forked from katzenpost/core
/
hkdf_expand.go
59 lines (52 loc) · 1.55 KB
/
hkdf_expand.go
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// hkdf_expand.go - HKDF-Expand.
// Copyright (C) 2015 Yawning Angel.
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as
// published by the Free Software Foundation, either version 3 of the
// License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.
package crypto
import (
"crypto/hmac"
"hash"
"github.com/katzenpost/core/utils"
)
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, with no way to separate the two steps.
h := hmac.New(hashFn, prk)
defer h.Reset()
digestSz := h.Size()
if l > 255*digestSz {
panic("hkdf: requested OKM length > 255*HashLen")
}
var t []byte
defer utils.ExplicitBzero(t)
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
}