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docs: be more specific about the limitations of AE-2 #97

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docs: be more specific about the limitations of AE-2 #97

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24 changes: 16 additions & 8 deletions docs/advanced-usage.md
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Expand Up @@ -45,7 +45,7 @@ This is useful to keep the total number of bytes down as much as possible. This

## Password protection / encryption

The data of ZIP files can be password protected / encrypted by passing a password as the `password` parameter to `stream_zip`.
The data of ZIP files can be password protected / encrypted by passing a password as the `password` parameter to `stream_zip`. This encrypts the data with AES-256, adhering to the [WinZip AE-2 specification](https://www.winzip.com/en/support/aes-encryption/).

```python
import secrets
Expand All @@ -54,10 +54,18 @@ password = secrets.token_urlsafe(32)
encrypted_zipped_chunks = stream_zip(member_files(), password=password)
```

Notes:

1. This encrypts the data with AES-256, adhering to the [WinZip AE-2 specification](https://www.winzip.com/en/support/aes-encryption/).

2. This is seen as more secure than ZipCrypto, the original mechanism of password protecting ZIP files, but fewer clients can open such ZIP files.

3. While a step forward from ZipCrypto, it has flaws that you should be aware of before using it. See ["Attacking and Repairing the WinZip Encryption Scheme" by Tadayoshi Kohno](https://homes.cs.washington.edu/~yoshi/papers/WinZip/winzip.pdf) and [fgrieu's answer to a question about WinZip's AE-1 and AE-2 on Crytography Stack Exchange](https://crypto.stackexchange.com/a/109269/113464).
You should use a long and random password, for example one generated by the [Python secrets module](https://docs.python.org/3/library/secrets.html).

> ### Warnings
>
> While AE-2 is seen as more secure than ZipCrypto, the original mechanism of password protecting ZIP files, fewer clients support AE-2 than ZipCrypto.
>
> More importantly, AE-2 has flaws. These include:
>
> - Not encrypting metadata, for example member file names, modification times, permissions, and sizes.
>
> - Not including sufficient mechanisms to alert recipients if data or metadata has been intercepted and changed. This can itself lead to information leakage.
>
> - A higher risk of information leakage when there's a higher number of member files in the ZIP encrypted with the same password, as stream-zip and most other ZIP writers do.
>
> See ["Attacking and Repairing the WinZip Encryption Scheme" by Tadayoshi Kohno](https://homes.cs.washington.edu/~yoshi/papers/WinZip/winzip.pdf) and [fgrieu's answer to a question about WinZip's AE-1 and AE-2 on Crytography Stack Exchange](https://crypto.stackexchange.com/a/109269/113464) for more information.