puttygen - Man Page

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