Fix PEM decoding issue for some KMS keys

[Kay-Zee]

Closes: N/A

Description

Was running into a PEM decode issue when working with some KMS keys. Kept getting cryptokms: failed to parse PEM public key: input has incorrect ECDSA_P256 key size.

The cause of this is sometimes, when calling .Bytes() on the X and Y points after decoding the PEM key, the corresponding byte slices were not the full 32 bytes, which was expected by the next function (expecting ECDSA_P256 keys to have a raw key of length 64)

This adds left padding (since the bytes are returned big endian) up to the expected length for ECDSA_P256 only. Can add other known lengths as we start to use them more.

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[turbolent]

@tarakby Can you please have a look here?

[tarakby]

I think there a few changes needed, some of them are related to the original function that existed before and I didn't notice.
Happy to help make the changes.

[tarakby]

I haven't noticed this before, we should test the type assertion here as other types of public keys may be returned.

goPublicKey, ok := publicKey.(*ecdsa.PublicKey)
if !ok {
   return PublicKey{}, fmt.Error("only ECDSA public keys are supported")
}

[tarakby]

Suggested change

	// DecodePublicKeyHex decodes a PEM public key with the given signature algorithm.
	// DecodePublicKeyHex decodes a PEM ECDSA public key with the given curve.

[tarakby]

I think we shouldn't have trailing data here.

Suggested change

	block, _ := pem.Decode([]byte(s))
	block, rest := pem.Decode([]byte(s))
	if len(rest)>0 {
	return PublicKey{}, fmt.Errorf("crypto: failed to parse PEM string: %w", err)
	}

[tarakby]

using ParsePKIXPublicKey only works in the case of curve P256. We will need to write a local ParsePKIXPublicKey function that supports secp256k1. I can help writing it.

[tarakby]

It's better to get that length this way:

Suggested change

	expectedLength := expectedPointByteLength[sigAlgo]
	expectedLength := bitsToBytes(goPublicKey.Params().P.BitLen())

With bitsToBytes the function converting bit size into byte size:

func bitsToBytes(bits int) int {
	return (bits + 7) >> 3
}

[tarakby]

We can remove this check and pad the expected length in all cases.

[tarakby]

Eventually, we won't need the map.