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/*
* Copyright (c) 2015, Yawning Angel <yawning at torproject dot org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* * Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
package scramblesuit
import (
"crypto/hmac"
"hash"
)
func hkdfExpand(hashFn func() hash.Hash, prk []byte, info []byte, l int) []byte {
// Why, yes. golang.org/x/crypto/hkdf exists, and is a fine
// implementation of HKDF. However it does both the extract
// and expand, while ScrambleSuit only does extract, with no
// way to separate the two steps.
h := hmac.New(hashFn, prk)
digestSz := h.Size()
if l > 255*digestSz {
panic("hkdf: requested OKM length > 255*HashLen")
}
var t []byte
okm := make([]byte, 0, l)
toAppend := l
ctr := byte(1)
for toAppend > 0 {
h.Reset()
h.Write(t)
h.Write(info)
h.Write([]byte{ctr})
t = h.Sum(nil)
ctr++
aLen := digestSz
if toAppend < digestSz {
aLen = toAppend
}
okm = append(okm, t[:aLen]...)
toAppend -= aLen
}
return okm
}
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