Wifi (802.11) ve Şifre

Giriş
Bu ailede frame'in zarf alanları (header) şifreli değildir. Sadece taşınan veri şifrelenir. Verinin şifrelin olduğu 802.11 Mac Frame Header zarfındaki Frame Control alanında Protected Frame Bit alanında belirtilir
1. WEP
2004 yılından itibaren yeterince güvenliği olmadığı için kullanılmıyor. Bir soru şöyle

Q : Have any cryptographic breaks been executed in the real world since World War II?

A : ... the attack on WEP, which is based on an unknown (to the designers at the time) related key attack on RC4 that lead to a key recovery.

2. TKIP
WEP'ten daha sonra geliştirilmiştir. WPA kullanmayan eski cihazlarda kullanılır.

3. WPA/WPA2
Bu doğrulama yöntemini diğer yöntemlere tercih etmek gerekir. Doğrulama için four-way handshake kullanılır.

In WPA2-PSK, both the station and the AP have to prove that they know the passphrase in the four-way handshake.

3.1 PSK Oluşturmak İçin Formül
WPA2 presharekey (PSK) oluşturmak için şu PBKDF2 formülünü kullanır. Bu formül şöyle.

psk = PBKDF2 (HMAC−SHA1, passphrase, ssid, 4096, 256)

Formülde şifrenin de (passpharase) de girdi olduğu görülebilir.

PSK'dan PTK (Pairwise Transient Key) üretilir. Açıklaması şöyle.

The password and the SSID are boiled down by a key derivation function (PBKDF2 for WPA2-PSK), from which we want 256 bits.

This key, called a PMK (Pairwise Master Key) is always 256 bits regardless of password length.

It is then used (along with nonces aNonce, sNonce and MAC addresses) in a 4-way handshake to generate the 512-bit Pairwise Transient Key (PTK), which is then split into multiple keys:

128-bit Temporal Key (TK) - Encrypts data itself
128-bit Key Encryption Key (KEK) - Encrypts keys (e.g. GTK) during transfer
128-bit Key Confirmation Key (KCK) - Involved in EAPoL MIC (part of authentication)
64-bit RX and TX keys, used for message integrity codes (MIC) on data frames
These multiple keys change periodically and are, again, fixed length regardless of the password. Specifically, your data is always encrypted by the Temporal Key (TK) and signed off by the TX/RX keys, which are always 128 bits and 64 bits respectively regardless of password length.

Password -> [PMK -> PTK -> Encryption keys]. Items in brackets are fixed length.

4. Krack saldırısı
Krack saldırısı four-way handshake'in 3. mesajı ile yapılıyor. Açıklaması şöyle. Bu açığı IEEE'in çalışma şekline bağlayanlar da var.

When a client joins a network, it [...] will install this key after receiving message 3 of the 4-way handshake. Once the key is installed, it will be used to encrypt normal data frames using an encryption protocol. However, because messages may be lost or dropped, the Access Point (AP) will retransmit message 3 if it did not receive an appropriate response as acknowledgment. [...] Each time it receives this message, it will reinstall the same encryption key, and thereby reset the incremental transmit packet number (nonce) and receive replay counter used by the encryption protocol. We show that an attacker can force these nonce resets by collecting and replaying retransmissions of message 3 of the 4-way handshake. By forcing nonce reuse in this manner, the encryption protocol can be attacked, e.g., packets can be replayed, decrypted, and/or forged.

WPA2 şifresini kırmak için şöyle yaparız.

$ hashcat -a 3 -m 2500 -2 ?u?d hashcat-wpa2.hccapx ?2?2?2?2?2?2?2?2?2?2?2
hashcat (v4.1.0) starting...