question: public OAuthRsaSha1Signer ( String privateKeyString ) throws OAuthException { if ( privateKeyString == null ) { throw new OAuthException ( "Private key string cannot be null" ) ; } else if ( privateKeyString . length ( ) == 0 ) { throw new OAuthException ( "Private key string cannot be empty" ) ; } try { setPrivateKey ( RsaSha1PrivateKeyHelper . getPrivateKey ( privateKeyString ) ) ; } catch ( Base64DecoderException e ) { throw new OAuthException ( "Invalid private key" , e ) ; } catch ( NoSuchAlgorithmException e ) { throw new OAuthException ( "Invalid private key" , e ) ; } catch ( InvalidKeySpecException e ) { throw new OAuthException ( "Invalid private key" , e ) ; } }
target:  Sets the RSA-SHA1 private key object used to sign this request.