Investigate Ed25519 Key Format Compatibility between Tendermint and libsodium

[torao]

Tendermint and libsodium generate different keys for 32-bytes fixed zeros seed.

tendermint: private key: 66687aadf862bd776c8fc18b8e9f8e20089714856ee233b3902a591d0d5f2925b1c4df1c17cce90a03cd4c057fc74d4e2ee24ddfe2a8c9c5fd8d0a45a1f082f3 (64 bytes)
libsodium : private key: 00000000000000000000000000000000000000000000000000000000000000003b6a27bcceb6a42d62a3a8d02a6f0d73653215771de243a63ac048a18b59da29 (64 bytes)
tendermint: public key: b1c4df1c17cce90a03cd4c057fc74d4e2ee24ddfe2a8c9c5fd8d0a45a1f082f3 (32 bytes)
libsodium : public key: 3b6a27bcceb6a42d62a3a8d02a6f0d73653215771de243a63ac048a18b59da29 (32 bytes)

Ed25519 is specified in RFC 8032, and its parameters are considered to be compatible. We need to convert them if we use the ED25519 key-pair of Tendermint. If it's not possible, another key must be used for the leader election.

[torao]

Tendermint and libsodium use the same Ed25519 key-pair format. Both are RFC 8032 compliant.

private key: [ seed (32-bytes) ][ public key (32-bytes) ]
public key : [ public key (32-bytes) ]

The function KeyPairFromSeed(seed) on libsodium uses the given value as a seed directly. On the other hand, ed25519.GenPrivKeyFromSecret(secret) of Tendermint uses the secret with SHA-256 as the seed.

Thus, by using SHA-256 applied secret as libsodium seed,

var secret [SEEDBYTES]byte
tsk := ed25519.GenPrivKeyFromSecret(secret[:])
tpk, _ := tsk.PubKey().(ed25519.PubKeyEd25519)

var seed [SEEDBYTES]byte
hashedSecret := sha256.Sum256(secret[:])
copy(seed[:], hashedSecret[:])
var spk, ssk = KeyPairFromSeed(seed)

I've seen that Tendermint and libsodium generate exactly identical key-pairs.

tendermint: private key: 66687aadf862bd776c8fc18b8e9f8e20089714856ee233b3902a591d0d5f2925b1c4df1c17cce90a03cd4c057fc74d4e2ee24ddfe2a8c9c5fd8d0a45a1f082f3 (64 bytes)
libsodium : private key: 66687aadf862bd776c8fc18b8e9f8e20089714856ee233b3902a591d0d5f2925b1c4df1c17cce90a03cd4c057fc74d4e2ee24ddfe2a8c9c5fd8d0a45a1f082f3 (64 bytes)
tendermint: public key : b1c4df1c17cce90a03cd4c057fc74d4e2ee24ddfe2a8c9c5fd8d0a45a1f082f3 (32 bytes)
libsodium : public key : b1c4df1c17cce90a03cd4c057fc74d4e2ee24ddfe2a8c9c5fd8d0a45a1f082f3 (32 bytes)

In conclusion, the Ed25519 key-pair of Tendermint and libsodium is binary compatible.

[torao]

Since the VRF API added in #1 is the same specification as libsodium, we need to change the private and public keys on them to ed25519.PrivKeyEd25519 and ed25519.PubKeyEd25519, respectively.