1// Copyright 2020 Google LLC 2// 3// Licensed under the Apache License, Version 2.0 (the "License"); 4// you may not use this file except in compliance with the License. 5// You may obtain a copy of the License at 6// 7// http://www.apache.org/licenses/LICENSE-2.0 8// 9// Unless required by applicable law or agreed to in writing, software 10// distributed under the License is distributed on an "AS IS" BASIS, 11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12// See the License for the specific language governing permissions and 13// limitations under the License. 14// 15//////////////////////////////////////////////////////////////////////////////// 16 17package subtle 18 19import ( 20 "crypto/subtle" 21 "fmt" 22 23 subtleprf "github.com/google/tink/go/prf/subtle" 24 25 // Placeholder for internal crypto/subtle allowlist, please ignore. 26) 27 28const ( 29 minCMACKeySizeInBytes = 16 30 recommendedCMACKeySizeInBytes = uint32(32) 31 minTagLengthInBytes = uint32(10) 32 maxTagLengthInBytes = uint32(16) 33) 34 35// AESCMAC represents an AES-CMAC struct that implements the MAC interface. 36type AESCMAC struct { 37 prf *subtleprf.AESCMACPRF 38 tagLength uint32 39} 40 41// NewAESCMAC creates a new AESCMAC object that implements the MAC interface. 42func NewAESCMAC(key []byte, tagLength uint32) (*AESCMAC, error) { 43 if len(key) < minCMACKeySizeInBytes { 44 return nil, fmt.Errorf("Only 256 but keys are allowed with AES-CMAC") 45 } 46 if tagLength < minTagLengthInBytes { 47 return nil, fmt.Errorf("Tag length %d is shorter than minimum tag length %d", tagLength, minTagLengthInBytes) 48 } 49 if tagLength > maxTagLengthInBytes { 50 return nil, fmt.Errorf("Tag length %d is longer than maximum tag length %d", tagLength, minTagLengthInBytes) 51 } 52 ac := &AESCMAC{} 53 var err error 54 ac.prf, err = subtleprf.NewAESCMACPRF(key) 55 if err != nil { 56 return nil, fmt.Errorf("Could not create AES-CMAC prf: %v", err) 57 } 58 ac.tagLength = tagLength 59 return ac, nil 60} 61 62// ComputeMAC computes message authentication code (MAC) for code data. 63func (a AESCMAC) ComputeMAC(data []byte) ([]byte, error) { 64 return a.prf.ComputePRF(data, a.tagLength) 65} 66 67// VerifyMAC returns nil if mac is a correct authentication code (MAC) for data, 68// otherwise it returns an error. 69func (a AESCMAC) VerifyMAC(mac, data []byte) error { 70 computed, err := a.prf.ComputePRF(data, a.tagLength) 71 if err != nil { 72 return fmt.Errorf("Could not compute MAC: %v", err) 73 } 74 if subtle.ConstantTimeCompare(mac, computed) != 1 { 75 return fmt.Errorf("CMAC: Invalid MAC") 76 } 77 return nil 78} 79 80// ValidateCMACParams validates the parameters for an AES-CMAC against the recommended parameters. 81func ValidateCMACParams(keySize, tagSize uint32) error { 82 if keySize != recommendedCMACKeySizeInBytes { 83 return fmt.Errorf("Only %d sized keys are allowed with Tink's AES-CMAC", recommendedCMACKeySizeInBytes) 84 } 85 if tagSize < minTagLengthInBytes { 86 return fmt.Errorf("Tag size too short") 87 } 88 if tagSize > maxTagLengthInBytes { 89 return fmt.Errorf("Tag size too long") 90 } 91 return nil 92} 93