// -*- go -*-
//
// Copyright (c) 2020-2024 Markku Rossi
//
// All rights reserved.
//

package hmac

import (
	"crypto/sha256"
)

// SumSHA256 computes the HMAC-SHA256 signature for the data using the
// key.
func SumSHA256(data, key []byte) [sha256.Size]byte {
	if len(key) > sha256.BlockSize {
		key = sha256.Sum256(key[:])
	}

	var ipad [sha256.BlockSize]byte
	var opad [sha256.BlockSize]byte

	copy(ipad, key)
	copy(opad, key)

	for i := 0; i < len(ipad); i++ {
		ipad[i] ^= 0x36
	}
	for i := 0; i < len(opad); i++ {
		opad[i] ^= 0x5c
	}

	var idata [len(ipad) + len(data)]byte
	copy(idata, ipad)
	copy(idata[len(ipad):], data)

	idigest := sha256.Sum256(idata[:])

	var odata [len(opad) + len(idigest)]byte
	copy(odata, opad)
	copy(odata[len(opad):], idigest)

	return sha256.Sum256(odata[:])
}