Decrypt files using AES method in Android

The original code posted in the question uses streams to read, decrypt and write to the corresponding files. This makes it possible to process files larger than available memory.

However, the originally posted code lacks Base64 decoding, which is required because the ciphertext of the PHP code is Base64 encoded.

BASE64 decoding can be comfortably implemented using Apache Encyclopedia-like codecs, which operate between and and are therefore easy to integrate:Base64InputStreamFileInputStreamCipherInputStream

import org.apache.commons.codec.binary.Base64InputStream;

...

public static void decrypt(String ciphertextFilepath, String decryptedFilePath) throws Exception {
    
    String password = "123";
    String initializationVector = "12dasdq3g5b2434b";

    byte[] key = new byte[16];
    byte[] passwordBytes = password.getBytes(StandardCharsets.UTF_8);
    System.arraycopy(passwordBytes, 0, key, 0, passwordBytes.length);
    SecretKeySpec secretKeySpec = new SecretKeySpec(key, "AES");
    IvParameterSpec ivParameterSpec = new IvParameterSpec(initializationVector.getBytes(StandardCharsets.UTF_8));
    
    Cipher cipher = Cipher.getInstance("AES/CBC/Pkcs5Padding");
    cipher.init(Cipher.DECRYPT_MODE, secretKeySpec, ivParameterSpec);
    
    try (FileInputStream fis = new FileInputStream(ciphertextFilepath);
         Base64InputStream b64is = new Base64InputStream(fis);
         CipherInputStream cis = new CipherInputStream(b64is, cipher);
         FileOutputStream fos = new FileOutputStream(decryptedFilePath)) {
            
            int read;
            byte[] buffersize = new byte[16]; // 16 bytes for testing, in practice use a suitable size (depending on your RAM size), e.g. 64 Mi
            while((read = cis.read(buffersize)) != -1) {
                fos.write(buffersize, 0, read);
        }
    }
}

Other fixed bugs/optimization points are (see also other answers/comments):

• Use CBC mode similar to PHP code
• Using the key from the PHP code
• A clear specification of the encoding used