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// Copyright 2024 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 outputprefix provides constants and shared utility functions for
// computing the prefix applied to the output of a cryptographic function.
package outputprefix
import (
"encoding/binary"
)
const (
// nonRawPrefixSize is the prefix size of Tink and Legacy key types.
nonRawPrefixSize = 5
// legacyStartByte is the first byte of the prefix of legacy key types.
legacyStartByte = byte(0)
// tinkStartByte is the first byte of the prefix of Tink key types.
tinkStartByte = byte(1)
)
// calculatePrefixBytes calculates the bytes prefixed to the output
// of a cryptographic function.
//
// The prefix of prefixSize bytes consists of a start byte and a 4-byte key id.
func calculatePrefixBytes(startByte byte, id uint32) []byte {
prefix := make([]byte, nonRawPrefixSize)
prefix[0] = startByte
binary.BigEndian.PutUint32(prefix[1:], id)
return prefix
}
// Tink returns the output prefix bytes from keyID for TINK keys.
func Tink(keyID uint32) []byte { return calculatePrefixBytes(tinkStartByte, keyID) }
// Legacy returns the output prefix bytes from keyID for LEGACY keys.
func Legacy(keyID uint32) []byte { return calculatePrefixBytes(legacyStartByte, keyID) }
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