Hash functions

A message's digest, the three resistances (preimage, second preimage, collisions), the avalanche effect, the birthday paradox and digest size, broken and recommended functions, file integrity and HMAC.

35 cards
6 topics
Free

Study this deck

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Concepts
Questions
Sort and link
The numbers
Scenarios
Understand

A few cards from the deck

Cryptographic hash function: what is it?
Show the answer
A public function that turns a message of any size into a fixed-size digest, easy to compute but infeasible to invert in practice.
No key is involved: anyone computes the same digest from the same message. SHA-256 always gives 256 bits, whether the input is one byte or one gigabyte.
What size is the digest of these two messages?

```python
import hashlib
a = hashlib.sha256(b"a").hexdigest()
b = hashlib.sha256(b"a" * 1000000).hexdigest()
print(len(a), len(b))
`
- 64 64
- 1 1000000
- 256 256
- 32 32
Show the answer
64 64
SHA-256 always gives 256 bits, that is 32 bytes, which hexdigest() writes as 64 hexadecimal characters (4 bits each), whatever the input size. With MD5 it would be 32 characters.
A cryptographic hash function is …: the same message always gives the same digest, and its size is ….
Show the answer
A cryptographic hash function is deterministic: the same message always gives the same digest, and its size is fixed.
Otherwise two digests could not be compared to check a file. SHA-256 digests are always 256 bits.
Preimage resistance: what is it?
Show the answer
A property of a hash function: given a digest, it is out of reach to find any message that produces it.
Against an n-bit digest, the generic attack tries about 2 to the power n messages. For SHA-256, that is 2 to the power 256 tries.

Study this deck