Module digest: Message Digests

• The same message always produces the same digest.
• The probability of two different random messages resulting in the same digest is close to 1 to the number of different digests.
• Deriving the original message (or set of messages) from the digest is computationally (nearly) equivalent to trying all possible messages and looking for a matching digest.

To compute a digest, proceed as follows:

1. Create a message digest for the desired algorithm with digest.new.
2. Add data to the digest ("digest" the data) with digest.add or digest.add16 (for 16-bit strings), until the entire data has been digested.
3. Obtain the message digest with digest.get.
4. Call digest.clear to reset the digest and start a new digestion with the same algorithm.

// a function to compute the MD5 of a file function filemd5(name)   d=digest.new("MD5");   f=io.open(name);   buf=io.read(f, 256);   while buf#null do     digest.add(d, buf); buf=io.read(f, 256)   end;   io.close(f);   return digest.get(d) end

// A function to compute the SHA1 digest of a string // (lower 8 bits only). // The digest is returned as a hexadecimal string. function sha1(s)   d=digest.new("SHA1");   digest.add(d, s);   return digest.get(d) end print encoding.tohex(sha1("Lucky Luke")) → d977763133f671049ca991a32e6a107b33be97d0

• digest.add
• digest.add16
• digest.clear
• digest.get
• digest.new

digest.add

Digest the low bytes of all characters in string data with digest, or digest the single byte with number data.

// Compute the MD5 of the bytes 0 to 255 d=digest.new("MD5") for b=0 to 255 do   digest.add(d, b) end print encoding.tohex(digest.get(d)) → e2c865db4162bed963bfaa9ef6ac18f0

digest.add16

Digest the low and high bytes of all characters in string data with digest. This function can be used to properly digest multi-byte strings (in UTF-16 little endian), as digest.add discards the high bytes.

digest.clear

Clear all data digested so far with digest, and start a new digestion.

digest.get

Finalize the current digestion with digest and get the result as a string of bytes.

Repeated calls to digest.get without digesting more data in between will return the same digest.

Note that calling digest.get may add padding bytes to be able to compute the digest. Thus, subsequent digests after adding more data will most likely be different from those obtained without intermediate calls to digest.get.

digest.new

Create a new digest with algorithm (a string, not case sensitive), and return it. Currently supported algorithms are:

Alg.	Description	RFC	Bits
MD2	Older message digest algorithm for general digital signature applications, and optimized for 8-bit machines.	1319	128
MD5	Widely used message digest algorithm for general digital signature applications.	1321	128
SHA1	Secure Hash Algorithm (version 1) for use with the Digital Signature Standard.	3174	180

Throws ErrArgument if algorithm is not the name of a supported algorithm.

// create an MD5 digest object d=digest.new("md5") // get the hash (hex) of the empty message print encoding.tohex(digest.get(d)) → d41d8cd98f00b204e9800998ecf8427e