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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 aescmac
import (
"fmt"
"github.com/tink-crypto/tink-go/v2/core/cryptofmt"
"github.com/tink-crypto/tink-go/v2/key"
)
// Variant is the prefix variant of AES-CMAC keys.
//
// It describes how the prefix of the tag is constructed. There are
// three options:
//
// - VariantTink: prepends '0x01<big endian key id>' to the tag.
// - VariantCrunchy: prepends '0x00<big endian key id>' to the tag.
// - VariantLegacy: appends a 0-byte to the input message before computing
// the tag, then prepends '0x00<big endian key id>' to the tag.
// - VariantNoPrefix: adds no prefix to the tag.
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 tag.
VariantTink
// VariantCrunchy prefixes '0x00<big endian key id>' to the tag.
VariantCrunchy
// VariantLegacy appends '0x00' to the input message BEFORE computing
// the tag, then prepends '0x00<big endian key id>' to the tag.
VariantLegacy
// VariantNoPrefix adds no prefix to the tag.
VariantNoPrefix
)
func (variant Variant) String() string {
switch variant {
case VariantTink:
return "TINK"
case VariantCrunchy:
return "CRUNCHY"
case VariantLegacy:
return "LEGACY"
case VariantNoPrefix:
return "NO_PREFIX"
default:
return "UNKNOWN"
}
}
// Parameters specifies an AES-CMAC key.
type Parameters struct {
keySizeInBytes int
tagSizeInBytes 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 }
// CryptographicTagSizeInBytes returns the size of the tag in bytes.
func (p *Parameters) CryptographicTagSizeInBytes() int { return p.tagSizeInBytes }
// TotalTagSizeInBytes returns the size of the tag in bytes plus the size of
// the prefix with which this key prefixes every cryptographic tag.
func (p *Parameters) TotalTagSizeInBytes() int {
if p.HasIDRequirement() {
return cryptofmt.TinkPrefixSize + p.CryptographicTagSizeInBytes()
}
return p.CryptographicTagSizeInBytes()
}
// Variant returns the variant of the key.
func (p *Parameters) Variant() Variant { return p.variant }
// 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.HasIDRequirement() == actualParams.HasIDRequirement() &&
p.keySizeInBytes == actualParams.keySizeInBytes &&
p.tagSizeInBytes == actualParams.tagSizeInBytes &&
p.variant == actualParams.variant
}
// ParametersOpts is the options for creating parameters.
type ParametersOpts struct {
KeySizeInBytes int
TagSizeInBytes int
Variant Variant
}
// NewParameters creates a new AES-CMAC parameters.
func NewParameters(opts ParametersOpts) (*Parameters, error) {
if opts.KeySizeInBytes != 16 && opts.KeySizeInBytes != 32 {
return nil, fmt.Errorf("key size must be between 16 and 32 bytes, got %d", opts.KeySizeInBytes)
}
if opts.TagSizeInBytes < 10 || opts.TagSizeInBytes > 16 {
return nil, fmt.Errorf("tag size must be between 10 and 16 bytes, got %d", opts.TagSizeInBytes)
}
if opts.Variant == VariantUnknown {
return nil, fmt.Errorf("variant must be specified")
}
return &Parameters{
keySizeInBytes: opts.KeySizeInBytes,
tagSizeInBytes: opts.TagSizeInBytes,
variant: opts.Variant,
}, nil
}
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