Update crypto.md to correct function description for decipher.setAAD

[jbuhacoff]

According to the NodeJS CCM example, when decrypting the plaintextLength parameter actually refers to the ciphertext length, not the plaintext length:

decipher.setAAD(aad, {
  plaintextLength: ciphertext.length
});

The same can be seen in the OpenSSL docs where a call to EVP_DecryptUpdate passes the ciphertext length:

/* Provide the total ciphertext length */
    if(1 != EVP_DecryptUpdate(ctx, NULL, &len, NULL, ciphertext_len))
        handleErrors();

This parameter probably should have been called inputLength or bufferLength instead of plaintextLength, so that it makes sense both when encrypting and decrypting, but at least we can correct the sentence in the documentation for now to refer to the correct value.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[addaleax]

@nodejs/crypto

[tniessen]

I am not sure if the PR in its current state is an improvement.

when decrypting the plaintextLength parameter actually refers to the ciphertext length, not the plaintext length

The plaintextLength option only applies to CCM ciphers. The ciphertext length produced by CCM ciphers is always equal to the plaintext length. So there is really no difference between "plaintext length" and "ciphertext length" (except for the caveat below).

This parameter probably should have been called inputLength or bufferLength instead of plaintextLength

When I implemented this, I called the option plaintextLength to avoid confusion as to whether it includes the length of the authentication tag. Many crypto libraries include the authentication tag in the ciphertext, which means that they produce ciphertexts of the length plaintextLength + authTagLength. Node.js does not include the authentication tag, so the ciphertext length is always plaintextLength.

---

I think a better approach would be to explain that the ciphertext length is equal to the plaintext length.

[jbuhacoff]

That's a great explanation, it would be helpful to add it to the documentation. Since there is already a link to the CCM Mode section, how about adding it there?

[addaleax]

@tniessen Do you want to take another look here?

[tniessen]

The entire paragraph is a verbatim quote from me, so I guess I'll have to approve it :)

[addaleax]

Landed in d093e78 🎉 Thanks for the PR again!