Truncate new SKIDs to 192 bits

More than long enough, and saves quite a bit of space especially for
SHA-512 certificates.

3 files changed, 14 insertions, 6 deletions

diff --git a/news.rst b/news.rst
index cb77a317f..573eb152c 100644
--- a/news.rst
+++ b/news.rst
@@ -13,6 +13,11 @@ Version 2.7.0, Not Yet Released
 * XMSS signature verification did not check that the signature was of
   the expected length which could lead to a crash. (GH #1537)
 
+* Botan generates X.509 subject key IDs by hashing the public key with
+  whatever hash function is being used to sign the certificate. However
+  especially for SHA-512 this caused SKIDs that were far longer than
+  necessary. Now all SKIDs are truncated to 192 bits.
+
 * Small optimizations for ECC (#1531)
 
 * In the test suite use ``mkstemp`` to create temporary files instead
diff --git a/src/lib/x509/x509_ext.cpp b/src/lib/x509/x509_ext.cpp
index b969ad7cf..c0fe904bc 100644
--- a/src/lib/x509/x509_ext.cpp
+++ b/src/lib/x509/x509_ext.cpp
@@ -434,6 +434,12 @@ Subject_Key_ID::Subject_Key_ID(const std::vector<uint8_t>& pub_key, const std::s
 
    hash->update(pub_key);
    hash->final(m_key_id.data());
+
+   // Truncate longer hashes, 192 bits here seems plenty
+   const size_t max_skid_len = (192 / 8);
+   if(m_key_id.size() > max_skid_len)
+      m_key_id.resize(max_skid_len);
+   m_key_id[0] = 0x44;
    }
 
 /*
diff --git a/src/lib/x509/x509self.cpp b/src/lib/x509/x509self.cpp
index 78bbe8615..32f21c101 100644
--- a/src/lib/x509/x509self.cpp
+++ b/src/lib/x509/x509self.cpp
@@ -82,13 +82,10 @@ X509_Certificate create_self_signed_cert(const X509_Cert_Options& opts,
       extensions.add_new(new Cert_Extension::Key_Usage(constraints), true);
       }
 
-   std::unique_ptr<HashFunction> hash(HashFunction::create_or_throw(hash_fn));
-   hash->update(pub_key);
-   std::vector<uint8_t> skid(hash->output_length());
-   hash->final(skid.data());
+   std::unique_ptr<Cert_Extension::Subject_Key_ID> skid(new Cert_Extension::Subject_Key_ID(pub_key, hash_fn));
 
-   extensions.add_new(new Cert_Extension::Subject_Key_ID(skid));
-   extensions.add_new(new Cert_Extension::Authority_Key_ID(skid));
+   extensions.add_new(new Cert_Extension::Authority_Key_ID(skid->get_key_id()));
+   extensions.add_new(skid.release());
 
    extensions.add_new(
       new Cert_Extension::Subject_Alternative_Name(subject_alt));