randombit/botan

Support ssh-rsa public key format (RFC 4253)

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#2,602 opened on Jan 23, 2021

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 (2 comments) (1 reaction) (0 assignees)C++ (654 forks)auto 404
enhancementhelp wanted

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Description

Currently the RSA class only supports the PKCS#1 (RFC 3447) standard. This is perfect for almost all cases except for when interfacing with SSH. RFC 4253 reverses the two public key parameters which makes using Botan with SSH keys impossible (without extensive workarounds).

This could be solved in a couple ways:

  1. Create a special SSHRSA_PublicKey class that extends the RSA_PublicKey classes. Invert the n and e reading and writing as necessary, including the fingerprint.
  2. Add provisions to the existing RSA_PublicKey class to handle an algorithm identifier of "ssh-rsa" differently than "rsa".
  3. Create stand-alone helper functions that use existing RSA_PublicKey but ensure the n and e are read/wrote in the correct order.

I am willing to do the legwork to make this possible, which is preferred by @randombit?

Contributor guide

Research direction
Study RFC 4253 and compare with Botan's current RSA public key handling. Implement a new SSHRSA PublicKey class or modify existing RSA PublicKey to support ssh rsa format. Ensure proper encoding/decoding of exponent and modulus in reverse order.
Tech stack
cpp
Domain
security
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C++cryptography basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65