Index: src/OFGZIPStream.h
==================================================================
--- src/OFGZIPStream.h
+++ src/OFGZIPStream.h
@@ -82,22 +82,28 @@
 
 /*!
  * @brief Creates a new OFGZIPStream with the specified underlying stream.
  *
  * @param stream The underlying stream for the OFGZIPStream
+ * @param mode The mode for the OFGZIPStream. Valid modes are "r" for reading
+ *	       and "w" for writing.
  * @return A new, autoreleased OFGZIPStream
  */
-+ (instancetype)streamWithStream: (OFStream *)stream;
++ (instancetype)streamWithStream: (OFStream *)stream
+			    mode: (OFString *)mode;
 
 - init OF_UNAVAILABLE;
 
 /*!
  * @brief Initializes an already allocated OFGZIPStream with the specified
  *	  underlying stream.
  *
  * @param stream The underlying stream for the OFGZIPStream
+ * @param mode The mode for the OFGZIPStream. Valid modes are "r" for reading
+ *	       and "w" for writing.
  * @return An initialized OFGZIPStream
  */
-- initWithStream: (OFStream *)stream OF_DESIGNATED_INITIALIZER;
+- initWithStream: (OFStream *)stream
+	    mode: (OFString *)mode OF_DESIGNATED_INITIALIZER;
 @end
 
 OF_ASSUME_NONNULL_END

Index: src/OFGZIPStream.m
==================================================================
--- src/OFGZIPStream.m
+++ src/OFGZIPStream.m
@@ -22,29 +22,38 @@
 
 #import "crc32.h"
 
 #import "OFChecksumFailedException.h"
 #import "OFInvalidFormatException.h"
+#import "OFNotImplementedException.h"
 #import "OFNotOpenException.h"
 #import "OFTruncatedDataException.h"
 
 @implementation OFGZIPStream
 + (instancetype)streamWithStream: (OFStream *)stream
+			    mode: (OFString *)mode
 {
-	return [[[self alloc] initWithStream: stream] autorelease];
+	return [[[self alloc] initWithStream: stream
+					mode: mode] autorelease];
 }
 
 - init
 {
 	OF_INVALID_INIT_METHOD
 }
 
 - initWithStream: (OFStream *)stream
+	    mode: (OFString *)mode
 {
 	self = [super init];
 
 	@try {
+		if (![mode isEqual: @"r"])
+			@throw [OFNotImplementedException
+			    exceptionWithSelector: _cmd
+					   object: self];
+
 		_stream = [stream retain];
 		_CRC32 = ~0;
 	} @catch (id e) {
 		[self release];
 		@throw e;

Index: src/OFZIPArchive.m
==================================================================
--- src/OFZIPArchive.m
+++ src/OFZIPArchive.m
@@ -168,11 +168,15 @@
 		if ([mode isEqual: @"r"]) {
 			_mode = OF_ZIP_ARCHIVE_MODE_READ;
 
 			[self of_readZIPInfo];
 			[self of_readEntries];
-		} else
+		} else if ([mode isEqual: @"w"] || [mode isEqual: @"a"])
+			@throw [OFNotImplementedException
+			    exceptionWithSelector: _cmd
+					   object: self];
+		else
 			@throw [OFInvalidArgumentException exception];
 	} @catch (id e) {
 		[self release];
 		@throw e;
 	}

Index: utils/ofzip/GZIPArchive.m
==================================================================
--- utils/ofzip/GZIPArchive.m
+++ utils/ofzip/GZIPArchive.m
@@ -53,11 +53,12 @@
 - initWithStream: (OF_KINDOF(OFStream *))stream
 {
 	self = [super init];
 
 	@try {
-		_stream = [[OFGZIPStream alloc] initWithStream: stream];
+		_stream = [[OFGZIPStream alloc] initWithStream: stream
+							  mode: @"r"];
 	} @catch (id e) {
 		[self release];
 		@throw e;
 	}
 

Index: utils/ofzip/OFZIP.m
==================================================================
--- utils/ofzip/OFZIP.m
+++ utils/ofzip/OFZIP.m
@@ -347,11 +347,12 @@
 			archive = [GZIPArchive archiveWithStream: file];
 		else if ([type isEqual: @"tar"])
 			archive = [TarArchive archiveWithStream: file];
 		else if ([type isEqual: @"tgz"])
 			archive = [TarArchive archiveWithStream:
-			    [OFGZIPStream streamWithStream: file]];
+			    [OFGZIPStream streamWithStream: file
+						      mode: @"r"]];
 		else if ([type isEqual: @"zip"])
 			archive = [ZIPArchive archiveWithStream: file];
 		else {
 			[of_stderr writeLine: OF_LOCALIZED(
 			    @"unknown_archive_type",