RSAEngine (Bouncy Castle Library 1.85 API Specification)
Date: 2026-07-12

org.bouncycastle.crypto.engines

## Class RSAEngine

- java.lang.Object
- - org.bouncycastle.crypto.engines.RSAEngine

- All Implemented Interfaces:
    AsymmetricBlockCipher

      public class RSAEngine
      extends java.lang.Object
      implements AsymmetricBlockCipher

  this does your basic RSA algorithm.

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `RSAEngine()`

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `int` | `getInputBlockSize()` Return the maximum size for an input block to this engine.
    `int` | `getOutputBlockSize()` Return the maximum size for an output block to this engine.
    `void` | `init(boolean forEncryption, CipherParameters param)` initialise the RSA engine.
    `byte[]` | `processBlock(byte[] in, int inOff, int inLen)` Process a single block using the basic RSA algorithm.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### RSAEngine

          public RSAEngine()

  - ### Method Detail

    - #### init

          public void init(boolean forEncryption,
                           CipherParameters param)

      initialise the RSA engine.
      Specified by:
        `init` in interface `AsymmetricBlockCipher`
      Parameters:
        `forEncryption` - true if we are encrypting, false otherwise.
        `param` - the necessary RSA key parameters.

    - #### getInputBlockSize

          public int getInputBlockSize()

      Return the maximum size for an input block to this engine. For RSA this is always one byte less than the key size on encryption, and the same length as the key size on decryption.
      Specified by:
        `getInputBlockSize` in interface `AsymmetricBlockCipher`
      Returns:
        maximum size for an input block.

    - #### getOutputBlockSize

          public int getOutputBlockSize()

      Return the maximum size for an output block to this engine. For RSA this is always one byte less than the key size on decryption, and the same length as the key size on encryption.
      Specified by:
        `getOutputBlockSize` in interface `AsymmetricBlockCipher`
      Returns:
        maximum size for an output block.

    - #### processBlock

          public byte[] processBlock(byte[] in,
                                     int inOff,
                                     int inLen)

      Process a single block using the basic RSA algorithm.
      Specified by:
        `processBlock` in interface `AsymmetricBlockCipher`
      Parameters:
        `in` - the input array.
        `inOff` - the offset into the input buffer where the data starts.
        `inLen` - the length of the data to be processed.
      Returns:
        the result of the RSA process.
      Throws:
        `DataLengthException` - the input block is too large.