**RSA Algorithm**
RSA is a public-key cryptosystem, which means it uses different keys for encryption and decryption. The core idea behind a public-key system is that it is computationally infeasible to derive the decryption key from the encryption key. This ensures a high level of security, as only the owner of the private key can decrypt the message.
In this system, the encryption key (also known as the public key) is shared openly, while the decryption key (or private key) must remain confidential. Both the encryption and decryption algorithms are publicly available. Although the private key is mathematically linked to the public key, it cannot be easily derived from it.
The RSA algorithm was introduced in 1978 and has since become one of the most widely used cryptographic methods. It involves a pair of keys: a public key that can be freely distributed and a private key that must be kept secure. To enhance security, RSA keys should be at least 500 bits long, with 1024 or 2048 bits being more common today. These longer keys make brute-force attacks significantly more difficult.
To improve efficiency, RSA is often combined with symmetric encryption techniques. For example, a session key (such as from DES or IDEA) is used to encrypt the actual data, and then this session key is encrypted using RSA. This hybrid approach reduces computational overhead while maintaining strong security.
One of the key features of RSA is its ability to support both encryption and digital signatures. This makes it particularly useful for securing communications and verifying the authenticity of messages. The RSA algorithm has been extensively studied and tested for over three decades, and it remains one of the most trusted public-key solutions in the industry.
**Encryption Process:**
When A wants to send a secure message to B, they first generate a digital signature. A computes the message digest of the message, then encrypts this digest using their own private key to create a signature. Next, A encrypts the original message along with the signature using B’s public key, resulting in ciphertext that is sent to B.
**Decryption Process:**
Upon receiving the ciphertext, B uses their private key to decrypt it and retrieves both the original message and the digital signature. Then, B uses A’s public key to decrypt the signature and compare the resulting message digest with one they compute independently. If the two digests match, the message is verified as authentic and unaltered.
This process ensures data integrity, confidentiality, and non-repudiation. Digital signatures play a crucial role in confirming the sender's identity and ensuring that the message has not been tampered with during transmission.
**RSA Encryption and Decryption Code**
Below is an example of how RSA encryption and decryption can be implemented in C#:
```csharp
///
/// RSA public key encryption
///
/// Content to be encrypted
/// Public key
/// Encrypted string
public static string EncryptByPublicKey(string content, string publicKey)
{
byte[] keyBytes = Convert.FromBase64String(publicKey);
AsymmetricKeyParameter publicKeyParam = PublicKeyFactory.CreateKey(keyBytes);
IBufferedCipher cipher = CipherUtilities.GetCipher("RSA/ECB/PKCS1Padding");
cipher.Init(true, publicKeyParam);
byte[] data = Encoding.UTF8.GetBytes(content);
byte[] encryptedData = cipher.DoFinal(data, 0, data.Length);
return Convert.ToBase64String(encryptedData);
}
///
/// RSA private key encryption
///
/// Plaintext to be encrypted
/// Private key
/// Ciphertext
public static string RSAEncry(string content, string privateKey)
{
byte[] keyBytes = Convert.FromBase64String(privateKey);
AsymmetricKeyParameter privateKeyParam = PrivateKeyFactory.CreateKey(keyBytes);
IBufferedCipher cipher = CipherUtilities.GetCipher("RSA/ECB/PKCS1Padding");
cipher.Init(true, privateKeyParam);
byte[] data = Encoding.UTF8.GetBytes(content);
byte[] encryptedData = cipher.DoFinal(data, 0, data.Length);
return Convert.ToBase64String(encryptedData);
}
///
/// RSA decryption
///
/// Ciphertext to be decrypted
/// Private key
/// Decrypted plaintext
public static string RSADeEncry(string content, string privateKey)
{
try
{
MemoryStream bufferStream = new MemoryStream();
byte[] bytData = Convert.FromBase64String(content);
int inputLength = bytData.Length;
AsymmetricKeyParameter privateKeyParam = PrivateKeyFactory.CreateKey(Convert.FromBase64String(privateKey));
IBufferedCipher cipher = CipherUtilities.GetCipher("RSA/ECB/PKCS1Padding");
cipher.Init(false, privateKeyParam);
int offSet = 0;
byte[] cache;
int i = 0;
while (inputLength - offSet > 0)
{
if (inputLength - offSet > 128)
{
cache = cipher.DoFinal(bytData, offSet, 128);
}
else
{
cache = cipher.DoFinal(bytData, offSet, inputLength - offSet);
}
bufferStream.Write(cache, 0, cache.Length);
i++;
offSet = i * 128;
}
byte[] decryptedData = bufferStream.ToArray();
return Encoding.UTF8.GetString(decryptedData);
}
catch (Exception e)
{
return e.Message;
}
}
```
This code demonstrates how RSA can be used to securely exchange information. By combining public and private keys, RSA ensures that only the intended recipient can access the message, while also allowing for secure digital signatures to verify the sender's identity.
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