get »fingerprint« from rsa key - node.js

Question

By using the crypto module, it is easy to create a private/public key Pair in node. But how can I compute the »fingerprint« of a key?
OpenCrypto has something like that:
crypt.getFingerprint(key, options).then(function (fingerprint) {
console.log(fingerprint)
})
What is the equivalent of nodes crypto module for that?

Answer 1

OpenCrypto.getFingerprint exports a public RSA key in the X509 DER format and creates a hash for these data. The same applies to a private RSA key with the difference that the private key is exported in the Pkcs8 DER format. The digest can be specified in the options (default: SHA 512) and also whether the data are returned as buffer or as hexadecimal string (default: hexadecimal string).
In the NodeJS code, keys can be generated with crypto.generateKeyPair, whereby the key format can be specified explicitly. If the keys aren't already in the appropriate formats (X509 DER and Pkcs8 DER) key conversions can be performed to produce the same fingerprints that OpenCrypto.getFingerprint creates. Suitable functions for these operations are crypto.createPublicKey or crypto.createPrivateKey. Finally the hash has to be generated with crypto.createHash.
Update:
The fingerprint is nothing else than a hash value, e.g. with the digest SHA-512 (as in OpenCrypto.getFingerprint):
var fingerprint = crypto.createHash('sha512').update(key).digest('hex'); // Fingerprint (hash) as hexadecimal string
where key is a public or private key in any format (string, Buffer, ...). If the key is given as X509 DER (public) or as PKCS8 DER (private), then the fingerprint matches that of OpenCrypto.getFingerprint.
If the key is in another format, the fingerprint can also be determined in this way. However, if the fingerprint should match the value provided by OpenCrypto.getFingerprint, the key must of course be converted into the formats used by OpenCrypto.getFingerprint before the hash is generated. This conversion isn't very complex, e.g. the conversion of a public PKCS1 PEM key (publicKey) into a X509 DER key (publicKeyDER) including the generation of the hash:
var publicKeyDER = crypto.createPublicKey(publicKey, { type: 'pkcs1', format: 'pem' }).export({ type: 'spki', format: 'der' }); // Convert a public PKCS1 PEM key into a X509 DER key
var fingerprint = crypto.createHash('sha512').update(publicKeyDER).digest('hex'); // Fingerprint (hash) as hexadecimal string
The bottom line is that these are functionally the same operations that are performed in OpenCrypto.getFingerprint. To my knowledge, there is no OpenCrypto.getFingerprint counterpart in the NodeJS crypto module. But with only little effort you can write your own function using the above crypto functions.