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Add blog about crypto key formats
Signed-off-by: Daniel Bevenius <[email protected]>
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‎_posts/2023-01-25-keys.md‎

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---
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title: "Is this a cryptographic key which I see before me?"
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date: 2023-01-25
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author: Daniel Bevenius
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---
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Yes, it is. Really? Then what format is it in and how can I tell?
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I've found myself in this situation a number of times and this post tries to
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provide some guidelines for figuring out the type and format of keys without
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having to go off and read some project's documentation.
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To start off we can try to determine if the key is in a PEM format, or in
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DER format.
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Keys in PEM format are in ascii and can be inspected from the
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command line using `cat`, or opened in any text editor. For example:
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```console
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$ cat pubkey.pem
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-----BEGIN PUBLIC KEY-----
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MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEDTvL0PRsxoxMXfSaXu+7w0ovVNzZ
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k/BAIoz2GL2cPY3qZENU/+YrR92AuZFXn0jSmmvOktpAzGhnDhtidonkyA==
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-----END PUBLIC KEY-----
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```
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If we try the same with DER format then we will get a bunch of strange
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characters printed. For example:
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```console
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$ cat pubkey.der
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;���lƌL]��^��J/T�ٓ�@"����=��dCT��+G݀��W�HҚkΒ�@�hgv���$
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```
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## PEM formatted keys
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So we have determined that the key we have in front of us is in PEM format.
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Now, if we take a look at the PEM output above again:
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```console
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-----BEGIN PUBLIC KEY-----
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MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEDTvL0PRsxoxMXfSaXu+7w0ovVNzZ
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k/BAIoz2GL2cPY3qZENU/+YrR92AuZFXn0jSmmvOktpAzGhnDhtidonkyA==
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-----END PUBLIC KEY-----
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```
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We can see that it has a header and the footer. Notice that there is no
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information about the type of public key that this file contains. This means
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that the information about the type of key in baked in there somewhere. So how
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can we find out what the type of the key?
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One option is to use the `openssl asn1parse` command:
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```
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$ openssl asn1parse -i -in pubkey.pem
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0:d=0 hl=2 l= 89 cons: SEQUENCE
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2:d=1 hl=2 l= 19 cons: SEQUENCE
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4:d=2 hl=2 l= 7 prim: OBJECT :id-ecPublicKey
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13:d=2 hl=2 l= 8 prim: OBJECT :prime256v1
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23:d=1 hl=2 l= 66 prim: BIT STRING
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```
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And we can see, that there is an id here which is `ecPublicKey`.
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As a rule of thumb, if there is no key type in the PEM header, then
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the format of the key is most probably in Subject Public Key Info (SPKI) if it
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is a public key, and in Public-Key Cryptography Standard 8 (pkcs8) format if it
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is a private key.
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With the knowledge that the key is an Elliptic Curve (EC) public key we can use
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the following openssl command to inspect it:
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```console
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$ openssl ec -pubin -in pubkey.pem --text --noout
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read EC key
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Public-Key: (256 bit)
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pub:
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04:0d:3b:cb:d0:f4:6c:c6:8c:4c:5d:f4:9a:5e:ef:
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bb:c3:4a:2f:54:dc:d9:93:f0:40:22:8c:f6:18:bd:
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9c:3d:8d:ea:64:43:54:ff:e6:2b:47:dd:80:b9:91:
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57:9f:48:d2:9a:6b:ce:92:da:40:cc:68:67:0e:1b:
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62:76:89:e4:c8
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ASN1 OID: prime256v1
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NIST CURVE: P-256
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```
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Some PEM keys can also be in a specific key format, in which case the type is
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in the header of the pem, for example:
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```
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-----BEGIN RSA PUBLIC KEY-----
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...
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-----END RSA PUBLIC KEY-----
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```
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And if needed we can use the `openssl rsa` command to inspect them further.
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The same reasoning can be applied to private keys as well with regards to the
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PEM header/footer information, and in the case of private keys the `-pubin`
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argument to the openssl commands should left out.
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## DER formatted keys
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As mentioned before we can't just print DER files as they are in binary format,
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but we can still use `openssl asn1parse`:
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```console
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$ openssl asn1parse -i -inform der -in pubkey.der
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0:d=0 hl=2 l= 89 cons: SEQUENCE
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2:d=1 hl=2 l= 19 cons: SEQUENCE
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4:d=2 hl=2 l= 7 prim: OBJECT :id-ecPublicKey
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13:d=2 hl=2 l= 8 prim: OBJECT :prime256v1
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23:d=1 hl=2 l= 66 prim: BIT STRING
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```
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And just like with the PEM example we can use other openssl tools to inspect the
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key.
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## When the guidelines fail
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The above seems to work for most situations, but it can fail.
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One example of this is when openssl cannot parse the key at all. I ran into this
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recently with [in-toto-rs](https://github.com/in-toto/in-toto-rs), which uses
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the Rust [ring](https://crates.io/crates/ring) crate to handle Ed25519 keys.
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The issue here is that `ring` supports pkcs8 version 2
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([RFC-5958](https://www.rfc-editor.org/rfc/rfc5958)), and OpenSSL currently only
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supports pkcs8 version 1 ([RFC-5208](https://www.rfc-editor.org/rfc/rfc5208)),
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so the openssl tools will not be able to parse keys in the version 2 format.
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Below is an example of trying to use a version 2 formatted Ed25519 key with
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openssl:
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```console
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$ openssl pkey -inform der -in ed25519-1 -pubout
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Could not read key from ed25519-1
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```
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There is an open [openssl issue](https://github.com/openssl/openssl/issues/10468)
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for this.
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Hopefully there will not be many cases like this, and we hope that the
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guidelines provided in this post are helpful.

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