ios - 快速将 ECPublicKey(SecKey) 转换为 PEM 字符串

Question

我想将 ECPublicKey 转换为 PEM 格式，如下所示

"-----BEGIN PUBLICKEY-----MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEo0WcFrFClvBE8iZ1Gdlj+mTgZoJWMNE3kDKTfRny2iaguwuSYxo+jCnXqzkR+kyFK9CR3D+pypD1sGb1BnfWAA==-----ENDPUBLIC KEY-----"

1. 生成的 ECPublicKey， key 类型为“ECC/ECDSA/spec256r1”。下面是打印内容

<SecKeyRef curve type: kSecECCurveSecp256r1, algorithm id: 3, keytype: ECPublicKey, version: 4, block size: 256 bits, y:B6EEBB3791933312E10BD7C3D65C950AB7E1C325BCDB57A8663EB7B1E29BFA77, x:1D41FDAD2137316D415B117227BCC465566E56A54E7B2B9B3A4B66C15A067611,addr: 0x7f818850f6d0>

据我所知，PEM 格式只不过是带有页眉和页脚的 Base64 编码字符串。尝试了下面的代码，但没有成功

var error:Unmanaged<CFError>?

guard let cfdata = SecKeyCopyExternalRepresentation(publicKey, &error)
else { return }

let data:Data = cfdata as Data

let b64String = data.base64EncodedString()

感谢此处的任何帮助

Answer 1

首先转换为 ASN.1 格式:(请参阅 ASN.1 Decoder 中的解码示例)

let publicKeyData: CFData = ...
let ecHeader: [UInt8] = [
    /* sequence          */ 0x30, 0x59,
    /* |-> sequence      */ 0x30, 0x13,
    /* |---> ecPublicKey */ 0x06, 0x07, 0x2A, 0x86, 0x48, 0xCE, 0x3D, 0x02, 0x01, // (ANSI X9.62 public key type)
    /* |---> prime256v1  */ 0x06, 0x08, 0x2A, 0x86, 0x48, 0xCE, 0x3D, 0x03, 0x01, // (ANSI X9.62 named elliptic curve)
    /* |-> bit headers   */ 0x07, 0x03, 0x42, 0x00
]

var asn1 = Data()
asn1.append(Data(ecHeader))
asn1.append(publicKeyData as Data)

然后进行 Base64 编码并添加 PEM 页眉和页脚:

let encoded = asn1.base64EncodedString(options: .lineLength64Characters)
let pemString = "-----BEGIN PUBLIC KEY-----\r\n\(encoded)\r\n-----END PUBLIC KEY-----\r\n"