HMAC for short messages I am using HMAC (hmac-sha256 to be precise) message authentication without encryption.  My question is, is having a short message make it easy to determine what my secret key is?

My C# code is as follows

HMACSHA256 hmacSha256 = new HMACSHA256(new byte[]{0xfa, 0x35, 0x24, ..., 0x5e}); // 64 byte secret key

byte[] authForMessageA = hmacSha256.ComputeHash(new byte[]{0x25}); // very short message
byte[] authForMessageB = hmacSha256.ComputeHash(new byte[]{0x66}); // another message, same key


I am exposing both the message (0x25 and 0x66) along with their authentication hashes to the public, does this make it simple for a hacker to determine my secret key (brute force or otherwise)?
No, you are not leaking any information except how to MAC those specific values with the specific key you are using. Using a short message is exactly as secure as using a long message.

For the following, remember the definition HMAC (K,m) = H((K ⊕ opad) || H((K ⊕ ipad) || m)). There are two hashes here, an outer hash and an inner hash nested inside the outer one. The message is inside the inner hash.

Suppose I am an attacker who has your message and MAC result, and I want to derive your key. In order to get your key, I will need to do a brute-force guess. I already know the function H, I know the constants ipad and opad, and I know your message m. I need to try building lots of hashes using different key values until I come up with the right HMAC. The hardness of this task is dependent entirely on the length of K; the length of m is not a factor.

Note, however, that if you are not using a temporary key, then any message/HMAC pairs you give out will be valid for your key, so an attacker could impersonate you in a replay attack for any messages she has seen from you.