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// Copyright 2024 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// Package hmac implements the hmac algorithm.
package hmac
import (
"crypto/hmac"
"errors"
"fmt"
"hash"
"github.com/tink-crypto/tink-go/v2/subtle"
)
const (
// Minimum key size in bytes.
minKeySizeInBytes = uint32(16)
// Minimum tag size in bytes. This provides minimum 80-bit security strength.
minTagSizeInBytes = uint32(10)
)
// HMAC implements the MAC interface.
type HMAC struct {
HashFunc func() hash.Hash
key []byte
tagSize uint32
}
// ValidateHMACParams validates parameters of HMAC constructor.
func ValidateHMACParams(hash string, keySize uint32, tagSize uint32) error {
// validate tag size
digestSize, err := subtle.GetHashDigestSize(hash)
if err != nil {
return err
}
if tagSize > digestSize {
return fmt.Errorf("tag size too big")
}
if tagSize < minTagSizeInBytes {
return fmt.Errorf("tag size too small")
}
// validate key size
if keySize < minKeySizeInBytes {
return fmt.Errorf("key too short")
}
return nil
}
// New returns a new HMAC instance.
func New(hashAlg string, key []byte, tagSize uint32) (*HMAC, error) {
if err := ValidateHMACParams(hashAlg, uint32(len(key)), tagSize); err != nil {
return nil, err
}
hashFunc := subtle.GetHashFunc(hashAlg)
if hashFunc == nil {
return nil, fmt.Errorf("hmac: invalid hash algorithm")
}
return &HMAC{
HashFunc: hashFunc,
key: key,
tagSize: tagSize,
}, nil
}
// ComputeMAC computes message authentication code (MAC) for the given data.
func (h *HMAC) ComputeMAC(data ...[]byte) ([]byte, error) {
if h.HashFunc == nil {
return nil, fmt.Errorf("hmac: invalid hash algorithm")
}
mac := hmac.New(h.HashFunc, h.key)
for _, d := range data {
if _, err := mac.Write(d); err != nil {
return nil, fmt.Errorf("hmac: failed to write data: %v", err)
}
}
tag := mac.Sum(nil)
return tag[:h.tagSize], nil
}
// VerifyMAC verifies whether the given MAC is a correct message authentication
// code (MAC) the given data.
func (h *HMAC) VerifyMAC(mac []byte, data ...[]byte) error {
expectedMAC, err := h.ComputeMAC(data...)
if err != nil {
return err
}
if hmac.Equal(expectedMAC, mac) {
return nil
}
return errors.New("HMAC: invalid MAC")
}
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