(Objective-C) Get Public Key in Format for ~/.ssh/authorized_keys

If you have an RSA private key, then by definition you also have the public key because the RSA public key is a subset of the private key. (The public key is just the modulus + exponent, whereas the private key contains additional items.)

This example converts an RSA private key to the OpenSSH format public-key (a single line of text) that can be inserted into the ~/.ssh/authorized_keys file on the SSH server to enable public-key authentication for a particular private key.

Note: On the SSH server, such as on a Ubuntu Linux computer, the sshd config file is located at /etc/ssh/sshd_config. It contains a line that indicates the authorized keys file, and the typical default is:

AuthorizedKeysFile      .ssh/authorized_keys

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#import <NSString.h>
#import <CkoStringBuilder.h>
#import <CkoSshKey.h>

// This example requires the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

// Load a private key from a file.  The format can be text format, such as .ppk (PuTTY private key) or .pem.
NSString *privateKeyPath = @"/myKeys/myRsaKey.ppk";

// Here's an example of a non-encrypted private key in ppk format:

// PuTTY-User-Key-File-3: ssh-rsa
// Encryption: none
// Comment: rsa-key-20220606
// Public-Lines: 6
// AAAAB3NzaC1yc2EAAAADAQABAAABAQDKQUdx8EU6qIpVcT1NbL/sSnELJdifQa+K
// kOqvgKIU30wIwGMfigSeiTFXw2jWsPcMyOIRKUvR3nf8wtR6NEj3qDfGUo3b/8Fx
// 5BHDKuR5uU0ZJmv3Am+/hSPWOJFltL8DkxDLEpGDujLwGiUbpD9SADYhhvk9OBA7
// 0l70hYs9gWJBjm9CR8hPg3b3F7gOYljAY2dk6W3PGmzQi+D7BTE0yYfhEOoDX7ah
// sgrUnWIiIWK9cqLT8XbAb8gitnCmP0LerxRAkX25O4Gx/uGSrMxO/cRwz5UpiNL2
// cciM7P4vzlbcpQiKrKaBYg5GB84QqE3xCt9cEr/qD235LpSMnWRR
// Private-Lines: 14
// AAABAQClKCE7PUSK3c34b3vrmX4vaapdvA3kHjNGJ4g8wAGaoazpCJDo1D9pZgZQ
// 8FTP27ohSniwItSzD8NTN5ViJQfglBDXddo5Z+ODKQYIJSJk85etjd5j2i1+ay4U
// ZCT2tF22gYUZDpScyJOH1RGwPLMoNtv9DMbB4uH+t46qhdJp6aWvJB7L/HUhnszc
// qbwTG//CDL2j8Y1Mre8zvWBA8cr0I0qCzWD/Xw5MgDgpA444CFGpYb1mt8ghLniT
// hMGgt6scUO/lnBEYo17a9N7wBexvyk0ZgEVo6nweRnCFHijNbG+C82svrsLQuL7g
// o65BcEqLo+wBO+FtutTf2gtevZcBAAAAgQDnt2Freyfn1V93qZar06w7dXn4yV4G
// cvQ3FmJBhk7akEFuZPOceDqrAgzS+TmsH4C96ssETdMxMO4vwEjSnZAatw6TKgnK
// 9aDJL6/uzhk/dmBmyp7iWttN9QW9cRQvLYtwJssNDPUmF1/1cGFMpLyLfTlRBCok
// 8dksYys4FTUfeQAAAIEA33N+LIO8i2U1NNfy2yU5GGJoFaXwPIa/CZNzZ8hbvLwm
// 5yay4MKxGgizX+FhQ/OiEiFDEs1GDssKTXaqhE9mI/vKKLGYj9wEiWhwnawx56GI
// WhdTZPtWkU4tYj/OkABD8vp973W1k4NEjYxVg8AoZZL1Kr4zturV8KJl9v4b/ZkA
// AACBANAijbBLPxSTdyG9duZT89+jIdgjdjVdV5UL/xZLEh6pWbHludZHItXl62G7
// luuJbiqhx2FANxhSu7/tAEX3z2jx7ari51Ddf/+sqfrLwc3dPkU8YXpUNYJIaNFR
// LyzMMZvL2sPxnE6dxt4nR761y+EnTZpZ2oeSC21Lzq2xKcBp
// Private-MAC: e0c3b5eba925944355d8d63fefcec02a982c7281a87e9994c04ce956b5c3e69b

CkoStringBuilder *sbKey = [[CkoStringBuilder alloc] init];
BOOL success = [sbKey LoadFile: privateKeyPath charset: @"utf-8"];
if (success == NO) {
    NSLog(@"%@",@"Failed to load the private key from a file.");
    return;
}

CkoSshKey *key = [[CkoSshKey alloc] init];

// If the private key file is encrypted, then you'll need to set a password.
// If it's not encrypted, the password here is ignored.
key.Password = @"private_key_password";

// The FromPuttyPrivateKey method will actually auto-detect the format and correctly load any string-based format.
// For example, you can pass in a PEM private key.
success = [key FromPuttyPrivateKey: [sbKey GetAsString]];
if (success == NO) {
    NSLog(@"%@",key.LastErrorText);
    return;
}

// Get the public key in OpenSSH format:
NSString *pubKeyStr = [key ToOpenSshPublicKey];
NSLog(@"%@",pubKeyStr);

// Sample output:
// ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDKQUdx8EU6qIpVcT1NbL/sSnELJdifQa+KkOqvgKIU30wIwGMfigSeiTFXw2jWsPcMyOIRKUvR3nf8wtR6NEj3qDfGUo3b/8Fx5BHDKuR5uU0ZJmv3Am+/hSPWOJFltL8DkxDLEpGDujLwGiUbpD9SADYhhvk9OBA70l70hYs9gWJBjm9CR8hPg3b3F7gOYljAY2dk6W3PGmzQi+D7BTE0yYfhEOoDX7ahsgrUnWIiIWK9cqLT8XbAb8gitnCmP0LerxRAkX25O4Gx/uGSrMxO/cRwz5UpiNL2cciM7P4vzlbcpQiKrKaBYg5GB84QqE3xCt9cEr/qD235LpSMnWRR rsa-key-20220606

// To authorize the private key for a particular SSH user account, copy the above public key into the ~/.ssh/authorized_keys file.
// If authorized_keys already contains some keys, you can append it to a new line in the file.