CreateSigningKeyImportBlob for ECC keys

[Lupus]

Is there any particular reason why CreateSigningKeyImportBlob works only with RSA keys? createPublicPrivateSign allows the key to be only RSA. Probably support for exporting ECC parameters can be added there?

[josephlr]

No particular reason. We would ideally support ECC, but it would require adding code in createPublicPrivateSign and I just didn't get around to it in #70

I don't think it would be too hard to implement.

[Lupus]

I've tried to do something along the lines of RSA example in the code, but it does not work unfortunately...

func createPublicPrivateSign(signingKey crypto.PrivateKey) (tpm2.Public, tpm2.Private, error) {
	switch key := signingKey.(type) {
	case *rsa.PrivateKey:
		rsaPub := key.PublicKey
		public := tpm2.Public{
			Type:       tpm2.AlgRSA,
			NameAlg:    client.DefaultEKTemplateRSA().NameAlg,
			Attributes: tpm2.FlagSign,
			RSAParameters: &tpm2.RSAParams{
				KeyBits:     uint16(rsaPub.N.BitLen()),
				ExponentRaw: uint32(rsaPub.E),
				ModulusRaw:  rsaPub.N.Bytes(),
				Sign: &tpm2.SigScheme{
					Alg:  tpm2.AlgRSASSA,
					Hash: tpm2.AlgSHA256,
				},
			},
		}
		private := tpm2.Private{
			Type:      tpm2.AlgRSA,
			AuthValue: nil,
			SeedValue: nil, // Only Storage Keys need a seed value. See part 3 TPM2_CREATE b.3.
			Sensitive: key.Primes[0].Bytes(),
		}

		return public, private, nil
	case *ecdsa.PrivateKey:
		eccPub := key.PublicKey
		public := tpm2.Public{
			Type:       tpm2.AlgECC,
			NameAlg:    client.DefaultEKTemplateECC().NameAlg,
			Attributes: tpm2.FlagSign,
			ECCParameters: &tpm2.ECCParams{
				CurveID: tpm2.CurveNISTP256,
				Point: tpm2.ECPoint{
					XRaw: make([]byte, 32),
					YRaw: make([]byte, 32),
				},
			},
		}
		private := tpm2.Private{
			Type:      tpm2.AlgECC,
			AuthValue: nil,
			SeedValue: nil,
			Sensitive: bytes.Join([][]byte{eccIntToBytes(eccPub, eccPub.X), eccIntToBytes(eccPub, eccPub.Y)}, []byte{}), // what to write here?
		}

		return public, private, nil
	default:
		return tpm2.Public{}, tpm2.Private{}, fmt.Errorf("unsupported signing key type: %T", signingKey)
	}
}

It fails with Error: import failed: handle 0, error code 0x7 : key size is not supported. Any pointers are greatly appreciated.

[Lupus]

Okay, this version seems to be working. Can you take a look plese if it makes sense?

func createPublicPrivateSign(signingKey crypto.PrivateKey) (tpm2.Public, tpm2.Private, error) {
	switch key := signingKey.(type) {
	case *rsa.PrivateKey:
		rsaPub := key.PublicKey
		public := tpm2.Public{
			Type:       tpm2.AlgRSA,
			NameAlg:    client.DefaultEKTemplateRSA().NameAlg,
			Attributes: tpm2.FlagSign,
			RSAParameters: &tpm2.RSAParams{
				KeyBits:     uint16(rsaPub.N.BitLen()),
				ExponentRaw: uint32(rsaPub.E),
				ModulusRaw:  rsaPub.N.Bytes(),
				Sign: &tpm2.SigScheme{
					Alg:  tpm2.AlgRSASSA,
					Hash: tpm2.AlgSHA256,
				},
			},
		}
		private := tpm2.Private{
			Type:      tpm2.AlgRSA,
			AuthValue: nil,
			SeedValue: nil, // Only Storage Keys need a seed value. See part 3 TPM2_CREATE b.3.
			Sensitive: key.Primes[0].Bytes(),
		}

		return public, private, nil
	case *ecdsa.PrivateKey:
		eccPub := key.PublicKey
		public := tpm2.Public{
			Type:       tpm2.AlgECC,
			NameAlg:    client.DefaultEKTemplateECC().NameAlg,
			Attributes: tpm2.FlagSign,
			ECCParameters: &tpm2.ECCParams{
				CurveID: fromGoCurve[eccPub.Curve],
				Point: tpm2.ECPoint{
					XRaw: eccIntToBytes(eccPub, eccPub.X),
					YRaw: eccIntToBytes(eccPub, eccPub.Y),
				},
				Sign: &tpm2.SigScheme{
					Alg:  tpm2.AlgECDSA,
					Hash: tpm2.AlgSHA256,
				},
			},
		}
		private := tpm2.Private{
			Type:      tpm2.AlgECC,
			AuthValue: nil,
			SeedValue: nil,
			Sensitive: key.D.Bytes(),
		}

		return public, private, nil
	default:
		return tpm2.Public{}, tpm2.Private{}, fmt.Errorf("unsupported signing key type: %T", signingKey)
	}
}

[josephlr]

@Lupus it seems roughly reasonable. I would need to double check the spec, but if it's working it's either exactly or almost exactly correct.

Care to open a PR so we can review, test, and add this functionality?

[Lupus]

PR would require a CLA, I'm figuring out the details with my employer's legal department, and it can take ages 😞 So if you have some time to look at it and contribute on your own, that would likely be much faster.

[josephlr]

Ahhhh, that makes sense. I can see if someone on our team has time to get a PR in soonish.