@@ -19,10 +19,11 @@
 
 #import "OFSHA1Hash.h"
 #import "OFSecureData.h"
 
 #import "OFHashAlreadyCalculatedException.h"
+#import "OFHashNotCalculatedException.h"
 #import "OFOutOfRangeException.h"
 
 static const size_t digestSize = 20;
 static const size_t blockSize = 64;
 
@@ -216,12 +217,21 @@
 	}
 }
 
 - (const unsigned char *)digest
 {
+	if (!_calculated)
+		@throw [OFHashNotCalculatedException exceptionWithObject: self];
+
+	return (const unsigned char *)_iVars->state;
+}
+
+- (void)calculate
+{
 	if (_calculated)
-		return (const unsigned char *)_iVars->state;
+		@throw [OFHashAlreadyCalculatedException
+		    exceptionWithObject: self];
 
 	_iVars->buffer.bytes[_iVars->bufferLength] = 0x80;
 	OFZeroMemory(_iVars->buffer.bytes + _iVars->bufferLength + 1,
 	    64 - _iVars->bufferLength - 1);
 
@@ -237,12 +247,10 @@
 
 	processBlock(_iVars->state, _iVars->buffer.words);
 	OFZeroMemory(&_iVars->buffer, sizeof(_iVars->buffer));
 	byteSwapVectorIfLE(_iVars->state, 5);
 	_calculated = true;
-
-	return (const unsigned char *)_iVars->state;
 }
 
 - (void)reset
 {
 	[self of_resetState];