Add max secret length

Author: thesimplekid

00.md

@@ -93,7 +93,7 @@ A `Proof` is also called an _input_ and is generated by `Alice` from a `BlindSig
 }
 ```
 
-`amount` is the amount of the `Proof`, `secret` is the secret message and is a utf-8 encoded string (the use of a 64 character hex string generated from 32 random bytes is recommended to prevent fingerprinting), `C` is the unblinded signature on `secret` (hex string), `id` is the [keyset id][02] of the mint public keys that signed the token (hex string).
+`amount` is the amount of the `Proof`, `secret` is the secret message and is a utf-8 encoded string that **MUST NOT** exceed 512 characters (the use of a 64 character hex string generated from 32 random bytes is recommended to prevent fingerprinting), `C` is the unblinded signature on `secret` (hex string), `id` is the [keyset id][02] of the mint public keys that signed the token (hex string).
 
 ## 0.2 - Protocol