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Truncate digests to the left most bits when signing with ECDSA #298

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merged 1 commit into from
Nov 15, 2022
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Truncate digests to the left most bits when signing with ECDSA #298

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20 changes: 17 additions & 3 deletions attest/wrapped_tpm20.go
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,7 @@ import (
"bytes"
"crypto"
"crypto/ecdsa"
"crypto/elliptic"
"crypto/rsa"
"encoding/asn1"
"errors"
Expand Down Expand Up @@ -500,16 +501,29 @@ func (k *wrappedKey20) sign(tb tpmBase, digest []byte, pub crypto.PublicKey, opt
if !ok {
return nil, fmt.Errorf("expected *wrappedTPM20, got %T", tb)
}
switch pub.(type) {
switch p := pub.(type) {
case *ecdsa.PublicKey:
return signECDSA(t.rwc, k.hnd, digest)
return signECDSA(t.rwc, k.hnd, digest, p.Curve)
case *rsa.PublicKey:
return signRSA(t.rwc, k.hnd, digest, opts)
}
return nil, fmt.Errorf("unsupported signing key type: %T", pub)
}

func signECDSA(rw io.ReadWriter, key tpmutil.Handle, digest []byte) ([]byte, error) {
func signECDSA(rw io.ReadWriter, key tpmutil.Handle, digest []byte, curve elliptic.Curve) ([]byte, error) {
// https://cs.opensource.google/go/go/+/refs/tags/go1.19.2:src/crypto/ecdsa/ecdsa.go;l=181
orderBits := curve.Params().N.BitLen()
orderBytes := (orderBits + 7) / 8
if len(digest) > orderBytes {
digest = digest[:orderBytes]
}
ret := new(big.Int).SetBytes(digest)
excess := len(digest)*8 - orderBits
if excess > 0 {
ret.Rsh(ret, uint(excess))
}
digest = ret.Bytes()

sig, err := tpm2.Sign(rw, key, "", digest, nil, nil)
if err != nil {
return nil, fmt.Errorf("cannot sign: %v", err)
Expand Down