From 0b26ed731ca990a38e7afa05cdb12d9dc280e28a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Matev=C5=BE=20Jekovec?= <matevz@oasisprotocol.org>
Date: Fri, 30 Jun 2023 12:01:11 +0200
Subject: [PATCH] feat(wallet): Add file-based sr25519-adr8 support

---
 wallet/file/sr25519.go | 84 ++++++++++++++++++++++++++++++++++++++----
 1 file changed, 76 insertions(+), 8 deletions(-)

diff --git a/wallet/file/sr25519.go b/wallet/file/sr25519.go
index 4bb27b73..250dd005 100644
--- a/wallet/file/sr25519.go
+++ b/wallet/file/sr25519.go
@@ -1,14 +1,19 @@
 package file
 
 import (
+	"crypto/hmac"
+	"crypto/sha512"
+	"encoding/binary"
 	"fmt"
 
+	"github.com/oasisprotocol/oasis-core/go/common/crypto/signature/signers/memory"
+	"github.com/tyler-smith/go-bip39"
+
+	"github.com/oasisprotocol/curve25519-voi/primitives/sr25519"
 	"github.com/oasisprotocol/oasis-core/go/common/crypto/sakg"
-	"github.com/oasisprotocol/oasis-core/go/common/crypto/signature"
 	"github.com/oasisprotocol/oasis-core/go/common/crypto/slip10"
 	sdkSignature "github.com/oasisprotocol/oasis-sdk/client-sdk/go/crypto/signature"
-	"github.com/oasisprotocol/oasis-sdk/client-sdk/go/crypto/signature/sr25519"
-	"github.com/tyler-smith/go-bip39"
+	sdkSr25519 "github.com/oasisprotocol/oasis-sdk/client-sdk/go/crypto/signature/sr25519"
 )
 
 // Sr25519FromMnemonic derives a signer using ADR-8 from given mnemonic.
@@ -27,7 +32,7 @@ func Sr25519FromMnemonic(mnemonic string, number uint32) (sdkSignature.Signer, e
 
 	seed := bip39.NewSeed(mnemonic, "")
 
-	signer, chainCode, err := slip10.NewMasterKey(seed)
+	_, chainCode, skBinary, err := newMasterKey(seed)
 	if err != nil {
 		return nil, fmt.Errorf("sakg: error deriving master key: %w", err)
 	}
@@ -38,16 +43,79 @@ func Sr25519FromMnemonic(mnemonic string, number uint32) (sdkSignature.Signer, e
 		return nil, fmt.Errorf("sakg: error creating BIP-0032 path %s: %w", pathStr, err)
 	}
 
+	var signer sdkSignature.Signer
 	for _, index := range path {
-		signer, chainCode, err = slip10.NewChildKey(signer, chainCode, index)
+		signer, chainCode, _, err = newChildKey(skBinary, chainCode, index)
 		if err != nil {
 			return nil, fmt.Errorf("sakg: error deriving child key: %w", err)
 		}
 	}
 
-	sr25519signer, err := sr25519.NewSigner(signer.(signature.UnsafeSigner).UnsafeBytes())
+	return signer, nil
+}
+
+func newMasterKey(seed []byte) (sdkSignature.Signer, slip10.ChainCode, []byte, error) {
+	// Let S be a seed byte sequence of 128 to 512 bits in length.
+	if sLen := len(seed); sLen < slip10.SeedMinSize || sLen > slip10.SeedMaxSize {
+		return nil, slip10.ChainCode{}, nil, fmt.Errorf("slip10: invalid seed")
+	}
+
+	// 1. Calculate I = HMAC-SHA512(Key = Curve, Data = S)
+	mac := hmac.New(sha512.New, []byte("ed25519 seed"))
+	_, _ = mac.Write(seed)
+	I := mac.Sum(nil)
+
+	// 2. Split I into two 32-byte sequences, IL and IR.
+	// 3. Use parse256(IL) as master secret key, and IR as master chain code.
+	return splitDigest(I)
+}
+
+func newChildKey(kPar []byte, cPar slip10.ChainCode, index uint32) (sdkSignature.Signer, slip10.ChainCode, []byte, error) {
+	if len(kPar) < memory.SeedSize {
+		return nil, slip10.ChainCode{}, nil, fmt.Errorf("slip10: invalid parent key")
+	}
+
+	// 1. Check whether i >= 2^31 (whether the child is a hardened key).
+	if index < 1<<31 {
+		// If not (normal child):
+		// If curve is ed25519: return failure.
+		return nil, slip10.ChainCode{}, nil, fmt.Errorf("slip10: non-hardened keys not supported")
+	}
+
+	// If so (hardened child):
+	// let I = HMAC-SHA512(Key = cpar, Data = 0x00 || ser256(kpar) || ser32(i)).
+	// (Note: The 0x00 pads the private key to make it 33 bytes long.)
+	var b [4]byte
+	mac := hmac.New(sha512.New, cPar[:])
+	_, _ = mac.Write(b[0:1])                 // 0x00
+	_, _ = mac.Write(kPar[:memory.SeedSize]) // ser256(kPar)
+	binary.BigEndian.PutUint32(b[:], index)  // Note: The spec neglects to define ser32.
+	_, _ = mac.Write(b[:])                   // ser32(i)
+	I := mac.Sum(nil)
+
+	// 2. Split I into two 32-byte sequences, IL and IR.
+	// 3. The returned chain code ci is IR.
+	// 4. If curve is ed25519: The returned child key ki is parse256(IL).
+	return splitDigest(I)
+}
+
+func splitDigest(digest []byte) (sdkSignature.Signer, slip10.ChainCode, []byte, error) {
+	IL, IR := digest[:32], digest[32:]
+
+	var chainCode slip10.ChainCode
+
+	msk, err := sr25519.NewMiniSecretKeyFromBytes(IL)
+	if err != nil {
+		return nil, chainCode, nil, err
+	}
+	sk := msk.ExpandEd25519()
+	skBinary, err := sk.MarshalBinary()
 	if err != nil {
-		return nil, err
+		return nil, chainCode, nil, err
 	}
-	return sr25519signer, nil
+
+	signer := sdkSr25519.NewSignerFromKeyPair(sk.KeyPair())
+	copy(chainCode[:], IR)
+
+	return signer, chainCode, skBinary, nil
 }