Implement pure Go fallback for P8K signer

- Enhanced the Signer struct to include a FallbackSigner for scenarios where the libsecp256k1 implementation is unavailable.
- Updated the New function to initialize the fallback signer when the context creation fails.
- Refactored methods in the Signer to delegate operations to the FallbackSigner when applicable, ensuring compatibility with pure Go implementations.
- Added comprehensive method implementations for the FallbackSigner, including key generation, signing, and verification.
- Improved error handling and memory management in the new implementation.

pkg/interfaces/signer/p8k/p8k.go

@@ -5,43 +5,71 @@ import (
 	"crypto/rand"
 
 	"lol.mleku.dev/errorf"
+	"next.orly.dev/pkg/crypto/ec/schnorr"
+	"next.orly.dev/pkg/crypto/ec/secp256k1"
 	secp "next.orly.dev/pkg/crypto/p8k"
 	"next.orly.dev/pkg/interfaces/signer"
 )
 
-// Signer implements the signer.I interface using p8k.mleku.dev
+// Signer implements the signer.I interface using p8k.mleku.dev or pure Go fallback
 type Signer struct {
+	// libsecp256k1 implementation
 	ctx     *secp.Context
 	secKey  []byte
 	pubKey  []byte
 	keypair secp.Keypair
+
+	// Pure Go fallback implementation
+	fallback *FallbackSigner
+}
+
+// FallbackSigner implements the signer.I interface using pure Go btcec/secp256k1
+type FallbackSigner struct {
+	privKey  *secp256k1.SecretKey
+	pubKey   *secp256k1.PublicKey
+	xonlyPub []byte
 }
 
 // Ensure Signer implements signer.I
 var _ signer.I = (*Signer)(nil)
 
-// New creates a new P8K signer
+// New creates a new P8K signer, falling back to pure Go implementation if libsecp256k1 is unavailable
 func New() (s *Signer, err error) {
 	var ctx *secp.Context
 	if ctx, err = secp.NewContext(secp.ContextSign | secp.ContextVerify); err != nil {
-		return
+		// Fallback to pure Go implementation
+		fallback, fallbackErr := newFallbackSigner()
+		if fallbackErr != nil {
+			return nil, fallbackErr
+		}
+		s = &Signer{fallback: fallback}
+		return s, nil
 	}
 	s = &Signer{ctx: ctx}
-	return
+	return s, nil
 }
 
 // MustNew creates a new P8K signer and panics on error
-func MustNew() (s *Signer) {
-	var err error
-	if s, err = New(); err != nil {
+func MustNew() *Signer {
+	s, err := New()
+	if err != nil {
 		panic(err)
 	}
-	return
+	return s
+}
+
+// newFallbackSigner creates a new fallback signer using pure Go implementation
+func newFallbackSigner() (*FallbackSigner, error) {
+	return &FallbackSigner{}, nil
 }
 
 // Generate creates a fresh new key pair from system entropy, and ensures it is even (so
 // ECDH works).
 func (s *Signer) Generate() (err error) {
+	if s.fallback != nil {
+		return s.fallback.Generate()
+	}
+
 	s.secKey = make([]byte, 32)
 	if _, err = rand.Read(s.secKey); err != nil {
 		return
@@ -70,6 +98,10 @@ func (s *Signer) Generate() (err error) {
 // InitSec initialises the secret (signing) key from the raw bytes, and also
 // derives the public key because it can.
 func (s *Signer) InitSec(sec []byte) (err error) {
+	if s.fallback != nil {
+		return s.fallback.InitSec(sec)
+	}
+
 	if len(sec) != 32 {
 		return errorf.E("secret key must be 32 bytes")
 	}
@@ -100,6 +132,10 @@ func (s *Signer) InitSec(sec []byte) (err error) {
 // InitPub initializes the public (verification) key from raw bytes, this is
 // expected to be an x-only 32 byte pubkey.
 func (s *Signer) InitPub(pub []byte) (err error) {
+	if s.fallback != nil {
+		return s.fallback.InitPub(pub)
+	}
+
 	if len(pub) != 32 {
 		return errorf.E("public key must be 32 bytes")
 	}
@@ -111,17 +147,31 @@ func (s *Signer) InitPub(pub []byte) (err error) {
 
 // Sec returns the secret key bytes.
 func (s *Signer) Sec() []byte {
+	if s.fallback != nil {
+		return s.fallback.Sec()
+	}
 	return s.secKey
 }
 
 // Pub returns the public key bytes (x-only schnorr pubkey).
 func (s *Signer) Pub() []byte {
+	if s.fallback != nil {
+		return s.fallback.Pub()
+	}
 	return s.pubKey
 }
 
 // PubCompressed returns the compressed public key (33 bytes with 0x02/0x03 prefix).
 // This is needed for ECDH operations like NIP-44.
 func (s *Signer) PubCompressed() (compressed []byte, err error) {
+	if s.fallback != nil {
+		// For fallback, we need to derive the compressed key from the x-only key
+		if s.fallback.pubKey == nil {
+			return nil, errorf.E("public key not initialized")
+		}
+		return s.fallback.pubKey.SerializeCompressed(), nil
+	}
+
 	if len(s.keypair) == 0 {
 		return nil, errorf.E("keypair not initialized")
 	}
@@ -142,6 +192,10 @@ func (s *Signer) PubCompressed() (compressed []byte, err error) {
 
 // Sign creates a signature using the stored secret key.
 func (s *Signer) Sign(msg []byte) (sig []byte, err error) {
+	if s.fallback != nil {
+		return s.fallback.Sign(msg)
+	}
+
 	if len(s.keypair) == 0 {
 		return nil, errorf.E("keypair not initialized")
 	}
@@ -162,6 +216,10 @@ func (s *Signer) Sign(msg []byte) (sig []byte, err error) {
 
 // Verify checks a message hash and signature match the stored public key.
 func (s *Signer) Verify(msg, sig []byte) (valid bool, err error) {
+	if s.fallback != nil {
+		return s.fallback.Verify(msg, sig)
+	}
+
 	if s.pubKey == nil {
 		return false, errorf.E("public key not initialized")
 	}
@@ -175,6 +233,11 @@ func (s *Signer) Verify(msg, sig []byte) (valid bool, err error) {
 
 // Zero wipes the secret key to prevent memory leaks.
 func (s *Signer) Zero() {
+	if s.fallback != nil {
+		s.fallback.Zero()
+		return
+	}
+
 	if s.secKey != nil {
 		for i := range s.secKey {
 			s.secKey[i] = 0
@@ -199,6 +262,10 @@ func (s *Signer) ECDH(pub []byte) (secret []byte, err error) {
 // - 32 bytes (x-only): will be converted to compressed format by trying 0x02 then 0x03
 // - 33 bytes (compressed): will be used as-is
 func (s *Signer) ECDHRaw(pub []byte) (sharedX []byte, err error) {
+	if s.fallback != nil {
+		return s.fallback.ECDHRaw(pub)
+	}
+
 	if s.secKey == nil {
 		return nil, errorf.E("secret key not initialized")
 	}
@@ -238,3 +305,173 @@ func (s *Signer) ECDHRaw(pub []byte) (sharedX []byte, err error) {
 
 	return
 }
+
+// FallbackSigner method implementations
+
+// Generate creates a fresh new key pair from system entropy
+func (s *FallbackSigner) Generate() (err error) {
+	// Generate a new private key
+	if s.privKey, err = secp256k1.GenerateSecretKey(); err != nil {
+		return errorf.E("failed to generate private key: %w", err)
+	}
+
+	// Derive public key
+	if s.pubKey = s.privKey.PubKey(); s.pubKey == nil {
+		return errorf.E("failed to derive public key")
+	}
+
+	// Get x-only public key (32 bytes) - compressed without the 0x02/0x03 prefix
+	compressed := s.pubKey.SerializeCompressed()
+	s.xonlyPub = make([]byte, 32)
+	copy(s.xonlyPub, compressed[1:])
+
+	return nil
+}
+
+// InitSec initializes the secret key from raw bytes
+func (s *FallbackSigner) InitSec(sec []byte) (err error) {
+	if len(sec) != 32 {
+		return errorf.E("secret key must be 32 bytes")
+	}
+
+	// Create private key from bytes
+	s.privKey = secp256k1.SecKeyFromBytes(sec)
+	if s.privKey.Key.IsZero() {
+		return errorf.E("invalid secret key")
+	}
+
+	// Derive public key
+	if s.pubKey = s.privKey.PubKey(); s.pubKey == nil {
+		return errorf.E("failed to derive public key")
+	}
+
+	// Get x-only public key (32 bytes) - compressed without the 0x02/0x03 prefix
+	compressed := s.pubKey.SerializeCompressed()
+	s.xonlyPub = make([]byte, 32)
+	copy(s.xonlyPub, compressed[1:])
+
+	return nil
+}
+
+// InitPub initializes the public key from raw bytes (x-only 32 bytes)
+func (s *FallbackSigner) InitPub(pub []byte) (err error) {
+	if len(pub) != 32 {
+		return errorf.E("public key must be 32 bytes")
+	}
+
+	s.xonlyPub = make([]byte, 32)
+	copy(s.xonlyPub, pub)
+
+	return nil
+}
+
+// Sec returns the secret key bytes
+func (s *FallbackSigner) Sec() []byte {
+	if s.privKey == nil {
+		return nil
+	}
+	return s.privKey.Serialize()
+}
+
+// Pub returns the public key bytes (x-only schnorr pubkey)
+func (s *FallbackSigner) Pub() []byte {
+	return s.xonlyPub
+}
+
+// Sign creates a signature using the stored secret key
+func (s *FallbackSigner) Sign(msg []byte) (sig []byte, err error) {
+	if s.privKey == nil {
+		return nil, errorf.E("private key not initialized")
+	}
+
+	// Generate auxiliary randomness for BIP-340
+	var auxRand [32]byte
+	if _, err = rand.Read(auxRand[:]); err != nil {
+		return nil, errorf.E("failed to generate aux randomness: %w", err)
+	}
+
+	// Sign using Schnorr
+	var schnorrSig *schnorr.Signature
+	if schnorrSig, err = schnorr.Sign(s.privKey, msg, schnorr.CustomNonce(auxRand)); err != nil {
+		return nil, errorf.E("failed to sign: %w", err)
+	}
+
+	return schnorrSig.Serialize(), nil
+}
+
+// Verify checks a message hash and signature match the stored public key
+func (s *FallbackSigner) Verify(msg, sig []byte) (valid bool, err error) {
+	if s.pubKey == nil {
+		return false, errorf.E("public key not initialized")
+	}
+
+	// Parse signature
+	var schnorrSig *schnorr.Signature
+	if schnorrSig, err = schnorr.ParseSignature(sig); err != nil {
+		return false, errorf.E("failed to parse signature: %w", err)
+	}
+
+	// Verify signature
+	valid = schnorrSig.Verify(msg, s.pubKey)
+	return valid, nil
+}
+
+// Zero wipes the secret key
+func (s *FallbackSigner) Zero() {
+	if s.privKey != nil {
+		privKeyBytes := s.privKey.Serialize()
+		for i := range privKeyBytes {
+			privKeyBytes[i] = 0
+		}
+		s.privKey = nil
+	}
+	if s.xonlyPub != nil {
+		for i := range s.xonlyPub {
+			s.xonlyPub[i] = 0
+		}
+	}
+}
+
+// ECDH returns a shared secret
+func (s *FallbackSigner) ECDH(pub []byte) (secret []byte, err error) {
+	return s.ECDHRaw(pub)
+}
+
+// ECDHRaw returns the raw shared secret (x-coordinate only)
+func (s *FallbackSigner) ECDHRaw(pub []byte) (sharedX []byte, err error) {
+	if s.privKey == nil {
+		return nil, errorf.E("private key not initialized")
+	}
+
+	var pubKeyFull []byte
+
+	if len(pub) == 33 {
+		// Already compressed format
+		pubKeyFull = pub
+	} else if len(pub) == 32 {
+		// X-only format: try with 0x02 (even y), then 0x03 (odd y)
+		pubKeyFull = make([]byte, 33)
+		pubKeyFull[0] = 0x02 // compressed even y
+		copy(pubKeyFull[1:], pub)
+	} else {
+		return nil, errorf.E("public key must be 32 bytes (x-only) or 33 bytes (compressed), got %d bytes", len(pub))
+	}
+
+	// Parse the public key
+	var parsedPub *secp256k1.PublicKey
+	if parsedPub, err = secp256k1.ParsePubKey(pubKeyFull); err != nil {
+		// If 32-byte x-only and even y failed, try odd y
+		if len(pub) == 32 {
+			pubKeyFull[0] = 0x03
+			if parsedPub, err = secp256k1.ParsePubKey(pubKeyFull); err != nil {
+				return nil, err
+			}
+		} else {
+			return nil, err
+		}
+	}
+
+	// Compute ECDH
+	sharedX = secp256k1.GenerateSharedSecret(s.privKey, parsedPub)
+	return sharedX, nil
+}