1.1. Manage RSA Keys

This chapter introduces how to generate, convert, encode, encrypt, decrypt RSA keys using OpenSSL command-line tools.

Generate RSA Private Key

Usually, the genrsa subcommand of OpenSSL is used to generate RSA private keys.

For example, to generate an RSA private key file named rsa-p1-raw.pem, the configuration is as follows:

1openssl genrsa -aes-256-cfb -out ./rsa-p1.pem 2048

If no encryption algorithm flag is passed (such as -aes-256-cfb), a raw private key without encryption protection will be generated.

Common Parameters

-outform

Specifies the output file format, which can be either PEM or DER.

-rand

Specifies the path to the random seed file used to generate random numbers.

Standard Conversion

PKCS#1 -> PKCS#8

1openssl pkcs8 \
2    -topk8 \
3    -inform PEM \
4    -outform PEM \
5    -in rsa-p1.pem \
6    -out rsa-p8.pem

By default, the -v2 aes256 parameter is used for encryption. If the converted key does not need encryption protection, add -nocrypt, for example:

1openssl pkcs8 \
2    -topk8 \
3    -inform PEM \
4    -outform PEM \
5    -nocrypt \
6    -in rsa-p1.pem \
7    -out rsa-p8-raw.pem

PKCS#8 -> PKCS#1

1openssl rsa \
2    -aes-256-cfb \
3    -in rsa-p8.pem \
4    -out rsa-p1.pem

If the converted key does not need encryption protection, remove the encryption algorithm flag -aes-256-cfb, such as:

1openssl rsa \
2    -in rsa-p8.pem \
3    -out rsa-p1-raw.pem

By default, both input and output encodings are PEM. If the input is DER, please specify -inform DER. If you need to output DER, please specify -outform DER.

Encoding Conversion

PKCS#1: PEM <=> DER

e.g. PEM -> DER, vice versa

1openssl rsa \
2    -inform     PEM \
3    -outform    DER \
4    -in         rsa-p1-raw.pem \
5    -out        rsa-p1-raw.der

PKCS#8: PEM <=> DER

The support for PKCS#8 DER encoding in OpenSSL is incomplete. Please do not use it.

Encryption and Decryption

Encrypt PKCS#1 Private Key

1openssl rsa \
2    -inform PEM \
3    -aes-256-cfb \
4    -in rsa-p1-raw.pem \
5    -out rsa-p1.pem

Decrypt PKCS#1 Private Key

1openssl rsa \
2    -inform PEM \
3    -in rsa-p1.pem \
4    -out rsa-p1-raw.pem

Encrypt PKCS#8 Private Key

1openssl pkcs8 \
2    -topk8 \
3    -inform PEM \
4    -v2 aes256 \
5    -in rsa-p8-raw.pem \
6    -out rsa-p8.pem

Decrypt PKCS#8 Private Key

1openssl pkcs8 \
2    -inform PEM \
3    -in rsa-p8.pem \
4    -out rsa-p8-raw.pem

Extract Public Key

1openssl rsa \
2    -inform PEM \
3    -in rsa-p1-raw.pem \
4    -pubout \
5    -out rsa-p1.pub

The -in parameter can pass either PKCS#1 or PKCS#8 keys.

View Public Key Information

1openssl rsa \
2    -inform PEM \
3    -pubin \
4    -text \
5    -noout \
6    -in rsa-p1.pub

View Private Key Information

1openssl rsa \
2    -inform PEM \
3    -noout \
4    -text \
5    -in rsa-p1.pem

Test Key

1# Generate a random file for testing
2openssl rand -out tmp.dat 4096
3
4# Sign the random file with the private key using RSA-SHA-256
5openssl dgst -sha256 -sign rsa-p1.pem -out tmp.dat.sig tmp.dat
6
7# Verify the signature
8openssl dgst -sha256 -verify rsa-p1.pub -signature tmp.dat.sig tmp.dat

Ok, all above are the common operations of RSA keys.