What Causes the RSA Decryption Error?

The error “input too large for RSA cipher” occurs when the size of the encrypted data exceeds the maximum block size that RSA can handle. This limitation arises because RSA encryption operates on fixed-size blocks determined by the key length.

Why Does This Error Occur in BouncyCastle?

BouncyCastle, a popular cryptographic library for Java and .NET, enforces RSA’s block size restrictions. If the input exceeds the allowed size, decryption fails with this error.

How to Fix the RSA Decryption Error?

• Use Hybrid Encryption: Encrypt the data using AES (a symmetric cipher) and then encrypt the AES key with RSA.
• Use Optimal Asymmetric Encryption Padding (OAEP): This reduces the likelihood of exceeding block size limitations.
• Split Large Data into Smaller Blocks: Manually divide data into chunks within RSA’s size limits.
• Ensure Correct Key Size: Use at least a 2048-bit RSA key for better encryption capacity.

Conclusion

To avoid the “RSA decryption error: input too large for RSA cipher” in BouncyCastle, always consider the size limitations of RSA and use best practices such as hybrid encryption or OAEP padding.