Cryptography I - 04: stream ciphers - Attack on stream ciphers and the one time pad

Attack1: two time pad is insecure!!

Never use stream cipher key more than once !!

$C_1 = m1 \oplus PRG(k)$

$C_2 = m2 \oplus PRG(k)$

Eavesdropper does:

$C_1 \oplus C_2 = m_1 \oplus m_2$

Enough redundancy English and ASCII encoding that:

$m_1 \oplus m_2$ => you can recover m1, m2

So Never use stream cipher key more than once !!

Real World Example

• project venona: they use same pads to encrypt multiple messages.
• MS-PPTP(windows NT): point to point transfer protocol, client and server using same key encrypt message.
• 802.11b.WEP:
• Disk encryption:

Two time pad: summary

Never use stream cipher key more than once!!

• Network traffic: negotiate new key for every session.
• Disk encryption: typically do not use a stream cipher.

Attack2: no integrity(OTP is malleable)

one time pad and stream cipher don’t provide integrity。

Modifications to ciphertext are undetected and have predictable impact on plaintext.

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