@@ -17,10 +17,11 @@
 
 #import "OFHMAC.h"
 #import "OFSecureData.h"
 
 #import "OFHashAlreadyCalculatedException.h"
+#import "OFHashNotCalculatedException.h"
 #import "OFInvalidArgumentException.h"
 
 @implementation OFHMAC
 @synthesize hashClass = _hashClass;
 @synthesize allowsSwappableMemory = _allowsSwappableMemory;
@@ -82,10 +83,11 @@
 		if (length > blockSize) {
 			id <OFCryptographicHash> hash = [_hashClass
 			    hashWithAllowsSwappableMemory:
 			    _allowsSwappableMemory];
 			[hash updateWithBuffer: key length: length];
+			[hash calculate];
 
 			length = hash.digestSize;
 			if OF_UNLIKELY (length > blockSize)
 				length = blockSize;
 
@@ -138,21 +140,30 @@
 		    exceptionWithObject: self];
 
 	[_innerHash updateWithBuffer: buffer length: length];
 }
 
-- (const unsigned char *)digest
+- (void)calculate
 {
+	if (_calculated)
+		@throw [OFHashAlreadyCalculatedException
+		    exceptionWithObject: self];
+
 	if (_outerHash == nil || _innerHash == nil)
 		@throw [OFInvalidArgumentException exception];
 
-	if (_calculated)
-		return _outerHash.digest;
-
+	[_innerHash calculate];
 	[_outerHash updateWithBuffer: _innerHash.digest
 			      length: _innerHash.digestSize];
+	[_outerHash calculate];
 	_calculated = true;
+}
+
+- (const unsigned char *)digest
+{
+	if (!_calculated)
+		@throw [OFHashNotCalculatedException exceptionWithObject: self];
 
 	return _outerHash.digest;
 }
 
 - (size_t)digestSize