66 lines
1.9 KiB
Go
66 lines
1.9 KiB
Go
// Copyright 2012 Aaron Jacobs. All Rights Reserved.
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// Author: aaronjjacobs@gmail.com (Aaron Jacobs)
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package cmac
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import (
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"bytes"
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"crypto/cipher"
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"../common"
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)
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var subkeyZero []byte
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var subkeyRb []byte
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func init() {
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subkeyZero = bytes.Repeat([]byte{0x00}, blockSize)
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subkeyRb = append(bytes.Repeat([]byte{0x00}, blockSize-1), 0x87)
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}
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// Given the supplied cipher, whose block size must be 16 bytes, return two
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// subkeys that can be used in MAC generation. See section 5.3 of NIST SP
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// 800-38B. Note that the other NIST-approved block size of 8 bytes is not
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// supported by this function.
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func generateSubkeys(ciph cipher.Block) (k1 []byte, k2 []byte) {
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if ciph.BlockSize() != blockSize {
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panic("generateSubkeys requires a cipher with a block size of 16 bytes.")
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}
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// Step 1
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l := make([]byte, blockSize)
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ciph.Encrypt(l, subkeyZero)
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// Step 2: Derive the first subkey.
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if common.Msb(l) == 0 {
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// TODO(jacobsa): Accept a destination buffer in ShiftLeft and then hoist
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// the allocation in the else branch below.
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k1 = common.ShiftLeft(l)
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} else {
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k1 = make([]byte, blockSize)
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common.Xor(k1, common.ShiftLeft(l), subkeyRb)
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}
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// Step 3: Derive the second subkey.
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if common.Msb(k1) == 0 {
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k2 = common.ShiftLeft(k1)
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} else {
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k2 = make([]byte, blockSize)
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common.Xor(k2, common.ShiftLeft(k1), subkeyRb)
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}
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return
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}
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