Several tools exist to generate SSH public/private key pairs. The following sections show how to generate an SSH key pair on UNIX, UNIX-like and Windows platforms.

Generating an SSH Key Pair on UNIX and UNIX-Like Platforms Using the ssh-keygen Utility

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UNIX and UNIX-like platforms (including Solaris and Linux) include the ssh-keygen utility to generate SSH key pairs.

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1. Navigate to your home directory:
2. Run the ssh-keygen utility, providing as filename your choice of file name for the private key:

   The ssh-keygen utility prompts you for a passphrase for the private key.

3. Enter a passphrase for the private key, or press Enter to create a private key without a passphrase:

   Note:

   While a passphrase is not required, you should specify one as a security measure to protect the private key from unauthorized use. When you specify a passphrase, a user must enter the passphrase every time the private key is used.

   The ssh-keygen utility prompts you to enter the passphrase again.

4. Enter the passphrase again, or press Enter again to continue creating a private key without a passphrase:
5. The ssh-keygen utility displays a message indicating that the private key has been saved as filename and the public key has been saved as filename.pub. It also displays information about the key fingerprint and randomart image.

Generating an SSH Key Pair on Windows Using the PuTTYgen Program

The PuTTYgen program is part of PuTTY, an open source networking client for the Windows platform.

1. Download and install PuTTY or PuTTYgen.

   To download PuTTY or PuTTYgen, go to http://www.putty.org/ and click the You can download PuTTY here link.

2. Run the PuTTYgen program.
3. Set the Type of key to generate option to SSH-2 RSA.
4. In the Number of bits in a generated key box, enter 2048.
5. Click Generate to generate a public/private key pair.

   As the key is being generated, move the mouse around the blank area as directed.

6. (Optional) Enter a passphrase for the private key in the Key passphrase box and reenter it in the Confirm passphrase box.

   Note:

   While a passphrase is not required, you should specify one as a security measure to protect the private key from unauthorized use. When you specify a passphrase, a user must enter the passphrase every time the private key is used.

7. Click Save private key to save the private key to a file. To adhere to file-naming conventions, you should give the private key file an extension of .ppk (PuTTY private key).

   Note:

   The .ppk file extension indicates that the private key is in PuTTY's proprietary format. You must use a key of this format when using PuTTY as your SSH client. It cannot be used with other SSH client tools. Refer to the PuTTY documentation to convert a private key in this format to a different format.
8. Select all of the characters in the Public key for pasting into OpenSSH authorized_keys file box.

   Make sure you select all the characters, not just the ones you can see in the narrow window. If a scroll bar is next to the characters, you aren't seeing all the characters.

9. Right-click somewhere in the selected text and select Copy from the menu.
10. Open a text editor and paste the characters, just as you copied them. Start at the first character in the text editor, and do not insert any line breaks.
11. Save the text file in the same folder where you saved the private key, using the .pub extension to indicate that the file contains a public key.
12. If you or others are going to use an SSH client that requires the OpenSSH format for private keys (such as the ssh utility on Linux), export the private key:
    1. On the Conversions menu, choose Export OpenSSH key.
    2. Save the private key in OpenSSH format in the same folder where you saved the private key in .ppk format, using an extension such as .openssh to indicate the file's content.

The calling application must specify the algorithm when calling this function. Because this algorithm type is kept bundled with the key, the application does not need to specify the algorithm later when the actual cryptographic operations are performed.

Syntax

Parameters

hProv

A handle to a cryptographic service provider (CSP) created by a call toCryptAcquireContext.

Algid

AnALG_ID value that identifies the algorithm for which the key is to be generated. Values for this parameter vary depending on the CSP used.

For ALG_ID values to use with the Microsoft Base Cryptographic Provider, seeBase Provider Algorithms.

For ALG_ID values to use with the Microsoft Strong Cryptographic Provider or the Microsoft Enhanced Cryptographic Provider, seeEnhanced Provider Algorithms.

For a Diffie-Hellman CSP, use one of the following values.

In addition to generating session keys for symmetric algorithms, this function can also generate public/private key pairs. Each CryptoAPI client generally possesses two public/private key pairs. To generate one of these key pairs, set the Algid parameter to one of the following values.

dwFlags

Specifies the type of key generated. The sizes of a session key, RSA signature key, and RSA key exchange keys can be set when the key is generated. The key size, representing the length of the key modulus in bits, is set with the upper 16 bits of this parameter. Thus, if a 2,048-bit RSA signature key is to be generated, the value 0x08000000 is combined with any other dwFlags predefined value with a bitwise-OR operation. The upper 16 bits of 0x08000000 is 0x0800, or decimal 2,048. The RSA1024BIT_KEY value can be used to specify a 1024-bit RSA key.

Due to changing export control restrictions, the default CSP and default key length may change between operating system versions. It is important that both the encryption and decryption use the same CSP and that the key length be explicitly set using the dwFlags parameter to ensure interoperability on different operating system platforms.

In particular, the default RSA Full Cryptographic Service Provider is the Microsoft RSA Strong Cryptographic Provider. The default DSS Signature Diffie-Hellman Cryptographic Service Provider is the Microsoft Enhanced DSS Diffie-Hellman Cryptographic Provider. Each of these CSPs has a default 128-bit symmetric key length for RC2 and RC4 and a 1,024-bit default key length for public key algorithms.

If the upper 16 bits is zero, the default key size is generated. If a key larger than the maximum or smaller than the minimum is specified, the call fails with the ERROR_INVALID_PARAMETER code.

The following table lists minimum, default, and maximum signature and exchange key lengths beginning with Windows XP.

For session key lengths, see CryptDeriveKey.

For more information about keys generated using Microsoft providers, seeMicrosoft Cryptographic Service Providers.

The lower 16-bits of this parameter can be zero or a combination of one or more of the following values.

phKey

Address to which the function copies the handle of the newly generated key. When you have finished using the key, delete the handle to the key by calling the CryptDestroyKey function.

Return value

Returns nonzero if successful or zero otherwise.

For extended error information, callGetLastError.

The error codes prefaced by 'NTE' are generated by the particular CSP being used. Some possible error codes are listed in the following table.

Remarks

If keys are generated for symmetricblock ciphers, the key, by default, is set up in cipher block chaining (CBC) mode with an initialization vector of zero. This cipher mode provides a good default method for bulk encrypting data. To change these parameters, use theCryptSetKeyParam function.

To choose an appropriate key length, the following methods are recommended:

Key Value Pairs

• Enumerate the algorithms that the CSP supports and get maximum and minimum key lengths for each algorithm. To do this, call CryptGetProvParam with PP_ENUMALGS_EX.
• Use the minimum and maximum lengths to choose an appropriate key length. It is not always advisable to choose the maximum length because this can lead to performance issues.
• After the desired key length has been chosen, use the upper 16 bits of the dwFlags parameter to specify the key length.

Examples

The following example shows the creation of a random session key. For an example that includes the complete context for this example, see Example C Program: Encrypting a File. For another example that uses this function, see Example C Program: Decrypting a File.

Requirements

Issues Of Key Pair Generation 6

Issues Of Key Pair Generation 1

See also