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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/hmac"
"fmt"
"hash"
"github.com/tink-crypto/tink-go/v2/subtle"
)
const (
minHMACKeySizeInBytes = uint32(16)
)
// HMACPRF is a type that can be used to compute several HMACs with the same key material.
type HMACPRF struct {
h func() hash.Hash
key []byte
}
// NewHMACPRF creates a new HMACPRF object and initializes it with the correct key material.
func NewHMACPRF(hashAlg string, key []byte) (*HMACPRF, error) {
h := &HMACPRF{}
hashFunc := subtle.GetHashFunc(hashAlg)
if hashFunc == nil {
return nil, fmt.Errorf("hmac: invalid hash algorithm")
}
h.h = hashFunc
h.key = key
return h, nil
}
// ValidateHMACPRFParams validates parameters of HMAC constructor.
func ValidateHMACPRFParams(hash string, keySize uint32) error {
// validate key size
if keySize < minHMACKeySizeInBytes {
return fmt.Errorf("key too short")
}
if subtle.GetHashFunc(hash) == nil {
return fmt.Errorf("invalid hash function")
}
return nil
}
// ComputePRF computes the HMAC for the given key and data, returning outputLength bytes.
func (h HMACPRF) ComputePRF(data []byte, outputLength uint32) ([]byte, error) {
mac := hmac.New(h.h, h.key)
if outputLength > uint32(mac.Size()) {
return nil, fmt.Errorf("outputLength must be between 0 and %d", mac.Size())
}
mac.Write(data)
return mac.Sum(nil)[:outputLength], nil
}
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