Add string utilities

ccryptolib/util.lua

@@ -0,0 +1,45 @@
+--- General utilities for handling byte strings.
+
+local expect = require "cc.expect".expect
+local random = require "cryptolib.random"
+local poly1305 = require "ccryptolib.poly1305"
+
+--- Returns the hexadecimal version of a string.
+--- @param str string A string.
+--- @return string hex The hexadecimal version of the string.
+local function toHex(str)
+    expect(1, str, "string")
+    return ("%02x"):rep(#str):format(str:byte(1, -1))
+end
+
+--- Converts back a string from hexadecimal.
+--- @param hex string A hexadecimal string.
+--- @return string? str The original string, or nil if the input is invalid.
+local function fromHex(hex)
+    expect(1, hex, "string")
+    local out = {}
+    local n = 0
+    for c in hex:gmatch("%x%x") do
+        n = n + 1
+        out[n] = tonumber(c, 16)
+    end
+    if 2 * n == #hex then return string.char(table.unpack(out)) end
+end
+
+--- Compares two strings while mitigating secret leakage through timing.
+--- @param a string
+--- @param b string
+--- @return boolean eq Whether a == b.
+local function compare(a, b)
+    expect(1, a, "string")
+    expect(2, b, "string")
+    if #a ~= #b then return false end
+    local kaux = random.random(32)
+    return poly1305.mac(kaux, a) == poly1305.mac(kaux, b)
+end
+
+return {
+    toHex = toHex,
+    fromHex = fromHex,
+    compare = compare,
+}