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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 (
"errors"
"fmt"
"io"
"golang.org/x/crypto/hkdf"
)
const (
// Minimum tag size in bytes. This provides minimum 80-bit security strength.
minTagSizeInBytes = uint32(10)
)
var errHKDFInvalidInput = errors.New("HKDF: invalid input")
// validateHKDFParams validates parameters of HKDF constructor.
func validateHKDFParams(hash string, keySize uint32, tagSize uint32) error {
// validate tag size
digestSize, err := GetHashDigestSize(hash)
if err != nil {
return err
}
if tagSize > 255*digestSize {
return fmt.Errorf("tag size too big")
}
if tagSize < minTagSizeInBytes {
return fmt.Errorf("tag size too small")
}
return nil
}
// ComputeHKDF extracts a pseudorandom key.
func ComputeHKDF(hashAlg string, key []byte, salt []byte, info []byte, tagSize uint32) ([]byte, error) {
keySize := uint32(len(key))
if err := validateHKDFParams(hashAlg, keySize, tagSize); err != nil {
return nil, fmt.Errorf("hkdf: %s", err)
}
hashFunc := GetHashFunc(hashAlg)
if hashFunc == nil {
return nil, fmt.Errorf("hkdf: invalid hash algorithm")
}
if len(salt) == 0 {
salt = make([]byte, hashFunc().Size())
}
result := make([]byte, tagSize)
kdf := hkdf.New(hashFunc, key, salt, info)
n, err := io.ReadFull(kdf, result)
if n != len(result) || err != nil {
return nil, fmt.Errorf("compute of hkdf failed")
}
return result, nil
}
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