Importing and exporting RSA/DSA/EC keys

The openssl package implements a modern interface to libssl and libcrypto for R. It builds on the new EVP api which was introduced in OpenSSL 1.0 and provides a unified API to the various methods and formats. OpenSSL supports three major public key crypto systems:

• RSA : Most popular method. Supports both encryption and signatures.
• DSA : Digital Signature Algorithm. Mostly for signatures, not very popular anymore.
• ECDSA : Elliptic Curve DSA. Supports signatures and encryption via Diffie Hellman. Gaining popularity.

For each type there are several common formats for storing keys and certificates:

• DER: binary format, serialized ASN.1 structure
• PEM: base64 encoded DER + header wrapped in ===
• SSH: single line of base64 with a header. Only for pubkeys.
• JWK: JSON Web key. Stores key data in a JSON object

The openssl package automatically detects the format when possible. However being able to recognize the various formats can be useful.

The DER format

DER is the standard binary format using by protocols for storing and exchanging keys and certificates. It consists of a serialized ASN.1 structure which hold the key’s (very large) prime numbers.

key <- ec_keygen()
pubkey <- key$pubkey
bin <- write_der(pubkey)
print(bin)

 [1] 30 59 30 13 06 07 2a 86 48 ce 3d 02 01 06 08 2a 86 48 ce 3d 03 01 07 03 42
[26] 00 04 c9 95 c0 9d 2a da f8 04 11 45 f0 8e 36 00 31 60 22 b0 6b 15 90 01 94
[51] 3f 39 e5 f0 b2 9e 86 39 3c 3d 65 9a 08 88 11 43 79 4f 8a 0e 55 9c f2 39 82
[76] 7c 0a 86 0e d2 45 d1 a5 4b d0 20 0a a6 ac 6b 8d

To read a DER key use read_key or read_pubkey with der = TRUE.

read_pubkey(bin, der = TRUE)

[256-bit ecdsa public key]
md5: 04f4ba7171c4fdececdb3394126e2bc6
sha256: bbaa337111faf05ff088595133ee5e70f0faa93198f71654f0754c5355c9ee67

Users typically don’t need to worry about the key’s underlying primes, but have a look at key$data if you are curious.

The PEM format

In practice the user rarely encounters DER because it is mainly for internal use. When humans exchange keys and certificates they typically use the PEM format. PEM is simply base64 encoded DER data, plus a header. The header identifies the key (and possibly encryption) type.

cat(write_pem(pubkey))

-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEyZXAnSra+AQRRfCONgAxYCKwaxWQ
AZQ/OeXwsp6GOTw9ZZoIiBFDeU+KDlWc8jmCfAqGDtJF0aVL0CAKpqxrjQ==
-----END PUBLIC KEY-----

cat(write_pem(key, password = NULL))

-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQghi7OheWhnPt3Wx/z
44bYM+A7DZwvfSHZ01vN6H0D2pyhRANCAATJlcCdKtr4BBFF8I42ADFgIrBrFZAB
lD855fCynoY5PD1lmgiIEUN5T4oOVZzyOYJ8CoYO0kXRpUvQIAqmrGuN
-----END PRIVATE KEY-----

The PEM format allows for protecting private keys with a password. R will prompt you for the password when reading such a protected key.

cat(write_pem(key, password = "supersecret"))

-----BEGIN ENCRYPTED PRIVATE KEY-----
MIHjME4GCSqGSIb3DQEFDTBBMCkGCSqGSIb3DQEFDDAcBAjXEaVpwNuwrAICCAAw
DAYIKoZIhvcNAgkFADAUBggqhkiG9w0DBwQIqwx1R0SAz7UEgZBHtB+nbHGZkIip
X92y6d5jlLhwJA6ALdoUmsW2f9XKW1ySXjwniv5QwXZYsf/LBIhKNHULIkQ9bk2C
WGVbdFYp1H2v96MpAbczpvP0S/vc3v+xCLLr1B7IXIgvtAMmdt3y/QRkeCvxrc2A
6odv2wGAFzjxHMZZsgqrEEVF6Dzkj20/7UFRTlbCa9yhlZ5E/Ok=
-----END ENCRYPTED PRIVATE KEY-----

The OpenSSH format

For better or worse, OpenSSH uses a custom format for public keys. The advantage of this format is that it fits on a single line which is nice for e.g. your ~/.ssh/known_hosts file. There is no special format for private keys, OpenSSH uses PEM as well.

str <- write_ssh(pubkey)
print(str)

[1] "ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBMmVwJ0q2vgEEUXwjjYAMWAisGsVkAGUPznl8LKehjk8PWWaCIgRQ3lPig5VnPI5gnwKhg7SRdGlS9AgCqasa40="

The read_pubkey function will automatically detect if a file contains a PEM or SSH key.

read_pubkey(str)

[256-bit ecdsa public key]
md5: 04f4ba7171c4fdececdb3394126e2bc6
sha256: bbaa337111faf05ff088595133ee5e70f0faa93198f71654f0754c5355c9ee67

The JSON Web Key (JWK) format

Yet another recent format to store RSA or EC keys are JSON Web Keys (JWK). JWK is part of the Javascript Object Signing and Encryption (JOSE) specification. The write_jwk and read_jwk functions are implemented in a separate package which uses the openssl package.

library(jose)
json <- write_jwk(pubkey)
jsonlite::prettify(json)

{
    "kty": "EC",
    "crv": "P-256",
    "x": "yZXAnSra-AQRRfCONgAxYCKwaxWQAZQ_OeXwsp6GOTw",
    "y": "PWWaCIgRQ3lPig5VnPI5gnwKhg7SRdGlS9AgCqasa40"
}
 

Keys from jose and openssl are the same.

mykey <- read_jwk(json)
identical(mykey, pubkey)

[1] TRUE

print(mykey)

[256-bit ecdsa public key]
md5: 04f4ba7171c4fdececdb3394126e2bc6
sha256: bbaa337111faf05ff088595133ee5e70f0faa93198f71654f0754c5355c9ee67