Re: Public key storage format in OpenSSH

On Fri, 24 Feb 2006 23:16:51 -0500, Richard E. Silverman wrote:

"JSS" == James S Singleton <pt109@xxxxxxxxxx> writes:

JSS> OpenSSH stores public keys in totally different formats, JSS>
depending on whether version 1.5 or version 2 of the protocol is JSS>
being used. Can anybody explain the difference?

The first format was defined by the first implementation of SSH-1 by Tatu
Ylönen in 1995. A new format was needed for SSH-2, as SSH-1 only allowed
RSA keys whereas SSH-2 can have any number of key types, and OpenSSH
implements two: RSA and DSA.

JSS> For 1.5 the format is obvious; for 2, it is not.

some datatype definitions:

length = 4 bytes, 32-bit integer (big-endian)

string = [length L] [L bytes: the string]

bignum = string N (binary, length <= 8192)
where N = BIGNUM type as defined in the OpenSSL crypto library
(openssl/bn.h) (multiprecision [large] integer)

An OpenSSH public key line consists of:

key-type [base64 encoded string] comment

This is the format of base64-decoded bytes:

[string: key type in ASCII ("ssh-rsa"|"ssh-dsa"|"rsa"|"dsa")]
(yes, the key type is repeated inside the encoded portion)
(case RSA key)
bignum: public exponent (e)
bignum: modulus (n)
(case DSA key)
bignum: p
bignum: q
bignum: g
bignum: y
(<p,q,g,y> DSA key parameters as defined e.g. here:
http://en.wikipedia.org/wiki/Digital_Signature_Algorithm)

OK, thanks so much.


.

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