Get key parameters from imported Elliptic Curve key in ASN.1 format

debugcn Published at Dev

Dropper

I need to write code that gets as input Elliptic Curve key in ASN.1 format.

The input byte array is next:

308187020100301306072A8648CE3D020106082A8648CE3D030107046D306B0201010420E699203AC5BCFE36402EBD0AC9E8E21CC6FAD5828A61297EA747468FFF4DBB20A144034200047E05188A03EA81E853B9F6AC5F20DCA1A1CA828FD7CD5D92161FB2120C35EAC52EAB079ED01A510123057C322DDFF95E239D6063055BC90858D161D71DE707F8

Online parser shows me the next structure:

To use key as I want I need to get public value X, public value Y and private value from this structure, at least I think so. But I do not know how.

I have searched information about OBJECT IDENTIFIER 1.2.840.10045.2.1 and OBJECT IDENTIFIER 1.2.840.10045.3.1.7. I've found this document. But there is no description of fields of ASN.1 structure.

How can I get required parameters from imported data?

Maarten Bodewes

It's commonly known as a PKCS#8 structure, which is the "Private-Key Information Syntax Specification". It only contains the unencrypted part of a PKCS#8 private key.

So this is in PKCS#8:

PrivateKeyInfo ::= SEQUENCE {
    version Version,
    privateKeyAlgorithm AlgorithmIdentifier {{PrivateKeyAlgorithms}},
    privateKey PrivateKey,
    attributes [0] Attributes OPTIONAL
}

The AlgorithmIdentifier is taken from PKCS#5

AlgorithmIdentifier { ALGORITHM-IDENTIFIER:InfoObjectSet } ::= SEQUENCE {
    algorithm ALGORITHM-IDENTIFIER.&id({InfoObjectSet}),
    parameters ALGORITHM-IDENTIFIER.&Type({InfoObjectSet}
    {@algorithm}) OPTIONAL
}

The Elliptic Curve Private Key structure:

ECPrivateKey ::= SEQUENCE {
    version        INTEGER { ecPrivkeyVer1(1) } (ecPrivkeyVer1),
    privateKey     OCTET STRING,
    parameters [0] ECParameters {{ NamedCurve }} OPTIONAL,
    publicKey  [1] BIT STRING OPTIONAL
}

Oh, and encoded in DER, the Distinguished Encoding Rules (study version) - you may not be able to rule out BER completely, which is a more loosely defined and therefore harder to parse structure). PKCS#8 defines BER unfortunately.

Oh, yeah, the public key is in uncompressed point format. Don't forget to strip away the 00 from the bit string.

Happy parsing.

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edited at2021-07-26

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