(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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#include <C_CkStringBuilder.h>
#include <C_CkSshKey.h>

void ChilkatSample(void)
    {
    const char *privateKeyPath;
    HCkStringBuilder sbKey;
    BOOL success;
    HCkSshKey key;
    const char *pubKeyStr;

    // 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.
    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

    sbKey = CkStringBuilder_Create();
    success = CkStringBuilder_LoadFile(sbKey,privateKeyPath,"utf-8");
    if (success == FALSE) {
        printf("Failed to load the private key from a file.\n");
        CkStringBuilder_Dispose(sbKey);
        return;
    }

    key = CkSshKey_Create();

    // 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.
    CkSshKey_putPassword(key,"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 = CkSshKey_FromPuttyPrivateKey(key,CkStringBuilder_getAsString(sbKey));
    if (success == FALSE) {
        printf("%s\n",CkSshKey_lastErrorText(key));
        CkStringBuilder_Dispose(sbKey);
        CkSshKey_Dispose(key);
        return;
    }

    // Get the public key in OpenSSH format:
    pubKeyStr = CkSshKey_toOpenSshPublicKey(key);
    printf("%s\n",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.


    CkStringBuilder_Dispose(sbKey);
    CkSshKey_Dispose(key);

    }