What's the use for the username and hostname at the endof the public RSA key

Question

What is the purpose of:

username@hostname

at the end of the RSA public key? I know that it matches the the generator of the key, but is it ever used for anything significant?

Answer 1

It is only a comment, to help you keep straight where each public key comes from.

In the openSSH source (v6.3,auth-rsa.c:57-65):

/*
 * The .ssh/authorized_keys file contains public keys, one per line, in the
 * following format:
 *   options bits e n comment
 * where bits, e and n are decimal numbers,
 * and comment is any string of characters up to newline.  The maximum
 * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
 * description of the options.
 */

And reading:

    case KEY_RSA1:
            /* Get number of bits. */
            if (*cp < '0' || *cp > '9')
                    return -1;      /* Bad bit count... */
            for (bits = 0; *cp >= '0' && *cp <= '9'; cp++)
                    bits = 10 * bits + *cp - '0';
            if (bits == 0)  
                    return -1;
            *cpp = cp;
            /* Get public exponent, public modulus. */
            if (!read_bignum(cpp, ret->rsa->e))
                    return -1;
            if (!read_bignum(cpp, ret->rsa->n))
                    return -1;
            /* validate the claimed number of bits */
            if ((u_int)BN_num_bits(ret->rsa->n) != bits) {
                    verbose("key_read: claimed key size %d does not match "
                       "actual %d", bits, BN_num_bits(ret->rsa->n));
                    return -1;
            }
            success = 1;
            break;

It doesn't even parse the comment.