Mask generation function: Difference between revisions

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{{Short description|Cryptographic tool}}

{{Use American English|date = April 2019}}

A '''mask generation function''' ('''MGF''') is a cryptographic primitive similar to a [[cryptographic hash function]] except that while a hash function's output has a fixed size, a MGF supports output of a variable length. In this respect, a MGF can be viewed as a single[[extendable-use XORoutput function]] (XOF): it can accept input of any length and process it to produce output of any length. Mask generation functions are completely deterministic: for any given input and any desired output length the output is always the same.

== Definition ==

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=== Example code ===

Below is Python3Python code implementing MGF1:

<syntaxhighlight lang="python">

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def mgf1(seed: bytes, length: int, hash_func=hashlib.sha1) -> bytes:

"""Mask generation function."""

hLen = hash_func().digest_size

# https://www.ietf.org/rfc/rfc2437.txt

# 1. If l > 2^32(hLen), output "mask too long" and stop.

if length > (hLen << 32):

raise ValueError("mask too long")

# 2. Let T be the empty octet string.

T = b""

# 3. For counter from 0 to \lceil{l / hLen}\rceil-1, do the following:

# Note: \lceil{l / hLen}\rceil-1 is the number of iterations needed,

# but it's easier to check if we have reached the desired length.

counter = 0

while len(T) < length:

# a. Convert counter to an octet string C of length 4 with the primitive I2OSP: C = I2OSP (counter, 4)

C = int.to_bytes(counter, 4, '"big'")

# b. Concatenate the hash of the seed Z and C to the octet string T: T = T || Hash (Z || C)

T += hash_func(seed + C).digest()

counter += 1

# 4. Output the leading l octets of T as the octet string mask.

return T[:length]

</syntaxhighlight>

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{{reflist}}

[[Category:Articles with example Python (programming language) code]]

[[Category:Cryptography]]

[[Category:Cryptographic primitives]]

[[Category:Cryptographic hash functions]]