PuTTY Key Generator

PuTTY Key Generator (PuTTYgen) creates RSA, DSA, ECDSA and Ed25519 key pairs for SSH authentication. During generation, it requires random mouse movement as entropy source to strengthen key security. It allows saving private keys with a passphrase, exporting public keys in OpenSSH format, and converting keys between PuTTY and ssh.com formats.

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• RSA key generation. PuTTYgen creates RSA keys with configurable lengths of 1024, 2048, 3072 or 4096 bits. The RSA algorithm relies on factoring large prime numbers and remains compatible with most older and modern SSH servers. The tool generates two random prime numbers to construct the modulus and exponent, producing a private and a public key pair.
• DSA key generation. This function produces DSA (Digital Signature Algorithm) keys of 1024 bits according to FIPS 186 standard. The DSA algorithm uses modular prime arithmetic and subgroups to sign data. PuTTYgen implements DSA with predefined parameters, ensuring compatibility with SSH clients requiring this signature method.
• ECDSA key generation. Generates keys based on elliptic curves using nistp256, nistp384 and nistp521 curves. ECDSA provides security levels equivalent to RSA but with smaller key lengths, accelerating cryptographic operations. The tool selects random curve points and computes the discrete logarithm as the basis for the private key.
• Ed25519 key generation. Supports the Ed25519 algorithm, a 255-bit Edwards curve offering high speed and resistance against side-channel attacks. Ed25519 uses deterministic signatures and is considered more secure against flawed random generation implementations. PuTTYgen generates complete Ed25519 keys in the specific PuTTY format.
• Mouse movement randomness entry. During key creation, PuTTYgen asks the user to move the mouse pointer inside a window area. The program captures coordinates, timings and movement trajectories, converting them into a seed for the random number generator. This method avoids algorithmic predictability and increases entropy of the key pair.
• Private key encryption with passphrase. Encrypts the private key using AES-256 in CBC mode with a user-provided passphrase. PuTTYgen applies PBKDF2 key derivation with thousands of iterations to resist brute force attacks. The private key remains unusable without the correct passphrase, even if the file is stolen.
• Key conversion to ssh.com format. Exports keys generated by PuTTYgen into the file format used by the ssh.com client. This conversion restructures private key fields, adapting RSA, DSA or ECDSA parameters to the commercial competitor specification. The converted key works on ssh.com servers and clients without loss of cryptographic functionality.
• Key import from OpenSSH format. PuTTYgen reads OpenSSH private key files (PEM format or new OpenSSH format) and internally transforms them into its .ppk structure. During import, it interprets Base64-encoded data blocks, extracts curve parameters or RSA primes, and reconstructs the key for use in PuTTY.
• Key fingerprint display. After generating or loading a key, the tool computes and displays the MD5 or SHA-256 fingerprint of the public key. The fingerprint allows unique identification of the key in authorized key lists or connection logs. PuTTYgen presents both hexadecimal representations for manual verification.
• Saving public key in authorized_keys format. Exports the public key as a text line compatible with the OpenSSH authorized_keys file. This line includes the key type (ssh-rsa, ssh-ed25519, etc.), Base64-encoded value and an optional comment. The user can copy this text directly to the server for configuring key-based authentication.
• Embedded comment generation within the key. PuTTYgen provides a text field for adding an arbitrary comment inside the .ppk private key file and the public key. The comment typically contains information such as user, machine or creation date, simplifying management of multiple key pairs. The comment does not affect security or signature calculation.
• Advanced prime number generation parameters. During RSA generation, the program applies Miller-Rabin tests to verify primality of candidate numbers. It runs multiple rounds of the test with random bases, achieving an error probability below 2⁻⁸⁰. PuTTYgen also forces prime numbers to have a specified length and not be easily factorable.
• Storage in proprietary .ppk format. Saves the private key in a file with .ppk extension, containing metadata such as encryption type, PBKDF2 iteration count and random salt. The .ppk format packages the key into plain text blocks with specific markers. Only PuTTY and compatible tools can directly read this format.

The development history of PuTTY Key Generator began in the year 2000, when Simon Tatham released the first versions of PuTTY and its accompanying utilities. The tool was originally written in the C language and remains in that language through current versions. Simon Tatham, a British developer, designed PuTTYgen as part of the PuTTY project for Windows systems. Over the years, contributors including Jacob Nevins, Ben Harris and other development team members added ECDSA support in 2011 and Ed25519 support in 2015.

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