Subject: Re: SSH2 host key length and hash compute

Re: SSH2 host key length and hash compute

From: Alexander Lamaison <>
Date: Wed, 3 Jul 2013 01:16:14 +0100

On 3 July 2013 01:13, Alexander Lamaison <> wrote:
> On 2 July 2013 15:03, Kalpesh Parekh <> wrote:
>>>From: Alexander Lamaison <>
>>>Date: Tue, 2 Jul 2013 09:57:53 +0100
>>>On 2 July 2013 07:34, Kalpesh Parekh <> >wrote:
>>> Hi Alex
>>> The APIs I am using are libssh2_session_hostkey to retreive the host >key
>>> and
>>> libssh2_hostkey_hash to compute the hash from the key.
>>> The first API returns the length of the host key in a variable passed to
>>> >it
>>> as a function argument. The variable is of size_t type and indicates >the
>>> size of host key. I need to convert this value to bits. Can you let me
>>> >know
>>> how can I do this?
>>>Why do you need to convert it to bits? What does that actually mean?
>>>The size_t length is just a number.
>> The requirement is to show the strength of the host key in bits. I assumed
>> the length of the host key should be indicating this value and tried to
>> convert it to bits from size_t.
> The length of the host key returned by session_hostkey is the exact
> size of the buffer holding the "server public host key and
> certificates (K_S)" in bytes (see RFC 4253 [1]). I'm not sure of the
> exact relationship between that and the key strength, but another part
> of RFC 4253 [2] indicated that that buffer may include a "format
> identifier" (presumably ssh-rsa or ssh-dsa) before the key data.
> Therefore, I wouldn't trust that they key strength is the returned
> length * 8.
>> How does ssh-keygen -l calcuate the strength
>> in bits?
> It extract the actual key data and counts the significant bits of one
> of the key fields. For DSA the prime, for RSA the modulus.
> [1]
> [2]

Could one of the crypto bods take a look at this? I've just done some
educated guesswork from reading the source and various RFCs, so plenty
of scope for error.


Swish - Easy SFTP for Windows Explorer (
Received on 2013-07-03