Generate Public Ssh Key from Private Key

Use RSA private key to generate public key?

openssl genrsa -out mykey.pem 1024

will actually produce a public - private key pair. The pair is stored in the generated mykey.pem file.

openssl rsa -in mykey.pem -pubout > mykey.pub

will extract the public key and print that out. Here is a link to a page that describes this better.

EDIT: Check the examples section here. To just output the public part of a private key:

openssl rsa -in key.pem -pubout -out pubkey.pem

To get a usable public key for SSH purposes, use ssh-keygen:

ssh-keygen -y -f key.pem > key.pub

Generate public ssh key from private key

The option -y outputs the public key. From the linux manual for the ssh-keygen command:

-y ---- This option will read a private OpenSSH format file and print
an
OpenSSH public key to stdout.

ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub

As a side note, the comment of the public key is lost. I've had a site which required the comment (Launchpad?), so you need to edit ~/.ssh/id_rsa.pub and append a comment to the first line with a space between the comment and key data. An example public key is shown truncated below.

ssh-rsa AAAA..../VqDjtS5 ubuntu@ubuntu

If you want a scripted way to add a comment and also add the pubkey to your authorized_keys file, you can do...

ssh-keygen -y -f ~/.ssh/id_rsa    | \
    sed 's/$/ comment-goes-here/' | \
    tee ~/.ssh/id_rsa.pub         | \
    tee -a authorized_keys

Can I generate private key from the last public key using puttygen?

You cannot generate private key from public key but you can generate public key from the private key using puttygen. As @alfasin mentioned if you could generate the private key from public key then RSA would be useless and this would make you vulnerable to attack.FYI the public key is used for encryption and private key is used for decryption.
this wikipedia page will help you better understand the reason.

C# create ssh-rsa public key from existing private key string

SSH key format is a bit complex. Moreover, .NET doesn't have a method to directly get the key in this format. However, something like this will work (I tried in a .NET 6 console application, and got the public key exactly how you want it.):

// See https://aka.ms/new-console-template for more information
using System.Security.Cryptography;
using System.Text;

static byte[] ToBytes(int i)
{
    byte[] bytes = BitConverter.GetBytes(i);

    if (BitConverter.IsLittleEndian)
    {
        Array.Reverse(bytes);
    }

    return bytes;
}

string privateKey = "-----BEGIN RSA PRIVATE KEY-----     MIIEogIBAAKCAQEAvyN0aQKoYl/LAZ/1dQt0rWuSNyOty88k3439HT3rcT/vhaSk     d5lbnNKiYTzdDEkAxAnx4rxw6bEdD/8A9ISs0jy3pFRORFdbgBVFjIPR2NKbwVbs     9fcQNOQHcNslAyHA/yy57ktw+/6VyHYnHfXFlhkt1Jx4A1ubFIGzXttnXkwuNhdn     2JLJ5+JA3zRDJNBZR7p7NHVu9cRBwADm/WSzPqI6Sgs8kkU0eBcfy7qJRao3cmR5     95lLxkhFARufSW8lD/tCs2k99T2ZwZpKJpliA5VGjIC3iHhck3tpXs5w9sQ5Axhv     n1kTq5GKNi48r132KgRNJO+jIY0QSI60A6akbwIDAQABAoIBACCB3SiG5TBl7lbG     Z66SVjOwWdu627IP9st2kJfKkiJep1PpXndgw632PNugyE9wkwrETjkrp2B3WOQB     kJ4Feob/AJSYKf+Bg/RSqdNuD+B6YTcOm5pxfHYiWgmdm7ven75GUxDuD7cr4zmG     rrxvsj0G5z6Dpf2cNNHWBTWaxwfITaC8yXp6dx8o8V86/T0qrsEl+S0YJ5VQWt6L     I5GzipFNhhjcaemkOxDJg2T/g0FbpBEuj3RnwWNfRiiTCt+AuROg6/4M2oyLBE9W     e8n8KAUhZvRJA2dFwzZY38U9MfX9k9zIkJXtpkeghGx3M2zG3cQcFOaly6aFNHjr     QuEd6kECgYEA7NQgfqxuJ34kMnmtZeYccFGI6WUosuXUlgMhqU8CUnjIaRX8u8Ho     UjvjbezHNsI8tyH3vopgHNqTkcuElyuxKZQBTtUOFGG4a1HUS5tlo913DcnuSVIa     qL8kn3XVDHvuTr8tJbsb4KXrEMFfGoJBemU4ixSDiYWk/FdXvyyEEbECgYEAzpx6     JPOktmdaLf8U7snvlRY9daBqKfPqtKDxYgsC3xOp90Z3FMWQi5OyPmBsLGmjHxhe     YrPYQ3lbRh2JuRgZ7rTAxXN9dnDNgrh4tFjEEqQiFBCGlhP6syNM9Kx0YYNAoJN4     U29Tv71rxHJiFaLiRTh3Nopdn5ir4Raoj2fQgB8CgYBxCCFmNAfzA2plSNuwia5D     ETcmJejR0Y2v91imhRYXpJwKQ7s3JaorLXgzq9G82eG+ihDDOSn8O3o5GIh02h6Z     OJGTPW6V3bn2RrzrRQSyu+2pgBohlnUw2uGw1b1UUwX/QZFbs7zvcGELwy8P6OE1     eIAPKUBKb6W55jnz/VwfUQKBgGPTpQyPkAj1vNO2iLWrag/dtApOXJ0yljd5/8cA     TP3dsWShbk3h+yoFTbznt7xpuf//NTN5c8d+LkSdZvrAk18LhIyidX8xl4pOeTui     G/JpzXFmXrDKrHm7V6ZsYLrwwNwVBLFDe/KLojNDlPKhRbRuSONYTU4cZQeXfA/1     9/6/AoGAarF4JSdpzMzfacpLy2nsOM6XmL76B218uKANSHQy9k1X/Hp1u1StY8tQ     H4+DSrRUQBb4sdxkCRXVvMH3zttDGoIrSUvDqN3k4opcP8nmzMc/EDwD3xFgri/p     yBXBhE99r1B0p7fneXt58tTqtcevk5dQPzyF9SdsfUxD5PrnZRI=     -----END RSA PRIVATE KEY-----";

var rsa = RSA.Create();
rsa.ImportFromPem(privateKey.ToCharArray());

byte[] sshrsaBytes = Encoding.Default.GetBytes("ssh-rsa");
byte[] n = rsa.ExportParameters(false).Modulus;
byte[] e = rsa.ExportParameters(false).Exponent;
string buffer64;

using (var ms = new MemoryStream())
{
    ms.Write(ToBytes(sshrsaBytes.Length), 0, 4);
    ms.Write(sshrsaBytes, 0, sshrsaBytes.Length);
    ms.Write(ToBytes(e.Length), 0, 4);
    ms.Write(e, 0, e.Length);
    ms.Write(ToBytes(n.Length + 1), 0, 4);
    ms.Write(new byte[] { 0 }, 0, 1);
    ms.Write(n, 0, n.Length);
    ms.Flush();
    buffer64 = Convert.ToBase64String(ms.ToArray());
}

string comment = "administrator@LovelyTrust";
string publicKey = $"ssh-rsa {buffer64} {comment}";
Console.WriteLine(publicKey);

ToBytes method is simply checks your machine's endianness and converts bytes into correct order if necessary.

We first create an RSA object and import the private key, just like you did. After that, the complex part I mentioned starts, and continues until the end of the using block. Finally, we concatenate that value with ssh-rsa word from the left and the comment (this is the value entered in Key comment text field if you use PuTTY Key Generator) from the right.

This code is heavily inspired from the code of SshKeyGenerator library. Unfortunately, the library itself does not support key import. On the other hand, if you want randomly generated SSH private and public key pairs, using directly the library will be probably easier and more convenient.

Generate private / public SSH key with Python

You did this to get the private key:

print crypto.dump_privatekey(crypto.FILETYPE_PEM, k)

Do this to get the public key:

print crypto.dump_publickey(crypto.FILETYPE_PEM, k)

Generate Private key with OpenSSL and Public key ssh-keygen for SSH

After doing some research and experiments I landed on the simplest solution.

Generate secure private key using openssl with a password length of 32 or more characters, then use ssh-keygen command to get my required output.

ssh-keygen  -y -f private.pem > public_key.pub

It works accurately!

How do I get the public key of a pem file?

Copy the public key to clipboard.

Linux

ssh-keygen -f private.pem -y | xclip

MacOS

ssh-keygen -f private.pem -y | pbcopy

Save to file

ssh-keygen -f private.pem -y > public.pub

Note that if your permissions are vague on the .pem file, then ssh-keygen will generate an empty .pub file.

You can usually fix this with:

chmod 400 private.pem 

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