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// Copyright 2020 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 subtle
import (
"crypto/subtle"
"fmt"
"github.com/tink-crypto/tink-go/v2/internal/mac/aescmac"
// Placeholder for internal crypto/subtle allowlist, please ignore.
)
const (
minCMACKeySizeInBytes = 16
recommendedCMACKeySizeInBytes = uint32(32)
minTagLengthInBytes = uint32(10)
maxTagLengthInBytes = aescmac.BlockSize
)
// AESCMAC represents an AES-CMAC struct that implements the MAC interface.
type AESCMAC struct {
cmac *aescmac.CMAC
tagLength uint32
}
// NewAESCMAC creates a new AESCMAC object that implements the MAC interface.
func NewAESCMAC(key []byte, tagLength uint32) (*AESCMAC, error) {
if len(key) < minCMACKeySizeInBytes {
return nil, fmt.Errorf("aescmac: invalid key size %d, want at least %d", len(key), minCMACKeySizeInBytes)
}
if tagLength < minTagLengthInBytes {
return nil, fmt.Errorf("aescmac: invalid tag length %d, want at least %d", tagLength, minTagLengthInBytes)
}
if tagLength > maxTagLengthInBytes {
return nil, fmt.Errorf("aescmac: invalid tag length %d, want at most %d", tagLength, maxTagLengthInBytes)
}
cmac, err := aescmac.New(key)
if err != nil {
return nil, fmt.Errorf("aescmac: %v", err)
}
return &AESCMAC{cmac: cmac, tagLength: tagLength}, nil
}
// ComputeMAC computes message authentication code (MAC) for code data.
func (a AESCMAC) ComputeMAC(data []byte) ([]byte, error) {
return a.cmac.Compute(data)[:a.tagLength], nil
}
// VerifyMAC returns nil if mac is a correct authentication code (MAC) for data,
// otherwise it returns an error.
func (a AESCMAC) VerifyMAC(mac, data []byte) error {
computed := a.cmac.Compute(data)[:a.tagLength]
if subtle.ConstantTimeCompare(mac, computed) != 1 {
return fmt.Errorf("aescmac: invalid MAC")
}
return nil
}
// ValidateCMACParams validates the parameters for an AES-CMAC against the
// recommended parameters.
func ValidateCMACParams(keySize, tagSize uint32) error {
if keySize != recommendedCMACKeySizeInBytes {
return fmt.Errorf("aescmac: key size %d is not the recommended size %d", keySize, recommendedCMACKeySizeInBytes)
}
if tagSize < minTagLengthInBytes {
return fmt.Errorf("aescmac: invalid tag length %d, want at least %d", tagSize, minTagLengthInBytes)
}
if tagSize > maxTagLengthInBytes {
return fmt.Errorf("aescmac: invalid tag length %d, want at most %d", tagSize, maxTagLengthInBytes)
}
return nil
}
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