constructor(hash) {
			const sha1 = this;
			/**
			 * The hash's block size, in bits.
			 * @constant
			 */
			sha1.blockSize = 512;
			/**
			 * The SHA-1 initialization vector.
			 * @private
			 */
			sha1._init = [0x67452301, 0xEFCDAB89, 0x98BADCFE, 0x10325476, 0xC3D2E1F0];
			/**
			 * The SHA-1 hash key.
			 * @private
			 */
			sha1._key = [0x5A827999, 0x6ED9EBA1, 0x8F1BBCDC, 0xCA62C1D6];
			if (hash) {
				sha1._h = hash._h.slice(0);
				sha1._buffer = hash._buffer.slice(0);
				sha1._length = hash._length;
			} else {
				sha1.reset();
			}
		}
Context for a SHA-1 operation in progress.
@constructor