App Crypto for Android

The application provides the following methods for encryption and decryption: affine cryptosystem for Latin text (26 letters), affine cryptosystem for Cyrillic text (30 letters), RSA cryptosystem and АSЕ cryptosystem.
Affine cryptosystem, are examples of private key cryptosystems. In a private key cryptosystem, once you know an encryption key, you can quickly find the decryption key. So, knowing how to encrypt messages using a particular key allows you to decrypt messages that were encrypted using this key.
The RSA  cryptosystem is a public-key cryptosystem, one of the oldest widely used for secure data transmission. In a public-key cryptosystem, the encryption key is public and distinct from the decryption key, which is kept secret (private). An RSA user creates and publishes a public key based on two large prime numbers, along with an auxiliary value. The prime numbers are kept secret. Messages can be encrypted by anyone, via the public key, but can only be decryption by someone who knows the private key.
The Advanced Encryption Standard (AES), also known by its original name Rijndael  is a specification for the encryption of electronic data established by the U.S. National Institute of Standards and Technology (NIST) in 2001. AES is a variant of the Rijndael block cipher.  Rijndael is a family of ciphers with different key and block sizes.
In app are used AES/CBC/PKCS5Padding which is a cryptographic mode of operation used for secure encryption and decryption of data. CBC (Cipher Block Chaining): this is an operating mode in which each block of data is combined with the previous block using the XOR operation before it is encrypted. The first block is combined with an initialization vector (IV), which must be unique for each encrypted message. The CBC mode provides better protection against attacks that attempt to alter the content of messages. PKCS5Padding: this is a padding scheme for the data that ensures that the input data is of a length that is a multiple of the block size (in this case 128 bits). PKCS5Padding adds bytes to the end of the last block so that it becomes full. These additional bytes contain information about the number of added bytes.
With all encryption methods in app, it is possible to store the encrypted files in the selected directory of the device where is encrypting file, in the names of which there is text „Encrypted...” plus name encrypting file, plus in brackets its extension and method of encryption like AES.
The encrypted text can be saved as files in Download device's folder.
In app private key for AES for saving are encrypted by RSA method and saved as separate file. So with AES encrypting are saved fore files with names:
EncryptedAes_xxx(.txt).bin – encrypted file xxx.txt;
EncryptedAesRSAPrivateKey_xxx.bin – private RSA key for encrypting private AES key for the same file xxx.txt;
EncryptedAesKey_xxx.bin – private AES key encrypted by RSAPrivate Key for the same file xxx.txt;
ivBin_xxx.bin – initialization vector for the same file xxx.txt;
So with RSA encrypting are saved three files with names:
EncryptedRSA_xxx(.txt).bin – encrypted file xxx.txt;
EncryptedRSAPrivateKey_xxx.bin - private RSA key;
EncryptedRSAPublicKey_xxx.bin - public RSA key;
With Affine Latin encrypting are saved two files with names:
EncryptedAffineLatin_xxx(.txt).bin – encrypted file xxx.txt;
EncryptedAffineLatinKeyB_xxx.bin - shifting b param;
Latin are changing whit Cyrillic with Affine Cyrillic encrypted files.
When decrypting, all files for the corresponding encryption method and the corresponding encrypted file (the file with the encrypted data and the corresponding keys) must be in the same folder.
When decrypting the method used to encrypt the file is selected first, the file with the encrypted data is selected too.
The application has ad banners that can activate or deactivate the display of ads.
The application has help and links to the of author's other apps.