Organize a clear list of all ZIP features and triage them.

[im7mortal]

There are many issues asking to implement some features from the standard, but some of them can be considered "Won't implement"

So I quickly asked AI to review APPNOTE.TXT and create a table. Here it is:

AI GENERATED UNTIL THE NEXT COMMENT. PROCEED TO THE NEXT COMMENT FOR CONTINUATION.

ZIP Format Feature Implementation Status

This document lists all major features from the ZIP File Format Specification (APPNOTE.TXT v6.3.10) and their implementation status in this library.

Compression Methods

Method ID	Method Name	Reading	Writing	Notes
0	Stored (no compression)	✅	✅	Fully supported
1	Shrink	✅ (legacy-zip)	❌	Legacy method, read-only with feature flag
2-5	Reduce (factors 1-4)	✅ (legacy-zip)	❌	Legacy method, read-only with feature flag
6	Implode	✅ (legacy-zip)	❌	Legacy method, read-only with feature flag
8	Deflate	✅	✅	Default compression method
9	Deflate64	✅	❌	Decompression only
10	PKWARE Data Compression Library Imploding	❌	❌	Not implemented
12	BZIP2	✅	✅	Fully supported
14	LZMA	✅	❌	Decompression only
16	IBM z/OS CMPSC Compression	❌	❌	Not implemented
18	IBM TERSE (new)	❌	❌	Not implemented
19	IBM LZ77 z Architecture	❌	❌	Not implemented
93	Zstandard (zstd)	✅	✅	Fully supported
94	MP3 Compression	❌	❌	Not implemented
95	XZ Compression	✅	✅	Fully supported
96	JPEG variant	❌	❌	Not implemented
97	WavPack compressed data	❌	❌	Not implemented
98	PPMd version I, Rev 1	✅	✅	Fully supported
99	AE-x encryption marker	✅ (aes-crypto)	✅ (aes-crypto)	See Encryption section

Encryption

Feature	Reading	Writing	Notes
Traditional PKWARE Encryption (ZipCrypto)	✅	✅	Weak encryption, supported for compatibility
Strong Encryption - AES	✅ (aes-crypto)	✅ (aes-crypto)	AES-128, AES-192, AES-256 (AE-1, AE-2)
Strong Encryption - DES	❌	❌	Not implemented
Strong Encryption - 3DES	❌	❌	Not implemented
Strong Encryption - RC2	❌	❌	Not implemented
Strong Encryption - RC4	❌	❌	Not implemented
Strong Encryption - Blowfish	❌	❌	Not implemented
Strong Encryption - Twofish	❌	❌	Not implemented
Certificate-based Encryption	❌	❌	Not implemented
Central Directory Encryption	❌	❌	Not implemented
Password-based Encryption	✅	✅	Via ZipCrypto and AES
X.509 Certificate Encryption	❌	❌	Not implemented

ZIP64 Extensions

Feature	Reading	Writing	Notes
ZIP64 Extended Information Extra Field (0x0001)	✅	✅	For files > 4GB
ZIP64 End of Central Directory Record	✅	✅	Fully supported
ZIP64 End of Central Directory Locator	✅	✅	Fully supported
ZIP64 Central Directory	✅	✅	Fully supported
ZIP64 Local Header	✅	✅	Fully supported
ZIP64 Data Descriptor	✅	✅	Fully supported

Extra Fields

Extra Field ID	Name	Reading	Writing	Notes
0x0001	Zip64 extended information	✅	✅	Fully supported
0x0007	AV Info	❌	❌	Recognized but not parsed
0x0008	Reserved for extended language encoding data (PFS)	❌	❌	Recognized but not parsed
0x0009	OS/2	❌	❌	Recognized but not parsed
0x000a	NTFS	✅	✅	Fully supported
0x000c	OpenVMS	❌	❌	Recognized but not parsed
0x000d	UNIX	❌	❌	Recognized but not parsed
0x000e	Reserved for file stream and fork descriptors	❌	❌	Recognized but not parsed
0x000f	Patch Descriptor	❌	❌	Recognized but not parsed
0x0014	PKCS#7 Store for X.509 Certificates	❌	❌	Recognized but not parsed
0x0015	X.509 Certificate ID and Signature for individual file	❌	❌	Recognized but not parsed
0x0016	X.509 Certificate ID for Central Directory	❌	❌	Recognized but not parsed
0x0017	Strong Encryption Header	❌	❌	Recognized but not parsed
0x0018	Record Management Controls	❌	❌	Recognized but not parsed
0x0019	PKCS#7 Encryption Recipient Certificate List	❌	❌	Recognized but not parsed
0x0020	Reserved for Timestamp record	❌	❌	Recognized but not parsed
0x0021	Policy Decryption Key Record	❌	❌	Recognized but not parsed
0x0022	Smartcrypt Key Provider Record	❌	❌	Recognized but not parsed
0x0023	Smartcrypt Policy Key Data Record	❌	❌	Recognized but not parsed
0x0065	IBM S/390 (Z390), AS/400 (I400) attributes	❌	❌	Recognized but not parsed
0x0066	Reserved for IBM S/390 (Z390), AS/400 (I400) attributes - compressed	❌	❌	Recognized but not parsed
0x5455	Extended Timestamp	✅	✅	Fully supported
0x6375	Info-ZIP Unicode Comment Extra Field	✅	✅	Fully supported
0x7075	Info-ZIP Unicode Path Extra Field	✅	✅	Fully supported
0x9901	AE-x encryption structure	✅	✅	Fully supported
0xa11e	Data Stream Alignment (Apache Commons-Compress)	✅	✅	Fully supported

File Structure Features

Feature	Reading	Writing	Notes
Local File Header	✅	✅	Fully supported
Central Directory Header	✅	✅	Fully supported
End of Central Directory Record	✅	✅	Fully supported
Data Descriptor	✅	✅	Fully supported (with and without signature)
Archive Decryption Header	❌	❌	Not implemented (Central Directory Encryption)
Archive Extra Data Record	❌	❌	Not implemented (Central Directory Encryption)
Digital Signature	❌	❌	Not implemented
File Comments	✅	✅	Fully supported
Archive Comments	✅	✅	Fully supported

General Purpose Bit Flags

Bit	Flag Name	Reading	Writing	Notes
0	Encrypted	✅	✅	Fully supported
1	Compression option (Deflate) / Dictionary size (Implode) / EOS marker (LZMA)	✅	✅	Supported per method
2	Compression option (Deflate) / Shannon-Fano trees (Implode)	✅	✅	Supported per method
3	Data Descriptor	✅	✅	Fully supported
4	Enhanced deflating	❌	❌	Not implemented
5	Compressed patched data	❌	❌	Not implemented
6	Strong encryption	✅ (aes-crypto)	✅ (aes-crypto)	AES encryption
7	Unused	N/A	N/A	Reserved
8	Unused	N/A	N/A	Reserved
9	Unused	N/A	N/A	Reserved
10	Unused	N/A	N/A	Reserved
11	Language encoding flag (EFS) - UTF-8	✅	✅	Fully supported
12	Reserved by PKWARE	❌	❌	Not implemented
13	Central Directory Encryption masking	❌	❌	Not implemented
14	Reserved by PKWARE	❌	❌	Not implemented
15	Reserved by PKWARE	❌	❌	Not implemented

Advanced Features

Feature	Reading	Writing	Notes
Multi-disk/Split Archives	❌	❌	Not implemented
Spanned Archives	❌	❌	Not implemented
Self-extracting Archives (PKSFX)	❌	❌	Not implemented
Streaming ZIP Creation	✅	✅	Via data descriptors
Streaming ZIP Reading	✅	❌	Partial support
Manifest Files	❌	❌	Not implemented (application-specific)
UTF-8 Filenames (Bit 11)	✅	✅	Fully supported
UTF-8 Comments (Bit 11)	✅	✅	Fully supported
MS-DOS Date/Time Format	✅	✅	Fully supported
CRC-32 Checksums	✅	✅	Fully supported
External File Attributes	✅	✅	Basic support (Unix permissions)
Internal File Attributes	✅	✅	Basic support

System Compatibility

System	Reading	Writing	Notes
MS-DOS / Windows (FAT/VFAT/FAT32)	✅	✅	Fully supported
UNIX	✅	✅	Unix permissions supported
OS/2	✅	✅	Basic support
Macintosh	✅	✅	Basic support
NTFS	✅	✅	NTFS extra field supported
OpenVMS	❌	❌	Not implemented
VM/CMS	❌	❌	Not implemented
Atari ST	❌	❌	Not implemented
Amiga	❌	❌	Not implemented
OS X (Darwin)	✅	✅	Basic support
IBM z/OS	❌	❌	Not implemented
AS/400	❌	❌	Not implemented

Summary

Fully Implemented Features

• ✅ Core ZIP file structure (local headers, central directory, EOCD)
• ✅ ZIP64 extensions for large files
• ✅ Data descriptors
• ✅ UTF-8 filename/comment support (Bit 11 and extra fields)
• ✅ Multiple compression methods (Stored, Deflate, Deflate64, BZIP2, Zstd, LZMA, XZ, PPMd)
• ✅ Traditional PKWARE encryption (ZipCrypto)
• ✅ AES encryption (AE-1, AE-2)
• ✅ Key extra fields (ZIP64, NTFS, Extended Timestamp, Unicode paths/comments, AES, Data Alignment)
• ✅ File and archive comments
• ✅ CRC-32 integrity checking

Partially Implemented Features

• ⚠️ Legacy compression methods (Shrink, Reduce, Implode) - read-only with feature flag
• ⚠️ LZMA and Deflate64 - decompression only
• ⚠️ Streaming - reading supported, writing via data descriptors

Not Implemented Features

• ❌ Multi-disk/split archives
• ❌ Strong encryption methods (DES, 3DES, RC2, RC4, Blowfish, Twofish)
• ❌ Certificate-based encryption
• ❌ Central Directory Encryption
• ❌ Digital signatures
• ❌ Most extra fields (recognized but not parsed)
• ❌ Self-extracting archives
• ❌ Platform-specific attributes (OpenVMS, VM/CMS, IBM z/OS, AS/400)

Legend

• ✅ = Fully implemented
• ⚠️ = Partially implemented
• ❌ = Not implemented
• N/A = Not applicable

[im7mortal]

I am looking for a good crypto-related feature that I could implement, and almost everything I check is actually outdated, not extendable, and not safe — so it probably shouldn’t be implemented at all.

For example, certificate encryption, which is not extendable and is not post-quantum safe — so it should be strictly discouraged and better not included in the library. Also because it is not really used.