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package handshake
import (
"crypto"
"encoding/binary"
"golang.org/x/crypto/hkdf"
)
// hkdfExpandLabel HKDF expands a label as defined in RFC 8446, section 7.1.
func hkdfExpandLabel(hash crypto.Hash, secret, context []byte, label string, length int) []byte {
b := make([]byte, 3, 3+6+len(label)+1+len(context))
binary.BigEndian.PutUint16(b, uint16(length))
b[2] = uint8(6 + len(label))
b = append(b, []byte("tls13 ")...)
b = append(b, []byte(label)...)
b = b[:3+6+len(label)+1]
b[3+6+len(label)] = uint8(len(context))
b = append(b, context...)
out := make([]byte, length)
n, err := hkdf.Expand(hash.New, secret, b).Read(out)
if err != nil || n != length {
panic("quic: HKDF-Expand-Label invocation failed unexpectedly")
}
return out
}
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