This is a fork of the Go archive/zip package to add support
for reading password protected AES encrypted files. Only supports
Winzip's AES extension: http://www.winzip.com/aes_info.htm. This
package DOES NOT intend to implement the encryption methods
mentioned in the original PKWARE spec (sections 6.0 and 7.0):
https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT

WinZip AES specifies
====================================================================
1. Encryption-Decryption w/ AES-CTR (128, 192, or 256 bits)
2. Key generation with PBKDF2-HMAC-SHA1 (1000 iteration count) that
generates a master key broken into the following:
    a. First m bytes is for the encryption key
    b. Next n bytes is for the authentication key
    c. Last 2 bytes is the password verification value.
3. Following salt lengths are used w/ password during keygen:
    ------------------------------
    AES Key Size    | Salt Size
    ------------------------------
    128bit(16bytes) | 8 bytes
    192bit(24bytes) | 12 bytes
    256bit(32bytes) | 16 bytes
    -------------------------------
4. Master key len = AESKeyLen + AuthKeyLen + PWVLen:
    a. AES 128 = 16 + 16 + 2 = 34 bytes of key material
    b. AES 192 = 24 + 24 + 2 = 50 bytes of key material
    c. AES 256 = 32 + 32 + 2 = 66 bytes of key material
5. Authentication Key is same size as AES key.
6. Authentication with HMAC-SHA1-80 (truncated to 80bits).
7. A new master key is generated for every file.
8. The file header and directory header compression method will
be 99 (decimal). The actual compression method will be in the
extra's payload at the end of the directory header.
9. A extra field will be added to the file header and directory
header identified by 0x9901 and contains the following info:
    a. Header ID (2 bytes)
    b. Data Size (2 bytes)
    c. Vendor Version (2 bytes)
    d. Vendor ID (2 bytes)
    e. AES Strength (1 byte)
    f. Compression Method (2 bytes)
10. The Data Size is always 7.
11. The Vendor Version can either be 0x0001 (AE-1) or
0x0002 (AE-2).
12. Vendor ID is ASCII "AE"
13. AES Strength:
    a. 0x01 - AES-128
    b. 0x02 - AES-192
    c. 0x03 - AES-256
14. Compression Method is the actual compression method
used that was replaced by the encryption process.
15. AE-1 keeps the CRC and should be verified after decompression.
16. AE-2 removes the CRC and shouldn't be verified after decompression.
Refer to http://www.winzip.com/aes_info.htm#winzip11 for the reasoning.