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#) ) (uri http://foo.bar x-foo:whatever_you_want) ) 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) ) (uri http://foo.bar x-foo:whatever_you_want) ) 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 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 and the algorithm keyword. (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#) ) 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) ) 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 ised with different protocols. PKCS-15 calls this a subjectKeyHash; it can be calculate 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.