I’m using AES GCM authentication in my android project and it works fine. But getting some issues with authentication tag when it compare with openssl API generate tag. Please find the java code below:
SecretKeySpec skeySpec = new SecretKeySpec(key, "AES"); byte iv = generateRandomIV(); IvParameterSpec ivspec = new IvParameterSpec(iv); Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding"); cipher.init(Cipher.ENCRYPT_MODE, skeySpec, ivspec); int outputLength = cipher.getOutputSize(data.length); // Prepare output buffer byte output = new byte[outputLength]; int outputOffset = cipher.update(data, 0, data.length, output, 0);// Produce cipher text outputOffset += cipher.doFinal(output, outputOffset);
I am using openssl for the same in iOS and generating authentication tag using below code
NSMutableData* tag = [NSMutableData dataWithLength:tagSize]; EVP_CIPHER_CTX_ctrl(&ctx, EVP_CTRL_GCM_GET_TAG, [tag length], [tag mutableBytes])
In java or bouncy castle, not able to get the exact authentication tag which openssl return and can you help me to resolve this issue. Thanks
In Java the tag is unfortunately added at the end of the ciphertext. You can configure the size (in bits, using a multiple of 8) using
GCMParameterSpec. You can therefore grab it using
Arrays.copyOfRange(ciphertext, ciphertext.length - (tagSize / Byte.SIZE), ciphertext.length) if you really want to.
It is unfortunate since the tag does not have to be put at the end, and it makes messes up the online nature of GCM decryption – requiring internal buffering instead of being able to directly return the plaintext. On the other hand, the tag is automatically verified during decryption.