mharj/mharj-jwt-util

// http://stackoverflow.com/questions/18835132/xml-to-pem-in-node-js

export function rsaPublicKeyPem(modulusB64: string, exponentB64: string) {
    const modulus = Buffer.from(modulusB64, 'base64');
    const exponent = Buffer.from(exponentB64, 'base64');

    let modulusHex = modulus.toString('hex');
    let exponentHex = exponent.toString('hex');

    modulusHex = prepadSigned(modulusHex);
    exponentHex = prepadSigned(exponentHex);

    const modlen = modulusHex.length / 2;
    const explen = exponentHex.length / 2;

    const encodedModlen = encodeLengthHex(modlen);
    const encodedExplen = encodeLengthHex(explen);
    const encodedPubkey =
        '30' +
        encodeLengthHex(modlen + explen + encodedModlen.length / 2 + encodedExplen.length / 2 + 2) +
        '02' +
        encodedModlen +
        modulusHex +
        '02' +
        encodedExplen +
        exponentHex;

    return Buffer.from(encodedPubkey, 'hex').toString('base64');
}

export function buildCertFrame(der: string | Buffer): Buffer | string {
    if (!Buffer.isBuffer(der)) {
        return der;
    }
    const match = der.toString().match(/.{1,64}/g);
    if (!match) {
        throw new Error('Cert data error');
    }
    return Buffer.from('-----BEGIN RSA PUBLIC KEY-----\r\n' + match.join('\r\n') + '\r\n-----END RSA PUBLIC KEY-----\r\n');
}

function prepadSigned(hexStr: string) {
    const msb = hexStr[0];
    if (msb < '0' || msb > '7') {
        return '00' + hexStr;
    } else {
        return hexStr;
    }
}

function toHex(numberValue: number) {
    const nstr = numberValue.toString(16);
    if (nstr.length % 2) {
        return '0' + nstr;
    }
    return nstr;
}

// encode ASN.1 DER length field
// if <=127, short form
// if >=128, long form
function encodeLengthHex(n: number) {
    if (n <= 127) {
        return toHex(n);
    } else {
        const nHex = toHex(n);
        const lengthOfLengthByte = 128 + nHex.length / 2; // 0x80+numbytes
        return toHex(lengthOfLengthByte) + nHex;
    }
}