Cryptographic - Private Key

About

This article talks about the secret known as the private key in a asymmetric cryptographic scheme.

A private key is a cryptographic key that is private (that you should keep secret).

This page is about how to manage/create a What is a Public Key Cryptography (known as Asymmetric Cipher) ? private key.

An What is a Public Key Cryptography (known as Asymmetric Cipher) ? private key is categorized as something you have authentication.

Management

Algorithm Scheme

There is two asymmetric cryptographic scheme that defines how to create a private key.

There is therefore two types of key. The most known being RSA

RSA Private Key
DSA

File Format

Different file formats are used to store keys.

Create

The creation of the private key is format dependent.

Pem

For a pem private key (the openssl format), you use:

the genrsa ¹⁾ utility to create a RSA private key.
the gendsa utility to create a DSA private key

Example: The below command ²⁾ creates a:

RSA private key with a key size of 4096 bit
encrypted using Triple-DES cipher with the passphrase yolo

openssl \
  genrsa \ 
  -des3 `# encrypt the key with DES in ede cbc mode (168 bit key)`  \
  -out private-key.pem `# the key file`  \
  -passout file:passphrase.txt `# the passphrase is in the file
  4096 `# num of bit default to 2048

Output Example:

Generating RSA private key, 4096 bit long modulus (2 primes)
.................................++++
.........................................................................++++
e is 65537 (0x010001)

-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: DES-EDE3-CBC,B03FF7BC9D49E89C
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..............
.............
-----END RSA PRIVATE KEY-----

Ppk

For a ppk private key (Putty), see ppk create

Decrypt / Suppress passphrase

With a:

PEM Format: Pem decrypt
With an openssh private key: OpenSsh keygen

-----BEGIN OPENSSH PRIVATE KEY-----
xxxxxxxxxx
-----END OPENSSH PRIVATE KEY-----

# with prompt of the passphrase
# where N is the new passphrase (ie empty)
# and f is the file
ssh-keygen -p  -N "" -f /path/keyfile.pem

¹⁾

genrsa man page

²⁾

Genrsa Syntax - Generate an RSA private key