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// Copyright 2012 Aaron Jacobs. All Rights Reserved.
// Author: aaronjjacobs@gmail.com (Aaron Jacobs)
//
// 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 siv
import (
"log"
"github.com/jacobsa/crypto/common"
)
// The xorend operator of RFC 5297.
//
// Given strings A and B with len(A) >= len(B), let D be len(A) - len(B). Write
// A[:D] followed by xor(A[D:], B) into dst. In other words, xor B over the
// rightmost end of A and write the result into dst.
func xorend(dst, a, b []byte) {
aLen := len(a)
bLen := len(b)
dstLen := len(dst)
if dstLen < aLen || aLen < bLen {
log.Panicf("Bad buffer lengths: %d, %d, %d", dstLen, aLen, bLen)
}
// Copy the left part.
difference := aLen - bLen
copy(dst, a[:difference])
// XOR in the right part.
common.Xor(dst[difference:difference+bLen], a[difference:], b)
}
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