Frequently Asked Questions

Is CTR cipher mode compatible with Java?

Yes. When you instantiate your AES cipher in Java:

Cipher  cipher = Cipher.getInstance("AES/CTR/NoPadding");

SecretKeySpec keySpec = new SecretKeySpec(new byte[16], "AES");
IvParameterSpec ivSpec = new IvParameterSpec(new byte[16]);

cipher.init(Cipher.ENCRYPT_MODE, keySpec, ivSpec);

You are effectively using CTR mode without a fixed nonce and with a 128-bit big endian counter starting at 0. The counter will wrap around only after 2¹²⁸ blocks.

You can replicate the same keystream in PyCryptodome with:

ivSpec = b'\x00' * 16
ctr = AES.new(keySpec, AES.MODE_CTR, initial_value=ivSpec)

Are RSASSA-PSS signatures compatible with Java or OpenSSL?

Yes. For Java, you must consider that by default the mask is generated by MGF1 with SHA-1 (regardless of how you hash the message) and the salt is 20 bytes long.

If you want to use another algorithm or another salt length, you must instantiate a PSSParameterSpec object, for instance:

Signature ss = Signature.getInstance("SHA256withRSA/PSS");
AlgorithmParameters pss1 = ss.getParameters();
PSSParameterSpec pssParameterSpec = new PSSParameterSpec("SHA-256", "MGF1", new MGF1ParameterSpec("SHA-256"), 32, 0xBC);
ss.setParameter(spec1);

On the other hand, a quirk of OpenSSL (and of a few other libraries, especially if they are wrappers to OpenSSL) is that the default salt length is maximized, and it does not match in size the digest applied to the message, as recommended in RFC8017. In PyCryptodome, you maximize the salt length with:

key = RSA.import_key(open('privkey.der').read())
h = SHA256.new(message)
salt_bytes = key.size_in_bytes() - h.digest_size - 2
signature = pss.new(key, salt_bytes=salt_bytes).sign(h)

Why do I get the error No module named Crypto on Windows?

Check the directory where Python packages are installed, like:

/path/to/python/Lib/site-packages/

You might find a directory named crypto, with all the PyCryptodome files in it.

The most likely cause is described here and you can fix the problem with:

pip uninstall crypto
pip uninstall pycryptodome
pip install pycryptodome

The root cause is that, in the past, you most likely have installed an unrelated but similarly named package called crypto, which happens to operate under the namespace crypto.

The Windows filesystem is case-insensitive so crypto and Crypto are effectively considered the same thing. When you subsequently install pycryptodome, pip finds that a directory named with the target namespace already exists (under the rules of the underlying filesystem), and therefore installs all the sub-packages of pycryptodome in it. This is probably a reasonable behavior, if it wansn’t that pip does not issue any warning even if it could detect the issue.