic curves only

Cryptography topics

cryptography

symmetric

asymmetric

stream

block different modes

key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Objectives

● Confidentiality● Authentication● Integrity● Non-repudiation

Kerckhoffs' Principle

The security of the encryption scheme must depend only on the secrecy of the key, and not on the secrecy of the algorithm.

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Symmetric cryptography

For symmetric (aka conventional, secret-key, single-key algorithms, or one-key) algorithms the encryption key can be calculated from the decryption key and vice versa.



Stream ciphers process plain text one unit (bit/byte/character) at a time



Stream ciphers process plain text one unit (bit/byte/character) at a time

Block ciphers process plain text in blocks

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Basics

XOR:

Stream ciphers overview

● A perfect encryption scheme: one-time pad


● A perfect encryption scheme: one-time pad● Closely tied with pseudo-random-sequence

generators



generators● The most well-known ciphers:

- RC4 (used by WEP)



generators● The most well-known ciphers:

- RC4 (used by WEP)

- A5/1 (used in GSM communications)

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Block ciphers

● A block cipher is an encryption function for fixed-size blocks of data.

● Block length is fixed, so padding is needed. Padding must be reversible.

Block ciphers



Example: block size is 16

42 42 42 42 42 42 42 42 42 42 06 06 06 06 06 06

42 42 42 42 42 42 42 42 42 42 42 05 05 05 05 05

Block ciphers



Side note: "Padding oracle" (nothing to do with Oracle DBMS) - it freely responds to queries about whether a message is correctly padded or not, and allows an attacker to use the padding validation of a cryptographic message to decrypt the ciphertext

Block ciphers



● Different modes exist to handle more than one block: ECB, CBC, OFB, CTR

Block ciphers



● Different modes exist to handle more than one block: ECB, CBC, OFB, CTR

● The most well-known ciphers: DES, 3-DES, AES, Blowfish

DES

● DES - Data Encryption Standard● Block length – 64 bits● Effective key length – 56 bits● 16 rounds (each consists of substitution and

permutation)● Used in: Microsoft LM, Net-NTLMv1, MS-

CHAPv2, linux crypt(), IPsec (if no better options)

DES

Feistel function (F)

DES and 3-DES

● Nowadays DES is not secure

DES and 3-DES


● 3-DES is better

● Effective key length is 112 or 168 bits

DES and 3-DES


● 3-DES is better

● Effective key length is 112 or 168 bits

In November 2017, NIST restricted usage to 220 64-bit blocks (8 MB of data) using a single key bundle, so it could no longer effectively be used for TLS, IPsec, or large file encryption.

AES

● AES - Advanced Encryption Standard● Block length – 128 bits● Key length – 128, 192, or 256 bits● 10-14 rounds (depending on the key length)● Used in: TLS, SSH, IPsec, PGP, S/MIME,

WPA2, Kerberos 5, SMB 3

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Block cipher modes

● Electronic Code Book (ECB) – each block is independent - bad

Block cipher modes


● Cipher Block Chaining (CBC) - XORing each plaintext block with the previous ciphertext block

Block cipher modes


● Cipher Block Chaining (CBC) - XORing each plaintext block with the previous ciphertext block

IV is for Initialization Vector – can be generated in different ways

Block cipher modes

● Output Feedback Mode (OFB) – block cipher is used to generate a pseudorandom key stream, which is XORed with the plaintext.

Block cipher modes

● Output Feedback Mode (OFB) – block cipher is used to generate a pseudorandom key stream, which is XORed with the plaintext.

● Counter Mode (CTR or CNT) – similar to OFB, but has a counter

IV or nonce MUST be unique every time

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Hash functions

● A hash (or message digest) function - a function that takes as an arbitrarily long input string and produces a fixed-size result.

Hash functions


● In cryptography must be one-way● Must be collision-resistant

Hash functions


● In cryptography must be one-way● Must be collision-resistant● Should be not too fast

Hash functions


● In cryptography must be one-way● Must be collision-resistant● Should be not too fast● The most well-known examples: MD4 (used in

NTLM), MD5, SHA-1, SHA-256, SHA-512, bcrypt

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Salt

● Cryptographic salt is a random string added to the password before hashing

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Message authentication codes

● A message authentication code (MAC) or a data authentication code (DAC), is a one-way hash function with the addition of a secret key.

● Only someone with the key can verify the hash value.


● CBC-MAC:


● CBC-MAC:

● HMAC:


● CBC-MAC:

● HMAC:

● Improve block ciphers modes:● Galois/Counter Mode (GCM) provides both encryption and

authentication for a block cipher, also used as MAC only as GMAC

● CCM mode = Counter mode with CBC-MAC



Problem: key management and distribution

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Asymmetric cryptography

Asymmetric (public-key) algorithms are designed so that the key used for encryption is different from the key used for decryption, and the decryption key cannot be calculated from the encryption key in any reasonable amount of time.

A bit of complexity theory

● P consists of all decision problems that can be solved on a deterministic Turing machine in an amount of time that is polynomial in the size of the input.

If n is the input length, the solving time is a polynom from n

time = aknk+a

k-1nk-1+...a

1n+a

0 – denoted as O(nk)




time = aknk+a

k-1nk-1+...a

1n+a


● Solutions to a NP problem can be verified on a deterministic Turing machine in polynomial time.

often the time to solve is exponential - O(2n)




time = aknk+a

k-1nk-1+...a

1n+a


● Solutions to a NP problem can be verified on a deterministic Turing machine in polynomial time.

often the time to solve is exponential - O(2n)

● P == NP ? - a Millennium Prize Problem, the classical asymmetric cryptography assumes that likely NO

Some of NP problems

● Prime Factorization:

given N=pq, p and q are prime, find p and q

Some of NP problems

● Prime Factorization:

given N=pq, p and q are prime, find p and q● Discrete Logarithm:

given p,g, and y=gx mod p, find x

x = logg(y)

A variant (for a different group) – Elliptic Curve Discrete Logarithm Problem

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Diffie-Hellman● Alice to Bob: X = gx mod p● Bob to Alice Y = gy mod p● Alice: k = Yx mod p● Bob: k` = Xy mod p

k=k`=gxy mod p

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

RSA encryption

● Ronald Rivest, Adi Shamir, and Leonard Adleman

t=(p-1)(q-1)cd mod n = med mod n = mkt+1 mod n = mkt m mod n = (mt)k m mod n = 1k m mod n = m

Cryptography topics

cryptography

symmetric

asymmetric

stream


key exchange

encryption

digital signatures

MAC

hashes

salt

cryptanalysis

Digital Signatures

● Verify authenticity● Can be created based on public-key encryption

Asymmetric cryptography

Asymmetric (public-key) algorithms are designed so that the key used for encryption is different from the key used for decryption, and the decryption key cannot be calculated from the encryption key in any reasonable amount of time.

Widespread algorithms: RSA, Diffie-Hellman key agreement protocol, Digital Signature Algorithm (DSA), Elliptic Curve Cryptography (ECC) including Elliptic Curve Digital Signature Algorithm (ECDSA)

All together (TLS example)

Thank youEugenia Potseluevskaya(Jennie)

@epotseluevskaya

cryptography basics - dc206cryptography basics eugenia potseluevskaya (jennie) @epotseluevskaya...

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