Serializing Keys

Import and export keys in raw bytes, PEM, DER, and other standard formats.

Overview

Key serialization comes up in wallet storage, protocol interop (EC PRIVATE KEY / PRIVATE KEY PEM envelopes, X.509 SubjectPublicKeyInfo DER), and on-the-wire Bitcoin/Lightning witness data. The sections below cover each supported format in the order you will typically encounter them.

Raw Bytes

The most common serialization. Use dataRepresentation to export and init(dataRepresentation:format:) to import:

// Export (33 bytes compressed, 65 bytes uncompressed)
let keyData = publicKey.dataRepresentation

// Import with explicit format
let restored = try P256K.Signing.PublicKey(
    dataRepresentation: keyData,
    format: .compressed
)

Private keys are always 32 bytes:

let privKeyData = privateKey.dataRepresentation  // 32 bytes
let restored = try P256K.Signing.PrivateKey(dataRepresentation: privKeyData)

PEM Encoding

Import keys from PEM-encoded strings. Both SEC1 (EC PRIVATE KEY) and PKCS#8 (PRIVATE KEY) formats are supported:

let pemString = """
-----BEGIN EC PRIVATE KEY-----
MHQCAQEEIBXwHPDpec6b07GeLbnwetT0dvWzp0nV3MR+4pPKXIc7oAcGBSuBBAAK
oUQDQgAEt2uDn+2GqqYs/fmkBr5+rCQ3oiFSIJMAcjHIrTDS6HEELgguOatmFBOp
2wU4P2TAl/0Ihiq+nMkrAIV69m2W8g==
-----END EC PRIVATE KEY-----
"""
let privateKey = try P256K.Signing.PrivateKey(pemRepresentation: pemString)

Public keys use the PUBLIC KEY PEM type:

let publicKey = try P256K.Signing.PublicKey(pemRepresentation: publicKeyPEM)

DER Encoding

Import from DER-encoded binary data:

let privateKey = try P256K.Signing.PrivateKey(derRepresentation: derBytes)
let publicKey = try P256K.Signing.PublicKey(derRepresentation: derBytes)

ECDSA signatures also support DER:

let signature = try P256K.Signing.ECDSASignature(derRepresentation: derData)
let derBytes = signature.derRepresentation

Compressed vs Uncompressed

Specify the format when creating keys:

// Compressed (default): 33-byte public key with 0x02 or 0x03 prefix
let compressed = try P256K.Signing.PrivateKey(format: .compressed)
compressed.publicKey.dataRepresentation.count  // 33

// Uncompressed: 65-byte public key with 0x04 prefix
let uncompressed = try P256K.Signing.PrivateKey(format: .uncompressed)
uncompressed.publicKey.dataRepresentation.count  // 65

Convert a compressed public key to its uncompressed form:

let fullBytes = compressedPublicKey.uncompressedRepresentation  // 65 bytes

X-Only Keys

Schnorr signatures (BIP-340) use 32-byte x-only public keys with implicit even parity:

let schnorrKey = try P256K.Schnorr.PrivateKey()
let xonlyBytes = schnorrKey.xonly.bytes  // [UInt8], 32 bytes

Convert between full public keys and x-only:

// Full key to x-only
let xonly = ecdsaPublicKey.xonly

// X-only back to full key (original parity preserved)
let fullKey = P256K.Signing.PublicKey(xonlyKey: xonly)

Format Comparison

Format	Size	Prefix	Use Case
Compressed	33 bytes	`0x02` / `0x03`	Default, blockchain storage
Uncompressed	65 bytes	`0x04`	Legacy compatibility
X-only	32 bytes	None	BIP-340 Schnorr, Taproot
DER (private)	~118 bytes	ASN.1	Interoperability
PEM (private)	~227 bytes	Base64 + header	Human-readable storage