What is a "key grip"?
wk at gnupg.org
Sat Jun 15 22:27:21 CEST 2013
On Sat, 15 Jun 2013 08:44, rick at openfortress.nl said:
> I'm having trouble understanding the concept of a "key grip", and I
> cannot find it documented anywhere. I'd like to generate keys in a
You find the description in Libgcrypt:
-- Function: unsigned char * gcry_pk_get_keygrip (gcry_sexp_t KEY,
unsigned char *ARRAY)
Return the so called "keygrip" which is the SHA-1 hash of the
public key parameters expressed in a way depended on the
algorithm. ARRAY must either provide space for 20 bytes or be
`NULL'. In the latter case a newly allocated array of that size is
returned. On success a pointer to the newly allocated space or to
ARRAY is returned. `NULL' is returned to indicate an error which
is most likely an unknown algorithm or one where a "keygrip" has
not yet been defined. The function accepts public or secret keys
It is a protocol independent way to identify a public key. pkcs#15 has
a similar concept but the exact details for the Keygrip are defined by
> As I understand it, it's a handle for the private key -- meaning it's
> not external stuff, but still useful to do well in a key store. It's
> a hex-denoted form
It is used for the public key. How it is formatted is not defined.
> I wonder if it should be derivable by GnuPG to a certain value (and if so, what is the calculation?) or if it is merely a value produced by the underlying SCD. In the latter case, I am confused that it is assumed to be 20 binary bytes long.
gpg2 --with-keygrip -k KEYID
> In short, if I store a public key and want it to be GnuPG-friendly, what should I use as the keygrip? For instance, do I write down an S-expression and determine it's SHA-1 and use it as the binary handle to the key?
You need to call the above Libgcrypt function (or your own implementation
of it). gcry_pk_get_keygrip takes an S-expression, but that is just a
part of the API and is not needed to define the keygrip.
If you look at libgcrypt/cipher/rsa.c you will find how we compute it
/* Compute a keygrip. MD is the hash context which we are going to
update. KEYPARAM is an S-expression with the key parameters, this
is usually a public key but may also be a secret key. An example
of such an S-expression is:
PKCS-15 says that for RSA only the modulus should be hashed -
however, it is not clear whether this is meant to use the raw bytes
(assuming this is an unsigned integer) or whether the DER required
0 should be prefixed. We hash the raw bytes. */
In short for RSA the keygrip is the SHA-1 value of the modulus given as
the unsigned integer with leading zero bytes removed.
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