Hi ssmith_c, Thanks for your patience and help so far. Just wanted to update you — the Bitvavo API is rejecting valid HMAC signatures generated from multiple independent implementations: Python, C++ (OpenSSL), and even raw curl requests all produce identical signatures, yet the API responds with a “signature is invalid” error. I even tried inspecting the traffic with Wireshark, but that didn’t shed any light — everything looks correct on the client side. I’ve contacted Bitvavo support with detailed logs and test cases. They’re investigating on their end now. So it seems the issue is definitely with Bitvavo’s signature verification rather than our code. I’ll post any updates when I get them. Thanks again for your support! regards, Sascha