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// Copyright 2025 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 aescmac
import (
"bytes"
"fmt"
"slices"
"github.com/tink-crypto/tink-go/v2/insecuresecretdataaccess"
"github.com/tink-crypto/tink-go/v2/internal/internalapi"
"github.com/tink-crypto/tink-go/v2/mac/subtle"
"github.com/tink-crypto/tink-go/v2/tink"
)
type fullMAC struct {
rawMAC tink.MAC
prefix []byte
variant Variant
}
var _ tink.MAC = (*fullMAC)(nil)
// NewMAC creates a new full AES-CMAC MAC primitive.
func NewMAC(key *Key, _ internalapi.Token) (tink.MAC, error) {
params := key.Parameters().(*Parameters)
if err := subtle.ValidateCMACParams(uint32(params.KeySizeInBytes()), uint32(params.CryptographicTagSizeInBytes())); err != nil {
return nil, fmt.Errorf("aescmac.NewMAC: invalid parameters: %v", err)
}
rawMAC, err := subtle.NewAESCMAC(key.KeyBytes().Data(insecuresecretdataaccess.Token{}), uint32(params.CryptographicTagSizeInBytes()))
if err != nil {
return nil, fmt.Errorf("aescmac.NewMAC: failed to create raw MAC: %v", err)
}
return &fullMAC{
rawMAC: rawMAC,
prefix: key.OutputPrefix(),
variant: params.Variant(),
}, nil
}
func (m *fullMAC) message(msg []byte) []byte {
if m.variant == VariantLegacy {
return slices.Concat(msg, []byte{0x00})
}
return msg
}
func (m *fullMAC) ComputeMAC(data []byte) ([]byte, error) {
rawMAC, err := m.rawMAC.ComputeMAC(m.message(data))
if err != nil {
return nil, err
}
return slices.Concat(m.prefix, rawMAC), nil
}
func (m *fullMAC) VerifyMAC(mac []byte, data []byte) error {
if len(mac) < len(m.prefix) {
return fmt.Errorf("aescmac: mac with size %d is too short", len(mac))
}
prefix := mac[:len(m.prefix)]
if !bytes.Equal(prefix, m.prefix) {
return fmt.Errorf("aescmac: mac prefix does not match")
}
return m.rawMAC.VerifyMAC(mac[len(m.prefix):], m.message(data))
}
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