-inform DER|NET|PEM

This specifies the input format. The DER option uses an ASN1 DER encoded form compatible with the PKCS#1 RSAPrivateKey or SubjectPublicKeyInfo format. The PEM form is the default format: it consists of the DER format base64 encoded with additional header and footer lines. On input PKCS#8 format private keys are also accepted. The NET form is a format is described in the NOTES section.

-outform DER|NET|PEM

This specifies the output format, the options have the same meaning as the -inform option.

-in filename

This specifies the input filename to read a key from or standard input if this option is not specified. If the key is encrypted a pass phrase will be prompted for.

-passin arg

the input file password source. For more information about the format of arg see the PASS PHRASE ARGUMENTS section in openssl(1).

-out filename

This specifies the output filename to write a key to or standard output if this option is not specified. If any encryption options are set then a pass phrase will be prompted for. The output filename should not be the same as the input filename.

-passout password

the output file password source. For more information about the format of arg see the PASS PHRASE ARGUMENTS section in openssl(1).

-sgckey

use the modified NET algorithm used with some versions of Microsoft IIS and SGC keys.

-des|-des3|-idea

These options encrypt the private key with the DES, triple DES, or the IDEA ciphers respectively before outputting it. A pass phrase is prompted for. If none of these options is specified the key is written in plain text. This means that using the rsa utility to read in an encrypted key with no encryption option can be used to remove the pass phrase from a key, or by setting the encryption options it can be use to add or change the pass phrase. These options can only be used with PEM format output files.

-text

prints out the various public or private key components in plain text in addition to the encoded version.

-noout

this option prevents output of the encoded version of the key.

-modulus

this option prints out the value of the modulus of the key.

-check

this option checks the consistency of an RSA private key.

-pubin

by default a private key is read from the input file: with this option a public key is read instead.

-pubout

by default a private key is output: with this option a public key will be output instead. This option is automatically set if the input is a public key.

-engine id

specifying an engine (by it's unique id string) will cause req to attempt to obtain a functional reference to the specified engine, thus initialising it if needed. The engine will then be set as the default for all available algorithms.

 -----BEGIN RSA PRIVATE KEY-----
 -----END RSA PRIVATE KEY-----

The PEM public key format uses the header and footer lines:

 -----BEGIN PUBLIC KEY-----
 -----END PUBLIC KEY-----

The NET form is a format compatible with older Netscape servers and Microsoft IIS .key files, this uses unsalted RC4 for its encryption. It is not very secure and so should only be used when necessary.

Some newer version of IIS have additional data in the exported .key files. To use these with the utility, view the file with a binary editor and look for the string ``private-key'', then trace back to the byte sequence 0x30, 0x82 (this is an ASN1 SEQUENCE). Copy all the data from this point onwards to another file and use that as the input to the rsa utility with the -inform NET option. If you get an error after entering the password try the -sgckey option.

 openssl rsa -in key.pem -out keyout.pem

To encrypt a private key using triple DES:

 openssl rsa -in key.pem -des3 -out keyout.pem

To convert a private key from PEM to DER format:

 openssl rsa -in key.pem -outform DER -out keyout.der

To print out the components of a private key to standard output:

 openssl rsa -in key.pem -text -noout

To just output the public part of a private key:

 openssl rsa -in key.pem -pubout -out pubkey.pem

There should be an option that automatically handles .key files, without having to manually edit them.