diff --git a/hmac.go b/hmac.go
index c6ed7ba..166f517 100644
--- a/hmac.go
+++ b/hmac.go
@@ -15,9 +15,10 @@ type SigningMethodHMAC struct {
 
 // Specific instances for HS256 and company
 var (
-	SigningMethodHS256 *SigningMethodHMAC
-	SigningMethodHS384 *SigningMethodHMAC
-	SigningMethodHS512 *SigningMethodHMAC
+	SigningMethodHS256  *SigningMethodHMAC
+	SigningMethodHS384  *SigningMethodHMAC
+	SigningMethodHS512  *SigningMethodHMAC
+	ErrSignatureInvalid = errors.New("signature is invalid")
 )
 
 func init() {
@@ -57,7 +58,7 @@ func (m *SigningMethodHMAC) Verify(signingString, signature string, key interfac
 			hasher.Write([]byte(signingString))
 
 			if !bytes.Equal(sig, hasher.Sum(nil)) {
-				err = errors.New("signature is invalid")
+				err = ErrSignatureInvalid
 			}
 		}
 		return err
diff --git a/jwt.go b/jwt.go
index 0f6cf0a..f7b4322 100644
--- a/jwt.go
+++ b/jwt.go
@@ -22,8 +22,9 @@ type Keyfunc func(*Token) (interface{}, error)
 
 // Error constants
 var (
-	ErrInvalidKey      = errors.New("key is invalid or of invalid type.")
-	ErrHashUnavailable = errors.New("the requested hash function is unavailable")
+	ErrInvalidKey       = errors.New("key is invalid or of invalid type")
+	ErrHashUnavailable  = errors.New("the requested hash function is unavailable")
+	ErrNoTokenInRequest = errors.New("no token present in request")
 )
 
 // A JWT Token.  Different fields will be used depending on whether you're
@@ -217,7 +218,7 @@ func ParseFromRequest(req *http.Request, keyFunc Keyfunc) (token *Token, err err
 		return Parse(tokStr, keyFunc)
 	}
 
-	return nil, errors.New("no token present in request.")
+	return nil, ErrNoTokenInRequest
 
 }
 
diff --git a/rsa_utils.go b/rsa_utils.go
index 427585f..6f3b6ff 100644
--- a/rsa_utils.go
+++ b/rsa_utils.go
@@ -7,6 +7,11 @@ import (
 	"errors"
 )
 
+var (
+	ErrKeyMustBePEMEncoded = errors.New("Invalid Key: Key must be PEM encoded PKCS1 or PKCS8 private key")
+	ErrNotRSAPrivateKey    = errors.New("Key is not a valid RSA private key")
+)
+
 // Parse PEM encoded PKCS1 or PKCS8 private key
 func ParseRSAPrivateKeyFromPEM(key []byte) (*rsa.PrivateKey, error) {
 	var err error
@@ -14,7 +19,7 @@ func ParseRSAPrivateKeyFromPEM(key []byte) (*rsa.PrivateKey, error) {
 	// Parse PEM block
 	var block *pem.Block
 	if block, _ = pem.Decode(key); block == nil {
-		return nil, errors.New("Invalid Key: Key must be PEM encoded PKCS1 or PKCS8 private key")
+		return nil, ErrKeyMustBePEMEncoded
 	}
 
 	var parsedKey interface{}
@@ -27,7 +32,7 @@ func ParseRSAPrivateKeyFromPEM(key []byte) (*rsa.PrivateKey, error) {
 	var pkey *rsa.PrivateKey
 	var ok bool
 	if pkey, ok = parsedKey.(*rsa.PrivateKey); !ok {
-		return nil, errors.New("Key is not a valid RSA private key")
+		return nil, ErrNotRSAPrivateKey
 	}
 
 	return pkey, nil
@@ -40,7 +45,7 @@ func ParseRSAPublicKeyFromPEM(key []byte) (*rsa.PublicKey, error) {
 	// Parse PEM block
 	var block *pem.Block
 	if block, _ = pem.Decode(key); block == nil {
-		return nil, errors.New("Invalid Key: Key must be PEM encoded PKCS1 or PKCS8 private key")
+		return nil, ErrKeyMustBePEMEncoded
 	}
 
 	// Parse the key
@@ -56,7 +61,7 @@ func ParseRSAPublicKeyFromPEM(key []byte) (*rsa.PublicKey, error) {
 	var pkey *rsa.PublicKey
 	var ok bool
 	if pkey, ok = parsedKey.(*rsa.PublicKey); !ok {
-		return nil, errors.New("Key is not a valid RSA public key")
+		return nil, ErrNotRSAPrivateKey
 	}
 
 	return pkey, nil