Use a diffrent Signer when interacting with Fulcio

ED25519-ph is not widely supported and it is not an accepted option in
x509 Certificates/CSR, so Fulcio does not accept them. Instead, clients
are supposed to use PureED25519 when interacting with Fulcio.

This commit provides to the Fulcio code a separate SignerVerifier
created from the one loaded from the private key. This SignerVerifier is
usually of the same type, except when dealing with ED25519ph.

Signed-off-by: Riccardo Schirone <riccardo.schirone@trailofbits.com>

Author: ret2libc

cmd/cosign/cli/fulcio/fulcio.go

@@ -109,7 +109,7 @@ type Signer struct {
 	signature.SignerVerifier
 }
 
-func NewSigner(ctx context.Context, ko options.KeyOpts, signer signature.SignerVerifier) (*Signer, error) {
+func NewSignerWithAdapter(ctx context.Context, ko options.KeyOpts, signer signature.SignerVerifier, fulcioSigner signature.SignerVerifier) (*Signer, error) {
 	fClient, err := NewClient(ko.FulcioURL)
 	if err != nil {
 		return nil, fmt.Errorf("creating Fulcio client: %w", err)
@@ -164,7 +164,7 @@ func NewSigner(ctx context.Context, ko options.KeyOpts, signer signature.SignerV
 		}
 		flow = flowNormal
 	}
-	Resp, err := GetCert(ctx, signer, idToken, flow, ko.OIDCIssuer, ko.OIDCClientID, ko.OIDCClientSecret, ko.OIDCRedirectURL, fClient) // TODO, use the chain.
+	Resp, err := GetCert(ctx, fulcioSigner, idToken, flow, ko.OIDCIssuer, ko.OIDCClientID, ko.OIDCClientSecret, ko.OIDCRedirectURL, fClient) // TODO, use the chain.
 	if err != nil {
 		return nil, fmt.Errorf("retrieving cert: %w", err)
 	}
@@ -179,6 +179,10 @@ func NewSigner(ctx context.Context, ko options.KeyOpts, signer signature.SignerV
 	return f, nil
 }
 
+func NewSigner(ctx context.Context, ko options.KeyOpts, signer signature.SignerVerifier) (*Signer, error) {
+	return NewSignerWithAdapter(ctx, ko, signer, signer)
+}
+
 func (f *Signer) PublicKey(opts ...signature.PublicKeyOption) (crypto.PublicKey, error) { //nolint: revive
 	return f.SignerVerifier.PublicKey()
 }

cmd/cosign/cli/fulcio/fulcioverifier/fulcioverifier.go

@@ -26,8 +26,8 @@ import (
 	"github.com/sigstore/sigstore/pkg/signature"
 )
 
-func NewSigner(ctx context.Context, ko options.KeyOpts, signer signature.SignerVerifier) (*fulcio.Signer, error) {
-	fs, err := fulcio.NewSigner(ctx, ko, signer)
+func NewSignerWithAdapter(ctx context.Context, ko options.KeyOpts, signer signature.SignerVerifier, fulcioSigner signature.SignerVerifier) (*fulcio.Signer, error) {
+	fs, err := fulcio.NewSignerWithAdapter(ctx, ko, signer, fulcioSigner)
 	if err != nil {
 		return nil, err
 	}
@@ -46,3 +46,7 @@ func NewSigner(ctx context.Context, ko options.KeyOpts, signer signature.SignerV
 
 	return fs, nil
 }
+
+func NewSigner(ctx context.Context, ko options.KeyOpts, signer signature.SignerVerifier) (*fulcio.Signer, error) {
+	return NewSignerWithAdapter(ctx, ko, signer, signer)
+}

cmd/cosign/cli/sign/sign.go

@@ -552,12 +552,17 @@ func keylessSigner(ctx context.Context, ko options.KeyOpts, sv *SignerVerifier)
 		err error
 	)
 
+	fulcioSV, err := AdaptSignerVerifierToFulcio(sv)
+	if err != nil {
+		return nil, fmt.Errorf("adapting signer verifier to Fulcio: %w", err)
+	}
+
 	if ko.InsecureSkipFulcioVerify {
-		if k, err = fulcio.NewSigner(ctx, ko, sv); err != nil {
+		if k, err = fulcio.NewSignerWithAdapter(ctx, ko, sv, fulcioSV); err != nil {
 			return nil, fmt.Errorf("getting key from Fulcio: %w", err)
 		}
 	} else {
-		if k, err = fulcioverifier.NewSigner(ctx, ko, sv); err != nil {
+		if k, err = fulcioverifier.NewSignerWithAdapter(ctx, ko, sv, fulcioSV); err != nil {
 			return nil, fmt.Errorf("getting key from Fulcio: %w", err)
 		}
 	}
@@ -624,6 +629,27 @@ func (c *SignerVerifier) Bytes(ctx context.Context) ([]byte, error) {
 	return pemBytes, nil
 }
 
+// AdaptSignerVerifierToFulcio adapts, if necessary, the SignerVerifier to be used to interact with Fulcio.
+//
+// This is needed in particular for ED25519 keys with the pre-hashed version of
+// the algorithm, which is not supported by Fulcio. This function creates a
+// ED25519 SignerVerifier based on that instead.
+func AdaptSignerVerifierToFulcio(sv *SignerVerifier) (*SignerVerifier, error) {
+	if ed25519phSV, ok := sv.SignerVerifier.(*signature.ED25519phSignerVerifier); ok {
+		signerVerifier, err := ed25519phSV.ConvertToED25519()
+		if err != nil {
+			return nil, err
+		}
+
+		return &SignerVerifier{
+			SignerVerifier: signerVerifier,
+			Cert:           sv.Cert,
+			Chain:          sv.Chain,
+		}, nil
+	}
+	return sv, nil
+}
+
 func fetchLocalSignedPayload(sig oci.Signature) (*cosign.LocalSignedPayload, error) {
 	signedPayload := &cosign.LocalSignedPayload{}
 	var err error