Encoding SSH RSA public key

[robert]

Thanks, Daniel. You were right.

On Wed, 2008-08-13 at 17:28 -0400, Daniel Kahn Gillmor wrote:
> On Wed 2008-08-13 10:54:02 -0400, robert wrote:
> 
> > There must be options (optional), bits, e, n and comments (optional),
> > but how are these represented before encoding? Are each of these data
> > encoded to base64 separately and then concatenated? What exactly is
> > encoded?
> >
> > Could anyone describe me the algorithm for obtaining the base64 string?
> > I couldn't find it anywhere.
> 
> The format for the base64-encded data (the unreadable stuff in the
> middle of the line) appears to be:
> 
> A series of length-prefixed bitstrings, where the length for each
> bitstring is encoded as a network-order, 32-bit unsigned integer
> representing the number of bytes in the following bitstring.
> 
> The first bitstring indicates the type of the key.  This can be used
> to determine the nature of the bitstrings which follow.  The type is
> represented by a 7-byte string ("ssh-rsa" or "ssh-dss"), so the first
> 4 bytes are 0x00,0x00,0x00,0x07 (this indicates the length of the
> type string).
> 
> For RSA keys, the exponent follows next as a multi-precision integer
> (MPI), and then the modulus (also an MPI).
> 
> So for example, for a 2048-bit key, you can unpack it this way:
> 
> [0 dkg at squeak ~]$ < ./.example/id_rsa.pub cut -f2 -d\  | base64 -d | hd | head -n2
> 00000000  00 00 00 07 73 73 68 2d  72 73 61 00 00 00 03 01  |....ssh-rsa.....|
> 00000010  00 01 00 00 01 01 00 c4  68 99 07 36 4f d4 7a 35  |........h..6O.z5|
> [0 dkg at squeak ~]$ 
> 
> the example above uses a 3-byte exponent of 0x10001 (65537), followed
> by a 257(==0x101)-byte modulus, which is the rest of the key.
> 
> Be careful that your MPIs all have the first bit set to 0, though!
> OpenSSH appears to treat the MPIs as a two's-complement signed
> representation, so if your first bit is a 1, ssh will think you're
> trying to provide a negative value.  If your calculations produce a
> number with the high bit set to 1, just increase the length by another
> byte and pad the beginning with 0x00 to keep it positive.  (this is
> why the modulus above is 257 bytes starting with 0x00,0xc4 instead of
> 256 starting with 0xc4,0x68).
> 
> Hope this is helpful,
> 
>      --dkg
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