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308 lines
9.5 KiB
Plaintext
308 lines
9.5 KiB
Plaintext
keyformat.txt (wk 2001-12-18)
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-----------------------------
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Some notes on the format of the secret keys used with gpg-agent.
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Location of keys
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================
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The secret keys[1] are stored on a per file basis in a directory below
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the ~/.gnupg home directory. This directory is named
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private-keys-v1.d
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and should have permissions 700.
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The secret keys are stored in files with a name matching the
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hexadecimal representation of the keygrip[2] and suffixed with ".key".
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Unprotected Private Key Format
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==============================
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The content of the file is an S-Expression like the ones used with
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Libgcrypt. Here is an example of an unprotected file:
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(private-key
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(rsa
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(n #00e0ce9..[some bytes not shown]..51#)
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(e #010001#)
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(d #046129F..[some bytes not shown]..81#)
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(p #00e861b..[some bytes not shown]..f1#)
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(q #00f7a7c..[some bytes not shown]..61#)
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(u #304559a..[some bytes not shown]..9b#)
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)
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(created-at timestamp)
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(uri http://foo.bar x-foo:whatever_you_want)
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(comment whatever)
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)
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"comment", "created-at" and "uri" are optional. "comment" is
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currently used to keep track of ssh key comments. "created-at" is used
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to keep track of the creation time stamp used with OpenPGP keys; it is
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optional but required for some operations to calculate the fingerprint
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of the key. This timestamp should be a string with the number of
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seconds since Epoch or an ISO time string (yyyymmddThhmmss).
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Actually this form should not be used for regular purposes and only
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accepted by gpg-agent with the configuration option:
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--allow-non-canonical-key-format. The regular way to represent the
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keys is in canonical representation[3]:
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(private-key
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(rsa
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(n #00e0ce9..[some bytes not shown]..51#)
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(e #010001#)
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(d #046129F..[some bytes not shown]..81#)
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(p #00e861b..[some bytes not shown]..f1#)
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(q #00f7a7c..[some bytes not shown]..61#)
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(u #304559a..[some bytes not shown]..9b#)
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)
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(uri http://foo.bar x-foo:whatever_you_want)
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)
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Protected Private Key Format
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==============================
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A protected key is like this:
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(protected-private-key
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(rsa
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(n #00e0ce9..[some bytes not shown]..51#)
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(e #010001#)
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(protected mode (parms) encrypted_octet_string)
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(protected-at <isotimestamp>)
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)
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(uri http://foo.bar x-foo:whatever_you_want)
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(comment whatever)
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)
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In this scheme the encrypted_octet_string is encrypted according to
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the algorithm described after the keyword protected; most protection
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algorithms need some parameters, which are given in a list before the
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encrypted_octet_string. The result of the decryption process is a
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list of the secret key parameters. The protected-at expression is
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optional; the isotimestamp is 15 bytes long (e.g. "19610711T172000").
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The only available protection mode for now is
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openpgp-s2k3-sha1-aes-cbc
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which describes an algorithm using using AES in CBC mode for
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encryption, SHA-1 for integrity protection and the String to Key
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algorithm 3 from OpenPGP (rfc2440).
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Example:
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(protected openpgp-s2k3-sha1-aes-cbc
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((sha1 16byte_salt no_of_iterations) 16byte_iv)
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encrypted_octet_string
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)
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The encrypted_octet string should yield this S-Exp (in canonical
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representation) after decryption:
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(
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(
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(d #046129F..[some bytes not shown]..81#)
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(p #00e861b..[some bytes not shown]..f1#)
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(q #00f7a7c..[some bytes not shown]..61#)
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(u #304559a..[some bytes not shown]..9b#)
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)
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(hash sha1 #...[hashvalue]...#)
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)
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For padding reasons, random bytes are appended to this list - they can
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easily be stripped by looking for the end of the list.
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The hash is calculated on the concatenation of the public key and
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secret key parameter lists: i.e it is required to hash the
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concatenation of these 6 canonical encoded lists for RSA, including
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the parenthesis, the algorithm keyword and (if used) the protected-at
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list.
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(rsa
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(n #00e0ce9..[some bytes not shown]..51#)
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(e #010001#)
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(d #046129F..[some bytes not shown]..81#)
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(p #00e861b..[some bytes not shown]..f1#)
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(q #00f7a7c..[some bytes not shown]..61#)
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(u #304559a..[some bytes not shown]..9b#)
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(protected-at "18950523T000000")
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)
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After decryption the hash must be recalculated and compared against
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the stored one - If they don't match the integrity of the key is not
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given.
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Shadowed Private Key Format
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============================
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To keep track of keys stored on IC cards we use a third format for
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private kyes which are called shadow keys as they are only a reference
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to keys stored on a token:
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(shadowed-private-key
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(rsa
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(n #00e0ce9..[some bytes not shown]..51#)
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(e #010001#)
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(shadowed protocol (info))
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)
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(uri http://foo.bar x-foo:whatever_you_want)
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(comment whatever)
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)
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The currently used protocol is "ti-v1" (token info version 1). The
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second list with the information has this layout:
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(card_serial_number id_string_of_key)
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More items may be added to the list.
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OpenPGP Private Key Transfer Format
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===================================
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This format is used to transfer keys between gpg and gpg-agent.
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(openpgp-private-key
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(version V)
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(algo PUBKEYALGO)
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(skey _ P1 _ P2 _ P3 ... e PN)
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(csum n)
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(protection PROTTYPE PROTALGO IV S2KMODE S2KHASH S2KSALT S2KCOUNT))
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* V is the packet version number (3 or 4).
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* PUBKEYALGO is a Libgcrypt algo name
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* P1 .. PN are the parameters; the public parameters are never encrypted
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the secrect key parameters are encrypted if the "protection" list is
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given. To make this more explicit each parameter is preceded by a
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flag "_" for cleartext or "e" for encrypted text.
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* CSUM is the depreciated 16 bit checksum as defined by OpenPGP. This
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is an optional element.
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* If PROTTYPE is "sha1" the new style SHA1 checksum is used if it is "sum"
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the old 16 bit checksum (above) is used and if it is "none" no
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protection at all is used.
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* PROTALGO is a Libgcrypt style cipher algorithm name
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* IV is the initialization verctor.
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* S2KMODE is the value from RFC-4880.
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* S2KHASH is a a libgcrypt style hash algorithm identifier.
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* S2KSALT is the 8 byte salt
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* S2KCOUNT is the count value from RFC-4880.
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Persistent Passphrase Format
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============================
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To allow persistent storage of cached passphrases we use a scheme
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similar to the private-key storage format. This is a master
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passphrase format where each file may protect several secrets under
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one master passphrase. It is possible to have several of those files
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each protected by a dedicated master passphrase. Clear text keywords
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allow to list the available protected passphrases.
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The name of the files with these protected secrets have this form:
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pw-<string>.dat. STRING may be an arbitrary string, as a default name
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for the passphrase storage the name "pw-default.dat" is suggested.
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(protected-shared-secret
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((desc descriptive_text)
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(key [key_1] (keyword_1 keyword_2 keyword_n))
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(key [key_2] (keyword_21 keyword_22 keyword_2n))
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(key [key_n] (keyword_n1 keyword_n2 keyword_nn))
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(protected mode (parms) encrypted_octet_string)
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(protected-at <isotimestamp>)
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)
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)
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After decryption the encrypted_octet_string yields this S-expression:
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(
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(
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(value key_1 value_1)
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(value key_2 value_2)
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(value key_n value_n)
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)
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(hash sha1 #...[hashvalue]...#)
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)
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The "descriptive_text" is displayed with the prompt to enter the
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unprotection passphrase.
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KEY_1 to KEY_N are unique identifiers for the shared secret, for
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example an URI. In case this information should be kept confidential
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as well, they may not appear in the unprotected part; however they are
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mandatory in the encrypted_octet_string. The list of keywords is
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optional. The oder of the "key" lists and the order of the "value"
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lists mut match, that is the first "key"-list is associated with the
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first "value" list in the encrypted_octet_string.
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The protection mode etc. is indentical to the protection mode as
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decribed for the private key format.
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list of the secret key parameters. The protected-at expression is
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optional; the isotimestamp is 15 bytes long (e.g. "19610711T172000").
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The "hash" in the encrypted_octet_string is calculated on the
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concatenation of the key list and value lists: i.e it is required to
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hash the concatenation of all these lists, including the
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parenthesis and (if used) the protected-at list.
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Example:
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(protected-shared-secret
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((desc "List of system passphrases")
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(key "uid-1002" ("Knuth" "Donald Ervin Knuth"))
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(key "uid-1001" ("Dijkstra" "Edsgar Wybe Dijkstra"))
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(key)
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(protected mode (parms) encrypted_octet_string)
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(protected-at "20100915T111722")
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)
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)
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with "encrypted_octet_string" decoding to:
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(
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(
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(value 4:1002 "signal flags at the lock")
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(value 4:1001 "taocp")
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(value 1:0 "premature optimization is the root of all evil")
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)
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(hash sha1 #0102030405060708091011121314151617181920#)
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)
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To compute the hash this S-expression (in canoncical format) was
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hashed:
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((desc "List of system passphrases")
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(key "uid-1002" ("Knuth" "Donald Ervin Knuth"))
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(key "uid-1001" ("Dijkstra" "Edsgar Wybe Dijkstra"))
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(key)
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(value 4:1002 "signal flags at the lock")
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(value 4:1001 "taocp")
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(value 1:0 "premature optimization is the root of all evil")
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(protected-at "20100915T111722")
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)
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Notes:
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======
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[1] I usually use the terms private and secret key exchangeable but prefer the
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term secret key because it can be visually be better distinguished
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from the term public key.
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[2] The keygrip is a unique identifier for a key pair, it is
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independent of any protocol, so that the same key can be used with
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different protocols. PKCS-15 calls this a subjectKeyHash; it can be
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calculated using Libgcrypt's gcry_pk_get_keygrip ().
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[3] Even when canonical representation are required we will show the
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S-expression here in a more readable representation.
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