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// Copyright 2022 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 streamingprf
import (
"fmt"
"hash"
"io"
"golang.org/x/crypto/hkdf"
"github.com/tink-crypto/tink-go/v2/subtle"
)
// minHKDFStreamingPRFKeySize is the minimum allowed key size in bytes.
const minHKDFStreamingPRFKeySize = 32
// hkdfStreamingPRF is a HKDF Streaming PRF that implements StreamingPRF.
type hkdfStreamingPRF struct {
h func() hash.Hash
key []byte
salt []byte
}
// Asserts that hkdfStreamingPRF implements the StreamingPRF interface.
var _ StreamingPRF = (*hkdfStreamingPRF)(nil)
// newHKDFStreamingPRF constructs a new hkdfStreamingPRF using hashName, key,
// and salt. Salt can be nil.
func newHKDFStreamingPRF(hashName string, key, salt []byte) (*hkdfStreamingPRF, error) {
if err := validateHKDFStreamingPRFParams(hashName, len(key)); err != nil {
return nil, err
}
return &hkdfStreamingPRF{
h: subtle.GetHashFunc(hashName),
key: key,
salt: salt,
}, nil
}
// Compute computes and returns the HKDF as a Reader.
func (h *hkdfStreamingPRF) Compute(data []byte) (io.Reader, error) {
return hkdf.New(h.h, h.key, h.salt, data), nil
}
func validateHKDFStreamingPRFParams(hash string, keySize int) error {
if hash != "SHA256" && hash != "SHA512" {
return fmt.Errorf("only SHA-256, SHA-512 allowed for HKDF")
}
if keySize < minHKDFStreamingPRFKeySize {
return fmt.Errorf("key too short, require %d-bytes: %d", minHKDFStreamingPRFKeySize, keySize)
}
return nil
}
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