Key Generation and Encryption Examples using OpenSSL
Generating RSA Keys
Generate a 1024-bit RSA private key using a public exponent (e) of 65537 (-F4 option). The key is NOT encrypted with DES (or other ciphers).
$ openssl genrsa -out privkeyA.pem -F4 1024
The output file (privkeyA.pem) is plaintext. It contains the private key, encoded as Base64, in between two lines indicating the begin and end of the key.
Setting up the Certificate Authority (CA Only)
The CA also has a certificate, which is created from a private key (same as using genrsa above), and a self-signed certificate. Since we don't have a higher-level CA to sign the certificate of our CA, we must sign the CAs certificate ourselve, as follows:
$ openssl req -new -x509 -key cakey.pem -out cacert.pem -days 1095
CA information can be stored in a set of directories. By default the openssl configration in /usr/lib/ssl/openssl.cnf assumes some directory structure. Hence we need to create it:
$ mkdir ./demoCA $ mkdir ./demoCA/certs $ mkdir ./demoCA/crl $ mkdir ./demoCA/newcerts $ mkdir ./demoCA/private $ touch ./demoCA/index.txt $ echo 02 > demoCA/serial $ mv cacert.pem ./demoCA $ mv cakey.pem ./demoCA/private
The CA's certificate (cacert.pem) is made available to all users.
Requesting a Certificate from CA
Create a certificate request that will be sent to the Certificate Authority (CA). This takes a private key as input (i.e. the file generated above) and produces a .csr certificate request file as output. This is a new certificate request. When prompted for information give the following responses:
- Country Name: TH
- State or Province: Pathumthani
- Locality: Bangkadi
- Organisation: SIIT
- Unit: ICT
- Common Name: FIRST_NAME LAST_NAME
- Email Address: EMAIL_FROM_MAILIST
- A challenge password: (nothing, just press enter)
- Optionaly company: (nothing, just press enter)
$ openssl req -new -key privkeyA.pem -out certA.csr
Send your certificate request to the CA.
Creating a Certificate (CA Only)
The CA will create a certificate (for the following to work, the appropriate directory structure at the CA must exist; see above):
$ openssl ca -in certA.csr -out cert-A.pem
and then return your certificate to you.
Verifying a Certificate
Upon receipt of the certificate, you should validate it:
$ openssl verify -CAfile cacert.pem cert-A.pem cert-A.pem: OK
If it is OK (and you trust the CA) then you are sure the received certificate came from the CA and was not modified along the way.
Encrypting with RSA
Now lets assume a set of users (A, B, C, ...) have completed the above steps and have their certificates (e.g. cert-A.pem, cert-B.pem). RSA can be used for encrypting (small) files. Consider a message that A wants to confidentially send to B:
$ cat message.txt A private message
Now user A encrypts the message using B's public key which is stored in B's certificate:
$ openssl rsautl -encrypt -certin -inkey cert-B.pem -in message.txt -out ciphertext.ssl
The output file ciphertext.ssl contains the encrypted message in binary:
$ ls -l ciphertext.ssl -rw-r--r-- 1 sgordon sgordon 128 2011-01-22 12:22 ciphertext.ssl
Now user B decrypts using their private key:
$ openssl rsautl -decrypt -inkey privkeyB.pem -in ciphertext.ssl -out decrypted.txt $ cat decrypted.txt A private message
Signing with RSA
The most common application of RSA is to sign messages (as opposed to encrypt them). User A signs a message with their private key:
$ openssl rsautl -sign -inkey privkeyA.pem -in message.txt -out message.sgn
User B that receives the signed message can verify the signature using A's certificate:
$ openssl rsautl -verify -in message.sgn -certin -inkey cert-B.pem A private message
If an incorrect key is used or the message has been modified an error should be reported. First consider a case where the message was signed with
Note to self: Where did the rest of this go?
Created on Sat, 22 Jan 2011, 8:48pm
Last changed on Mon, 03 Nov 2014, 11:02am