6 Yes, plaintext is spelled like this edited Apr 2 '18 at 8:42 Discrete lizard 1,4941010 silver badges3535 bronze badges I suppose there is a type of encryption that is not crackable using quantum computers: a one-time pad such as the Vigenère cipher. This is a cipher with a keypad that has at least the length of the encoded string and will be used only once. This cipher is impossible to crack even with a quantum computer. I will explain why: Let's assume our plain textplaintext is ABCD. The corresponding key could be 1234. If you encode it then you get XYZW. Now you can use 1234 to get ABCD or 4678 to get EFGH what might be a valid sentence too. So the problem is that nobody can decide whether you meant ABCD or EFGH without knowing your key. The only reason this kind of encryption can be cracked is that the users are lazy and use a key twice. And then you can try to crack it. Other problems are, as @peterh stated that one-time-pads require a secret channel to be shared I suppose there is a type of encryption that is not crackable using quantum computers: a one-time pad such as the Vigenère cipher. This is a cipher with a keypad that has at least the length of the encoded string and will be used only once. This cipher is impossible to crack even with a quantum computer. I will explain why: Let's assume our plain text is ABCD. The corresponding key could be 1234. If you encode it then you get XYZW. Now you can use 1234 to get ABCD or 4678 to get EFGH what might be a valid sentence too. So the problem is that nobody can decide whether you meant ABCD or EFGH without knowing your key. The only reason this kind of encryption can be cracked is that the users are lazy and use a key twice. And then you can try to crack it. Other problems are, as @peterh stated that one-time-pads require a secret channel to be shared I suppose there is a type of encryption that is not crackable using quantum computers: a one-time pad such as the Vigenère cipher. This is a cipher with a keypad that has at least the length of the encoded string and will be used only once. This cipher is impossible to crack even with a quantum computer. I will explain why: Let's assume our plaintext is ABCD. The corresponding key could be 1234. If you encode it then you get XYZW. Now you can use 1234 to get ABCD or 4678 to get EFGH what might be a valid sentence too. So the problem is that nobody can decide whether you meant ABCD or EFGH without knowing your key. The only reason this kind of encryption can be cracked is that the users are lazy and use a key twice. And then you can try to crack it. Other problems are, as @peterh stated that one-time-pads require a secret channel to be shared 5 grammar and spelling edit approved Apr 2 '18 at 8:42 ItamarG3 46544 silver badges2020 bronze badges I suppose there is a type of encryption that is not crackable also using quantum computers: a one-time pad like for examplesuch as the Vigenère cipher. This is a cipher with a keypad that has at least the length of the encoded string and will be used only once. This cipher is impossible to crack, even when using anwith a quantum computer. I will explain why: Let's assume our plaintextplain text is ABCD. The corresponding key could be 1234. If you encode it then you get XYZW. Now you can use 1234 to get ABCD or 4678 to get EFGH what might be ana valid sentence too. So the problem is that nobody can decide whether you meant ABCD or EFGH without knowing your key. The only reason this kind of encryption can be cracked is that the users are lazy and use a key twice. And then you can try to crack it. Other problems are, as @peterh stated that one-time-pads require a secret channel to be shared I suppose there is a type of encryption that is not crackable also using quantum computers: a one-time pad like for example the Vigenère cipher. This is a cipher with a keypad that has at least the length of the encoded string and will be used only once. This is impossible to crack, even when using an quantum computer. I will explain why: Let's assume our plaintext is ABCD. The corresponding key could be 1234. If you encode it then you get XYZW. Now you can use 1234 to get ABCD or 4678 to get EFGH what might be an valid sentence too. So the problem is that nobody can decide whether you meant ABCD or EFGH without knowing your key. The only reason this kind of encryption can be cracked is that the users are lazy and use a key twice. And then you can try to crack it. Other problems are, as @peterh stated that one-time-pads require a secret channel to be shared I suppose there is a type of encryption that is not crackable using quantum computers: a one-time pad such as the Vigenère cipher. This is a cipher with a keypad that has at least the length of the encoded string and will be used only once. This cipher is impossible to crack even with a quantum computer. I will explain why: Let's assume our plain text is ABCD. The corresponding key could be 1234. If you encode it then you get XYZW. Now you can use 1234 to get ABCD or 4678 to get EFGH what might be a valid sentence too. So the problem is that nobody can decide whether you meant ABCD or EFGH without knowing your key. The only reason this kind of encryption can be cracked is that the users are lazy and use a key twice. And then you can try to crack it. Other problems are, as @peterh stated that one-time-pads require a secret channel to be shared 4 presenting a one-time pad as a Vigenère cipher is more confusing than helpful edited Mar 20 '18 at 7:33 MEE - Reinstate Monica 63777 silver badges2626 bronze badges I suppose there is a type of encryption that is not crackable also using quantum computers: thea one-time pad like for example the Vigenère cipher. This is a cipher with a keypad that has at least the length of the encoded string and will be used only once. This is impossible to crack, even when using an quantum computer. I will explain why: Let's assume our plaintext is ABCD. The corresponding key could be 1234. If you encode it then you get XYZW. Now you can use 1234 to get ABCD or 4678 to get EFGH what might be an valid sentence too. So the problem is that nobody can decide whether you meant ABCD or EFGH without knowing your key. The only reason this kind of encryption can be cracked is that the users are lazy and use a key twice. And then you can try to crack it. Other problems are, as @peterh stated that one-time-pads require a secret channel to be shared I suppose there is a type of encryption that is not crackable also using quantum computers: the one-time pad. This is a cipher with a keypad that has at least the length of the encoded string and will be used only once. This is impossible to crack, even when using an quantum computer. I will explain why: Let's assume our plaintext is ABCD. The corresponding key could be 1234. If you encode it then you get XYZW. Now you can use 1234 to get ABCD or 4678 to get EFGH what might be an valid sentence too. So the problem is that nobody can decide whether you meant ABCD or EFGH without knowing your key. The only reason this kind of encryption can be cracked is that the users are lazy and use a key twice. And then you can try to crack it. Other problems are, as @peterh stated that one-time-pads require a secret channel to be shared I suppose there is a type of encryption that is not crackable also using quantum computers: a one-time pad like for example the Vigenère cipher. This is a cipher with a keypad that has at least the length of the encoded string and will be used only once. This is impossible to crack, even when using an quantum computer. I will explain why: Let's assume our plaintext is ABCD. The corresponding key could be 1234. If you encode it then you get XYZW. Now you can use 1234 to get ABCD or 4678 to get EFGH what might be an valid sentence too. So the problem is that nobody can decide whether you meant ABCD or EFGH without knowing your key. The only reason this kind of encryption can be cracked is that the users are lazy and use a key twice. And then you can try to crack it. Other problems are, as @peterh stated that one-time-pads require a secret channel to be shared 3 presenting a one-time pad as a Vigenère cipher is more confusing than helpful edit approved Mar 20 '18 at 7:33 Gilles 'SO- stop being evil' 10344 bronze badges 2 added information given from @peterh edited Mar 12 '18 at 18:03 MEE - Reinstate Monica 63777 silver badges2626 bronze badges 1 answered Mar 12 '18 at 17:40 MEE - Reinstate Monica 63777 silver badges2626 bronze badges