Section: OpenSSL (1SSL)
The pkcs8 command processes private keys in PKCS#8 format. It can handle both unencrypted PKCS#8 PrivateKeyInfo format and EncryptedPrivateKeyInfo format with a variety of PKCS#5 (v1.5 and v2.0) and PKCS#12 algorithms.
- Normally a PKCS#8 private key is expected on input and a traditional format private key will be written. With the -topk8 option the situation is reversed: it reads a traditional format private key and writes a PKCS#8 format key.:
- -inform DER|PEM
- This specifies the input format. If a PKCS#8 format key is expected on input then either a DER or PEM encoded version of a PKCS#8 key will be expected. Otherwise the DER or PEM format of the traditional format private key is used.:
- -outform DER|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 by default. 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 arg
- the output file password source. For more information about the format of arg see the PASS PHRASE ARGUMENTS section in openssl?(1).:
- PKCS#8 keys generated or input are normally PKCS#8 EncryptedPrivateKeyInfo structures using an appropriate password based encryption algorithm. With this option an unencrypted PrivateKeyInfo structure is expected or output. This option does not encrypt private keys at all and should only be used when absolutely necessary. Certain software such as some versions of Java code signing software used unencrypted private keys.:
- This option generates RSA private keys in a broken format that some software uses. Specifically the private key should be enclosed in a OCTET STRING but some software just includes the structure itself without the surrounding OCTET STRING.:
- This option generates DSA keys in a broken format. The DSA parameters are embedded inside the PrivateKey structure. In this form the OCTET STRING contains an ASN1 SEQUENCE consisting of two structures: a SEQUENCE containing the parameters and an ASN1 INTEGER containing the private key.:
- This option generates DSA keys in a broken format compatible with Netscape private key databases. The PrivateKey contains a SEQUENCE consisting of the public and private keys respectively.:
- -v2 alg
- This option enables the use of PKCS#5 v2.0 algorithms. Normally PKCS#8 private keys are encrypted with the password based encryption algorithm called pbeWithMD5AndDES-CBC this uses 56 bit DES encryption but it was the strongest encryption algorithm supported in PKCS#5 v1.5. Using the -v2 option PKCS#5 v2.0 algorithms are used which can use any encryption algorithm such as 168 bit triple DES or 128 bit RC2 however not many implementations support PKCS#5 v2.0 yet. If you are just using private keys with OpenSSL then this doesn't matter.
The alg argument is the encryption algorithm to use, valid values include des, des3 and rc2. It is recommended that des3 is used.
- -v1 alg
- This option specifies a PKCS#5 v1.5 or PKCS#12 algorithm to use. A complete list of possible algorithms is included below.:
- -engine id
- specifying an engine (by its unique id string) will cause pkcs8 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.:
The encrypted form of a PEM encode PKCS#8 files uses the following headers and footers:
-----BEGIN ENCRYPTED PRIVATE KEY----- -----END ENCRYPTED PRIVATE KEY-----
The unencrypted form uses:
-----BEGIN PRIVATE KEY----- -----END PRIVATE KEY-----
Private keys encrypted using PKCS#5 v2.0 algorithms and high iteration counts are more secure that those encrypted using the traditional SSLeay compatible formats. So if additional security is considered important the keys should be converted.
The default encryption is only 56 bits because this is the encryption that most current implementations of PKCS#8 will support.
Some software may use PKCS#12 password based encryption algorithms with PKCS#8 format private keys: these are handled automatically but there is no option to produce them.
It is possible to write out DER encoded encrypted private keys in PKCS#8 format because the encryption details are included at an ASN1 level whereas the traditional format includes them at a PEM level.
PKCS#5 v1.5 and PKCS#12 algorithms.
Various algorithms can be used with the -v1 command line option, including PKCS#5 v1.5 and PKCS#12. These are described in more detail below.
- PBE-MD2-DES PBE-MD5-DES
- These algorithms were included in the original PKCS#5 v1.5 specification. They only offer 56 bits of protection since they both use DES.:
- PBE-SHA1-RC2-64 PBE-MD2-RC2-64 PBE-MD5-RC2-64 PBE-SHA1-DES
- These algorithms are not mentioned in the original PKCS#5 v1.5 specification but they use the same key derivation algorithm and are supported by some software. They are mentioned in PKCS#5 v2.0. They use either 64 bit RC2 or 56 bit DES.:
- PBE-SHA1-RC4-128 PBE-SHA1-RC4-40 PBE-SHA1-3DES PBE-SHA1-2DES PBE-SHA1-RC2-128 PBE-SHA1-RC2-40
- These algorithms use the PKCS#12 password based encryption algorithm and allow strong encryption algorithms like triple DES or 128 bit RC2 to be used.:
Convert a private from traditional to PKCS#5 v2.0 format using triple DES:
openssl pkcs8 -in key.pem -topk8 -v2 des3 -out enckey.pem
Convert a private key to PKCS#8 using a PKCS#5 1.5 compatible algorithm (DES):
openssl pkcs8 -in key.pem -topk8 -out enckey.pem
Convert a private key to PKCS#8 using a PKCS#12 compatible algorithm (3DES):
openssl pkcs8 -in key.pem -topk8 -out enckey.pem -v1 PBE-SHA1-3DES
Read a DER unencrypted PKCS#8 format private key:
openssl pkcs8 -inform DER -nocrypt -in key.der -out key.pem
Convert a private key from any PKCS#8 format to traditional format:
openssl pkcs8 -in pk8.pem -out key.pem
Test vectors from this PKCS#5 v2.0 implementation were posted to the pkcs-tng mailing list using triple DES, DES and RC2 with high iteration counts, several people confirmed that they could decrypt the private keys produced and Therefore it can be assumed that the PKCS#5 v2.0 implementation is reasonably accurate at least as far as these algorithms are concerned.
The format of PKCS#8 DSA (and other) private keys is not well documented: it is hidden away in PKCS#11 v2.01, section 11.9. OpenSSL's default DSA PKCS#8 private key format complies with this standard.
There should be an option that prints out the encryption algorithm in use and other details such as the iteration count.