dertoraw.swift

static func convertDERtoRAW(_ signature: Data) throws -> Data {
    // Check minimum length and sequence tag
    guard signature.count >= 2, signature[0] == 0x30 else {
        throw JWTSigningError.signatureConversionFailed
    }
    
    // Verify sequence length
    let sequenceLength = Int(signature[1])
    guard signature.count >= sequenceLength + 2 else {
        throw JWTSigningError.signatureConversionFailed
    }
    
    var index = 2
    
    // Extract R value
    guard index < signature.count, signature[index] == 0x02 else {
        throw JWTSigningError.signatureConversionFailed
    }
    index += 1
    
    guard index < signature.count else {
        throw JWTSigningError.signatureConversionFailed
    }
    let rLength = Int(signature[index])
    index += 1
    
    guard index + rLength <= signature.count else {
        throw JWTSigningError.signatureConversionFailed
    }
    var rValue = signature[index..<(index + rLength)]
    
    // Handle DER leading zero byte if present
    if rValue.first == 0x00 {
        rValue = rValue.dropFirst()
    }
    
    index += rLength
    
    // Extract S value with same checks
    guard index < signature.count, signature[index] == 0x02 else {
        throw JWTSigningError.signatureConversionFailed
    }
    index += 1
    
    guard index < signature.count else {
        throw JWTSigningError.signatureConversionFailed
    }
    let sLength = Int(signature[index])
    index += 1
    
    guard index + sLength <= signature.count else {
        throw JWTSigningError.signatureConversionFailed
    }
    var sValue = signature[index..<(index + sLength)]
    
    // Handle DER leading zero byte if present
    if sValue.first == 0x00 {
        sValue = sValue.dropFirst()
    }
    
    // Create fixed-length 32-byte components
    var rawSignature = Data(count: 64)
    
    // Copy R value (right-aligned in first 32 bytes)
    let rBytes = Array(rValue.suffix(min(32, rValue.count)))
    let rStart = 32 - rBytes.count  // Right align
    rawSignature.replaceSubrange(rStart..<(rStart + rBytes.count), with: rBytes)
    
    // Copy S value (right-aligned in second 32 bytes)
    let sBytes = Array(sValue.suffix(min(32, sValue.count)))
    let sStart = 64 - sBytes.count  // Right align
    rawSignature.replaceSubrange(sStart..<(sStart + sBytes.count), with: sBytes)
    
    return rawSignature
}