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<a href="https://gitlab.com/andrew-aladev">Andrew Aladjev</a>
commented:
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<p dir="auto"><code>lib/nettle/cipher.c</code>:</p>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">if (gnutls_memcmp(((uint8_t*)encr)+encr_size, tag, tag_size) != 0) {</span>
<span id="LC2" class="line" lang="plaintext"> fprintf(stderr, "algo: %d\n", ctx->cipher->algo);</span>
<span id="LC3" class="line" lang="plaintext"> return gnutls_assert_val(GNUTLS_E_DECRYPTION_FAILED);</span>
<span id="LC4" class="line" lang="plaintext">}</span></code></pre>
<p dir="auto">This is the place where <code>GNUTLS_E_DECRYPTION_FAILED</code> was born. <code>algo</code> equals <code>11 GNUTLS_CIPHER_AES_256_GCM</code>.</p>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">static void</span>
<span id="LC2" class="line" lang="plaintext">_gcm_decrypt(struct nettle_cipher_ctx *ctx, size_t length, uint8_t * dst,</span>
<span id="LC3" class="line" lang="plaintext"> const uint8_t * src)</span>
<span id="LC4" class="line" lang="plaintext">{</span>
<span id="LC5" class="line" lang="plaintext"> gcm_decrypt(GCM_CTX_GET_CTX(ctx->ctx_ptr), GCM_CTX_GET_KEY(ctx->ctx_ptr),</span>
<span id="LC6" class="line" lang="plaintext"> GCM_CTX_GET_CIPHER(ctx->ctx_ptr), ctx->cipher->encrypt_block,</span>
<span id="LC7" class="line" lang="plaintext"> length, dst, src);</span>
<span id="LC8" class="line" lang="plaintext">}</span></code></pre>
<p dir="auto">This is the function called as <code>ctx->cipher->decrypt(ctx, encr_size, plain, encr);</code> before <code>gnutls_memcmp</code> in <code>lib/nettle/cipher.c</code>. This function comes from <code>nettle</code> library and it is broken.</p>
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