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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 keyderivation
import (
"errors"
"fmt"
"github.com/tink-crypto/tink-go/v2/internal/internalregistry"
"github.com/tink-crypto/tink-go/v2/keyderivation/internal/streamingprf"
"github.com/tink-crypto/tink-go/v2/keyset"
tinkpb "github.com/tink-crypto/tink-go/v2/proto/tink_go_proto"
)
const hkdfPRFTypeURL = "type.googleapis.com/google.crypto.tink.HkdfPrfKey"
// prfBasedDeriver uses prf and the Tink registry to derive a keyset handle as
// described by derivedKeyTemplate.
type prfBasedDeriver struct {
prf streamingprf.StreamingPRF
derivedKeyTemplate *tinkpb.KeyTemplate
}
// Asserts that prfBasedDeriver implements the KeysetDeriver interface.
var _ KeysetDeriver = (*prfBasedDeriver)(nil)
func newPRFBasedDeriver(prfKeyData *tinkpb.KeyData, derivedKeyTemplate *tinkpb.KeyTemplate) (*prfBasedDeriver, error) {
// Obtain Streaming PRF from PRF key data.
if prfKeyData == nil {
return nil, errors.New("PRF key data is nil")
}
if prfKeyData.GetTypeUrl() != hkdfPRFTypeURL {
return nil, fmt.Errorf("PRF key data with type URL %q is not supported", prfKeyData.GetTypeUrl())
}
// For HKDF PRF keys, create a local instance of the HKDF Streaming PRF key
// manager and obtain the Streaming PRF interface through it, instead of
// obtaining it through the registry. This allows us to keep the HKDF
// Streaming PRF key manager out of the registry for smoother deprecation.
//
// TODO(b/260619626): Remove this once PRF and Streaming PRF share the same
// type URL and registry.Primitive() can return multiple interfaces per
// primitive.
hkdfStreamingPRFKeyManager := streamingprf.HKDFStreamingPRFKeyManager{}
p, err := hkdfStreamingPRFKeyManager.Primitive(prfKeyData.GetValue())
if err != nil {
return nil, fmt.Errorf("failed to retrieve StreamingPRF primitive from key manager: %v", err)
}
prf, ok := p.(streamingprf.StreamingPRF)
if !ok {
return nil, errors.New("primitive is not StreamingPRF")
}
// Validate derived key template.
if !internalregistry.CanDeriveKeys(derivedKeyTemplate.GetTypeUrl()) {
return nil, errors.New("derived key template is not a derivable key type")
}
return &prfBasedDeriver{
prf: prf,
derivedKeyTemplate: derivedKeyTemplate,
}, nil
}
func (p *prfBasedDeriver) DeriveKeyset(salt []byte) (*keyset.Handle, error) {
randomness, err := p.prf.Compute(salt)
if err != nil {
return nil, fmt.Errorf("compute randomness from PRF failed: %v", err)
}
keyData, err := internalregistry.DeriveKey(p.derivedKeyTemplate, randomness)
if err != nil {
return nil, fmt.Errorf("derive key failed: %v", err)
}
// Fill in placeholder values for key ID, status, and output prefix type.
// The placeholder values are s.t. the keyset is valid and can be used to
// create a keyset handle.
// These will be populated with the correct values in the keyset deriver
// factory. This is acceptable because the keyset as-is will never leave
// Tink, and the user only interacts via the keyset deriver factory.
var primaryKeyID uint32 = 0
return keysetHandle(&tinkpb.Keyset{
PrimaryKeyId: primaryKeyID,
Key: []*tinkpb.Keyset_Key{
&tinkpb.Keyset_Key{
KeyData: keyData,
Status: tinkpb.KeyStatusType_ENABLED,
KeyId: primaryKeyID,
OutputPrefixType: tinkpb.OutputPrefixType_RAW,
},
},
})
}
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