Mask generation function: Difference between revisions

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{{Short description|cryptographicCryptographic 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 ishas a fixed size, a MGF supports output of a variable length. In this respect, a MGF can be viewed as a single-use [[spongeextendable-output function]] (XOF): it can absorbaccept any lengthinput of inputany length and process it to produce any lengthoutput of outputany length. Mask generation functions are completely deterministic: for any given input and any desired output length the output is always the same.

{{Short description|cryptographic tool}}

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

== Definition ==

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== Applications ==

Mask generation functions, as generalizations of hash functions, are useful wherever hash functions are. However, use of a MGF is desirable in cases where a fixed-size hash would be inadequate. Examples include generating [[Padding (cryptography)|padding]], producing [[Oneone-time pad|one time pads]]s or [[keystream|keystreams]] in [[Symmetric-key_algorithm|symmetric -key encryption]], and yielding outputs for [[pseudorandom number generator|pseudorandom number generators]]s.

=== Padding schemes ===

Mask generation functions were first proposed as part of the specification for padding in the [[Optimal_asymmetric_encryption_padding|RSA-OAEP]] algorithm. The OAEP algorithm required a cryptographic hash function that could generate an output equal in size to a "data block" whose length was proportional to arbitrarily sized input message.<ref name="rsa"/>

=== Keyed encryption ===

The [[Salsa20]] stream cipher may be viewed as a mask generation function as its [[keystream]] is produced by hashing the key and nonce with a counter, to yield an arbitrarily long output.<ref>{{cite web |url=https://cr.yp.to/snuffle/salsafamily-20071225.pdf | title=The Salsa20 family of stream ciphers |author=Daniel J. Bernstein}}</ref>

<blockquote>

Salsa20 generates the stream in 64-byte (512-bit) blocks. Each block is an independent hash of the key, the nonce, and a 64-bit block number; there is no chaining from one block to the next. The Salsa20 output stream can therefore be accessed randomly, and any number of blocks can be computed in parallel.

</blockquote>

=== Random number generators ===

NIST Special Publication 800-90A<ref>{{cite webjournal |url=http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf | title=Recommendation for Random Number Generation Using Deterministic Random Bit Generators |author=National Institute of Standards and Technology|year=2012 |doi=10.6028/NIST.SP.800-90A }}</ref> defines a class of cryptographically secure random number generators, one of which is the "Hash &nbsp;DRBG", which uses a hash function with a counter to produce a requested sequence of random bits equal in size to the requested number of random bits.

== Examples ==

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=== MGF1 ===

MGF1 is a mask generation function defined in the Public Key Cryptography Standard #1 published by RSA Laboratories:<ref name="rsa"/>:

<blockquote>

====Options====

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</blockquote>

=== Example Codecode ===

Below is pythonPython code implementing MGF1:

<sourcesyntaxhighlight lang="python">

import hashlib

def mgf1(inputseed: bytes, length: int, hashhash_func=hashlib.sha1) -> bytes:

def i2osp(integer, size=4):

"""Mask generation function."""

return ''.join([chr((integer >> (8 * i)) & 0xFF) for i in reversed(range(size))])

hLen = hash_func().digest_size

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

def mgf1(input, length, hash=hashlib.sha1):

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

counter = 0

if length > (hLen << 32):

output = ''

raise ValueError("mask too long")

while (len(output) < length):

# 2. Let T be the empty octet string.

C = i2osp(counter, 4)

T = b""

output += hash(input + C).digest()

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

counter += 1

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

return output[:length]

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

</source>

counter = 0

while (len(outputT) < 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)

output T += hashhash_func(inputseed + C).digest()

C = i2osp( counter, 4)+= 1

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

return outputT[:length]

</syntaxhighlight>

Example outputs of MGF1:

<sourcesyntaxhighlight lang="pythonpycon">

Python 23.710.64 (defaultmain, SepApr 16 9 20142022, 1516:0428:3641) [GCC 8.3.0] on linux

[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin

Type "help", "copyright", "credits" or "license" for more information.

>>> from mgf1 import mgf1

>>> from binascii import hexlify

>>> from hashlib import sha256

>>> hexlify(mgf1('b"foo'", 3).hex()

'1ac907'

>>> hexlify(mgf1('b"foo'", 5).hex()

'1ac9075cd4'

>>> hexlify(mgf1('b"bar'", 5).hex()

'bc0c655e01'

>>> hexlify(mgf1('b"bar'", 50).hex()

'bc0c655e016bc2931d85a2e675181adcef7f581f76df2739da74faac41627be2f7f415c89e983fd0ce80ced9878641cb4876'

>>> hexlify(mgf1('b"bar'", 50, sha256).hex()

'382576a7841021cc28fc4c0948753fb8312090cea942ea4c4e735d10dc724b155f9f6069f289d61daca0cb814502ef04eae1'

</syntaxhighlight>

</source>

== References ==

{{reflist}}

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

[[Category:Cryptography]]

[[Category:Cryptographic primitives]]

[[Category:Cryptographic hash functions]]