rsac is software that performs RSA public key cipher. It uses a key of 2,048 bits in length. (For readability, it also supports a key of 1,024 bits in length.) As a method of RSA public key cipher, RSA-KEM PKCS#1 v.1.5 it supports these two methods. RSA-KEM, it works as following 1. First, encrypt the object to be encrypted with the block cipher (AES-256-CBC). 2. Next, the encryption key used in the block cipher is encrypted with RSA-KEM. Since public key cryptography and block cipher are used together, it is sometimes called hybrid cipher. Also, for encrypting block encryption, always use the newly generated encryption key. Therefore, it is sometimes called one-time pad encryption. When using RSA-KEM, there is no limit on the length of encryption target. PKCS#1 v.1.5 is the most basic method for RSA public key cipher. On this method, the encryption target is encrypted directly using RSA public key cipher. When using PKCS#1 v.1.5, the length of the encryption target is limited to 245 bytes. If the length of the encryption target exceeds 245 bytes, it can not be encrypted. "Encryption key, ciphertext input/output format" of rsac conform to the specifications of PKCS, RFC, etc. Therefore, it is possible to handle encryption keys and ciphertexts created by other public key cryptographic software. rsac supports encryption and decryption using X.509 public key certificate, PKCS#12. Although it is of simple format, rsac can also creates X.509 public key certificate, PKCS#12.