Applied Cryptography

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Applied Cryptography
Classical (Symmetric) Cryptography
Modern Symmetric Cryptography
Cipher Modes
Cryptographic Hashing
RSA & Related Subjects
Asymmetric Key Management
Digital Signatures

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Approaches
Merkle-Damgård construction
Sponge functions
Common Algorithms

Digest functions

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Last updated 1 year ago

Give a fixed-length value from a variable-length text.

Sort of text “fingerprint”

Produce very different values for similar texts.

Cryptographic one-way hash functions.

Relevant properties:

Preimage resistance.
- Given a digest, it is infeasible to find an original text producing it.
2nd-preimage resistance.
- Given a text, it is infeasible to find another one with the same digest.
Collision resistance.
- It is infeasible to find any two texts with the same digest.
Birthday paradox.

Approaches

Merkle-Damgård construction

Iterative compression.
Collision-resistent, one-way compression functions.
Length padding (1, followed by zeros, followed by length).

Sponge functions

Absorption: update a finite internal state (entropy pool) from a variable-length, padded input stream.
Squeezing: produce an arbitrary-length output from the internal state.

Common Algorithms

MD5 (128 bits)
- No longer secure! It’s easy to find collisions!
- Disclaimer: it can be used when collisions are not an issue.
SHA-1 (Secure Hash Algorithm, 160 bits)
- Also no longer secure ... (collisions found in 2017).
RIPEMD (128 and 160)
SHA-2, aka SHA-256 / SHA-348 / SHA-512
SHA-3 (Keccak)