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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 aessiv
import (
"fmt"
"github.com/tink-crypto/tink-go/v2/key"
)
// Variant is the prefix variant of AES-SIV keys.
//
// It describes how the prefix of the ciphertext is constructed. For AEAD there
// are three options:
//
// * TINK: prepends '0x01<big endian key id>' to the ciphertext.
// * CRUNCHY: prepends '0x00<big endian key id>' to the ciphertext.
// * NO_PREFIX: adds no prefix to the ciphertext.
type Variant int
const (
// VariantUnknown is the default and invalid value of Variant.
VariantUnknown Variant = iota
// VariantTink prefixes '0x01<big endian key id>' to the ciphertext.
VariantTink
// VariantCrunchy prefixes '0x00<big endian key id>' to the ciphertext.
VariantCrunchy
// VariantNoPrefix adds no prefix to the ciphertext.
VariantNoPrefix
)
func (variant Variant) String() string {
switch variant {
case VariantTink:
return "TINK"
case VariantCrunchy:
return "CRUNCHY"
case VariantNoPrefix:
return "NO_PREFIX"
default:
return "UNKNOWN"
}
}
// Parameters specifies an AES-SIV key.
type Parameters struct {
keySizeInBytes int
variant Variant
}
var _ key.Parameters = (*Parameters)(nil)
// KeySizeInBytes returns the size of the key in bytes.
func (p *Parameters) KeySizeInBytes() int { return p.keySizeInBytes }
// Variant returns the variant of the key.
func (p *Parameters) Variant() Variant { return p.variant }
func validateParams(params *Parameters) error {
// AES-SIV key sizes specified in
// https://www.rfc-editor.org/rfc/rfc5297#section-2.2.
if params.KeySizeInBytes() != 32 && params.KeySizeInBytes() != 48 && params.KeySizeInBytes() != 64 {
return fmt.Errorf("unsupported key size; want 32, 48, or 64, got: %v", params.KeySizeInBytes())
}
if params.Variant() == VariantUnknown {
return fmt.Errorf("unsupported variant: %v", params.Variant())
}
return nil
}
// NewParameters creates a new AES-SIV Parameters object.
func NewParameters(keySizeInBytes int, variant Variant) (*Parameters, error) {
p := &Parameters{
keySizeInBytes: keySizeInBytes,
variant: variant,
}
if err := validateParams(p); err != nil {
return nil, fmt.Errorf("aessiv.NewParameters: %v", err)
}
return p, nil
}
// HasIDRequirement returns whether the key has an ID requirement.
func (p *Parameters) HasIDRequirement() bool { return p.variant != VariantNoPrefix }
// 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.variant == actualParams.variant
}
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