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Update crypto.md to correct function description for decipher.setAAD #33095

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Update crypto.md
jbuhacoff Apr 27, 2020
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Update crypto.md
jbuhacoff Apr 27, 2020
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Merge branch 'patch-1' of github.com:jbuhacoff/node into patch-1
jbuhacoff Apr 27, 2020
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Update crypto.md 
Add explanation in [CCM mode](https://nodejs.org/docs/latest-v14.x/api/crypto.html#crypto_ccm_mode], that ciphertext length is equal to plaintext length in Node.js crypto output because the authentication tag is returned separately:

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

The same can be seen in the [OpenSSL docs](https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption) 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();
```
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jbuhacoff committed Apr 27, 2020
commit 7fda32fbd7eddde9171c28a9511b3ae2061a28f1
7 changes: 6 additions & 1 deletion doc/api/crypto.md
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Expand Up @@ -3095,7 +3095,12 @@ mode must adhere to certain restrictions when using the cipher API:
mode might fail as CCM cannot handle more than one chunk of data per instance.
* When passing additional authenticated data (AAD), the length of the actual
message in bytes must be passed to `setAAD()` via the `plaintextLength`
option. This is not necessary if no AAD is used.
option.
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`.
This is not necessary if no AAD is used.
* As CCM processes the whole message at once, `update()` can only be called
once.
* Even though calling `update()` is sufficient to encrypt/decrypt the message,
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