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<p class="MsoNormal">Hi,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">We have an application that uses OpenSSL 1.1.1k to receive AES256-GCM encrypted messages. The streamed data starts with a 12 byte IV, then the ciphertext and ends with the 16 byte tag. Our decryption code is based
directly on the sample code in the Wiki and the distribution.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><a href="https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Decryption_using_GCM_mode">https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Decryption_using_GCM_mode</a><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><a href="https://github.com/openssl/openssl/blob/9b06ebb1edfddffea083ba36090af7eb7cad207b/demos/evp/aesgcm.c#L82-L115">https://github.com/openssl/openssl/blob/9b06ebb1edfddffea083ba36090af7eb7cad207b/demos/evp/aesgcm.c#L82-L115</a><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">First call EVP_DecryptInit_ex, set the IV length, provide key and IV. Some number of EVP_DecryptUpdate calls. Finally set the tag and call EVP_DecryptFinal_ex.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">An external code review has now pointed out to us that we should set the tag before the IV, according to OpenSSL documentation.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><a href="https://www.openssl.org/docs/man1.1.1/man3/EVP_CIPHER_CTX_ctrl.html#GCM-and-OCB-Modes">https://www.openssl.org/docs/man1.1.1/man3/EVP_CIPHER_CTX_ctrl.html#GCM-and-OCB-Modes</a><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">They must be referring to the sentence “The tag length can only be set before specifying an IV.” But what exactly does this mean? EVP_CTRL_AEAD_SET_TAG sets both the tag length and the value in a single call, unlike
EVP_CTRL_AEAD_SET_IVLEN which only sets the IV length. We do not have the tag value until the end of the message has been received.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I guess the sample code happens to work because 16 byte is the default tag size, so it is already “set” correctly before the IV is specified.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">How can we convince the reviewers that this is a non-issue?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Andrew.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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