code: def mgf1(mgf_seed, mask_len, hash_class=hashlib.sha1):
      '''
         Mask Generation Function v1 from the PKCS#1 v2.0 standard.
  
         mgs_seed - the seed, a byte string
         mask_len - the length of the mask to generate
         hash_class - the digest algorithm to use, default is SHA1
  
         Return value: a pseudo-random mask, as a byte string
         '''
      h_len = hash_class().digest_size
      if mask_len > 0x10000:
          raise ValueError('mask too long')
      T = b''
      for i in range(0, integer_ceil(mask_len, h_len)):
          C = i2osp(i, 4)
          T = T + hash_class(mgf_seed + C).digest()
      return T[:mask_lenf mgf1(mgf_seed, mask_len, hash_class=hashlib.sha1):
      '''
         Mask Generation Function v1 from the PKCS#1 v2.0 standard.
  
         mgs_seed - the seed, a byte string
         mask_len - the length of the mask to generate
         hash_class - the digest algorithm to use, default is SHA1
  
         Return value: a pseudo-random mask, as a byte string
         '''
      h_len = hash_class().digest_size
      if mask_len > 0x10000:
          raise ValueError('mask too long')
      T = b''
      for i in range(0, integer_ceil(mask_len, h_len)):
          C = i2osp(i, 4)
          T = T + hash_class(mgf_seed + C).digest()
      return T[:mask_len]
docs: Mask Generation Function v1 from the PKCS#1 v2.0 standard.
  
         mgs_seed - the seed, a byte string
         mask_len - the length of the mask to generate
         hash_class - the digest algorithm to use, default is SHA1
  
         Return value: a pseudo-random mask, as a byte string