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/*
HKDF is a simple key derivation function (KDF) based on
a hash-based message authentication code (HMAC). It was initially proposed by its authors as a building block in
various protocols and applications, as well as to discourage the proliferation of multiple KDF mechanisms.
The main approach HKDF follows is the "extract-then-expand" paradigm, where the KDF logically consists of two modules:
the first stage takes the input keying material and "extracts" from it a fixed-length pseudorandom key, and then the
second stage "expands" this key into several additional pseudorandom keys (the output of the KDF).
*/
package hkdf
import (
"crypto/hmac"
"crypto/sha256"
"fmt"
"golang.org/x/crypto/hkdf"
"io"
)
/*
Expand expands a given key with the HKDF algorithm.
*/
func Expand(key []byte, length int, info string) ([]byte, error) {
if info == "" {
keyBlock := hmac.New(sha256.New, key)
var out, last []byte
var blockIndex byte = 1
for i := 0; len(out) < length; i++ {
keyBlock.Reset()
//keyBlock.Write(append(append(last, []byte(info)...), blockIndex))
keyBlock.Write(last)
keyBlock.Write([]byte(info))
keyBlock.Write([]byte{blockIndex})
last = keyBlock.Sum(nil)
blockIndex += 1
out = append(out, last...)
}
return out[:length], nil
} else {
h := hkdf.New(sha256.New, key, nil, []byte(info))
out := make([]byte, length)
n, err := io.ReadAtLeast(h, out, length)
if err != nil {
return nil, err
}
if n != length {
return nil, fmt.Errorf("new key to short")
}
return out[:length], nil
}
}
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