handle unsupported AES-CTR (#221)

aes.go

@@ -20,7 +20,7 @@ type extraModes interface {
 	NewGCMTLS() (cipher.AEAD, error)
 }
 
-var _ extraModes = (*aesCipher)(nil)
+var _ extraModes = (*aesWithCTR)(nil)
 
 func NewAESCipher(key []byte) (cipher.Block, error) {
 	var kind cipherKind
@@ -38,19 +38,32 @@ func NewAESCipher(key []byte) (cipher.Block, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &aesCipher{c}, nil
+	ac := aesCipher{c}
+	// The SymCrypt provider doesn't support AES-CTR.
+	// Prove that the provider supports AES-CTR before
+	// returning an aesWithCTR.
+	if loadCipher(kind, cipherModeCTR) != nil {
+		return &aesWithCTR{ac}, nil
+	}
+	return &ac, nil
 }
 
 // NewGCMTLS returns a GCM cipher specific to TLS
 // and should not be used for non-TLS purposes.
 func NewGCMTLS(c cipher.Block) (cipher.AEAD, error) {
-	return c.(*aesCipher).NewGCMTLS()
+	if c, ok := c.(*aesCipher); ok {
+		return c.NewGCMTLS()
+	}
+	return c.(*aesWithCTR).NewGCMTLS()
 }
 
 // NewGCMTLS13 returns a GCM cipher specific to TLS 1.3 and should not be used
 // for non-TLS purposes.
 func NewGCMTLS13(c cipher.Block) (cipher.AEAD, error) {
-	return c.(*aesCipher).NewGCMTLS13()
+	if c, ok := c.(*aesCipher); ok {
+		return c.NewGCMTLS13()
+	}
+	return c.(*aesWithCTR).NewGCMTLS13()
 }
 
 type aesCipher struct {
@@ -83,10 +96,6 @@ func (c *aesCipher) NewCBCDecrypter(iv []byte) cipher.BlockMode {
 	return c.newCBC(iv, cipherOpDecrypt)
 }
 
-func (c *aesCipher) NewCTR(iv []byte) cipher.Stream {
-	return c.newCTR(iv)
-}
-
 func (c *aesCipher) NewGCM(nonceSize, tagSize int) (cipher.AEAD, error) {
 	return c.newGCMChecked(nonceSize, tagSize)
 }
@@ -98,3 +107,11 @@ func (c *aesCipher) NewGCMTLS() (cipher.AEAD, error) {
 func (c *aesCipher) NewGCMTLS13() (cipher.AEAD, error) {
 	return c.newGCM(cipherGCMTLS13)
 }
+
+type aesWithCTR struct {
+	aesCipher
+}
+
+func (c *aesWithCTR) NewCTR(iv []byte) cipher.Stream {
+	return c.newCTR(iv)
+}