Delegate some MAC operations to the provisioning context
The MAC key used to authenticate the public keys between the GenerateRkp
and GenerateCsr calls is supposed to be known _only_ to the secure
element. The code was passing the MAC key up from the context, which
makes the security guarantee difficult or impossible on some platforms.
Generate the MAC by delegating to the provisioning context. This required
refactoring a bit of code to use a callback for MAC calculation instead
of passing the raw MAC key around.
Bug: 182928606
Test: VtsHalRemotelyProvisionedComponentTargetTest
Test: RemoteProvisionerUnitTests
Change-Id: Id9a04ee0e0b1abc11d616ef53f41569cdaae3182
Merged-In: Id9a04ee0e0b1abc11d616ef53f41569cdaae3182
diff --git a/cppcose/cppcose.cpp b/cppcose/cppcose.cpp
index e17f273..2d8652e 100644
--- a/cppcose/cppcose.cpp
+++ b/cppcose/cppcose.cpp
@@ -53,12 +53,22 @@
} // namespace
-ErrMsgOr<bytevec> generateCoseMac0Mac(const bytevec& macKey, const bytevec& externalAad,
- const bytevec& payload) {
- if (macKey.empty()) {
- return "Empty MAC key";
- }
+ErrMsgOr<HmacSha256> generateHmacSha256(const bytevec& key, const bytevec& data) {
+ HmacSha256 digest;
+ unsigned int outLen;
+ uint8_t* out = HMAC(EVP_sha256(), //
+ key.data(), key.size(), //
+ data.data(), data.size(), //
+ digest.data(), &outLen);
+ if (out == nullptr || outLen != digest.size()) {
+ return "Error generating HMAC";
+ }
+ return digest;
+}
+
+ErrMsgOr<HmacSha256> generateCoseMac0Mac(HmacSha256Function macFunction, const bytevec& externalAad,
+ const bytevec& payload) {
auto macStructure = cppbor::Array()
.add("MAC0")
.add(cppbor::Map().add(ALGORITHM, HMAC_256).canonicalize().encode())
@@ -66,32 +76,24 @@
.add(payload)
.encode();
- bytevec macTag(SHA256_DIGEST_LENGTH);
- uint8_t* out = macTag.data();
- unsigned int outLen;
- out = HMAC(EVP_sha256(), //
- macKey.data(), macKey.size(), //
- macStructure.data(), macStructure.size(), //
- out, &outLen);
-
- assert(out != nullptr && outLen == macTag.size());
- if (out == nullptr || outLen != macTag.size()) {
+ auto macTag = macFunction(macStructure);
+ if (!macTag) {
return "Error computing public key MAC";
}
- return macTag;
+ return *macTag;
}
-ErrMsgOr<cppbor::Array> constructCoseMac0(const bytevec& macKey, const bytevec& externalAad,
- const bytevec& payload) {
- auto tag = generateCoseMac0Mac(macKey, externalAad, payload);
+ErrMsgOr<cppbor::Array> constructCoseMac0(HmacSha256Function macFunction,
+ const bytevec& externalAad, const bytevec& payload) {
+ auto tag = generateCoseMac0Mac(macFunction, externalAad, payload);
if (!tag) return tag.moveMessage();
return cppbor::Array()
.add(cppbor::Map().add(ALGORITHM, HMAC_256).canonicalize().encode())
.add(cppbor::Map() /* unprotected */)
.add(payload)
- .add(tag.moveValue());
+ .add(std::pair(tag->begin(), tag->end()));
}
ErrMsgOr<bytevec /* payload */> verifyAndParseCoseMac0(const cppbor::Item* macItem,
@@ -118,7 +120,10 @@
return "Unsupported Mac0 algorithm";
}
- auto macTag = generateCoseMac0Mac(macKey, {} /* external_aad */, payload->value());
+ auto macFunction = [&macKey](const bytevec& input) {
+ return generateHmacSha256(macKey, input);
+ };
+ auto macTag = generateCoseMac0Mac(macFunction, {} /* external_aad */, payload->value());
if (!macTag) return macTag.moveMessage();
if (macTag->size() != tag->value().size() ||