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Synchronous stream cipher, or just stream cipher. In a synchronous stream cipher a stream of pseudo-random digits is generated independently of the plaintext and ciphertext messages, and then combined with the plaintext (to encrypt) or the ciphertext (to decrypt). In the most common form, binary digits are used (bits), and the keystream is combined with ...


In OTP you have $C=K\oplus{M}$. If the key is the same $C1=K\oplus M1$ and $C2=K\oplus M2$, => $C1 \oplus C2$ = $K\oplus M1 \oplus K\oplus M2$ = $M1 \oplus M2$ If you have the same ciphertext, that means that the same message was encrypted. E.g. messages 4,6,7,11 have the same first 6 symbols.

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