CoseToKey()

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Converts a COSE key into a usable Java security key. COSE (CBOR Object Signing and Encryption) is the key format used by WebAuthn/passkeys — when a user registers a passkey, the browser sends their public key in COSE format.

Accepts either a struct (e.g. from CborDecode()) or raw CBOR bytes. Returns a key pair struct with public (and private if the COSE key includes private material). Supports EC (P-256, P-384, P-521) and Ed25519 keys.

Requires Extension: Crypto Extension

CoseToKey( coseKey=any );

Returns: Struct

Argument Description
coseKey
any, required 		edit

COSE key as a struct (from CborDecode) or raw CBOR-encoded binary

Usage Notes

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COSE key format: COSE keys use integer keys to identify fields (defined in RFC 9052):

  • 1 = key type (2 for EC, 1 for OKP/EdDSA)
  • 3 = algorithm (-7 for ES256, -35 for ES384, -36 for ES512, -8 for EdDSA)
  • -1 = curve (1 for P-256, 2 for P-384, 3 for P-521, 6 for Ed25519)
  • -2 = x coordinate (binary)
  • -3 = y coordinate (binary, EC only)
  • -4 = private key (binary, only if private material is present)

You don't need to construct these structs yourself — they come from CborDecode() when decoding WebAuthn authenticator data, or from KeyToCose() when converting existing keys.

Return value: Always returns a struct with a public key. If the COSE key contains private material (key -4), the struct also includes a private key.

WebAuthn: The vast majority of passkeys use EC P-256 (ES256), but Ed25519 support is growing. P-384 and P-521 are rare in practice but supported.

Examples

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// Roundtrip: generate a key pair, convert to COSE and back
keyPair = GenerateKeyPair( "P-256" );
cose = KeyToCose( keyPair );
keys = CoseToKey( cose );

// The roundtripped key works for signature verification
sig = GenerateSignature( "test data", keyPair.private );
isValid = VerifySignature( "test data", sig, keys.public ); // true

// CoseToKey also accepts raw CBOR bytes — it decodes them internally
cborBytes = CborEncode( cose );
keys = CoseToKey( cborBytes );
// Same result as CoseToKey( CborDecode( cborBytes ) )

// Ed25519 keys work too
edKp = GenerateKeyPair( "Ed25519" );
edCose = KeyToCose( edKp );
edKeys = CoseToKey( edCose );
sig = GenerateSignature( "test data", edKp.private );
isValid = VerifySignature( "test data", sig, edKeys.public ); // true

// WebAuthn registration: extract the public key from authenticator data
attestationObject = CborDecode( Base64UrlDecode( response.attestationObject ) );
authData = attestationObject.authData;
// Parse authData to extract the COSE key bytes (after rpIdHash, flags, counter, credId)
coseKeyStruct = CborDecode( coseKeyBytes );
keys = CoseToKey( coseKeyStruct );
publicKey = keys.public;
// Store KeyToPem( publicKey ) alongside the credential ID

// WebAuthn authentication: verify a passkey signature
isValid = VerifySignature(
	data = signedData,
	signature = Base64UrlDecode( response.signature ),
	publicKey = storedPublicKeyPem,
	algorithm = "SHA256withECDSA"
);

See also