implement draft/webpush (#2205)

vendor/github.com/ergochat/webpush-go/v2/legacy.go

@@ -0,0 +1,76 @@
+package webpush
+
+import (
+	"crypto/ecdh"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"encoding/base64"
+	"fmt"
+	"math/big"
+)
+
+// ecdhPublicKeyToECDSA converts an ECDH key to an ECDSA key.
+// This is deprecated as per https://github.com/golang/go/issues/63963
+// but we need to do it in order to parse the legacy private key format.
+func ecdhPublicKeyToECDSA(key *ecdh.PublicKey) (*ecdsa.PublicKey, error) {
+	rawKey := key.Bytes()
+	switch key.Curve() {
+	case ecdh.P256():
+		return &ecdsa.PublicKey{
+			Curve: elliptic.P256(),
+			X:     big.NewInt(0).SetBytes(rawKey[1:33]),
+			Y:     big.NewInt(0).SetBytes(rawKey[33:]),
+		}, nil
+	case ecdh.P384():
+		return &ecdsa.PublicKey{
+			Curve: elliptic.P384(),
+			X:     big.NewInt(0).SetBytes(rawKey[1:49]),
+			Y:     big.NewInt(0).SetBytes(rawKey[49:]),
+		}, nil
+	case ecdh.P521():
+		return &ecdsa.PublicKey{
+			Curve: elliptic.P521(),
+			X:     big.NewInt(0).SetBytes(rawKey[1:67]),
+			Y:     big.NewInt(0).SetBytes(rawKey[67:]),
+		}, nil
+	default:
+		return nil, fmt.Errorf("cannot convert non-NIST *ecdh.PublicKey to *ecdsa.PublicKey")
+	}
+}
+
+func ecdhPrivateKeyToECDSA(key *ecdh.PrivateKey) (*ecdsa.PrivateKey, error) {
+	// see https://github.com/golang/go/issues/63963
+	pubKey, err := ecdhPublicKeyToECDSA(key.PublicKey())
+	if err != nil {
+		return nil, fmt.Errorf("converting PublicKey part of *ecdh.PrivateKey: %w", err)
+	}
+	return &ecdsa.PrivateKey{
+		PublicKey: *pubKey,
+		D:         big.NewInt(0).SetBytes(key.Bytes()),
+	}, nil
+}
+
+// DecodeLegacyVAPIDPrivateKey decodes the legacy string private key format
+// returned by GenerateVAPIDKeys in v1.
+func DecodeLegacyVAPIDPrivateKey(key string) (*VAPIDKeys, error) {
+	bytes, err := decodeSubscriptionKey(key)
+	if err != nil {
+		return nil, err
+	}
+
+	ecdhPrivKey, err := ecdh.P256().NewPrivateKey(bytes)
+	if err != nil {
+		return nil, err
+	}
+
+	ecdsaPrivKey, err := ecdhPrivateKeyToECDSA(ecdhPrivKey)
+	if err != nil {
+		return nil, err
+	}
+
+	publicKey := base64.RawURLEncoding.EncodeToString(ecdhPrivKey.PublicKey().Bytes())
+	return &VAPIDKeys{
+		privateKey: ecdsaPrivKey,
+		publicKey:  publicKey,
+	}, nil
+}