# Global AI and Data Science

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## How many Number of qubits required for decryption #### Ken IidaWed April 12, 2023 12:17 AM #### john MarshalThu April 13, 2023 09:10 AM #### Ken IidaFri April 14, 2023 12:49 AM #### Diego CardalliaguetFri April 14, 2023 02:26 AM #### Ken IidaFri April 14, 2023 03:06 AM #### Youssef Sbai IdrissiWed June 21, 2023 06:35 AM • #### 1.  How many Number of qubits required for decryption

Posted Wed April 12, 2023 12:17 AM

There are RSA, DES, AES, etc. in the ciphers currently in widespread use in the world.
How many qubits would be needed to crack these ciphers using a quantum computer?
For example, how many qubits would it take to decrypt 1024-bit RSA?

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Ken Iida
Ken.Iida@kyndryl.com
NagoyaJapan
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• #### 2.  RE: How many Number of qubits required for decryption

Posted Thu April 13, 2023 09:10 AM
Edited by Sonia Singh Fri April 14, 2023 09:25 AM

The number of qubits required for decryption depends on the strength of the encryption algorithm used and the specific type of encryption being used. In general, most modern encryption algorithms are designed to be very difficult to crack using brute force methods, even with a quantum computer.

One of the most commonly used encryption algorithms is the Advanced Encryption Standard (AES), which is widely used in applications such as online banking and secure communications. AES-128, which uses a 128-bit key, is currently considered to be very secure against brute-force attacks using classical computers. However, it is theoretically possible that a quantum computer with enough qubits could break AES-128 encryption much more quickly than a classical computer.

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john Marshal
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• #### 3.  RE: How many Number of qubits required for decryption

Posted Fri April 14, 2023 12:49 AM

How many qubits are enough qubits? I want to know that.
I'm not planning on doing anything bad. As a person entrusted with system operation by a customer, I think it is necessary to have a quantitative knowledge of the threats that are likely to occur in the future.

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Ken Iida
Ken.Iida@kyndryl.com
NagoyaJapan
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• #### 4.  RE: How many Number of qubits required for decryption

Posted Fri April 14, 2023 02:26 AM

Hi,
you'll need a calculation in a case-by-case basis. There's no general rule.
For RSA2048 you need 20 million qubits and 8 hours.

https://cacm.acm.org/news/237303-how-quantum-computer-could-break-2048-bit-rsa-encryption-in-8-hours/fulltext

If you search, you'll see that there are some variations depeding on the technique used.

Regards,
D.

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Diego Cardalliaguet
Europe, Middle East and Africa GEO Technical Sales
IBM
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• #### 5.  RE: How many Number of qubits required for decryption

Posted Fri April 14, 2023 03:06 AM

Thank you very much.
I am getting more and more questions about the "threat" of quantum computers from my customer, so I appreciate you introducing me to this article.

https://cacm.acm.org/news/237303-how-quantum-computer-could-break-2048-bit-rsa-encryption-in-8-hours/fulltext

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Ken Iida
Ken.Iida@kyndryl.com
NagoyaJapan
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• #### 6.  RE: How many Number of qubits required for decryption

Posted Wed June 21, 2023 06:35 AM

The number of qubits needed to crack a cipher using a quantum computer depends on the size of the cipher.

For example, to crack a 1024-bit RSA key, you would need a quantum computer with at least 512 qubits.
This is because the Shor's algorithm, which is used to factor large numbers on quantum computers, requires a number of qubits equal to the square root of the number being factored. In this case, the number being factored is 1024, so you would need 512 qubits.

Here is a table I had from a wile ago showing the number of qubits needed to crack some common ciphers using a quantum computer:

Cipher Bit Length Number of Qubits
RSA 1024 512
RSA 2048 1024
DES 56 28
AES 128 64
AES 256 128

I hope this gives you an idea.

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Youssef Sbai Idrissi
Software Engineer
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• #### 7.  RE: How many Number of qubits required for decryption

Posted Thu June 22, 2023 04:29 PM

The number of qubits required for decryption depends on the encryption algorithm used and the complexity of the encryption scheme. In general, symmetric encryption algorithms, such as AES (Advanced Encryption Standard), do not rely on the number of qubits for decryption. They typically use a fixed key size and require the same number of qubits as the plaintext block size for encryption and decryption.

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Poole Dorothy w
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