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.

RSA-PSS

RSA-PSS is a probabilistic signature padding scheme. Compared with the RSA PKCS#1 v1.5 padding scheme commonly used in RSA, it provides better security (at the cost of higher computational complexity, i.e., performance loss).

If you need to generate an RSA private key with RSA-PSS signature padding scheme parameters, you can use the following command:

1openssl genpkey \
2    -algorithm rsa-pss \
3    -pkeyopt rsa_keygen_bits:2048 \
4    -pkeyopt rsa_pss_keygen_md:sha256 \
5    -pkeyopt rsa_pss_keygen_mgf1_md:sha256 \
6    -pkeyopt rsa_pss_keygen_saltlen:32 \
7    -out rsa-pss.pem

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.