Compute an HMAC — crypt_hmac • zucrypt

Computes a keyed message authentication code (RFC 2104) over a raw vector, or over everything a connection yields.

Usage

crypt_hmac(data, key, algorithm = "sha256")

Arguments

data: A raw vector, or a connection to read to its end (see "Files and connections" below). Character input is never accepted: this package does not guess a text encoding and never treats a string as a file name. Convert deliberately with charToRaw() or serialize(), or pass file(path) to hash a file.
key: A raw vector holding the key. Any length is accepted, including zero, as the standard specifies. This is a key, not a password: deriving a key from a password is the caller's responsibility and is not something this package does for you.
algorithm: A single algorithm name: one of "sha1", "sha256", "sha384" or "sha512", and whatever crypt_info()$algorithms reports is what this build actually accepts. Matched exactly – there is no partial matching and no fallback, so a typo is an error rather than a quietly different algorithm.

Value

A raw vector the length of the underlying digest.

Files and connections

data may be a connection, read in chunks exactly as for crypt_hash(); see "Files and connections" there. The key is always a raw vector.

Verifying a MAC

Compare with crypt_equal(), never with identical() or ==. An ordinary comparison stops at the first differing byte, and the time it takes is therefore a measurement of how much of the expected value an attacker has guessed.

Examples

key <- as.raw(rep(0x0b, 20))
tag <- crypt_hmac(charToRaw("Hi There"), key)
tag
#>  [1] b0 34 4c 61 d8 db 38 53 5c a8 af ce af 0b f1 2b 88 1d c2 00 c9 83 3d a7 26
#> [26] e9 37 6c 2e 32 cf f7

# Verification, done properly.
crypt_equal(tag, crypt_hmac(charToRaw("Hi There"), key))
#> [1] TRUE