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PV181 Laboratory of security and applied cryptography
Symmetric cryptography
•Marek Sýs, Zdeněk Říha
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Cryptography - brief overview
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Concept, goals, primitives, brief,

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Goals of Cryptography
•Confidentiality (privacy) - preventing open access
–ciphers
•Authentication:
1.Entity – identity verification – various (password, MAC, …)
2.Data origin – identity of message originator – MAC
•Integrity - preventing unauthorized modification
–hash functions
•Non-repundation - preventing denial of actions
–digital signature
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Crypto primitives
•Ciphers – encryption/decryption of data using key
–Symmetric ciphers – same key for enc/dec
–Asymmetric ciphers – different key for enc/dec
•Random number generators (RNGs)
–Key generation
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•Hash functions – “unique” fingerprint of data
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•Based on previous: MAC, PBKDF, Digital signature
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encryption
decryption
message
Encrypted
message
Alice
Shared secret key
Bob
Adapted Network and
Internetwork Security (Stallings)
Decrypted
original
 message
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Symmetric cryptosystem

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encryption

decryption
message
Alice
Public key of Bob
Bob
Adapted Source: Network and
Internetwork Security (Stallings)
Private key of Alice
Encrypted
message
Decrypted
original
 message
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Asymmetric cryptosystem

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Random number generators
•Used to generate: keys, IV, …
1.Truly RNG - physical process
–aperiodic, slow
2.Pseudo RNG (PRNG) – software function
–deterministic, periodic, fast
– initialized by seed – fully determines random data
•Combination often used:
–truly RNG used to generate seed for PRNG
–dev/urandom, dev/random in Linux, Fortuna scheme
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Hash function
•Cryptographic hash function
•Input of arbitrary size
•Output of fixed size: n bits (e.g. 256 bits).
•Function is not injective (there are “collisions”).
•Hash is a compact representative of input (also called imprint, (digital) fingerprint or message
digest).
•Hash functions often used to protect integrity. First the has is computed and then only the hash
is protected (e.g. digitally signed).
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Standards
•Everything defined in standard:
•implementation, settings, usage, etc.
•If you need something look into standard
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•Different types:
•FIPS PUB 197 – AES block cipher
•RFC1321 – md5 hash function
•NIST SP,…
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Implementation testing - test vectors
•Examples of input/output (and also intermediate) for the reference implementation
•MD5 defined in RFC 1321:
–MD5(“”) = d41d8cd98f00b204e9800998ecf8427e
–MD5(“message digest”) = f96b697d7cb7938d525…
•AES defined in FIPS197:
–Plaintext: 00112233445566778899aabbccddeeff
–Key 000102030405060708090a0b0c0d0e0f
–Ciphertext 69c4e0d86a7b0430d8cdb78070b4c55a
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Symmetric cryptography
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Block cipher
•Input divided into blocks of fixed size (e.g 256 bits)
–Padding - message is padded to complete last block
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•Different modes of operation:
–Insecure basic ECB mode – leaks info
–Secure modes: CBC, OFB,CFB,CTR,…
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•CBC, OFB,CFB need initialization
–Initialization vector (IV) – must be known
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https://upload.wikimedia.org/wikipedia/commons/f/f0/Tux_ecb.jpg
https://upload.wikimedia.org/wikipedia/commons/5/56/Tux.jpg
Source: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation

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Block ciphers - padding
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•  Standard        method
•ANSI X.923
•ISO 10126
•PKCS7
•ISO/IEC 7816-4
•Zero padding
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... | DD DD DD DD DD DD DD DD | DD DD DD DD 81 A6 23 04 |
... | DD DD DD DD DD DD DD DD | DD DD DD DD 04 04 04 04 |
... | DD DD DD DD DD DD DD DD | DD DD DD DD 00 00 00 04 |
... | DD DD DD DD DD DD DD DD | DD DD DD DD 00 00 00 00 |
... | DD DD DD DD DD DD DD DD | DD DD DD DD 80 00 00 00 |
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Block ciphers: ECB vs CBC mode
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ECB encryption.svg CBC encryption.svg
Source: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation

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Hash functions - examples
•MD5
–Input: „Autentizace“.
–Output: 2445b187f4224583037888511d5411c7 .
–Output 128 bits, written in hexadecimal notation.
–Input: „Cutentizace“.
–Output: cd99abbba3306584e90270bf015b36a7.
–A single bit changed in input → big change in output,
so called “Avalanche effect”
•SHA-1
–Input: „Autentizace“.
–Output: 647315cd2a6c953cf5c29d36e0ad14e395ed1776
•SHA-256
–Input: „Autentizace“.
–Output: a2eb4bc98a5f71a4db02ed4aed7f12c4ead1e7c98323fda8ecbb69282e4df584
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Secure Hash Algorithm (SHA)
•SHA-1
– NIST standard, collision found in 2016, 160 bits hash
•SHA-2
–function family: SHA-256, SHA-384, SHA-512, SHA-224
–defined in FIPS 180-2
–Recommended
•SHA-3
–New standard 2015
–Keccak sponge function family: SHAKE-128, SHA3-224, …
–defined in FIPS 202, used in FIPS-202, SP 800-185
–Recomended
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Password protection
password hashing & salting
1.Clear password could be stolen:
–store hash of password
hash = H(password)
–Checking: password is correct if hash matches
2.Attack (brute force or dictionary)
–trying possible passwords “aaa”, “aab”…“zzz” – N tests
–N test for single but also for 2,3,… passwords !!!
3.Slow down attack - increase password size:
–random “salt” is added to password,
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Password protection
password hashing & salting
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Výsledek obrázku pro password hashing
Source: http://blog.conviso.com.br/worst-and-best-practices-for-secure-password-storage/

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Key/password protection
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PBKDF2
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Salt &
# iterations
KEY
PBKDF2
key
Password / Passphrase
<cipher>
encrypted
KEY

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Message authentication code (MAC)
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Výsledek obrázku pro message authentication code
Source: https://www.tutorialspoint.com/cryptography/message_authentication.htm

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Instructions for the seminar
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OpenSSL access
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OpenSSL materials
•Books:
–free ebook OpenSSL cookbook
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•Useful links:
–Manual: https://www.openssl.org/docs/man1.0.2
–https://wiki.openssl.org/index.php/Command_Line_Utilities
–https://www.madboa.com/geek/openssl/
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OpenSSL basics
•Usage:
–With cmd history(prefered): openssl commands params
–Without:
•openssl (enter)
•commands params (without openssl)
•Typical problems:
–order of parameters
–string: str or “str” (may represent string of 5 chars)
•Help = non-recognized command
– e.g. blablabla or help
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Symmetric crypto
•OpenSSL commands:
•dgst – hashing, MAC, signature
•enc – encryption, base 64 encoding,…
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•Hashed, encrypted, … data are not readable (not ASCII characters) hence we will use rather:
–hex – byte encoded 2 chars from charset (0-9, A-F)
–or base64 – 6 bits encoded by 1 char (A-Z, a-z, 0-9, +, /)
•Padding is used (no padding, “=“, “==“)
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Practical part
•See instructions in Tasks01.txt in IS
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