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An exercise in cryptographic minimlism
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CLMM - the Crypto Library for Mere Mortals Based on TweetNaCl (http://tweetnacl.cr.yp.to/) NOTES AND DISCLAIMERS: 1. CLMM is not actually a library, it's a collection of utility. But "Crypto Utilities for Mere Mortals" has a rather unfortunate acronym. 2. CLMM is a prototype. Its purpose is mainly pedagogical. It was an exercise for me to figure out how to use tweetnacl, and I'm publishing it to get feedback and in the hopes that it might be useful as reference code. The tweetnacl crypto core was written by someone who knew what they were doing and so it's a sound foundation to build on, but CLMM itself has a number of design issues that make it unsuitable for serious security applications in its current form. Maybe some day. In the meantime, comments and constructive criticisms are welcome. ------------------------------------------------------------------- CLMM is a thin wrapper for tweetnacl intended to make it easier for people who are not familiar with the ins and outs of crypto code to be able to use it. CLMM consists of five unix utilities, each of which exposes a piece of tweetnacl's functionality in an easy-to-use manner. Each utility consits of less than 100 lines of source code. To build, simply run make. To use it, you first have to generate some keys. This is done using the genkey utility. By default, genkey generates keys for the current user, but you can also pass it another name as an optional argument to generate additional keys for testing. CLMM stores keys in the ~/.clmm/ directory, which CLMM creates if it does not already exist. There are four keys in each set, one public/secret keypair for encryption and another one for signing. Secret keys are stored in binary format to make it a little less tempting for naive users to share them. Public keys are stored in base64 format. Once you have a set of keys, the following operations are availble: encrypt file_name recipient_key_id [sender_key_id] The encrypt utility encrypts a file using authenticated encryption from the sender_key_id to the recipient_key_id. To encrypt a file you must have the recipient's public key in your ~/.clmm/ directory. (To encrypt a file for your own use you can, of course, use your own key as the recipeint key.) decrypt file_name [recipient_key_id] This utility decrypts a file that has been encrypted by encrypt. sign file_name [key_id] This utility produces an ed25519 signature for the given file. The output is a self-contained datum that can be verified independent of the original file. verify [signature_file] Displays 'valid' or 'NOT valid' as the case may be, and also returns a status code of 0 if the signature is valid, -1 if it is not. ------------------------------------------------------------------- DESIGN NOTES: The format for signatures is a base64 representation of the public key, followed by the 64-byte signature in hexadecimal, followed by the 64-byte sha512 hash of the document being signed, also in hex. The signature is actually not of the original document, but rather of the sha512 hash of the documnet (so the documnt is hashed twice, once by CLMM, and then again by the ed25519 algorithm). The reason for this is that tweetnacl has a very weird signing API that copes the documnet being signed. By hashing the document first, this (gratuitous AFAICT) copy operation is constant-time and not horribly expensive. The original document only needs to be traversed once, to compute the original hash. The hash is stored in hexadecimal format to make it easy to visually compare it to the output of the unix shasum utility. The signature is stored in hex for aesthetic reasons: so it will match the document hatch. The signature comes before the document id to make it easier to parse: tweetnacl expects the input to its signature verification routine to be in that order.