Hi,

rene@frodo:~/libapache2-mod-auth-mellon-0.19.1$ grep -r xmlsec
doc/user_guide/mellon_user_guide.adoc:===== Using xmlsec to sign metadata [[xmlsec_metadata_signing]]
doc/user_guide/mellon_user_guide.adoc:The `xmlsec` tools are commonly available on most Linux based
doc/user_guide/mellon_user_guide.adoc:SAML implementation uses the `xmlsec` library to perform all of its
doc/user_guide/mellon_user_guide.adoc:XML signing and signature verification. `xmlsec` usually ships with an
doc/user_guide/mellon_user_guide.adoc:`xmlsec` command line utility, which can perform XML signing
doc/user_guide/mellon_user_guide.adoc:NOTE: `xmlsec` may be packaged under the name `xmlsec1` in your
doc/user_guide/mellon_user_guide.adoc:To sign Mellon metadata using `xmlsec` you need to add a signature
doc/user_guide/mellon_user_guide.adoc:template to the Mellon metadata. When `xmlsec` reads the input
doc/user_guide/mellon_user_guide.adoc:document will be signed. In order for `xmlsec` to generate a signature
doc/user_guide/mellon_user_guide.adoc:xmlsec \\ <1>
doc/user_guide/mellon_user_guide.adoc:<1> `xmlsec` command may be named `xmlsec1` on your system
doc/user_guide/mellon_user_guide.adoc:xmlsec \\ <1>
doc/user_guide/mellon_user_guide.adoc:<1> `xmlsec` command may be named `xmlsec1` on your system
configure:#include <xmlsec/xmlenc.h>
configure:{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for xmlsec1-openssl" >&5
configure:printf %s "checking for xmlsec1-openssl... " >&6; }
configure:    { { printf "%s\n" "$as_me:${as_lineno-$LINENO}: \$PKG_CONFIG --exists --print-errors \"xmlsec1-openssl\""; } >&5
configure:  ($PKG_CONFIG --exists --print-errors "xmlsec1-openssl") 2>&5
configure:  pkg_cv_XMLSEC_CFLAGS=`$PKG_CONFIG --cflags "xmlsec1-openssl" 2>/dev/null`
configure:    { { printf "%s\n" "$as_me:${as_lineno-$LINENO}: \$PKG_CONFIG --exists --print-errors \"xmlsec1-openssl\""; } >&5
configure:  ($PKG_CONFIG --exists --print-errors "xmlsec1-openssl") 2>&5
configure:  pkg_cv_XMLSEC_LIBS=`$PKG_CONFIG --libs "xmlsec1-openssl" 2>/dev/null`
configure:                XMLSEC_PKG_ERRORS=`$PKG_CONFIG --short-errors --print-errors --cflags --libs "xmlsec1-openssl" 2>&1`
configure:                XMLSEC_PKG_ERRORS=`$PKG_CONFIG --print-errors --cflags --libs "xmlsec1-openssl" 2>&1`
configure:        as_fn_error $? "Package requirements (xmlsec1-openssl) were not met:
configure.ac:#include <xmlsec/xmlenc.h>
configure.ac:PKG_CHECK_MODULES(XMLSEC, xmlsec1-openssl)

Since libapache2-mod-auth-mellon uses libxmlsec1 directly it should also
directly build-depend on it, regardless of whether lasso already does

Patch:

$ debdiff libapache2-mod-auth-mellon_0.19.1-1.dsc libapache2-mod-auth-mellon_0.19.1-1.1.dsc
diff -Nru libapache2-mod-auth-mellon-0.19.1/debian/changelog libapache2-mod-auth-mellon-0.19.1/debian/changelog
--- libapache2-mod-auth-mellon-0.19.1/debian/changelog	2024-07-16 15:51:04.000000000 +0200
+++ libapache2-mod-auth-mellon-0.19.1/debian/changelog	2025-08-16 10:57:43.000000000 +0200
@@ -1,3 +1,10 @@
+libapache2-mod-auth-mellon (0.19.1-1.1) UNRELEASED; urgency=medium
+
+  * Non-maintainer upload.
+  * build-depend on libxmlsec1-dev (closes: #-1)
+
+ -- Rene Engelhard <rene@debian.org>  Sat, 16 Aug 2025 10:57:43 +0200
+
 libapache2-mod-auth-mellon (0.19.1-1) unstable; urgency=medium

   * New upstream release.
diff -Nru libapache2-mod-auth-mellon-0.19.1/debian/control libapache2-mod-auth-mellon-0.19.1/debian/control
--- libapache2-mod-auth-mellon-0.19.1/debian/control	2024-07-16 15:51:04.000000000 +0200
+++ libapache2-mod-auth-mellon-0.19.1/debian/control	2025-08-16 10:57:42.000000000 +0200
@@ -4,7 +4,7 @@
 Maintainer: Thijs Kinkhorst <thijs@debian.org>
 Build-Depends: debhelper-compat (= 13), dh-exec,
  dh-apache2, apache2-dev,
- libcurl3-dev, liblasso3-dev
+ libcurl3-dev, liblasso3-dev, libxmlsec1-dev
 Standards-Version: 4.7.0
 Vcs-Git: https://salsa.debian.org/debian/modmellon.git
 Vcs-Browser: https://salsa.debian.org/debian/modmellon

Given Vcs-*, I can also do a MR in salsa if you wish

Regards,

Rene