What is the PEM encoding for RSA public keys?

[erickt]

In section 4.2, the spec states that RSA public keys are stored in PEM format, but may be vague. RSA PEM keys support two forms of encoding, RSAPublicKey form, as in:

-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEA4XLc9x7DX00eSQ4WvlqyojG1D/hr+X5Sn7dtd0Lq2MUarOqCRNDC
liKVni4ljwOADTn/+/JOgyS7Qf9RSi3KhKFMqEO83vKrbxJQHI4jK9kXlDtTPWmb
fdPzAMkc3tMNmp7M1DrYRUKR1++z5rb1AKCLIylhMp6j4oxOhKAc5ySKppMdr0EU
5Yih2kcqF/BmcIc6h/XNXS+iLOnOq9uT0+1VBhQ1iK542AM6XpXg3VADgq9PUzRf
ZfU4hVgbL6nTLH9j/lTyRn6Rl/rWgKSHo2xoAXrRTNJ5IEjfrSpsWrI4c9nSHs07
JosZDpmwJlC0GDET0ps18m1X8BTfj55UxQIDAQAB
-----END RSA PUBLIC KEY-----

And the newer SubjectPublicKeyInfo form, as in:

-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA4XLc9x7DX00eSQ4Wvlqy
ojG1D/hr+X5Sn7dtd0Lq2MUarOqCRNDCliKVni4ljwOADTn/+/JOgyS7Qf9RSi3K
hKFMqEO83vKrbxJQHI4jK9kXlDtTPWmbfdPzAMkc3tMNmp7M1DrYRUKR1++z5rb1
AKCLIylhMp6j4oxOhKAc5ySKppMdr0EU5Yih2kcqF/BmcIc6h/XNXS+iLOnOq9uT
0+1VBhQ1iK542AM6XpXg3VADgq9PUzRfZfU4hVgbL6nTLH9j/lTyRn6Rl/rWgKSH
o2xoAXrRTNJ5IEjfrSpsWrI4c9nSHs07JosZDpmwJlC0GDET0ps18m1X8BTfj55U
xQIDAQAB
-----END PUBLIC KEY-----

Should we support both forms of key encoding, or just one? Presumably we need to at least support SubjectPublicKeyInfo for ECDSA keys.

[trishankatdatadog]

I vote for only one: SubjectPublicKeyInfo (looks like what we do, but needs to be confirmed)

[lukpueh]

Confirming what @trishankatdatadog says:
We, i.e. the TUF reference implementation via securesystemslib, expect the -----BEGIN PUBLIC KEY----- header when parsing RSA pubkeys.

The issue of different PEM formats (IIRC there are more than two) and how to parse them is discussed in secure-systems-lab/securesystemslib#54.

That said (and mildly in contrast to it), I don't think the spec needs to be more specific about the PEM format. The format is only important to parse out a key, in order to use it for signature creation/verification, which to me seems an implementation detail.

OTOH, when it comes to creating/verifying a signature over a piece of metadata that may contain public keys, the value of PUBLIC in { ..., "keyval" : {"public" : PUBLIC}} is just an opaque blob.