crypto/rsa: add PublicKey.Size accessor

Provide the fixed size from the key pair.

Change-Id: I365c8d0f7d915229ef089e46458d4c83273fc648
Reviewed-on: https://go-review.googlesource.com/103876Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/rsa/pkcs1v15.go

@@ -38,7 +38,7 @@ func EncryptPKCS1v15(rand io.Reader, pub *PublicKey, msg []byte) ([]byte, error)
 	if err := checkPub(pub); err != nil {
 		return nil, err
 	}
-	k := (pub.N.BitLen() + 7) / 8
+	k := pub.Size()
 	if len(msg) > k-11 {
 		return nil, ErrMessageTooLong
 	}
@@ -106,7 +106,7 @@ func DecryptPKCS1v15SessionKey(rand io.Reader, priv *PrivateKey, ciphertext []by
 	if err := checkPub(&priv.PublicKey); err != nil {
 		return err
 	}
-	k := (priv.N.BitLen() + 7) / 8
+	k := priv.Size()
 	if k-(len(key)+3+8) < 0 {
 		return ErrDecryption
 	}
@@ -134,7 +134,7 @@ func DecryptPKCS1v15SessionKey(rand io.Reader, priv *PrivateKey, ciphertext []by
 // in order to maintain constant memory access patterns. If the plaintext was
 // valid then index contains the index of the original message in em.
 func decryptPKCS1v15(rand io.Reader, priv *PrivateKey, ciphertext []byte) (valid int, em []byte, index int, err error) {
-	k := (priv.N.BitLen() + 7) / 8
+	k := priv.Size()
 	if k < 11 {
 		err = ErrDecryption
 		return
@@ -232,7 +232,7 @@ func SignPKCS1v15(rand io.Reader, priv *PrivateKey, hash crypto.Hash, hashed []b
 	}
 
 	tLen := len(prefix) + hashLen
-	k := (priv.N.BitLen() + 7) / 8
+	k := priv.Size()
 	if k < tLen+11 {
 		return nil, ErrMessageTooLong
 	}
@@ -268,7 +268,7 @@ func VerifyPKCS1v15(pub *PublicKey, hash crypto.Hash, hashed []byte, sig []byte)
 	}
 
 	tLen := len(prefix) + hashLen
-	k := (pub.N.BitLen() + 7) / 8
+	k := pub.Size()
 	if k < tLen+11 {
 		return ErrVerification
 	}

src/crypto/rsa/rsa.go

@@ -42,6 +42,11 @@ type PublicKey struct {
 	E int      // public exponent
 }
 
+// Size returns the number of bytes for signatures from this key.
+func (pub *PublicKey) Size() int {
+	return (pub.N.BitLen() + 7) / 8
+}
+
 // OAEPOptions is an interface for passing options to OAEP decryption using the
 // crypto.Decrypter interface.
 type OAEPOptions struct {
@@ -373,7 +378,7 @@ func EncryptOAEP(hash hash.Hash, random io.Reader, pub *PublicKey, msg []byte, l
 		return nil, err
 	}
 	hash.Reset()
-	k := (pub.N.BitLen() + 7) / 8
+	k := pub.Size()
 	if len(msg) > k-2*hash.Size()-2 {
 		return nil, ErrMessageTooLong
 	}
@@ -587,7 +592,7 @@ func DecryptOAEP(hash hash.Hash, random io.Reader, priv *PrivateKey, ciphertext
 	if err := checkPub(&priv.PublicKey); err != nil {
 		return nil, err
 	}
-	k := (priv.N.BitLen() + 7) / 8
+	k := priv.Size()
 	if len(ciphertext) > k ||
 		k < hash.Size()*2+2 {
 		return nil, ErrDecryption