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// Copyright 2025 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 aesgcmhkdf
import (
"fmt"
"github.com/tink-crypto/tink-go/v2/key"
)
// HashType is the hash function used in HKDF.
type HashType int
const (
// HashTypeUnknown is the default and invalid value of HashType.
HashTypeUnknown HashType = iota
// SHA1 specifies SHA-1.
SHA1
// SHA256 specifies SHA-256.
SHA256
// SHA512 specifies SHA-512.
SHA512
)
func (ht HashType) String() string {
switch ht {
case SHA1:
return "SHA1"
case SHA256:
return "SHA256"
case SHA512:
return "SHA512"
default:
return "UNKNOWN"
}
}
// Parameters specifies an AES-GCM-HKDF streaming key.
type Parameters struct {
keySizeInBytes int
derivedKeySizeInBytes int
hkdfHashType HashType
segmentSizeInBytes int32 // For compatibility with Tink Java.
}
var _ key.Parameters = (*Parameters)(nil)
// KeySizeInBytes returns the size of the key in bytes.
func (p *Parameters) KeySizeInBytes() int { return p.keySizeInBytes }
// DerivedKeySizeInBytes returns the size of the derived key in bytes.
func (p *Parameters) DerivedKeySizeInBytes() int { return p.derivedKeySizeInBytes }
// HKDFHashType returns the hash function for HKDF.
func (p *Parameters) HKDFHashType() HashType { return p.hkdfHashType }
// SegmentSizeInBytes returns the ciphertext segment size in bytes.
func (p *Parameters) SegmentSizeInBytes() int32 { return p.segmentSizeInBytes }
// ParametersOpts specifies options for creating AES-GCM-HKDF parameters.
type ParametersOpts struct {
KeySizeInBytes int
DerivedKeySizeInBytes int
HKDFHashType HashType
SegmentSizeInBytes int32
}
func validateOpts(opts *ParametersOpts) error {
if opts.DerivedKeySizeInBytes != 16 && opts.DerivedKeySizeInBytes != 32 {
return fmt.Errorf("unsupported derived key size; want 16 or 32, got: %v", opts.DerivedKeySizeInBytes)
}
if opts.KeySizeInBytes < opts.DerivedKeySizeInBytes {
return fmt.Errorf("key size must be at least %v, got: %v", opts.DerivedKeySizeInBytes, opts.KeySizeInBytes)
}
if opts.HKDFHashType == HashTypeUnknown {
return fmt.Errorf("unsupported HKDF hash type: %v", opts.HKDFHashType)
}
// See
// https://developers.google.com/tink/streaming-aead/aes_gcm_hkdf_streaming#key_and_parameters.
minSegmentSize := int32(opts.DerivedKeySizeInBytes + 24 + 1)
if opts.SegmentSizeInBytes < minSegmentSize {
return fmt.Errorf("ciphertext segment size must be at least %d, got %d", minSegmentSize, opts.SegmentSizeInBytes)
}
return nil
}
// NewParameters creates a new AES-GCM-HKDF [Parameters] object.
func NewParameters(opts ParametersOpts) (*Parameters, error) {
if err := validateOpts(&opts); err != nil {
return nil, fmt.Errorf("aesgcmhkdf.NewParameters: %v", err)
}
return &Parameters{
keySizeInBytes: opts.KeySizeInBytes,
derivedKeySizeInBytes: opts.DerivedKeySizeInBytes,
hkdfHashType: opts.HKDFHashType,
segmentSizeInBytes: opts.SegmentSizeInBytes,
}, nil
}
// HasIDRequirement returns false because AES-GCM-HKDF keys do not have an ID
// requirement.
func (p *Parameters) HasIDRequirement() bool { return false }
// Equal returns whether this Parameters object is equal to other.
func (p *Parameters) Equal(other key.Parameters) bool {
actualParams, ok := other.(*Parameters)
return ok &&
p.keySizeInBytes == actualParams.keySizeInBytes &&
p.derivedKeySizeInBytes == actualParams.derivedKeySizeInBytes &&
p.hkdfHashType == actualParams.hkdfHashType &&
p.segmentSizeInBytes == actualParams.segmentSizeInBytes
}
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