Mask generation function: Difference between revisions

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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"/> The algorithm is described as follows:

<blockquote>

====Options====

<math>\mathsf{Hash}</math>

Hash : hash function (<math>\mathsf{hLen}</math> denotes the length in octets of the hash function output)

====Input====

<pre>

<math>Z</math>

Hash hash function (hLen denotes the length in octets of the hash

Z :seed from which mask is generated, an octet string

function output)

<math>l</math>

l :intended length in octets of the mask, at most <math>2^{32}(\mathsf{hLen})</math>

====Output:====

Input:

<math>\mathsf{mask}</math>

Z seed from which mask is generated, an octet string

mask :mask, an octet string of length <math>l</math>; or "mask too long"

l intended length in octets of the mask, at most 2^32(hLen)

====Steps====

Output:

{{Ordered list | list_style_type=decimal

mask mask, an octet string of length l; or "mask too long"

| 1.If <math>l > 2^{32}(\mathsf{hLen})</math>, output "mask too long" and stop.

| 2.Let <math>T</math> be the empty octet string.

Steps:

| 3.For <math>\mathsf{counter}</math> from <math>0</math> to <math>\left\lceil{\tfrac{l / }{\mathsf{hLen}}}\right\rceil-1</math>, do the following:

{{Ordered list | list_style_type=lower-alpha

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

a. | Convert <math>\mathsf{counter}</math> to an octet string <math>C</math> of length <math>4</math> with the primitive <math>\mathsf{I2OSP}</math>:

: I2OSP: <math>C = \mathsf{I2OSP} (\mathsf{counter}, 4)</math>

2.Let T be the empty octet string.

b. | Concatenate the hash of the seed <math>Z</math> and <math>C</math> to the octet string <math>T</math>: T =

: <math>T = T \shortparallel \mathsf{Hash} (Z \shortparallel C)</math>

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

}}

| 4.Output the leading <math>l</math> octets of <math>T</math> as the octet string mask.

a.Convert counter to an octet string C of length 4 with the primitive

}}

I2OSP: C = I2OSP (counter, 4)

</blockquote>

b.Concatenate the hash of the seed Z and C to the octet string T: T =

T || Hash (Z || C)

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

</pre>

=== Example Code ===