通过证书拿到公钥，进行加密、解密

InputStream is = this.getAssets().open(fileName);//证书读取流

CertificateFactory cf = CertificateFactory.getInstance("X.509");//获取X.509的证书工厂

X509Certificate certificate = (X509Certificate) cf.generateCertificate(is);//获取证书实例

PublicKey publicKey = certificate.getPublicKey();//获取证书公钥

Cipher tcsCipher = Cipher.getInstance(publicKey.getAlgorithm());//通过公钥的加密算法获取加解密实例

　　　　tcsCipher.init(Cipher.ENCRYPT_MODE, publicKey);//通过公钥初始化实例

byte[] encode = Base64.encode("123".getBytes(),Base64.DEFAULT);/* 要加密的字符串进行编码 */
byte[] doFinal = tcsCipher.doFinal(encode);/*加密*/
String tcsEncryptPassword = Base64.encodeToString(doFinal, Base64.DEFAULT);/* 将加密后的字符串进行编码 */