puttygen - public-key generator for the PuTTY tools
puttygen ( keyfile | -t keytype [ -b bits ] [ --primes method ] [ -q ] )
[ -C new-comment ] [ -P ] [ --reencrypt ]
[ --certificate cert-file | --remove-certificate ]
[ -O output-type | -l | -L | -p | --dump | --cert-info ]
[ --ppk-param key=value,... | -E fptype ]
[ -o output-file ]
puttygen is a tool to generate and manipulate SSH public
and private key pairs. It is part of the PuTTY suite, although it can also
interoperate with the key formats used by some other SSH clients.
When you run puttygen, it does three things. Firstly, it
either loads an existing key file (if you specified keyfile), or
generates a new key (if you specified keytype). Then, it optionally
makes modifications to the key (such as changing the comment and/or the
passphrase); finally, it outputs the key, or some information about the key,
to a file.
All three of these phases are controlled by the options described
in the following section.
In the first phase, puttygen either loads or generates a
key. Note that generating a key requires random data, which can cause
puttygen to pause, possibly for some time if your system does not
have much randomness available.
The options to control this phase are:
- keyfile
- Specify a key file to be loaded. (Use `-' to read a key file from
standard input.)
Usually this will be a private key, which can be in the (de facto
standard) SSH-1 key format, or in PuTTY's SSH-2 key format, or in either of
the SSH-2 private key formats used by OpenSSH and ssh.com's
implementation.
You can also specify a file containing only a public key
here. The operations you can do are limited to outputting another public key
format (possibly removing an attached certificate first), or a fingerprint.
Public keys can be in RFC 4716 or OpenSSH format, or the standard SSH-1
format.
- -t keytype
- Specify a type of key to generate. The acceptable values here are
rsa, dsa, ecdsa, eddsa, ed25519, and
ed448 (to generate SSH-2 keys), and rsa1 (to generate SSH-1
keys).
- -b bits
- Specify the size of the key to generate, in bits. Default for rsa
and dsa keys is 2048.
- --primes
method
- Method for generating prime numbers. The acceptable values here are
probable (the default), proven, and proven-even; the
later methods are slower. (Various synonyms for these method names are
also accepted.)
The `probable primes' method sounds unsafe, but it's the most
commonly used prime-generation strategy. There is in theory a possibility
that it might accidentally generate a number that isn't prime, but the
software does enough checking to make that probability vanishingly small
(less than 1 in 2^80, or 1 in 10^24). So, in practice, nobody worries about
it very much.
The other methods cause PuTTYgen to use numbers that it is
sure are prime, because it generates the output number together with
a proof of its primality. This takes more effort, but it eliminates that
theoretical risk in the probabilistic method.
You might choose to switch from probable to proven primes if you
have a local security standard that demands it, or if you don't trust the
probabilistic argument for the safety of the usual method.
- --strong-rsa
- When generating an RSA key, make sure the prime factors of the key modulus
are `strong primes'. A strong prime is a prime number chosen to have a
particular structure that makes certain factoring algorithms more
difficult to apply, so some security standards recommend their use.
However, the most modern factoring algorithms are unaffected, so this
option is probably not worth turning on unless you have a local
standard that recommends it.
- -q
- Suppress the progress display when generating a new key.
- --old-passphrase
file
- Specify a file name; the first line will be read from this file (removing
any trailing newline) and used as the old passphrase. CAUTION: If
the passphrase is important, the file should be stored on a temporary
filesystem or else securely erased after use.
- --random-device
device
- Specify device to read entropy from. By default, puttygen uses
/dev/urandom, falling back to /dev/random if it has to.
In the second phase, puttygen optionally alters properties
of the key it has loaded or generated. The options to control this are:
- -C
new-comment
- Specify a comment string to describe the key. This comment string will be
used by PuTTY to identify the key to you (when asking you to enter the
passphrase, for example, so that you know which passphrase to type).
- -P
- Indicate that you want to change the key's passphrase. This is automatic
when you are generating a new key, but not when you are modifying an
existing key.
- --certificate
certificate-file
- Adds an OpenSSH-style certificate to the public half of the key, so that
the output file contains a certified public key with the same private key.
If the input file already contained a certificate, it will be replaced
with the new one. (Use `-' to read a certificate from standard
input.)
- --remove-certificate
- Removes any certificate that was part of the key, to recover the
uncertified version of the underlying key.
- --reencrypt
- For an existing private key saved with a passphrase, refresh the
encryption without changing the passphrase.
This is most likely to be useful with the --ppk-param
option, to change some aspect of the key file's format or encryption.
- --ppk-param
key=value,...
- When saving a PPK file (the default private output type for SSH-2
keys), adjust details of the on-disk format.
Aspects to change are specified as a series of
key=value pairs separated by commas. The keys
are:
- version
- The PPK format version. Possible values are 3 (the default) and
2 (which is less resistant to brute-force decryption, but which you
might need if your key needs to be used by old versions of PuTTY tools, or
other PPK consumers).
The following keys only affect PPK version 3 files.
- kdf
- The variant of the Argon2 key derivation function to use. Options are
argon2id (default, and recommended), argon2i, and
argon2d.
You might change this if you consider your exposure to
side-channel attacks to be different to the norm.
- memory
- The amount of memory needed to decrypt the key, in Kbyte. Default is 8192
(i.e., 8 Mbyte).
- time
- Approximate time, on this machine, required to attempt decrypting the key,
in milliseconds. Default is 100 (ms).
- passes
- Alternative to time: explicitly specify the number of hash passes
required to attempt decrypting the key.
- parallelism
- Number of parallelisable threads that can be used to decrypt the key.
Default is 1 (force decryption to run single-threaded).
In the third phase, puttygen saves the key or information
about it. The options to control this are:
- -O
output-type
- Specify the type of output you want puttygen to produce. Acceptable
options are:
- private
- Save the private key in a format usable by PuTTY. This will either be the
standard SSH-1 key format, or PuTTY's own SSH-2 key format (`PPK'). This
is the default.
- public
- Save the public key only. For SSH-1 keys, the standard public key format
will be used (`1024 37 5698745...'). For SSH-2 keys, the public key
will be output in the format specified by RFC 4716, which is a multi-line
text file beginning with the line `---- BEGIN SSH2 PUBLIC KEY
----'.
- public-openssh
- Save the public key only, in a format usable by OpenSSH. For SSH-1 keys,
this output format behaves identically to public. For SSH-2 keys,
the public key will be output in the OpenSSH format, which is a single
line (`ssh-rsa AAAAB3NzaC1yc2...').
- fingerprint
- Print a fingerprint of the public key. The -E option lets you
specify which fingerprinting algorithm to use. All algorithms are believed
compatible with OpenSSH.
- private-openssh
- Save an SSH-2 private key in OpenSSH's format, using the oldest format
available to maximise backward compatibility. This option is not permitted
for SSH-1 keys.
- private-openssh-new
- As private-openssh, except that it forces the use of OpenSSH's
newer format even for RSA, DSA, and ECDSA keys.
- private-sshcom
- Save an SSH-2 private key in ssh.com's format. This option is not
permitted for SSH-1 keys.
- cert-info
- Save a textual dump of information about the certificate on the key, if
any: whether it's a host or a user certificate, what host(s) or user(s)
it's certified to be, its validity period, ID and serial number, and the
fingerprint of the signing CA.
- text
- Save a textual dump of the numeric components comprising the key (both the
public and private parts, if present). Useful for debugging, or for using
PuTTYgen as a key generator for applications other than SSH.
The output consists of a series of name=value lines, where
each value is either a C-like string literal in double quotes, a
hexadecimal number starting with 0x..., or a binary blob encoded with
base64, denoted by b64("...").
If no output type is specified, the default is private.
- -o
output-file
- Specify the file where puttygen should write its output. If this
option is not specified, puttygen will assume you want to overwrite
the original file if the input and output file types are the same
(changing a comment or passphrase), and will assume you want to output to
stdout if you are asking for a public key, fingerprint, or one of the
textual dump types. Otherwise, the -o option is required.
- -l
- Synonym for `-O fingerprint'.
- -L
- Synonym for `-O public-openssh'.
- -p
- Synonym for `-O public'.
- --cert-info
- Synonym for `-O cert-info'.
- --dump
- Synonym for `-O text'.
- -E fptype
- Specify the algorithm to use if generating a fingerprint. The available
algorithms are are sha256 (the default) and md5.
By default, when showing the fingerprint of a public key that
includes a certificate, puttygen will not include the certificate, so
that the fingerprint shown will be the same as the underlying public key. If
you want the fingerprint including the certificate (for example, so as to
tell two certified keys apart), you can specify sha256-cert or
md5-cert as the fingerprint type.
- --new-passphrase
file
- Specify a file name; the first line will be read from this file (removing
any trailing newline) and used as the new passphrase. If the file is empty
then the saved key will be unencrypted. CAUTION: If the passphrase
is important, the file should be stored on a temporary filesystem or else
securely erased after use.
The following options do not run PuTTYgen as normal, but print
informational messages and then quit:
- -h, --help
- Display a message summarizing the available options.
- -V, --version
- Display the version of PuTTYgen.
- --pgpfp
- Display the fingerprints of the PuTTY PGP Master Keys, to aid in verifying
new files released by the PuTTY team.
To generate an SSH-2 RSA key pair and save it in PuTTY's own
format (you will be prompted for the passphrase):
puttygen -t rsa -C "my home key" -o mykey.ppk
To generate a larger (4096-bit) key:
puttygen -t rsa -b 4096 -C "my home key" -o mykey.ppk
To change the passphrase on a key (you will be prompted for the
old and new passphrases):
puttygen -P mykey.ppk
To change the comment on a key:
puttygen -C "new comment" mykey.ppk
To convert a key into OpenSSH's private key format:
puttygen mykey.ppk -O private-openssh -o my-openssh-key
To convert a key from another format (puttygen will
automatically detect the input key type):
puttygen my-ssh.com-key -o mykey.ppk
To display the SHA-256 fingerprint of a key (some key types
require a passphrase to extract even this much information):
puttygen -l mykey.ppk
To add the OpenSSH-format public half of a key to your authorised
keys file:
puttygen -L mykey.ppk >> $HOME/.ssh/authorized_keys