Sample code for 30+ languages & platforms
Swift

Duplicate openssl smime -decrypt -in some_file.dat.enc -binary -inform DER -inkey private.key -out some_file.dat

See more OpenSSL Examples

Demonstrates how to decrypt binary DER that was encrypted using the following openssl command:
openssl smime -encrypt -binary -aes-256-cbc -in some_file.dat -out some_file.dat.enc -outform DER cert.crt

Chilkat Swift Downloads

Swift

func chilkatTest() {
    var success: Bool = false

    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    // Duplicates the following openssl command:
    // openssl smime -decrypt -in hello.txt.enc -binary -inform DER -inkey private.key -out hello.txt

    let cert = CkoCert()!
    success = cert.load(fromFile: "qa_data/openssl/EE.cer")
    if success == false {
        print("\(cert.lastErrorText!)")
        return
    }

    let bd = CkoBinData()!
    success = bd.loadFile(path: "qa_data/openssl/EE.key")
    // Assuming success..

    let privKey = CkoPrivateKey()!
    success = privKey.loadAnyFormat(privKeyData: bd, password: "")
    if success == false {
        print("\(privKey.lastErrorText!)")
        return
    }

    let crypt = CkoCrypt2()!
    success = crypt.setDecryptCert2(cert: cert, key: privKey)
    if success == false {
        print("\(crypt.lastErrorText!)")
        return
    }

    crypt.cryptAlgorithm = "PKI"
    success = crypt.ckDecryptFile(srcFile: "qa_data/openssl/hello.txt.enc", destFile: "qa_output/hello.txt")
    if success == false {
        print("\(crypt.lastErrorText!)")
        return
    }

    print("Success.")

}