def mgf1(seed, length, hasher='SHA-1'): """ MGF1 is a Mask Generation Function based on a hash function. A mask generation function takes an octet string of variable length and a desired output length as input, and outputs an octet string of the desired length. The plaintext-awareness of RSAES-OAEP relies on the random nature of the output of the mask generation function, which in turn relies on the random nature of the underlying hash. :param bytes seed: seed from which mask is generated, an octet string :param int length: intended length in octets of the mask, at most 2^32(hLen) :param str hasher: hash function (hLen denotes the length in octets of the hash function output) :return: mask, an octet string of length `length` :rtype: bytes :raise OverflowError: when `length` is too large for the specified `hasher` :raise ValueError: when specified `hasher` is invalid """ try: hash_length = pkcs1.HASH_METHODS[hasher]().digest_size except KeyError: raise ValueError( 'Invalid `hasher` specified. Please select one of: {hash_list}'.format( hash_list=', '.join(sorted(pkcs1.HASH_METHODS.keys())) ) ) # If l > 2^32(hLen), output "mask too long" and stop. if length > (2**32 * hash_length): raise OverflowError( "Desired length should be at most 2**32 times the hasher's output " "length ({hash_length} for {hasher} function)".format( hash_length=hash_length, hasher=hasher, ) ) # Looping `counter` from 0 to ceil(l / hLen)-1, build `output` based on the # hashes formed by (`seed` + C), being `C` an octet string of length 4 # generated by converting `counter` with the primitive I2OSP output = b''.join( pkcs1.compute_hash( seed + transform.int2bytes(counter, fill_size=4), method_name=hasher, ) for counter in range(common.ceil_div(length, hash_length) + 1) ) # Output the leading `length` octets of `output` as the octet string mask. return output[:length]
def mgf1(seed, length, hasher='SHA-1'): """ MGF1 is a Mask Generation Function based on a hash function. A mask generation function takes an octet string of variable length and a desired output length as input, and outputs an octet string of the desired length. The plaintext-awareness of RSAES-OAEP relies on the random nature of the output of the mask generation function, which in turn relies on the random nature of the underlying hash. :param bytes seed: seed from which mask is generated, an octet string :param int length: intended length in octets of the mask, at most 2^32(hLen) :param str hasher: hash function (hLen denotes the length in octets of the hash function output) :return: mask, an octet string of length `length` :rtype: bytes :raise OverflowError: when `length` is too large for the specified `hasher` :raise ValueError: when specified `hasher` is invalid """ try: hash_length = pkcs1.HASH_METHODS[hasher]().digest_size except KeyError: raise ValueError( 'Invalid `hasher` specified. Please select one of: {hash_list}'.format( hash_list=', '.join(sorted(pkcs1.HASH_METHODS.keys())) ) ) # If l > 2^32(hLen), output "mask too long" and stop. if length > (2**32 * hash_length): raise OverflowError( "Desired length should be at most 2**32 times the hasher's output " "length ({hash_length} for {hasher} function)".format( hash_length=hash_length, hasher=hasher, ) ) # Looping `counter` from 0 to ceil(l / hLen)-1, build `output` based on the # hashes formed by (`seed` + C), being `C` an octet string of length 4 # generated by converting `counter` with the primitive I2OSP output = b''.join( pkcs1.compute_hash( seed + transform.int2bytes(counter, fill_size=4), method_name=hasher, ) for counter in range(common.ceil_div(length, hash_length) + 1) ) # Output the leading `length` octets of `output` as the octet string mask. return output[:length]