How to convert PKCS#8-formatted PEM private key to the traditional format?

Question

From OpenSSL 1.0 change log:

Make PKCS#8 the default write format for private keys, replacing the traditional format. This form is standardised, more secure and doesn't include an implicit MD5 dependency. [Steve Henson]

However, I need the private key file in the previous, traditional format. Is it possible to convert the pem file from PKCS#8 to the traditional format (using OpenSSL.exe app)?

Thank you very much!

Answer 1

Succeeded to solve that in that way - the request:

openssl req -configconfigfile.cfg -newkey rsa:2048 -keyout newkey.pem -out newreq.pem 365

Then, I converted it to RSA format:

openssl rsa -in newkey.pem -out newkey.pem

Hope that it will help someone.

Answer 2

Using Openssl 3.0 :https://www.openssl.org/docs/man3.0/man1/openssl-pkey.html

Convert from PKCS#1 to PKCS#8:

$ cat pkcs1.pem
-----BEGIN RSA PRIVATE KEY-----
...
-----END RSA PRIVATE KEY-----
$ openssl pkey -in pkcs1.pem
-----BEGIN PRIVATE KEY-----
...
-----END PRIVATE KEY-----

Convert from PKCS#8 to PKCS#1:

$ cat pkcs8.pem
-----BEGIN PRIVATE KEY-----
...
-----END PRIVATE KEY-----
$ openssl pkey -in pkcs8.pem -traditional
-----BEGIN RSA PRIVATE KEY-----
...
-----END RSA PRIVATE KEY-----

See https://stackoverflow.com/a/20065522/2162144 for a description of PKCS#1 vs PKCS#8.

Answer 3

RSA private key

To convert from PKCS#1 to PKCS#8:

openssl pkcs8 -topk8 -inform pem -in private_pkcs1.pem -outform pem -nocrypt \
 -out private_pkcs8.pem

To convert from PKCS#8 to PKCS#1:

openssl rsa -in private_pkcs8.pem -out private_pkcs1.pem

RSA public key

To convert from PKCS#8 to PKCS#1:

openssl rsa -pubin -in public_pkcs8.pem -RSAPublicKey_out -out public_pkcs1.pem

To convert from PKCS#1 to PKCS#8:

openssl rsa -RSAPublicKey_in -in public_pkcs1.pem -pubout -out public_pkcs8.pem