diff --git a/README.md b/README.md
index 086833a42a91af54255c0f97141f424352049cd8..80994fd8b1cfb3b9ac7a708a2adbe56fb1dc0273 100644
--- a/README.md
+++ b/README.md
@@ -23,4 +23,13 @@ An S/MIME certificate has both a serial number (arbitrarily assigned by the issu
 
 ```bash
 gpg --show-keys --with-fingerprint fs-coli@cl.uni-heidelberg.de.pgp.pub.asc 
-```
\ No newline at end of file
+```
+
+## Mutual signatures
+
+You trust one key (S/MIME or OpenPGP), but not the other? For this case, we have mutually signed each key with the other one. The directory `Signatures` contains the following two files:
+
+* The S/MIME key signed with the OpenPGP key: [`Signatures/fs-coli@cl.uni-heidelberg.de.smime.pub.pem.sig`](Signatures/fs-coli@cl.uni-heidelberg.de.smime.pub.pem.sig)
+* The OpenPGP key signed with the S/MIME key: [`Signatures/fs-coli@cl.uni-heidelberg.de.pgp.pub.asc.sig`](Signatures/fs-coli@cl.uni-heidelberg.de.pgp.pub.asc.sig)
+
+By verifying the signature made with the key you already trust, you can verify that you can also trust the other key.
diff --git a/Signatures/fs-coli@cl.uni-heidelberg.de.pgp.pub.asc.sig b/Signatures/fs-coli@cl.uni-heidelberg.de.pgp.pub.asc.sig
new file mode 100644
index 0000000000000000000000000000000000000000..0a566481b3e01e2d17dd2619d9f4abecf94d18a2
--- /dev/null
+++ b/Signatures/fs-coli@cl.uni-heidelberg.de.pgp.pub.asc.sig
@@ -0,0 +1,87 @@
+MIME-Version: 1.0
+Content-Type: multipart/signed; protocol="application/x-pkcs7-signature"; micalg="sha-256"; boundary="----7114FE60DBBFA0477D3F7E749D204BAF"
+
+This is an S/MIME signed message
+
+------7114FE60DBBFA0477D3F7E749D204BAF
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+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+=9b2a
+-----END PGP PUBLIC KEY BLOCK-----
+
+------7114FE60DBBFA0477D3F7E749D204BAF
+Content-Type: application/x-pkcs7-signature; name="smime.p7s"
+Content-Transfer-Encoding: base64
+Content-Disposition: attachment; filename="smime.p7s"
+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+
+------7114FE60DBBFA0477D3F7E749D204BAF--
+
diff --git a/Signatures/fs-coli@cl.uni-heidelberg.de.smime.pub.pem.sig b/Signatures/fs-coli@cl.uni-heidelberg.de.smime.pub.pem.sig
new file mode 100644
index 0000000000000000000000000000000000000000..0b7f78524d757b8cdfaf454fb3def364138764f9
--- /dev/null
+++ b/Signatures/fs-coli@cl.uni-heidelberg.de.smime.pub.pem.sig
@@ -0,0 +1,56 @@
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA512
+
+Issuer ...: /CN=GEANT Personal CA 4/O=GEANT Vereniging/C=NL
+Serial ...: 076407AE88FD1AD97435B13A03EB04B0
+Subject ..: /O=Universitaet Heidelberg/ST=Baden-Wuerttemberg/C=DE/EMail=fs-coli@cl.uni-heidelberg.de/2.5.4.97=GOVDE+BW
+    aka ..: fs-coli@cl.uni-heidelberg.de
+Keygrip ..: 8CC6C7ED16AF341184BA3B87DC74062A924E1E20
+
+- -----BEGIN CERTIFICATE-----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- -----END CERTIFICATE-----
+-----BEGIN PGP SIGNATURE-----
+
+iHUEARYKAB0WIQSgx3O/2VQpEM3Mr8GeimFVM+SIegUCZZFiNQAKCRCeimFVM+SI
+elWhAQDq49ftb+SSHNjQg7jDQvCVWmGgASMVJWYE3ZvCsTBLIwEA5lYpQAoiI+VJ
+G6dONh+vgZ3YjNgmIlLbdeSxH6xKSA0=
+=Dat8
+-----END PGP SIGNATURE-----