Normally buttress dams are made of concrete and feature a series braces called buttresses along the downstream side of the dam to prevent the natural flow of water. The Daniel-Johnson Dam in Quebec, Canada is a multiple arch buttress dam.
SHA-1 HMAC Generation. In cryptography, a keyed-hash message authentication code (HMAC) is a specific construction for calculating a message authentication code (MAC) involving a cryptographic hash function in combination with a secret cryptographic key.he following is a Python implementation of HMAC-MD5: Python includes an hmac module, so the function above can be replaced by a shorter version. The following is a PHP implementation of HMAC-SHA1: PHP includes an hmac function, so the example above can be replaced by a shorter version.
An HMAC is a MAC which is based on a hash function. The basic idea is to concatenate the key and the message, and hash them together. Since it is impossible, given a cryptographic hash, to find out what it is the hash of, knowing the hash (or even a collection of such hashes) does not make it possible to find the key. The key is used in generating the HMAC from a message. 2  The key used for generating HMACS is not anyones public key. 3  Its more like a shared secret between two parties. 4  You could check out the REST API authentication method for AWS stuff to better understand how HMACS are used for URL-signing.