Index: src/OFASN1BitString.h
==================================================================
--- src/OFASN1BitString.h
+++ src/OFASN1BitString.h
@@ -39,10 +39,15 @@
 /*!
  * @brief The length of the BitString in bits.
  */
 @property (readonly, nonatomic) size_t bitStringLength;
 
+/*!
+ * @brief The BitString in DER encoding.
+ */
+@property (readonly, nonatomic) OFData *DEREncodedValue;
+
 /*!
  * @brief Creates an ASN.1 BitString with the specified BitString value and
  *	  length.
  *
  * @param bitStringValue The value of the BitString

Index: src/OFASN1BitString.m
==================================================================
--- src/OFASN1BitString.m
+++ src/OFASN1BitString.m
@@ -42,11 +42,11 @@
 {
 	self = [super init];
 
 	@try {
 		if (bitStringValue.count * bitStringValue.itemSize !=
-		    bitStringLength / 8)
+		    OF_ROUND_UP_POW2(8, bitStringLength) / 8)
 			@throw [OFInvalidFormatException exception];
 
 		_bitStringValue = [bitStringValue copy];
 		_bitStringLength = bitStringLength;
 	} @catch (id e) {
@@ -78,20 +78,29 @@
 			@throw [OFInvalidFormatException exception];
 
 		lastByteBits =
 		    *(unsigned char *)[DEREncodedContents itemAtIndex: 0];
 
+		if (lastByteBits > 7)
+			@throw [OFInvalidFormatException exception];
+
+		/*
+		 * Can't have any bits of the last byte used if we have no byte.
+		 */
 		if (count == 1 && lastByteBits != 0)
 			@throw [OFInvalidFormatException exception];
 
 		if (SIZE_MAX / 8 < count - 1 ||
 		    SIZE_MAX - (count - 1) * 8 < lastByteBits)
 			@throw [OFOutOfRangeException exception];
 
-		bitStringLength = (count - 1) * 8 + lastByteBits;
+		bitStringLength = (count - 1) * 8;
 		bitStringValue = [[DEREncodedContents
 		    subdataWithRange: of_range(1, count - 1)] copy];
+
+		if (lastByteBits != 0)
+			bitStringLength -= (8 - lastByteBits);
 	} @catch (id e) {
 		[self release];
 		@throw e;
 	}
 
@@ -112,10 +121,32 @@
 {
 	[_bitStringValue release];
 
 	[super dealloc];
 }
+
+- (OFData *)DEREncodedValue
+{
+	size_t bitStringValueCount = [_bitStringValue count];
+	unsigned char lastByteBits = _bitStringLength % 8;
+	unsigned char header[] = { 3, bitStringValueCount + 1, lastByteBits };
+	OFMutableData *data;
+
+	if (bitStringValueCount + 1 > UINT8_MAX ||
+	    bitStringValueCount != OF_ROUND_UP_POW2(8, _bitStringLength) / 8)
+		@throw [OFInvalidFormatException exception];
+
+	data = [OFMutableData dataWithCapacity: 3 + bitStringValueCount];
+	[data addItems: header
+		 count: 3];
+	[data addItems: [_bitStringValue items]
+		 count: bitStringValueCount];
+
+	[data makeImmutable];
+
+	return data;
+}
 
 - (bool)isEqual: (id)object
 {
 	OFASN1BitString *bitString;
 

Index: tests/OFASN1DEREncodedValueTests.m
==================================================================
--- tests/OFASN1DEREncodedValueTests.m
+++ tests/OFASN1DEREncodedValueTests.m
@@ -23,10 +23,33 @@
 
 @implementation TestsAppDelegate (OFASN1DEREncodedValueTests)
 - (void)ASN1DEREncodedValueTests
 {
 	OFAutoreleasePool *pool = [[OFAutoreleasePool alloc] init];
+	OFData *data;
+
+	module = @"OFASN1BitString";
+	TEST(@"-[DEREncodedValue]",
+	    (data = [OFData dataWithItems: "\xFF\x00\xF8"
+				    count: 3]) &&
+	    [[[OFASN1BitString bitStringWithBitStringValue: data
+					   bitStringLength: 20] DEREncodedValue]
+	    isEqual: [OFData dataWithItems: "\x03\x04\x04\xFF\x00\xF8"
+				     count: 6]] &&
+	    (data = [OFData dataWithItems: "abcdefäöü"
+				    count: 12]) &&
+	    [[[OFASN1BitString bitStringWithBitStringValue: data
+					   bitStringLength: 12 * 8]
+	    DEREncodedValue] isEqual:
+	    [OFData dataWithItems: "\x03\x0D\x00" "abcdefäöü"
+			    count: 15]] &&
+	    (data = [OFData dataWithItems: ""
+				    count: 0]) &&
+	    [[[OFASN1BitString bitStringWithBitStringValue: data
+					   bitStringLength: 0] DEREncodedValue]
+	    isEqual: [OFData dataWithItems: "\x03\x01\x00"
+				     count: 3]])
 
 	module = @"OFASN1Boolean";
 	TEST(@"-[DEREncodedValue]",
 	    [[[OFASN1Boolean booleanWithBooleanValue: false] DEREncodedValue]
 	    isEqual: [OFData dataWithItems: "\x01\x01\x00"

Index: tests/OFASN1DERValueTests.m
==================================================================
--- tests/OFASN1DERValueTests.m
+++ tests/OFASN1DERValueTests.m
@@ -102,11 +102,11 @@
 	    (bitString = [[OFData dataWithItems: "\x03\x0D\x01Hello World\x80"
 					  count: 15] ASN1DERValue]) &&
 	    [bitString.bitStringValue
 	    isEqual: [OFData dataWithItems: "Hello World\x80"
 				     count: 12]] &&
-	    bitString.bitStringLength == 97 &&
+	    bitString.bitStringLength == 89 &&
 	    (bitString = [[OFData dataWithItems: "\x03\x81\x80\x00xxxxxxxxxxxxx"
 						 "xxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
 						 "xxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
 						 "xxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
 						 "xxxxxxxxxxxxxxxxxxxxxxxxxxx"