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 89 f8 ae 89 21 04 2e 6b 3d 8c 82 dc b9 1f 44 da 1c 74 e7 a4 8e e7 70
[51] 7d 58 b0 b4 5a 79 64 47 21 ab 45 af 39 f1 6e 5d e6 b0 28 d9 88 a0 20 a2 ce
[76] ba ec 03 33 44 bf 23 d4 38 97 9b b4 63 5e 0d 34

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: 7b0ce70b8c1dfbb171390903bb94bc0a
sha256: 12c0832dbe7c8e255851a5a39fe32e6586a3d5b3d1a8f75babf3a4a5ba784d8d

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-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEifiuiSEELms9jILcuR9E2hx056SO
53B9WLC0WnlkRyGrRa858W5d5rAo2YigIKLOuuwDM0S/I9Q4l5u0Y14NNA==
-----END PUBLIC KEY-----

cat(write_pem(key, password = NULL))

-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQguTgOJ2ZiQyYoFPrZ
9kL8HcqPlReOluQ2bC55S9L85jihRANCAASJ+K6JIQQuaz2Mgty5H0TaHHTnpI7n
cH1YsLRaeWRHIatFrznxbl3msCjZiKAgos667AMzRL8j1DiXm7RjXg00
-----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-----
MIHrMFYGCSqGSIb3DQEFDTBJMDEGCSqGSIb3DQEFDDAkBBCbympe+174aWBepshl
absVAgIIADAMBggqhkiG9w0CCQUAMBQGCCqGSIb3DQMHBAhWAFHPlfoongSBkB/I
IleLw2OG9PyCSokOEYARLA30q2ObxGUYLxaJBrahBcn3a5stpvXxMC4+JsnmWRyl
4mdrZ01A99L6l9OLlZH2Jb3WsEFI3W6dJYiQjNG4wyIdHPfItu/WrBY4VbTAlsvv
c7LkmJp04ke7lfWnKuKCBOv4ge43paIDlklmhzl8spP8bmScvMOi6B11oKbGpQ==
-----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 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBIn4rokhBC5rPYyC3LkfRNocdOekjudwfViwtFp5ZEchq0WvOfFuXeawKNmIoCCizrrsAzNEvyPUOJebtGNeDTQ="

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: 7b0ce70b8c1dfbb171390903bb94bc0a
sha256: 12c0832dbe7c8e255851a5a39fe32e6586a3d5b3d1a8f75babf3a4a5ba784d8d

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": "ifiuiSEELms9jILcuR9E2hx056SO53B9WLC0WnlkRyE",
    "y": "q0WvOfFuXeawKNmIoCCizrrsAzNEvyPUOJebtGNeDTQ"
}
 

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: 7b0ce70b8c1dfbb171390903bb94bc0a
sha256: 12c0832dbe7c8e255851a5a39fe32e6586a3d5b3d1a8f75babf3a4a5ba784d8d