Transit: Add public key format

[cipherboy]

Per @SchulzeStTSI on hashicorp/vault#25141:

Describe the bug When retrieving the public key of an ed25519 key pair from the transit API the key is formatted in a strange encoding. After checking it, i found out that the key is simply a base64 encoding of ed25519.PublicKey which is the internal go type representation.

To Reproduce Generate ed25519 keypair and use the private one to extract the public:

p := key.Public().(ed25519.PublicKey)
b := base64.StdEncoding.EncodeToString(p)

Output is an base64 string which is not a PEM or anything else "usable" by openssl. Something like this: DixHOIHXtDW0l6DHE2qGCRCfC/XjS5CHxHS+sEUICH0=

Expected behavior Convert the key into a PEM format/standard encoding that it matches with the other key pair types in format (they use all pub keys in PEM format) to have this format:

-----BEGIN PUBLIC KEY----- MCowBQYDK2VwAyEA8eIBpK/74foJBEzRn2kCImhWyVnOcqV5aX1L8DSHyxA= -----END PUBLIC KEY-----

This is then readable by openssl.

bytes, err := x509.MarshalPKIXPublicKey(key.PublicKey())

if err != nil {
return nil, err
}

pemBlock := &pem.Block{
Type:  "PUBLIC KEY",
Bytes: bytes,
}

pubkey_bytes := pem.EncodeToMemory(pemBlock)

---

Since changing the key format now will likely break compatibility, we should probably add a new format parameter to the transit/export/:type/:key API. This would allow callers to specify the format (defaulting perhaps to native).

However, if the OpenSSL application is modifiable, I believe the EVP_PKEY_new_raw_private_key and EVP_PKEY_new_raw_public_key API calls are useful for parsing Vault keys. Not sure if there's a comparable OpenSSL 3.0 API though.

[naphelps]

Migration guide: https://www.openssl.org/docs/man3.1/man7/migration_guide.html

The OpenSSL 1.1.1 apis went EOL back in November. We should probably work on migrating these to the current 3.2 version.

[cipherboy]

@naphelps Note that we do not use OpenSSL; this was advice I was giving to OP on how they could call a different function for Ed25519 keys (in their app, which they say uses OpenSSL) to support the format Go uses, described in https://pkg.go.dev/crypto/ed25519

The public key, as discussed in RFC 8032 is 32 bytes. It is not a PEM blob, like they requested.

It looks like these functions are still present in OpenSSL 3.1 though: https://www.openssl.org/docs/man3.1/man3/EVP_PKEY_get_raw_public_key.html so OP should be good even on a later version.

I'd still probably implement both formats for all keys for consistency.