gnupg/agent/keyformat.txt

keyformat.txt emacs, please switch to -- org -- mode

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] and suffixed with ".key".

• Extended Private Key Format

GnuPG 2.3+ will use a new format to store private keys that is both more flexible and easier to read and edit by human beings. The new format stores name,value-pairs using the common mail and http header convention. Example (here indented with two spaces):

Description: Key to sign all GnuPG released tarballs. The key is actually stored on a smart card. Use-for-ssh: yes OpenSSH-cert: long base64 encoded string wrapped so that this key file can be easily edited with a standard editor. Key: (shadowed-private-key (rsa (n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e #00010001#) (shadowed t1-v1 (#D2760001240102000005000011730000# OPENPGP.1) )))

GnuPG 2.2 is able to read and update keys using the new format, but will not create new files using the new format. Furthermore, it only makes use of the value stored under the name 'Key:'.

Keys in the extended format can be recognized by looking at the first byte of the file. If it starts with a '(' it is a naked S-expression, otherwise it is a key in extended format.

** Names

A name must start with a letter and end with a colon. Valid characters are all ASCII letters, numbers and the hyphen. Comparison of names is done case insensitively. Names may be used several times to represent an array of values.

The name "Key:" is special in that it may occur only once and the associated value holds the actual S-expression with the cryptographic key. The S-expression is formatted using the 'Advanced Format' (GCRYSEXP_FMT_ADVANCED) that avoids non-printable characters so that the file can be easily inspected and edited. See section 'Private Key Format' below for details.

** Values

Values are UTF-8 encoded strings. Values can be wrapped at any point, and continued in the next line indicated by leading whitespace. A continuation line with one leading space does not introduce a blank so that the lines can be effectively concatenated. A blank line as part of a continuation line encodes a newline.

** Comments

Lines containing only whitespace, and lines starting with whitespace followed by '#' are considered to be comments and are ignored.

• Private Key Format ** 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).

** 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 ) ) (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 currently defined protection modes are:

*** openpgp-s2k3-sha1-aes-cbc

This describes an algorithm using AES in CBC mode for encryption, SHA-1 for integrity protection and the String to Key algorithm 3 from OpenPGP (rfc4880).

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.

*** openpgp-s2k3-ocb-aes

This describes an algorithm using AES-128 in OCB mode, a nonce of 96 bit, a taglen of 128 bit, and the String to Key algorithm 3 from OpenPGP (rfc4880).

Example:

(protected openpgp-s2k3-ocb-aes ((sha1 16byte_salt no_of_iterations) 12byte_nonce) 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#) ) )

For padding reasons, random bytes may be appended to this list - they can easily be stripped by looking for the end of the list.

The associated data required for this protection mode is the list formiing the public key parameters. For the above example this is is this canonical encoded S-expression:

(rsa (n #00e0ce9..[some bytes not shown]..51#) (e #010001#) (protected-at "18950523T000000") )

*** openpgp-native

This is a wrapper around the OpenPGP Private Key Transport format which resembles the standard OpenPGP format and allows the use of an existing key without re-encrypting to the default protection format.

Example:

(protected openpgp-native (openpgp-private-key (version V) (algo PUBKEYALGO) (skey _ P1 _ P2 _ P3 ... e PN) (csum n) (protection PROTTYPE PROTALGO IV S2KMODE S2KHASH S2KSALT S2KCOUNT)))

Note that the public key parameters in SKEY are duplicated and should be identical to their copies in the standard parameter elements. Here is an example of an entire protected private key using this format:

(protected-private-key (rsa (n #00e0ce9..[some bytes not shown]..51#) (e #010001#) (protected openpgp-native (openpgp-private-key (version 4) (algo rsa) (skey _ #00e0ce9..[some bytes not shown]..51# _ #010001# e #.........................#) (protection sha1 aes #aabbccddeeff00112233445566778899# 3 sha1 #2596f93e85f41e53# 3:190)))) (uri http://foo.bar x-foo:whatever_you_want) (comment whatever))

** 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 "t1-v1" (token info version 1). The second list with the information has this layout:

(card_serial_number id_string_of_key fixed_pin_length)

FIXED_PIN_LENGTH is optional. It can be used to store the length of the PIN; a value of 0 indicates that this information is not available. The rationale for this field is that some pinpad equipped readers don't allow passing a variable length PIN.

More items may be added to the list.

** OpenPGP Private Key Transfer Format

This format is used to transfer keys between gpg and gpg-agent.

(openpgp-private-key (version V) (algo PUBKEYALGO) (curve CURVENAME) (skey _ P1 _ P2 _ P3 ... e PN) (csum n) (protection PROTTYPE PROTALGO IV S2KMODE S2KHASH S2KSALT S2KCOUNT))

• V is the packet version number (3 or 4).
• PUBKEYALGO is a Libgcrypt algo name
• CURVENAME is the name of the curve - only used with ECC.
• P1 .. PN are the parameters; the public parameters are never encrypted the secrect key parameters are encrypted if the "protection" list is given. To make this more explicit each parameter is preceded by a flag "_" for cleartext or "e" for encrypted text.
• CSUM is the deprecated 16 bit checksum as defined by OpenPGP. This is an optional element.
• If PROTTYPE is "sha1" the new style SHA1 checksum is used if it is "sum" the old 16 bit checksum (above) is used and if it is "none" no protection at all is used.
• PROTALGO is a Libgcrypt style cipher algorithm name
• IV is the initialization verctor.
• S2KMODE is the value from RFC-4880.
• S2KHASH is a libgcrypt style hash algorithm identifier.
• S2KSALT is the 8 byte salt
• S2KCOUNT is the count value from RFC-4880.

** Persistent Passphrase Format

Note: That this has not yet been implemented.

To allow persistent storage of cached passphrases we use a scheme similar to the private-key storage format. This is a master passphrase format where each file may protect several secrets under one master passphrase. It is possible to have several of those files each protected by a dedicated master passphrase. Clear text keywords allow listing the available protected passphrases.

The name of the files with these protected secrets have this form: pw-.dat. STRING may be an arbitrary string, as a default name for the passphrase storage the name "pw-default.dat" is suggested.

(protected-shared-secret ((desc descriptive_text) (key [key_1] (keyword_1 keyword_2 keyword_n)) (key [key_2] (keyword_21 keyword_22 keyword_2n)) (key [key_n] (keyword_n1 keyword_n2 keyword_nn)) (protected mode (parms) encrypted_octet_string) (protected-at ) ) )

After decryption the encrypted_octet_string yields this S-expression:

( ( (value key_1 value_1) (value key_2 value_2) (value key_n value_n) ) (hash sha1 #...[hashvalue]...#) )

The "descriptive_text" is displayed with the prompt to enter the unprotection passphrase.

KEY_1 to KEY_N are unique identifiers for the shared secret, for example an URI. In case this information should be kept confidential as well, they may not appear in the unprotected part; however they are mandatory in the encrypted_octet_string. The list of keywords is optional. The order of the "key" lists and the order of the "value" lists must match, that is the first "key"-list is associated with the first "value" list in the encrypted_octet_string.

The protection mode etc. is identical to the protection mode as described for the private key format.

list of the secret key parameters. The protected-at expression is optional; the isotimestamp is 15 bytes long (e.g. "19610711T172000").

The "hash" in the encrypted_octet_string is calculated on the concatenation of the key list and value lists: i.e it is required to hash the concatenation of all these lists, including the parenthesis and (if used) the protected-at list.

Example:

(protected-shared-secret ((desc "List of system passphrases") (key "uid-1002" ("Knuth" "Donald Ervin Knuth")) (key "uid-1001" ("Dijkstra" "Edsger Wybe Dijkstra")) (key) (protected mode (parms) encrypted_octet_string) (protected-at "20100915T111722") ) )

with "encrypted_octet_string" decoding to:

( ( (value 4:1002 "signal flags at the lock") (value 4:1001 "taocp") (value 1:0 "premature optimization is the root of all evil") ) (hash sha1 #0102030405060708091011121314151617181920#) )

To compute the hash this S-expression (in canoncical format) was hashed:

((desc "List of system passphrases") (key "uid-1002" ("Knuth" "Donald Ervin Knuth")) (key "uid-1001" ("Dijkstra" "Edsger Wybe Dijkstra")) (key) (value 4:1002 "signal flags at the lock") (value 4:1001 "taocp") (value 1:0 "premature optimization is the root of all evil") (protected-at "20100915T111722") )

• 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.