Create Mask Generation Function (MGF1) from PKCS #1 v2.2 standard (B.2.1 page 50) at below link. Input will be character array (mgfSeed) which will need to be converted to octet array using UTF-8 representation. Output must be octet array (uint8) of length, maskLen (input). Hashing must use SHA-1 (160 - 20 byte).

For example:

mgfSeed = 'I like to swim.';%input
maskLen = 20;%input
mask = [170,251,101,210,23,101,10,242,193,163,174,148,104,138,228,245,52,234,0,195];%output

https://www.emc.com/collateral/white-papers/h11300-pkcs-1v2-2-rsa-cryptography-standard-wp.pdf