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<p class="MsoNormal"><span lang="EN-US">Hi all,<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 need to implement support for X52219/X448 for DH key exchange (and Ed52219/Ed448 for DSA) elliptic curves in our project. I need to export public key for DH exchange in form of DER encoded chunk in form tag+X-coordinate+Y-coordinate.
Thus I need to get EC_POINT from EVP_PKEY and encode it as needed. I understand that those key types differs from EC types in way that I need just X coordinate and a flag bit to reconstruct the key, but still, how do I get the X coordinate?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">My solution works for all other EC types such as SecpX and Brainpool families, but not for X52219/X448 keys and I do not completely understand why. Specifically when I decode public key previously encoded with i2d_PUBKEY()
to EVP_PEKY and try to get EC_KEY by calling EVP_PKEY_get0_EC_KEY(), it returns NULL and issues an error that it’s not an EC key…<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’m using following code:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="p1"><span class="s1">EVP_PKEY</span> *key = <span lang="EN-US">… // Decode from DER encoded public key<o:p></o:p></span></p>
<p class="p2"><o:p> </o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s2">if</span>(key !=
<span class="s2">nil</span>) {<o:p></o:p></p>
<p class="p2"><o:p> </o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s1">EC_KEY</span> *ecKey =
<span class="s3">EVP_PKEY_get0_EC_KEY</span>(key);<o:p></o:p></p>
<p class="p2"> <span lang="EN-US">/// When X52219 or X448 key is passed, ecKey is NULL<o:p></o:p></span></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s2">if</span>(ecKey !=
<span class="s2">NULL</span>) {<o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s2">const</span>
<span class="s1">EC_POINT</span> *point = <span class="s3">EC_KEY_get0_public_key</span>(ecKey);<o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s2">const</span>
<span class="s1">EC_GROUP</span> *group = <span class="s3">EC_KEY_get0_group</span>(ecKey);<o:p></o:p></p>
<p class="p2"><span class="apple-converted-space"> </span><o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s2">if</span>(point !=
<span class="s2">NULL</span> && group != <span class="s2">NULL</span>) {<o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s1">BIGNUM</span> *bnX =
<span class="s3">BN_new</span>();<o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s1">BIGNUM</span> *bnY =
<span class="s3">BN_new</span>();<o:p></o:p></p>
<p class="p2"><span class="apple-converted-space"> </span><o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s2">if</span>(<span class="s3">EC_POINT_get_affine_coordinates</span>(group, point, bnX, bnY,
<span class="s2">NULL</span>)) {<o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s2">char</span> *hexX =
<span class="s3">BN_bn2hex</span>(bnX);<o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s2">char</span> *hexY =
<span class="s3">BN_bn2hex</span>(bnY);<o:p></o:p></p>
<p class="p2"><o:p> </o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s4"><span lang="EN-US">// Convert to custom data structures<o:p></o:p></span></span></p>
<p class="p1"><span class="s4"><span lang="EN-US"> …</span></span><span lang="EN-US" style="color:#D0A8FF"><o:p></o:p></span></p>
<p class="p1"><span class="apple-converted-space"> </span>}<o:p></o:p></p>
<p class="p2"><span class="apple-converted-space"> </span><o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s3">BN_free</span>(bnX);<o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span><span class="s3">BN_free</span>(bnY);<o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span>}<o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span>}<o:p></o:p></p>
<p class="p1"><span class="apple-converted-space"> </span>}<o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US"><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">Is there any way how to export those key types in desired format? I’m using OpenSSL version 1.1.1q.<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">Thank you very much for any hint<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Matt<o:p></o:p></span></p>
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