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keyformat.txt (wk 2001-12-18)
-----------------------------
Some notes on the format of the secret keys used with gpg-agent.
Location of keys
================
The secret keys[1] are stored on a per file basis in a directory below
the ~/.gnupg home directory. This directory is named
private-keys-v1.d
and should have permissions 700.
The secret keys are stored in files with a name matching the
hexadecimal representation of the keygrip[2].
Unprotected Private Key Format
==============================
The content of the file is an S-Expression like the ones used with
Libgcrypt. Here is an example of an unprotected file:
(private-key
(rsa
(n #00e0ce9..[some bytes not shown]..51#)
(e #010001#)
(d #046129F..[some bytes not shown]..81#)
(p #00e861b..[some bytes not shown]..f1#)
(q #00f7a7c..[some bytes not shown]..61#)
(u #304559a..[some bytes not shown]..9b#)
)
(created-at timestamp)
(uri http://foo.bar x-foo:whatever_you_want)
(comment whatever)
)
"comment", "created-at" and "uri" are optional. "comment" is
currently used to keep track of ssh key comments. "created-at" is used
to keep track of the creation time stamp used with OpenPGP keys; it is
optional but required for some operations to calculate the fingerprint
of the key. This timestamp should be a string with the number of
seconds since Epoch or an ISO time string (yyyymmddThhmmss).
Actually this form should not be used for regular purposes and only
accepted by gpg-agent with the configuration option:
--allow-non-canonical-key-format. The regular way to represent the
keys is in canonical representation[3]:
(private-key
(rsa
(n #00e0ce9..[some bytes not shown]..51#)
(e #010001#)
(d #046129F..[some bytes not shown]..81#)
(p #00e861b..[some bytes not shown]..f1#)
(q #00f7a7c..[some bytes not shown]..61#)
(u #304559a..[some bytes not shown]..9b#)
)
(uri http://foo.bar x-foo:whatever_you_want)
)
Protected Private Key Format
==============================
A protected key is like this:
(protected-private-key
(rsa
(n #00e0ce9..[some bytes not shown]..51#)
(e #010001#)
(protected mode (parms) encrypted_octet_string)
(protected-at <isotimestamp>)
)
(uri http://foo.bar x-foo:whatever_you_want)
(comment whatever)
)
In this scheme the encrypted_octet_string is encrypted according to
the algorithm described after the keyword protected; most protection
algorithms need some parameters, which are given in a list before the
encrypted_octet_string. The result of the decryption process is a
list of the secret key parameters. The protected-at expression is
optional; the isotimestamp is 15 bytes long (e.g. "19610711T172000").
The only available protection mode for now is
openpgp-s2k3-sha1-aes-cbc
which describes an algorithm using using AES in CBC mode for
encryption, SHA-1 for integrity protection and the String to Key
algorithm 3 from OpenPGP (rfc2440).
Example:
(protected openpgp-s2k3-sha1-aes-cbc
((sha1 16byte_salt no_of_iterations) 16byte_iv)
encrypted_octet_string
)
The encrypted_octet string should yield this S-Exp (in canonical
representation) after decryption:
(
(
(d #046129F..[some bytes not shown]..81#)
(p #00e861b..[some bytes not shown]..f1#)
(q #00f7a7c..[some bytes not shown]..61#)
(u #304559a..[some bytes not shown]..9b#)
)
(hash sha1 #...[hashvalue]...#)
)
For padding reasons, random bytes are appended to this list - they can
easily be stripped by looking for the end of the list.
The hash is calculated on the concatenation of the public key and
secret key parameter lists: i.e it is required to hash the
concatenation of these 6 canonical encoded lists for RSA, including
the parenthesis, the algorithm keyword and (if used) the protected-at
list.
(rsa
(n #00e0ce9..[some bytes not shown]..51#)
(e #010001#)
(d #046129F..[some bytes not shown]..81#)
(p #00e861b..[some bytes not shown]..f1#)
(q #00f7a7c..[some bytes not shown]..61#)
(u #304559a..[some bytes not shown]..9b#)
(protected-at "18950523T000000")
)
After decryption the hash must be recalculated and compared against
the stored one - If they don't match the integrity of the key is not
given.
Shadowed Private Key Format
============================
To keep track of keys stored on IC cards we use a third format for
private kyes which are called shadow keys as they are only a reference
to keys stored on a token:
(shadowed-private-key
(rsa
(n #00e0ce9..[some bytes not shown]..51#)
(e #010001#)
(shadowed protocol (info))
)
(uri http://foo.bar x-foo:whatever_you_want)
(comment whatever)
)
The currently used protocol is "ti-v1" (token info version 1). The
second list with the information has this layout:
(card_serial_number id_string_of_key)
More items may be added to the list.
Notes:
======
[1] I usually use the terms private and secret key exchangeable but prefer the
term secret key because it can be visually be better distinguished
from the term public key.
[2] The keygrip is a unique identifier for a key pair, it is
independent of any protocol, so that the same key can be used with
different protocols. PKCS-15 calls this a subjectKeyHash; it can be
calculated using Libgcrypt's gcry_pk_get_keygrip ().
[3] Even when canonical representation are required we will show the
S-expression here in a more readable representation.
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