diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2e95893ae98..cacaf51495a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Changed
 
 - The `read_ledger.py` tool now has a `--quiet` option which avoids printing anything per-transaction, as well as other performance improvements, which should make it more useful in verifying the integrity of large ledgers.
+- COSE signatures now set a kid that is a hex-encoded SHA-256 of the DER representation of the key used to produce them (#6703).
 
 ## [6.0.0-dev8]
 
diff --git a/cddl/ccf-merkle-tree-cose-signature.cddl b/cddl/ccf-merkle-tree-cose-signature.cddl
index 7ad74096b34..60171302ff0 100644
--- a/cddl/ccf-merkle-tree-cose-signature.cddl
+++ b/cddl/ccf-merkle-tree-cose-signature.cddl
@@ -13,7 +13,7 @@ unprotected-headers = {
 
 protected-headers = {
   &(alg: 1) => int,                     ; signing algoritm ID, as per RFC8152
-  &(kid: 4) => bstr,                    ; signing key hash
+  &(kid: 4) => bstr,                    ; Opaque key identifier, hex-encoded SHA-256 of the public key encoded as DER.
   &(cwt: 15) => cwt-map,                ; CWT claims, as per RFC8392
   &(vds: 395) => int,                   ; verifiable data structure, as per COSE Receipts (draft) RFC (https://datatracker.ietf.org/doc/draft-ietf-cose-merkle-tree-proofs/)
   "ccf.v1" => ccf-map                   ; a set of CCF-specific parameters
diff --git a/python/src/ccf/cose.py b/python/src/ccf/cose.py
index 573e2c13f44..553dbff90f3 100644
--- a/python/src/ccf/cose.py
+++ b/python/src/ccf/cose.py
@@ -211,9 +211,12 @@ def verify_receipt(
     # Extract the expected KID from the public key used for verification,
     # and check it against the value set in the COSE header before using
     # it to verify the proofs.
-    expected_kid = sha256(
-        key.public_bytes(Encoding.DER, PublicFormat.SubjectPublicKeyInfo)
-    ).digest()
+    expected_kid = (
+        sha256(key.public_bytes(Encoding.DER, PublicFormat.SubjectPublicKeyInfo))
+        .digest()
+        .hex()
+        .encode()
+    )
     receipt = Sign1Message.decode(receipt_bytes)
     cose_key = from_cryptography_eckey_obj(key)
     assert receipt.phdr[pycose.headers.KID] == expected_kid
diff --git a/src/crypto/openssl/cose_sign.cpp b/src/crypto/openssl/cose_sign.cpp
index 5b6f421181e..047e117e13e 100644
--- a/src/crypto/openssl/cose_sign.cpp
+++ b/src/crypto/openssl/cose_sign.cpp
@@ -200,7 +200,7 @@ namespace ccf::crypto
   }
 
   std::shared_ptr<COSEParametersFactory> cose_params_int_bytes(
-    int64_t key, const std::vector<uint8_t>& value)
+    int64_t key, std::span<const uint8_t> value)
   {
     const size_t args_size = sizeof(key) + value.size() +
       +extra_size_for_int_tag + extra_size_for_seq_tag;
@@ -213,7 +213,7 @@ namespace ccf::crypto
   }
 
   std::shared_ptr<COSEParametersFactory> cose_params_string_bytes(
-    const std::string& key, const std::vector<uint8_t>& value)
+    const std::string& key, std::span<const uint8_t> value)
   {
     const size_t args_size = key.size() + value.size() +
       extra_size_for_seq_tag + extra_size_for_seq_tag;
diff --git a/src/crypto/openssl/cose_sign.h b/src/crypto/openssl/cose_sign.h
index a8513cca91b..5300962ca45 100644
--- a/src/crypto/openssl/cose_sign.h
+++ b/src/crypto/openssl/cose_sign.h
@@ -129,10 +129,10 @@ namespace ccf::crypto
     const std::string& key, const std::string& value);
 
   std::shared_ptr<COSEParametersFactory> cose_params_int_bytes(
-    int64_t key, const std::vector<uint8_t>& value);
+    int64_t key, std::span<const uint8_t> value);
 
   std::shared_ptr<COSEParametersFactory> cose_params_string_bytes(
-    const std::string& key, const std::vector<uint8_t>& value);
+    const std::string& key, std::span<const uint8_t> value);
 
   class COSEParametersPair : public COSEParametersFactory
   {
diff --git a/src/node/history.h b/src/node/history.h
index d0a91cb3f11..0dcd0fd420a 100644
--- a/src/node/history.h
+++ b/src/node/history.h
@@ -379,8 +379,8 @@ namespace ccf
       constexpr int64_t vds_merkle_tree = 2;
 
       const auto& service_key_der = service_kp.public_key_der();
-      std::vector<uint8_t> kid(SHA256_DIGEST_LENGTH);
-      SHA256(service_key_der.data(), service_key_der.size(), kid.data());
+      auto kid = ccf::crypto::Sha256Hash(service_key_der).hex_str();
+      std::span<const uint8_t> kid_span{(uint8_t*)kid.data(), kid.size()};
 
       const auto time_since_epoch =
         std::chrono::duration_cast<std::chrono::seconds>(
@@ -415,7 +415,7 @@ namespace ccf
       const auto pheaders = {
         // Key digest
         ccf::crypto::cose_params_int_bytes(
-          ccf::crypto::COSE_PHEADER_KEY_ID, kid),
+          ccf::crypto::COSE_PHEADER_KEY_ID, kid_span),
         // VDS
         ccf::crypto::cose_params_int_int(
           ccf::crypto::COSE_PHEADER_KEY_VDS, vds_merkle_tree),