proposal for func (b *LockedBuffer) ByteArray56() *[56]byte

[david415]

I'd like to use memguard in combination with this X448 ECDH library: https://pkg.go.dev/github.com/cloudflare/circl@v1.0.1-0.20210104183656-96a0695de3c3/dh/x448#example-package-X448

which uses pointers to 56 byte arrays as it's key type in all it's function signatures. Thus the most elegant solution would be for memguard's LockedBuffer to return a pointer to a 56 byte array.

Without this proposed solution using memguard with x448 is very awkward. Here's a simple example:

package x448

import (
    "crypto/rand"
    "testing"

    "github.com/awnumar/memguard"
    "github.com/cloudflare/circl/dh/x448"
    "github.com/stretchr/testify/require"
)

func TestX448(t *testing.T) {
    alicePrivate, err := memguard.NewBufferFromReader(rand.Reader, x448.Size)
    require.NoError(t, err)
    var aliceSecret x448.Key
    copy(aliceSecret[:], alicePrivate.Bytes())
    var alicePublic x448.Key
    x448.KeyGen(&alicePublic, &aliceSecret)

    bobPrivate, err := memguard.NewBufferFromReader(rand.Reader, x448.Size)
    require.NoError(t, err)
    var bobSecret x448.Key
    copy(bobSecret[:], bobPrivate.Bytes())
    var bobPublic x448.Key
    x448.KeyGen(&bobPublic, &bobSecret)

    // Deriving Alice's shared key
    var aliceSharedSecret x448.Key
    ok := x448.Shared(&aliceSharedSecret, &aliceSecret, &bobPublic)
    require.True(t, ok)

    // Deriving Bob's shared key
    var bobSharedSecret x448.Key
    ok = x448.Shared(&bobSharedSecret, &bobSecret, &alicePublic)
    require.True(t, ok)

    // Shared secrets are equal, of course.
    require.Equal(t, bobSharedSecret, aliceSharedSecret)
}