decrypt.rb

[David Salisbury]

>>>
>> Yes. I gave it a try here, and it seems to work. Does it give any extra
>> information if you include -i flag?
>>
>> Aki
>>
>
> Yes, I had tried that, and it doesn't give much extra information, at 
> least to my eye, that seems to help my issue.  Above the previous 
> output it outputs the Version, Flags, Header length, Cipher algo, and 
> Digest algo, and then the Key derivation Rounds. Then it does the 
> previous output and exits as before.
>
> I tried using pry to debug through the script a little, and strace as 
> well, but have not found anything pointing me in the direction of a 
> solution or what may be causing it not to work for me yet. Will keep 
> looking.
>
> Out of curiosity, what version of ruby were you using to run the 
> script?  My ruby version is 2.5.1p57.
>
> -Dave

So, I found that in decrypt.rb there is a point where this section is 
reached:

[code]
unless our_key == nil
    # decrypt data!
[/code]

While testing I discovered that, for me, our_key was apparently equal to 
nil because the code was never even making it into that block.  There 
was a block right above that that was setting our_key to nil if a 
certain condition happened, but I could tell that condition wasn't 
happening as the accompanying error message wasn't printing.  Looking 
farther up, I found:

[code]
our_key = key if key[:digest] == options[:key_digest]
[/code]

I printed the values of key[:digest] and options[:key_digest], and they 
are in fact different.  Since our_key is nil by default, our_key was 
just remaining nil, hence no decryption for me.

The key[:digest] variable is filled a little above that part of the code:

[code]
(key[:type],key[:digest]) = options[:input].read(33).unpack('Ca*')
[/code]

and options[:key_digest] is filled as the private key option is passed in:

[code]
opts.on("-k","--key KEY", "Private key to decrypt file") do |k|
   options[:key] = OpenSSL::PKey.read(File.open(k))
   options[:key_digest] = get_pubid_priv(options[:key])
end
[/code]

It's apparently using the key from the command line to get the key 
digest with the get_pubid_priv() function, and for some reason that 
value is coming back as different than the key digest that is 
ascertained by the "options[:input].read" line.

Out of curiosity, and since I know I'm using the correct key, I 
commented out the if statement in the our_key line so as not to make the 
comparison between the digests:

[code]
our_key = key #if key[:digest] == options[:key_digest]
[/code]

.... and then it worked!  The script successfully decrypted the message!

So, not being an expert at encryption, what are the ramifications of 
those digests being read as different values in the two different 
places??   I do notice that the get_pubid_priv() function is internal to 
the decrypt.rb script and calls several OpenSSL functions.

-Dave